CA2053398C - Optical analytical instrument and method - Google Patents
Optical analytical instrument and methodInfo
- Publication number
- CA2053398C CA2053398C CA002053398A CA2053398A CA2053398C CA 2053398 C CA2053398 C CA 2053398C CA 002053398 A CA002053398 A CA 002053398A CA 2053398 A CA2053398 A CA 2053398A CA 2053398 C CA2053398 C CA 2053398C
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- Prior art keywords
- fluid
- calibration
- optical
- instrument
- reading
- Prior art date
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
- G01N21/85—Investigating moving fluids or granular solids
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/27—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands using photo-electric detection ; circuits for computing concentration
- G01N21/274—Calibration, base line adjustment, drift correction
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
- G01N21/49—Scattering, i.e. diffuse reflection within a body or fluid
- G01N21/53—Scattering, i.e. diffuse reflection within a body or fluid within a flowing fluid, e.g. smoke
- G01N21/532—Scattering, i.e. diffuse reflection within a body or fluid within a flowing fluid, e.g. smoke with measurement of scattering and transmission
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2201/00—Features of devices classified in G01N21/00
- G01N2201/08—Optical fibres; light guides
- G01N2201/0826—Fibre array at source, distributing
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2201/00—Features of devices classified in G01N21/00
- G01N2201/08—Optical fibres; light guides
- G01N2201/0833—Fibre array at detector, resolving
Abstract
Optical analytical instruments to determine a physical parameter of a fluid, and methods of operation of such instruments, are shown. These employ a source of suitable optical radiation, a detector means, means defining a zone for the fluid, means defining an optical path from the radiation source through the fluid zone to the detector means, and reading and control circuitry. At least two readings of optical energy that has been influenced by the fluid are taken without there having occurred substantial change to the fluid. During one of the readings a filter of known absorbance is included in the optical path so that the respective reading represents a calibration reading. A
calibration-value-determining means is constructed first to compare, effectively, the two readings to remove the effect of the fluid from the value of the calibration reading, second, to compare, effectively the residual value of the calibration reading to a known value based on the known absorbance of the filter, and, third, on the basis of the second comparison, to make a calibration adjustment based on values derived while the fluid sample remained unchanged in the fluid zone. Novel software implementations of the determinations are shown. Specific examples of instruments and methods implementing these features shown are a dual lamp, no-moving part in-line spectrophotometer, a no-moving part reflection colorimeter capable of on-line or off-line operation, a scatter or fluorescence detecting implementation and a turbidimeter.
calibration-value-determining means is constructed first to compare, effectively, the two readings to remove the effect of the fluid from the value of the calibration reading, second, to compare, effectively the residual value of the calibration reading to a known value based on the known absorbance of the filter, and, third, on the basis of the second comparison, to make a calibration adjustment based on values derived while the fluid sample remained unchanged in the fluid zone. Novel software implementations of the determinations are shown. Specific examples of instruments and methods implementing these features shown are a dual lamp, no-moving part in-line spectrophotometer, a no-moving part reflection colorimeter capable of on-line or off-line operation, a scatter or fluorescence detecting implementation and a turbidimeter.
Description
OPTICAL ANALYTICAL INSTRUMENT AND METHOD
Backqround of the Invention The present invention concerns photometers, i.e.
optical analytical instruments that employ optical energy to determine physical parameters of fluids such as liquids or gasses found in effluents and in industrial processes.
There are many instances in which it would be useful to have a relatively low cost analytical instrument that could be depended upon to monitor a process or environmental condition at a desired level of accuracy and which did not require highly trained personnel for its operation and maintenance. There is a special need for a low cost instrument in the case of monitoring waste water and effluent gasses before release to the environment. The present invention addresses these needs, and provides general improvements in photometers as well.
Summary of the Invention In accordance with the present inventionl there is provided an optical analytical instrument for quantitative measurements capable of producing amplitude readings based upon the effect of a fluld upon a selected fixed wavelength in a bearn of optical radiation to determine a quantitative physical parameter of the fluid, said instrument comprlsing optical source means of suitable optical radiation, detector means responsive to said selectlve fixed wavelength within a predetermined amplitude measurernent range of the instrument, a fluld zone for a sample fluid, optical path means from said optlcal source means throuqh said fluid zone to said detector means, reading and control circuitry for taking from said detector means at l.east two amplitude readings of optical energy of said selected fixed wavelength that result from the influence of the fluid on sa1.d optical radiation without there having occurred substantial change to the fluid, means during one of said readings for incl.uding ln said optlcal path a fi].ter of known absorbance that absorbs a predetermined portion of the energy at said fixed wavelength, said portion representin~ a substantial fraction of said amplitude measurement range of the instrument so that the respective reading represents an ampl.i.tude Galibration reading within sald measurement range of the instrument, calibration-value-determining means operable while a sample of said fluid remains unchanged in sald fll].id zone, first to compare, effectively, said two amplitude readings at said selected fixed wavelength to remove said effect of said fluid from the value of said callbration reading, second, to compare, effectively the residl~al value of said calibration reading to a known value based on the known ahsorbance of said filter, and, thlrd, on the basis of said second comparison, to determine an amplitude calibration value for said instrument based on a.mplitude values derived while said fluid sample remains unchan~ed in said fluid zone.
In accordance with the present invention, there is also provided an optical analytical instrument capable of producing readings based upon the effect of a fluid upon a beam of optical energy to determine a physical parameter of - la -6041~-2180 - 205~98 the fluid, said instrument comprising optical source means of suitable optical radiatlon, detector means, a fluid zone for a sample fluid, optical path means from sald optical source means through said fluld zone to sald detector means, readlng and control clrcuitry for taking from sald detector means at least two readlngs of optlcal energy that has been lnfluenced by the fluid wlthout there having occurred substantlal change to the fluid, means during one of said readings for including in said optical path a filter of known absorbance so that the respective reading represents a callbratlon readlng, calibratlon-value-determining means constructed first to compare, effectively, said two readings to remove sald effect of sald fluid from the value of said callbration reading, second, to compare effectively the residual value of said calibration readlng to a known value based on the known absorbance of sald fllter, and, third, on the basis of said second comparison, to make a calibration adiustment based on values derived while said fluld sample remained unchanged in said fluid zone, wherein sald optical source means comprise two separately en.ergizable radiation emitters, said optical path means from said source having two branches, one defining a respectlve optical path from each radiation emitter, with both branches transmitting via a substantially common path through said sample, one of said emitters, denoted the "measure" emitter, energiæed by said control means during the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energized by sald control means during the taking of a callbration reading, sald filter - lb -~~ 60412-2180 . . , 20533g8 of known absorbance being included in the optical path from sald calibration emitter to said fluid zone.
In accordance w.ith the present lnventlon, there is further provided an optical analytical instrument capable of producing readings based upon the effect of a fluld upon a beam of optical energy to determine a physical parameter of the fluid, said instrument compriæing optical source means of suitable optical radiation, detector means, a fluid zone for a sample fluid, optlcal path means from sald radiation source through said fluid zone to sald detector means, reading and control circuitry for taking from sald detector means at least two readings of optical energy that has been influenced by the fluid wlthout there having occurred substantial change to the fluid, means during one of said readlngs for includlng in said optical path a fllter of known absorbance so that the respective reading represents a calibration reading, calibration-value-determining means constructed first to compare, effectively, said two readings to remove sald effect of said fluld from the value of said calibration reading, second, to compare, effectively the residual value of said callbration reading to a known value based on the known absorbance of said filter, and, third, on the basis of said second comparison, to make a callbration adjustment based on values derived while said fluid sample remained unchanged in said fluld zone, wherein said optical source means comprises two separately energizable radiation emltters, said optical path from said source havlng two branches, one defining a respective optical path from each radlation emitter, with both -- l.c --B 6o4l2-2l8o 2~53398 branches transmltting via a substantially common path through sald sample, one of said emitters, denoted the "measure"
emltter, energized by said control means durlng the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energlzed by said control means during the taking of a ca]lbration reading, said filter of known absorbance being included in the optical path from said calibration emitter to said fluid zone, and wherein said instrument has no moving parts for accomplishing calibration, said control means constructed and arranged such that normal and calibration readings for use by said instrument in establishing said calibration ad~ustment are effected by selective energization of said emitters during operation.
In accordance with the present invention, there is further provided a method of calibrating an optical analytical instrument capable of producing amplitude readings based upon the effect of a fluid upon a selected fixed wavelength in a beam of optical radiation to determine a quantitative physical parameter of the fluid, comprising providing an optical source means, optical path means, and optical detector means responsive to said selected fixed wavelength, taking from said detector means at least two amplitude readings at said selected fixed wavelength and within a predetermined amplitude measurement range of the instrument, of optical energy that result from the influence of the fluid on the optical radiation in a fluid zone without there having occurred substantial change to the fluid, including in said optical path means a filter of known absorbance that absorbs a - ld -~.~ 60412-2180 predetermlned portion of the energy at sald flxed wavelength that represents a substantlal fractlon of sald amplltude measurement range of the lnstrument so that one of sald amplltude readlngs represents a callbratlon readlng wlthln sald measurement range of the lnstrument, flrst, comparlng, effectlvely, sald two amplltude readlngs to remove sald effect of sald fluld from the value of sald callbratlon readlng, second, comparlng, effectlvely, the resldual value of sald calibratlon reading to a known value based on the known absorbance of sald fllter, and, on the basls of sald second comparlson, determlnlng an amplltude callbratlon value for the lnstrument based on values derlved whlle sald fluld sample remalns unchanged in sald fluld zone.
In accordance wlth the present lnventlon, there ls further provlded a method of callbratlng an optical analytlcal lnstrument capable of produclng readlngs based upon the effect of a fluid on a beam of optical energy to determlne a physlcal parameter of the fluld, comprlslng provldlng optical radlatlon source rneans that lncludes a "measure" emltter and a "callbratlon" emltter separately energlzable by control means, optlcal path means, and optlcal detector means, taklng from sald detector means at least two readlngs of optlcal energy, that has been influenced by the fluld in a fluid zone without there havlng occurred substantlal change to the fluld, by energizlng said "measure" emitter by said control means during the taklng of a normal measurement, and by energlzlng sald "callbratlon" emitter by said control means during the taking of a calibratlon readlng whlle a fllter of known absorbance ls - le -B
- 20~3~38 included in sald optlcal path means from said callbratlon emltter to sald fluld, flrst, comparlng, effectlvely, sald two readlngs to remove sald effect of sald fluld from the value of sald callbratlon readlng, second, comparlng, effectlvely, the resldual value of sald callbratlon readlng to a known value based on the known absorbance of sald fllter, and, on the basls of sald second comparlson, maklng a callbratlon ad~ustment based on values derlved whlle said fluld sample remalned unchanged in sald fluld zone.
In accordance wlth the present lnventlon, there ls further provlded a rnethod of callbratlng an optical analytical lnstrument capable of produclng readlngs based upon the effect of a fluid on a beam of optlcal energy to determlne a physlcal parameter of the fluld comprlslng provldlng optlcal radlatlon source means that lncludes a "measure" emltter and a "callbratlon" emltter separately energlzable by control means, optlcal path means, and optlcal detector means, taklng from sald detector means at least two readlngs of optlcal energy, that has been influenced by the fluld ln a fluld zone wlthout there havlng occurred substantlal change to the fluld, by energlzing sald "measure" emltter by sald control means durlng the taklng of a normal measurement, and by energlzlng a "calibratlon" emitter by sald control means durlng the taking of a calibration reading while a filter of known absorbance ls included in the optlcal path from sald callbratlon emitter to said fluid, flrst, comparlng, effectively, sald two readlngs to remove said effect of said fluld from the value of said callbratlon readlng, second, comparlng, effectlvely, the - lf -2~53398 resldual value of said calibratlon readlng to a known value based on the known absorbance of sald fllter, and, on the basis of said second comparison, maklng a callbratlon ad~ustment based on values derived whlle said fluld sample remained unchanged in said fluid zone, wherein said calibration ad~ustment is effected by selective energization of said emitters during operation without moving any part of said lnstrument.
According to the invention a photometer or optical analytical instrument is provided that is relatlvely low cost to construct but still has the means to correct its calibration or report the accuracy of its measurements during use, without need for a skilled operator.
Such an lnstrument is realized according to the invention by means that permit construction with no moving parts, in implementations that are relatively simple, compact, easy to assemble, and requlres no alignment.
A principal feature of the invention is an optical analytical instrument capable of producing readings based upon the effect of a beam of optical energy on a fluid to determine a physical parameter of the fluid, the instrument comprising a source of suitable optical radiation, a - lg --detector means, means defining a zone for the fluid, means defining an optical path from the radiation source through the fluid zone to the detector means, reading and control circuitry for taking from the detector means at least two readings of optical energy that has been influenced by the fluid without there having occurred substantial change to the fluid, means during one of the readings for including in the optical path a filter of known absorbance so that the respective reading represents a calibration reading, calibration-value-determining means constructed first to compare, effectively, the two readings to remove the effect of the fluid from the value of the calibration reading, second, to compare, effectively the residual value of the calibration reading to a known value based on the known absorbance of the filter, and, third, on the basis of the second comparison, to make a calibration adjustment based on values derived while the fluid sample remained unchanged in the fluid zone.
Preferred embodiments of the aspect of the invention have the following features.
The optical radiation source comprises two separately energizable radiation emitters, the optical path from the source having two branches, one defining a respective optical path from each radiation emitter, with both branches transmitting via a substantially common path through the sample, one of the emitters, denoted the "measure" emitter, energized by the control means during the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energized by the control means during the taking of a calibration reading, the filter of known absorbance being included in the optical path from the calibration emitter to the fluid zone.
20~3398 The instrument has no moving parts for accomplishing calibration, the control means constructed and arranged such that normal and calibration readings for use by the instrument in establishing the calibration adjustment are effected by selective energization of the emitters during operation.
The optical analytical instrument is in the form of an in-line analytical instrument in which the means defining the fluid zone comprises a conduit for at least part of a process stream through which process fluid flows, the process stream having a characteristic maximum rate at which the parameter can change, and the reading and control means constructed, in relation to the process stream, to take the two readings within such a short time interval that there is insufficient passage of time to allow substantial change to occur in the parameter of the fluid in the process stream.
The optical analytical instrument is in the form of a titration chamber constructed to be filled by a liquid process stream and, after titration, flushed only by the process stream.
The optical analytical instrument is in the form of a titration chamber constructed to receive a known repeatatable small sample of process stream liquid and after titration said chamber is constructed to be flushed with a suitable liquid.
The optical analytical instrument is in the form of a colorimeter.
The optical analytical instrument is in the form of a colorimeter, the portion of the optical path in the fluid zone being defined by a probe which includes a reflector whereby energy which enters the fluid zone is reflected to pass back through the fluid zone to the detector means.
20~3398 The optical analytical instrument is in the form of an absorption spectrophotometer, the detector means comprising at least two detectors sensitized to respectiv~ly different wavelengths, one wavelength corresponding to a characteristic absorption of a component of interest in the fluid zone, the detected signal varying with the concentration of the component, and the other wavelength being a reference wave-length for which the component of interest has relatively low absorbance.
The optical analytical instrument has the detector means positioned to receive scattered radiant energy from the sample, preferably the filter belng positioned to filter the scattered radiant energy and preferably the detector is postioned and adapted to receive Raleigh scattering or fluorescence.
The optical analytical instrument is in the form of a turbidimeter, preferably with an optical reference path bypassing the sample.
Also preferred embodiments of the instrument have the following features.
The means defining the optical path from the radiation source to the fluid zone is comprised of optical fibers.
The means to make a calibration adjustment is constructed to calibrate the instrument, preferably the adjustment comprising means to adjust the sensitivity of the instrument and means to adjust the zero reference of the instrument.
The optical analytical instrument including means to provide the known value in the form of means-to store a - -reading of the detector means taken with the filter of known absorbance in place and with the fluid zone empty.
Another principal feature of the invention is a method of calibrating an optical analytical instrument capable of producing readings based upon the effect of a beam of optical energy on a fluid to determine a physical parameter of the fluid, comprising taking from a detector means at least two readings of optical energy that has been influenced by the fluid in a fluid zone without there having occurred substantial change to the fluid, including in the optical path a filter of known absorbance so that one of the readings represents a calibration reading, first, comparing, effectively, the two readings to remove the effect of the fluid from the value of the calibration reading, second, comparing, effectively, the residual value of the calibration reading to a known value based on the known absorbance of the filter, and, on the basis of the second comparison, making a calibration adjustment based on values derived while the fluid sample remained unchanged in the fluid zone.
Preferred embodiments of this aspect of the invention have the following features.
The optical radiation source comprises two separately energizable radiation emitters, further comprising energizing a "measure" emitter by the control means during the taking of a normal measurement, and energizing a "calibration" emitter, by the control means during the taking of a calibration reading while the filter of known absorbance is included in the optical path from the calibration emitter to the fluid.
The calibration adjustment is effected by selective energization of the emitters during operation without moving any parts of the instrument.
The fluid flows in a process stream further comprising taking two readings by the control means within 2~3398 such a short time interval that there is insufficient passage of time to allow substantial change to occur in the parameter of the fluid in the process stream.
The method includes the steps of filling a titration chamber by a liquid process stream and, after titration, flushing the titration chamber only by the process stream.
The method includes the steps of filling a titration chamber by a liquid process stream and, after titration, flushing the titration chamber by a suitable liquid.
The calibration is performed for a colorimeter, an absorption spectrophotometer, a turbidimeter, or an instrument capable of detecting fluorescence or Raleigh scattering.
The step of making a calibration adjustment calibrates the instrument and preferably adjusts the zero reference of the instrument.
The method includes the step of storing a reading of the detector means taken with the filter of known absorbance in place and with the fluid zone empty.
The step of making a calibration adjustment includes updating a current filter absorbance value.
The calibration adjustment step is periodically repeated and the current filter value is successively updated during these repetitions.
Past trends in the current absorbance value are taken into consideration in updating the current absorbance value.
Another principal feature of the invention is apparatus or method wherein, respectively, the instrument is constructed or the method is conducted to measure~
concentration or the like, C, according substantially to the relationship:
8(n) C8td X (A~(n) ~ Az) 20a3398 in which the values are substantially as defined in the following specification.
Description of the Preferred Embodiments FIG. 1 is a schematic illustration of a dual lamp spectrophotometer that has no moving parts.
FIG. 2 is a plot of absorbance against wavele~gth in the spectral region of interest ~or a sample to bc mc~urcd.
FIG. 3 is a flowchart illustrating an initialization routine for the embodiment of FIG. 1.
FIG. 4 is a flowchart illustrating a sensitivity correction routine for the embodiment of FIG. 1.
FIG. 5 is a flowchart illustrating a zero correction routine for the embodiment of FIG. 1.
FIG. 6 is an idealized response plot for the instrument of the invention.
FIG. 7 is a set of graphs showing the voltage applied to the sample and calibration lamps and the concentration of the sample against time for a single calibration operation.
FIG. 8 is a plot of absorbance against wavelength for a measurement situation where the baseline absorption is sloping.
FIG. 9 is a plot of absorbance against wavelength for a measurement situation where the baseline absorption is sloping, showing two reference wavelengths.
FIG. 10 is a schematic illustration of a colorimeter embodiment.
FIG. 11 is a schematic illustration of an embodiment of the invention that measures concentration by detecting scattering effects within the sample.
FIG. 12 is a schematic illustration of a turbidimeter embodiment.
20~33398 Referring to FIG. 1, a dual lamp spectrophotometer for monitoring a continuous flow includes a sample lamp or emitter 2 and a calibration lamp or emitter 4 that are each linked to a central processing unit 22 or other form of calbration-value-determining and control circuitry via a control line 26. The sample and calibration lamps are located in front of respective fiber optic branches 14a and 14b which join to form a random-ordered first fiber optics bundle 14 which provides a common path for energy from the two branches. A calibration filter 10 is disposed between lamp 4 and its fiber optic branch. The calibration filter 10 has a known absorbance at the wavelength to be measured.
The fiber optics bundle leads to a sample cell 12 or other zone for confining a sample 18. Also adjacent to the sample cell, behind the first fiber optics bundle, is a second fiber optics bundle 16 which provides a common path for energy transmitted through the cell. This second fiber optics bundle is split into branches 16a and 16b to provide optical paths through a measure filter 7 and a reference filter 9.
A measure detector 6 and a reference detector 8 are placed behind the measure and reference filters, respectively. The outputs of these detectors are connected to a data acquisition module 20 that is, in turn, connected to the central processing unit 22. A display 24 is also connected to the central processing unit.
The lamps or emitters are capable of emitting optical radiation at a wavelength that is absorbed (or, in other ebodiments, otherwise influenced) by the substance to be detected or monitored, where "optical" radiation is defined, for the purposes of this application, as radiation in the visible, infrared and ultraviolet regions of the spectrum.
_ 60412-2180 In operatlon, a contlnuous flow of a sample fluld passes through the cell, whlch acts as condult for at least a part of a process stream. Alternatlvely, the cell may be fllled wlth a statlc sample of a fluld whlch may be replaced from tlme to tlme.
The former of these methods enables the lnstrument to be used, e.g., ln ln-llne monltorlng of waste water or lndustrlal processes ln whlch the fluld flows steadlly through the cell. The latter enables the lnstrument to be used, e.g., ln on-llne monltorlng, ln whlch fluld ls repeatedly drawn from the process llne and used to provlde samples, e.g., for tltratlon or colorlmetrlc measurement, and to provlde the flushlng medlum.
An lnstrument wlth whlch present lnventlon ls useful ls shown ln Unlted States Patents 4,911,891 and 4,910,151, both entltled "Sample Monltorlng Instrument for On-Llne Appllcatlon".
There are two types of operatlons that may be performed wlth thls apparatus; "measure" operatlons and "callbratlon"
operatlons. The flrst step ln a measure operatlon ls for the central processlng unlt 22 to cause the sample lamp 2 to lllumlnate. The sample lamp emlts polychromatlc llght that passes from branch 14a through the flrst flber optlcs bundle 14 and lnto the cell. In the cell, the llght lnteracts wlth the sample and certaln characterlstlc wavelengths of the llght are absorbed by the constltuent materlals ln the sample.
A portlon of the unabsorbed llght passes along the common path through the cell lnto the second flber optlcs bundle 16 and ` -~ 60412-2180 - ~a -exlts the bundle ln two beams. The flrst beam passes through a measure fllter 7 that attenuates all wavelengths except that of the spectral llne of lnterest for the materlal belng monltored ln the spectrophotometer. The spectral llne typlcally corresponds to a characterlstlc absorption of the compound to be measured. The second beam passes through a reference filter 9 that attenuates all wavelengths except for a reference wavelength that is generally close to the measured wavelength but which does not correspond to a characteristic absorption line of the compound of interest. The relationship between the wavelength at the spectral line of interest and the reference wavelength are shown in FIG. 2. By using the two wavelengths in this manner, the effect of the illumination level of the lamps may be eliminated by performing a differential measurement using two detectors.
These measure and reference detectors 6, 8 detect the light passing through the measure and reference filters, respectively. Each of these detectors produces an electrical signal that has a magnitude related to the amount of light incident upon it and provides that signal to a data acquisition module 20. The response of the sample 18 to the measure wavelength may be determined by Vr A = log10 --Vm where A is the absorbance of the sample, V~ is the voltage of the measure detector 6 and Vr is the voltage on reference detector 8. The data acquisition module digitizes the voltage signals from the detectors and provides the digital values to the CPU for processing.
In normal "measure" operation the instrument provides readings for the substance of interest in the sample in the units desired by the user. Typically the reading for a sample is in terms of concentration, C" for example, percent or ppm. The concentration Cs(~) of sample 20533~8 -at time n is implemented in the present embodiment according to the relationship:
C~ (n) = Cstd X (A~ (n) A, ) where: AF
C~(~) = calculated nth sample concentration, C8td = concentration value corresponding to the known absorbance of the filter selected, A. (n) = measured absorbance value with nth sample in situ Az = previously measured and stored zero absorbance value AF = previously measured and stored filter absorbance value, all will be explained below. It will be observed that the above equation normalizes the sample concentration to the corresponding ratio of C~+d (the concentration represented by the absorbance of the filter) vs. the measured calibration filter absorbance value AF' thus taking advantage of updates as to any detected drift of the instrument that have been determined by prior calibration readings.
The routine implemented by the instrument in a normal measurement operation is shown in the first two blocks of Fig. 4.
The second operation, the "calibration" operation is performed with the sample 18 in place and may be performed for all measurements or at selected intervals. In this step, the central processing unit causes the calibration lamp 4 to illuminate while the sample lamp remains de-energized. The calibration lamp emits polychromatic light that passes through the calibration filter 10. The calibration filter absorbs light at the measurement wavelength to a degree which is precisely known, e.g. 30 20~3398 percent of the measurement range. It may also absorb some energy at the reference wavelength; this should be significantly less than the degree to which the measurement wavelength is absorbed.
Unabsorbed light from the calibration filter 10 passes into the first fiber optics bundle 14 and on through the sample cell where portions of it are absorbed by constituents of the sample being monitored. Unabsorbed portions of the light then pass into the second fiber optics bundle 16 and spectral portions are detected and corresponding values are provided to the central processing unit in the manner as they were in the measure operation.
The central processing unit divides the digitized measure value by its corresponding reference value and takes the logarithm of the result to generate an absorbance value for each measurement. This "calibration" reading is used in subsequent calibration calculations, as described below in connection with FIGS. 3-5. Basically, a prior "measure"
reading and a "calibration" reading, both taken with the same fluid sample present, are effectively compared to remove the effect of the fluid sample from the "calibration"
reading. Then the residual value is effectively compared to the known, i.e. previously determined, actual absorbance of the calibration filter. From this, or a series of such measurements, a calibration adjustment can be determined, or equivalently, a report may be generated as to the degree to which the instrument has drifted from the initial state.
The values given in Figs. 3-5 are defined as follows:
2û~3~98 A F - FILrER obaorbanca valua ua-d In calculatlona A FO - Inltlal abaorbanca volu- ~or FILTER
A F(n) - Calcualtod abaorbonc- valu- ot FILTER
wlth nth aampla In ~Itu A Fmln - Mtnlmum llmlt tor FILTER obaorbonc- volua A Fmax - Ma~lmum ll~lt ~or FILTER ab~orbanCO valv-A r~M(n)- Abaorbanco voluo o~ nth aampla wlth CALIeRATION mlttar A M - ~b~orbanco m o~ur~d A ~ax - Maxlmumn llmlt tor mcaaurod abaorbanc-a A S(n) - Abaorbanca valuo wlth nth aampl- In ~Itu A z - ZERO obaorbanca valua uaod In colculatlon-A zo - Inltlal abaorbanc- valu- tor ZERO concantratlan A Z(n) - Colculatod ab~orbanco valuo ~or ZERO
wlth nth Jam41- In altu A Z(n) - ~Inlmum llm~t ~or ZERO abaorbanc- valu~
A Z(n) - M~xlmum llmlt ~or ZERO abaorbanc- valuo C S(n) - Calculat-d nth aampla concantratlon C STD - Concontrotlon volua corroaponalna to FIETER aol-cted (Valu- ~ntoraa by oparator) When the instrument is first turned on, the central processing unit performs an initialization routine, as shown in FIG. 3. This initialization routine is performed 30 on an empty cell 12 or a cell filled with "zero" sample (sample containing "zero" concentration of the component of interest, e.g., for color in liquids, distilled water is used as a "zero" sample) in order to obtain an initial zero value for the instrument. Thus an initial "measure"
operation is performed by turning the measure lamp on 32, calculating and storing the resulting absorbance value for 2D~3398 zero sample present (Azo) 34 and turning the measure lamp off 36.
Likewise, an initial "calibration" operation for the filter is performed by turning the calibration lamp on 36 and performing a further measure operation with the circuitry to obtain an absorbance measurement, AM. The initial measured absorbance value of the calibration filter AFO, is then obtained by subtracting the absorbance value for zero Azo from the measured absorbance AM' this value AFO
then being stored.
Now, with the calibration lamp off 40, and the measure emitter on, a further measurement 40 is performed, the zero values AFO and Azo are respectively stored as the filter and absorbance values AF and A~ to be used in further calculations, and using these values the zero value of the scale reading instrument is set. Here the known absorption value of the calibration filter, i.e. the concentration level of the compound of interest that it represents, is used to standardize the readings of the instrument. This completes the initialization routine 44.
Once the initialization routine is complete, the instrument may be used to perform absorbance measurements, and to display these values on the display 24 or otherwise present them to the user or another piece of equipment.
These values may be expressed to the user as concentration values. After a certain period, which is user-definable, the instrument performs a self calibration.
The self calibration routine, which is &hown in flow diagram in FIGS. 4 and 5, includes a sensitivity correction routine that is used to adjust the sensitivity of the instrument and a zero correction routine that adjusts the zero value of the instrument. As may be seen in FIG. 6, a 20~3398 sensitivity change corresponds to a change in the gain of the instrument or ~y/~x, while a change in zero value corresponds to a change in the zero intercept b of the instrument.
S The sensitivity correction routine (FIG. 4) begins with a measure operation 50 that reads the absorbance A~
for sample n. The central processing unit calculates a corresponding sample concentration value using the formula shown at step 52 of FIG. 4 and displays it.
The self calibration routine then determines whether the calibration is within range by determining if the absorbance value measured for the current sample A~(n) added to the current filter value used in calculations Ap is within the range of measurement of the system 54. If it is not, it is probable that the operating range for the automatic calibration has been exceeded. In this case, calibration can not take place, and the central processor unit flags this condition to the user.
If the unit can be calibrated, the central processing unit turns off the measure lamp and turns on the calibration lamp 56 to obtain and store 58 an absorbance value for the sample and the filter together. From this value, the central processing unit computes a calculated absorbance value for the filter AF(n) 60. If this absorbance value is the same AF that currently is used 62, the system is properly calibrated, and the calibration routine is complete 76.
If the measured absorbance value of the calibration filter is different, then it is known that the instrument has drifted since the actual absorbance of the calibration - -filter itself is a known, fixed quantity. To correct for this instrument drift, the central processing unit generates 20~33~8 -a new current filter absorbance value AFI 64, taking into consideration past trends of the value. The new value will be determined by the average of a set of most recent values, unless there is an appreciable trend in one direction or the other, which might be caused by a system malfunction, for example, component failure.
The central processing unit then tests the new filter absorbance value to see if it is within the allowed range 60. If it is outside the range, the instrument displays an error indication 74 and ceases its attempt at calibration 76. If the new, adjusted, filter absorbance value AF is withln the proper range, it is stored for further use.
The central processing unit then reports to the user a sensitivity value ~S that indicates to the user that the calibration has shifted. This value is obtained from the equation in block 70 of FIG. 4. By comparison of the value of new and previous determinations of AF~ the performance history of the sensitivity of the instrument can be determined and reported. Execution of the "calibration"
operation proceeds 72 to the zero correction routine.
The zero correction routine, which is shown in flow diagram in FIG. 5, generates a zero value for use in further measurements. This routine begins with a calculation of a zero value computed by subtracting the current filter absorption value AF and the current measured sample absorbance A~n) from the measured absorbance of the filter and sample together A~(n) for the current sample 80. If this value is determined to be within an acceptable range of zero absorbance values, the current zero value is updated'''~'''''''''''' 84, the zero shift is reported to the operator 86, and the calibration routine is completed 90. If the value is outside of the acceptable range, an error condition is flagged 88.
Once the calibration routines are completed, the instrument is ready to perform further measurements, until it is recalibrated again. The above method may thus repetitively recalibrate the instrument by simply performing successive updates of the Ap variable.
It will be observed that one assumption inherent in the above discussion in respect of in-line operation, is that the flow of fluid must have an absorbance that is substantially constant during a measure operation and the following calibration operation (i.e., its characteristic maximum rate of change is small), or the calibration may be performed incorrectly. This is illustrated in FIG. 7, where lS it can be seen that between the measurement points To and T
when the detector measures unabsorbed light, the concentration of the solution must remain stable. It is found in practice that the time interval for making such measurements is sufficiently short that concentrations in many processes to be monitored do not vary significantly, so that valid readings in-line can be obtained.
For on-line analysis, i.e. when a sample is captured and retained in the sample cell, the same sample remains in the cell for both "measure" and "calibration" operations so that this is of no concern.
In some cases, constituents of the sample may cause the baseline absorption around the wavelength of interest to be skewed by an amount X, as shown in FIG. 8. This can cause the reference value to depend on the concentration of constituents that are not important to the measurement and thus affect the accuracy of measurements. In this situation, two or more reference detectors may be used, and 20~3398 their average value used to determine the effective reference value, as illustrated in FIG. 9.
By use of the instrument as described, the calibration of the instrument of Fig. 1 can be automatically checked and corrected during continuing operations without evacuating the sample or using any test samples. The system also contains no moving parts which contributes to compactness as well as to reliability and less need for maintenance. This instrument, therefore, is particularly well suited for use in rugged field applications such as waste water monitoring where dependable operation without the need for regular support by highly qualified technicians is important.
Furthermore, this optical apparatus and associated method are equally useful for measurements in the visible, infrared and ultra-violet regions of the spectrum, when supplied with the appropriate emitters, filters, and detectors.
In certain instances, it may be useful to replace the two lamps with a single source and a shutter mechanism or its equivalent thereby allowing the same source to illuminate the sample via the sample and filter paths. This requires the addition of a moving part that can be less reliable or more expensive, but may be warranted in certain situations. For example, when using an arc source or other embodiment in which the emitted radiant energy from the emitter source is optimized at its maximum, a single emitter and suitable means for switching between from an optical path with and without the calibration filter present can be employed.
Numerous other instruments may be manufactured to measure absorbance or other physical parameters using the calibration method of the invention.
20~3398 For example, FIG. lo shows a probe-based configuration suitable for use as a titrometer or a colorimeter. This instrument operates in much the same way as the spectrophotometer embodiment, with the exception that the light passes through the sample fluid 100 in the probe 102 and is reflected off of a reflecting surface 104 back through the probe before the resulting absorbance is measured. The sampling technique of the above-mentioned patent may be employed to especial advantage, or other techniques may be used.
An instrument for measuring concentration by detecting scattered radiation is shown in FIG. 11. This instrument includes an emitter, such as an ultraviolet arc source 110 that may emit light into a sample cell 118 and excite the sample 116 and cause a certain degree of scattering. Detectors directly opposite the incident path 122 of the exciting radiation and at right angles thereto 120 measure transmitted and scattered radiation, respectively. The ratio of the amplitude of detected scattered radiation and the transmitted scattered radiation is proportional to the concentration of the material being measured.
In this instrument, the calibration filter 112 of known absorbance that is translated into and out of the optical path of the detector provides the ability to perform alternate "measure" operations and "calibration" operations.
This instrument may be calibrated in essentially the same way as the dual lamp spectrophotometer of Fig. 1. The basis structure of this embodiment is thus applicable to scattering effects such as Raleigh scattering and to fluorescence measurements.
Referring to FIG. 12, a turbidimeter embodiment includes a source 120 that provides optical radiation along -two optical paths 122, 124. The sample path brings a portion of the radiation through a sample cell 126 and a sample filter 128 to a sample detector 130. The reference path brings a portion of the radiation through a reference filter 132 to a reference detector. A calibration filter driving mechanism 138 may translate a neutral density calibration filter 136 of known absorbance into and out of the optical path in series with the sample cell.
The ratio of the received radiation energy at the sample detector to that received at the reference detector is related to the transmission through the sample. Higher values of transmission are indicated as lower values of turbidity and vice-versa. The neutral density calibration filter, of known absorbance, provides the ability to perform measure operations and calibration operations. This instrument may therefore be calibrated in much the same way as the dual lamp spectrophotometer.
In the embodiments shown, stored measurements are used to effectively adjust the instrument performance. In other embodiments, calibration errors kept in memory without updating working values, and the instrument performance may readily be revised and used, e.g. by computer using the raw instrument data for measure and calibration readings together with stored values representing an initialized condition of the instrument.
Other embodiments are within the scope of the following claims.
What is claimed is:
Backqround of the Invention The present invention concerns photometers, i.e.
optical analytical instruments that employ optical energy to determine physical parameters of fluids such as liquids or gasses found in effluents and in industrial processes.
There are many instances in which it would be useful to have a relatively low cost analytical instrument that could be depended upon to monitor a process or environmental condition at a desired level of accuracy and which did not require highly trained personnel for its operation and maintenance. There is a special need for a low cost instrument in the case of monitoring waste water and effluent gasses before release to the environment. The present invention addresses these needs, and provides general improvements in photometers as well.
Summary of the Invention In accordance with the present inventionl there is provided an optical analytical instrument for quantitative measurements capable of producing amplitude readings based upon the effect of a fluld upon a selected fixed wavelength in a bearn of optical radiation to determine a quantitative physical parameter of the fluid, said instrument comprlsing optical source means of suitable optical radiation, detector means responsive to said selectlve fixed wavelength within a predetermined amplitude measurernent range of the instrument, a fluld zone for a sample fluid, optical path means from said optlcal source means throuqh said fluid zone to said detector means, reading and control circuitry for taking from said detector means at l.east two amplitude readings of optical energy of said selected fixed wavelength that result from the influence of the fluid on sa1.d optical radiation without there having occurred substantial change to the fluid, means during one of said readings for incl.uding ln said optlcal path a fi].ter of known absorbance that absorbs a predetermined portion of the energy at said fixed wavelength, said portion representin~ a substantial fraction of said amplitude measurement range of the instrument so that the respective reading represents an ampl.i.tude Galibration reading within sald measurement range of the instrument, calibration-value-determining means operable while a sample of said fluid remains unchanged in sald fll].id zone, first to compare, effectively, said two amplitude readings at said selected fixed wavelength to remove said effect of said fluid from the value of said callbration reading, second, to compare, effectively the residl~al value of said calibration reading to a known value based on the known ahsorbance of said filter, and, thlrd, on the basis of said second comparison, to determine an amplitude calibration value for said instrument based on a.mplitude values derived while said fluid sample remains unchan~ed in said fluid zone.
In accordance with the present invention, there is also provided an optical analytical instrument capable of producing readings based upon the effect of a fluid upon a beam of optical energy to determine a physical parameter of - la -6041~-2180 - 205~98 the fluid, said instrument comprising optical source means of suitable optical radiatlon, detector means, a fluid zone for a sample fluid, optical path means from sald optical source means through said fluld zone to sald detector means, readlng and control clrcuitry for taking from sald detector means at least two readlngs of optlcal energy that has been lnfluenced by the fluid wlthout there having occurred substantlal change to the fluid, means during one of said readings for including in said optical path a filter of known absorbance so that the respective reading represents a callbratlon readlng, calibratlon-value-determining means constructed first to compare, effectively, said two readings to remove sald effect of sald fluid from the value of said callbration reading, second, to compare effectively the residual value of said calibration readlng to a known value based on the known absorbance of sald fllter, and, third, on the basis of said second comparison, to make a calibration adiustment based on values derived while said fluld sample remained unchanged in said fluid zone, wherein sald optical source means comprise two separately en.ergizable radiation emitters, said optical path means from said source having two branches, one defining a respectlve optical path from each radiation emitter, with both branches transmitting via a substantially common path through said sample, one of said emitters, denoted the "measure" emitter, energiæed by said control means during the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energized by sald control means during the taking of a callbration reading, sald filter - lb -~~ 60412-2180 . . , 20533g8 of known absorbance being included in the optical path from sald calibration emitter to said fluid zone.
In accordance w.ith the present lnventlon, there is further provided an optical analytical instrument capable of producing readings based upon the effect of a fluld upon a beam of optical energy to determine a physical parameter of the fluid, said instrument compriæing optical source means of suitable optical radiation, detector means, a fluid zone for a sample fluid, optlcal path means from sald radiation source through said fluid zone to sald detector means, reading and control circuitry for taking from sald detector means at least two readings of optical energy that has been influenced by the fluid wlthout there having occurred substantial change to the fluid, means during one of said readlngs for includlng in said optical path a fllter of known absorbance so that the respective reading represents a calibration reading, calibration-value-determining means constructed first to compare, effectively, said two readings to remove sald effect of said fluld from the value of said calibration reading, second, to compare, effectively the residual value of said callbration reading to a known value based on the known absorbance of said filter, and, third, on the basis of said second comparison, to make a callbration adjustment based on values derived while said fluid sample remained unchanged in said fluld zone, wherein said optical source means comprises two separately energizable radiation emltters, said optical path from said source havlng two branches, one defining a respective optical path from each radlation emitter, with both -- l.c --B 6o4l2-2l8o 2~53398 branches transmltting via a substantially common path through sald sample, one of said emitters, denoted the "measure"
emltter, energized by said control means durlng the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energlzed by said control means during the taking of a ca]lbration reading, said filter of known absorbance being included in the optical path from said calibration emitter to said fluid zone, and wherein said instrument has no moving parts for accomplishing calibration, said control means constructed and arranged such that normal and calibration readings for use by said instrument in establishing said calibration ad~ustment are effected by selective energization of said emitters during operation.
In accordance with the present invention, there is further provided a method of calibrating an optical analytical instrument capable of producing amplitude readings based upon the effect of a fluid upon a selected fixed wavelength in a beam of optical radiation to determine a quantitative physical parameter of the fluid, comprising providing an optical source means, optical path means, and optical detector means responsive to said selected fixed wavelength, taking from said detector means at least two amplitude readings at said selected fixed wavelength and within a predetermined amplitude measurement range of the instrument, of optical energy that result from the influence of the fluid on the optical radiation in a fluid zone without there having occurred substantial change to the fluid, including in said optical path means a filter of known absorbance that absorbs a - ld -~.~ 60412-2180 predetermlned portion of the energy at sald flxed wavelength that represents a substantlal fractlon of sald amplltude measurement range of the lnstrument so that one of sald amplltude readlngs represents a callbratlon readlng wlthln sald measurement range of the lnstrument, flrst, comparlng, effectlvely, sald two amplltude readlngs to remove sald effect of sald fluld from the value of sald callbratlon readlng, second, comparlng, effectlvely, the resldual value of sald calibratlon reading to a known value based on the known absorbance of sald fllter, and, on the basls of sald second comparlson, determlnlng an amplltude callbratlon value for the lnstrument based on values derlved whlle sald fluld sample remalns unchanged in sald fluld zone.
In accordance wlth the present lnventlon, there ls further provlded a method of callbratlng an optical analytlcal lnstrument capable of produclng readlngs based upon the effect of a fluid on a beam of optical energy to determlne a physlcal parameter of the fluld, comprlslng provldlng optical radlatlon source rneans that lncludes a "measure" emltter and a "callbratlon" emltter separately energlzable by control means, optlcal path means, and optlcal detector means, taklng from sald detector means at least two readlngs of optlcal energy, that has been influenced by the fluld in a fluid zone without there havlng occurred substantlal change to the fluld, by energizlng said "measure" emitter by said control means during the taklng of a normal measurement, and by energlzlng sald "callbratlon" emitter by said control means during the taking of a calibratlon readlng whlle a fllter of known absorbance ls - le -B
- 20~3~38 included in sald optlcal path means from said callbratlon emltter to sald fluld, flrst, comparlng, effectlvely, sald two readlngs to remove sald effect of sald fluld from the value of sald callbratlon readlng, second, comparlng, effectlvely, the resldual value of sald callbratlon readlng to a known value based on the known absorbance of sald fllter, and, on the basls of sald second comparlson, maklng a callbratlon ad~ustment based on values derlved whlle said fluld sample remalned unchanged in sald fluld zone.
In accordance wlth the present lnventlon, there ls further provlded a rnethod of callbratlng an optical analytical lnstrument capable of produclng readlngs based upon the effect of a fluid on a beam of optlcal energy to determlne a physlcal parameter of the fluld comprlslng provldlng optlcal radlatlon source means that lncludes a "measure" emltter and a "callbratlon" emltter separately energlzable by control means, optlcal path means, and optlcal detector means, taklng from sald detector means at least two readlngs of optlcal energy, that has been influenced by the fluld ln a fluld zone wlthout there havlng occurred substantlal change to the fluld, by energlzing sald "measure" emltter by sald control means durlng the taklng of a normal measurement, and by energlzlng a "calibratlon" emitter by sald control means durlng the taking of a calibration reading while a filter of known absorbance ls included in the optlcal path from sald callbratlon emitter to said fluid, flrst, comparlng, effectively, sald two readlngs to remove said effect of said fluld from the value of said callbratlon readlng, second, comparlng, effectlvely, the - lf -2~53398 resldual value of said calibratlon readlng to a known value based on the known absorbance of sald fllter, and, on the basis of said second comparison, maklng a callbratlon ad~ustment based on values derived whlle said fluld sample remained unchanged in said fluid zone, wherein said calibration ad~ustment is effected by selective energization of said emitters during operation without moving any part of said lnstrument.
According to the invention a photometer or optical analytical instrument is provided that is relatlvely low cost to construct but still has the means to correct its calibration or report the accuracy of its measurements during use, without need for a skilled operator.
Such an lnstrument is realized according to the invention by means that permit construction with no moving parts, in implementations that are relatively simple, compact, easy to assemble, and requlres no alignment.
A principal feature of the invention is an optical analytical instrument capable of producing readings based upon the effect of a beam of optical energy on a fluid to determine a physical parameter of the fluid, the instrument comprising a source of suitable optical radiation, a - lg --detector means, means defining a zone for the fluid, means defining an optical path from the radiation source through the fluid zone to the detector means, reading and control circuitry for taking from the detector means at least two readings of optical energy that has been influenced by the fluid without there having occurred substantial change to the fluid, means during one of the readings for including in the optical path a filter of known absorbance so that the respective reading represents a calibration reading, calibration-value-determining means constructed first to compare, effectively, the two readings to remove the effect of the fluid from the value of the calibration reading, second, to compare, effectively the residual value of the calibration reading to a known value based on the known absorbance of the filter, and, third, on the basis of the second comparison, to make a calibration adjustment based on values derived while the fluid sample remained unchanged in the fluid zone.
Preferred embodiments of the aspect of the invention have the following features.
The optical radiation source comprises two separately energizable radiation emitters, the optical path from the source having two branches, one defining a respective optical path from each radiation emitter, with both branches transmitting via a substantially common path through the sample, one of the emitters, denoted the "measure" emitter, energized by the control means during the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energized by the control means during the taking of a calibration reading, the filter of known absorbance being included in the optical path from the calibration emitter to the fluid zone.
20~3398 The instrument has no moving parts for accomplishing calibration, the control means constructed and arranged such that normal and calibration readings for use by the instrument in establishing the calibration adjustment are effected by selective energization of the emitters during operation.
The optical analytical instrument is in the form of an in-line analytical instrument in which the means defining the fluid zone comprises a conduit for at least part of a process stream through which process fluid flows, the process stream having a characteristic maximum rate at which the parameter can change, and the reading and control means constructed, in relation to the process stream, to take the two readings within such a short time interval that there is insufficient passage of time to allow substantial change to occur in the parameter of the fluid in the process stream.
The optical analytical instrument is in the form of a titration chamber constructed to be filled by a liquid process stream and, after titration, flushed only by the process stream.
The optical analytical instrument is in the form of a titration chamber constructed to receive a known repeatatable small sample of process stream liquid and after titration said chamber is constructed to be flushed with a suitable liquid.
The optical analytical instrument is in the form of a colorimeter.
The optical analytical instrument is in the form of a colorimeter, the portion of the optical path in the fluid zone being defined by a probe which includes a reflector whereby energy which enters the fluid zone is reflected to pass back through the fluid zone to the detector means.
20~3398 The optical analytical instrument is in the form of an absorption spectrophotometer, the detector means comprising at least two detectors sensitized to respectiv~ly different wavelengths, one wavelength corresponding to a characteristic absorption of a component of interest in the fluid zone, the detected signal varying with the concentration of the component, and the other wavelength being a reference wave-length for which the component of interest has relatively low absorbance.
The optical analytical instrument has the detector means positioned to receive scattered radiant energy from the sample, preferably the filter belng positioned to filter the scattered radiant energy and preferably the detector is postioned and adapted to receive Raleigh scattering or fluorescence.
The optical analytical instrument is in the form of a turbidimeter, preferably with an optical reference path bypassing the sample.
Also preferred embodiments of the instrument have the following features.
The means defining the optical path from the radiation source to the fluid zone is comprised of optical fibers.
The means to make a calibration adjustment is constructed to calibrate the instrument, preferably the adjustment comprising means to adjust the sensitivity of the instrument and means to adjust the zero reference of the instrument.
The optical analytical instrument including means to provide the known value in the form of means-to store a - -reading of the detector means taken with the filter of known absorbance in place and with the fluid zone empty.
Another principal feature of the invention is a method of calibrating an optical analytical instrument capable of producing readings based upon the effect of a beam of optical energy on a fluid to determine a physical parameter of the fluid, comprising taking from a detector means at least two readings of optical energy that has been influenced by the fluid in a fluid zone without there having occurred substantial change to the fluid, including in the optical path a filter of known absorbance so that one of the readings represents a calibration reading, first, comparing, effectively, the two readings to remove the effect of the fluid from the value of the calibration reading, second, comparing, effectively, the residual value of the calibration reading to a known value based on the known absorbance of the filter, and, on the basis of the second comparison, making a calibration adjustment based on values derived while the fluid sample remained unchanged in the fluid zone.
Preferred embodiments of this aspect of the invention have the following features.
The optical radiation source comprises two separately energizable radiation emitters, further comprising energizing a "measure" emitter by the control means during the taking of a normal measurement, and energizing a "calibration" emitter, by the control means during the taking of a calibration reading while the filter of known absorbance is included in the optical path from the calibration emitter to the fluid.
The calibration adjustment is effected by selective energization of the emitters during operation without moving any parts of the instrument.
The fluid flows in a process stream further comprising taking two readings by the control means within 2~3398 such a short time interval that there is insufficient passage of time to allow substantial change to occur in the parameter of the fluid in the process stream.
The method includes the steps of filling a titration chamber by a liquid process stream and, after titration, flushing the titration chamber only by the process stream.
The method includes the steps of filling a titration chamber by a liquid process stream and, after titration, flushing the titration chamber by a suitable liquid.
The calibration is performed for a colorimeter, an absorption spectrophotometer, a turbidimeter, or an instrument capable of detecting fluorescence or Raleigh scattering.
The step of making a calibration adjustment calibrates the instrument and preferably adjusts the zero reference of the instrument.
The method includes the step of storing a reading of the detector means taken with the filter of known absorbance in place and with the fluid zone empty.
The step of making a calibration adjustment includes updating a current filter absorbance value.
The calibration adjustment step is periodically repeated and the current filter value is successively updated during these repetitions.
Past trends in the current absorbance value are taken into consideration in updating the current absorbance value.
Another principal feature of the invention is apparatus or method wherein, respectively, the instrument is constructed or the method is conducted to measure~
concentration or the like, C, according substantially to the relationship:
8(n) C8td X (A~(n) ~ Az) 20a3398 in which the values are substantially as defined in the following specification.
Description of the Preferred Embodiments FIG. 1 is a schematic illustration of a dual lamp spectrophotometer that has no moving parts.
FIG. 2 is a plot of absorbance against wavele~gth in the spectral region of interest ~or a sample to bc mc~urcd.
FIG. 3 is a flowchart illustrating an initialization routine for the embodiment of FIG. 1.
FIG. 4 is a flowchart illustrating a sensitivity correction routine for the embodiment of FIG. 1.
FIG. 5 is a flowchart illustrating a zero correction routine for the embodiment of FIG. 1.
FIG. 6 is an idealized response plot for the instrument of the invention.
FIG. 7 is a set of graphs showing the voltage applied to the sample and calibration lamps and the concentration of the sample against time for a single calibration operation.
FIG. 8 is a plot of absorbance against wavelength for a measurement situation where the baseline absorption is sloping.
FIG. 9 is a plot of absorbance against wavelength for a measurement situation where the baseline absorption is sloping, showing two reference wavelengths.
FIG. 10 is a schematic illustration of a colorimeter embodiment.
FIG. 11 is a schematic illustration of an embodiment of the invention that measures concentration by detecting scattering effects within the sample.
FIG. 12 is a schematic illustration of a turbidimeter embodiment.
20~33398 Referring to FIG. 1, a dual lamp spectrophotometer for monitoring a continuous flow includes a sample lamp or emitter 2 and a calibration lamp or emitter 4 that are each linked to a central processing unit 22 or other form of calbration-value-determining and control circuitry via a control line 26. The sample and calibration lamps are located in front of respective fiber optic branches 14a and 14b which join to form a random-ordered first fiber optics bundle 14 which provides a common path for energy from the two branches. A calibration filter 10 is disposed between lamp 4 and its fiber optic branch. The calibration filter 10 has a known absorbance at the wavelength to be measured.
The fiber optics bundle leads to a sample cell 12 or other zone for confining a sample 18. Also adjacent to the sample cell, behind the first fiber optics bundle, is a second fiber optics bundle 16 which provides a common path for energy transmitted through the cell. This second fiber optics bundle is split into branches 16a and 16b to provide optical paths through a measure filter 7 and a reference filter 9.
A measure detector 6 and a reference detector 8 are placed behind the measure and reference filters, respectively. The outputs of these detectors are connected to a data acquisition module 20 that is, in turn, connected to the central processing unit 22. A display 24 is also connected to the central processing unit.
The lamps or emitters are capable of emitting optical radiation at a wavelength that is absorbed (or, in other ebodiments, otherwise influenced) by the substance to be detected or monitored, where "optical" radiation is defined, for the purposes of this application, as radiation in the visible, infrared and ultraviolet regions of the spectrum.
_ 60412-2180 In operatlon, a contlnuous flow of a sample fluld passes through the cell, whlch acts as condult for at least a part of a process stream. Alternatlvely, the cell may be fllled wlth a statlc sample of a fluld whlch may be replaced from tlme to tlme.
The former of these methods enables the lnstrument to be used, e.g., ln ln-llne monltorlng of waste water or lndustrlal processes ln whlch the fluld flows steadlly through the cell. The latter enables the lnstrument to be used, e.g., ln on-llne monltorlng, ln whlch fluld ls repeatedly drawn from the process llne and used to provlde samples, e.g., for tltratlon or colorlmetrlc measurement, and to provlde the flushlng medlum.
An lnstrument wlth whlch present lnventlon ls useful ls shown ln Unlted States Patents 4,911,891 and 4,910,151, both entltled "Sample Monltorlng Instrument for On-Llne Appllcatlon".
There are two types of operatlons that may be performed wlth thls apparatus; "measure" operatlons and "callbratlon"
operatlons. The flrst step ln a measure operatlon ls for the central processlng unlt 22 to cause the sample lamp 2 to lllumlnate. The sample lamp emlts polychromatlc llght that passes from branch 14a through the flrst flber optlcs bundle 14 and lnto the cell. In the cell, the llght lnteracts wlth the sample and certaln characterlstlc wavelengths of the llght are absorbed by the constltuent materlals ln the sample.
A portlon of the unabsorbed llght passes along the common path through the cell lnto the second flber optlcs bundle 16 and ` -~ 60412-2180 - ~a -exlts the bundle ln two beams. The flrst beam passes through a measure fllter 7 that attenuates all wavelengths except that of the spectral llne of lnterest for the materlal belng monltored ln the spectrophotometer. The spectral llne typlcally corresponds to a characterlstlc absorption of the compound to be measured. The second beam passes through a reference filter 9 that attenuates all wavelengths except for a reference wavelength that is generally close to the measured wavelength but which does not correspond to a characteristic absorption line of the compound of interest. The relationship between the wavelength at the spectral line of interest and the reference wavelength are shown in FIG. 2. By using the two wavelengths in this manner, the effect of the illumination level of the lamps may be eliminated by performing a differential measurement using two detectors.
These measure and reference detectors 6, 8 detect the light passing through the measure and reference filters, respectively. Each of these detectors produces an electrical signal that has a magnitude related to the amount of light incident upon it and provides that signal to a data acquisition module 20. The response of the sample 18 to the measure wavelength may be determined by Vr A = log10 --Vm where A is the absorbance of the sample, V~ is the voltage of the measure detector 6 and Vr is the voltage on reference detector 8. The data acquisition module digitizes the voltage signals from the detectors and provides the digital values to the CPU for processing.
In normal "measure" operation the instrument provides readings for the substance of interest in the sample in the units desired by the user. Typically the reading for a sample is in terms of concentration, C" for example, percent or ppm. The concentration Cs(~) of sample 20533~8 -at time n is implemented in the present embodiment according to the relationship:
C~ (n) = Cstd X (A~ (n) A, ) where: AF
C~(~) = calculated nth sample concentration, C8td = concentration value corresponding to the known absorbance of the filter selected, A. (n) = measured absorbance value with nth sample in situ Az = previously measured and stored zero absorbance value AF = previously measured and stored filter absorbance value, all will be explained below. It will be observed that the above equation normalizes the sample concentration to the corresponding ratio of C~+d (the concentration represented by the absorbance of the filter) vs. the measured calibration filter absorbance value AF' thus taking advantage of updates as to any detected drift of the instrument that have been determined by prior calibration readings.
The routine implemented by the instrument in a normal measurement operation is shown in the first two blocks of Fig. 4.
The second operation, the "calibration" operation is performed with the sample 18 in place and may be performed for all measurements or at selected intervals. In this step, the central processing unit causes the calibration lamp 4 to illuminate while the sample lamp remains de-energized. The calibration lamp emits polychromatic light that passes through the calibration filter 10. The calibration filter absorbs light at the measurement wavelength to a degree which is precisely known, e.g. 30 20~3398 percent of the measurement range. It may also absorb some energy at the reference wavelength; this should be significantly less than the degree to which the measurement wavelength is absorbed.
Unabsorbed light from the calibration filter 10 passes into the first fiber optics bundle 14 and on through the sample cell where portions of it are absorbed by constituents of the sample being monitored. Unabsorbed portions of the light then pass into the second fiber optics bundle 16 and spectral portions are detected and corresponding values are provided to the central processing unit in the manner as they were in the measure operation.
The central processing unit divides the digitized measure value by its corresponding reference value and takes the logarithm of the result to generate an absorbance value for each measurement. This "calibration" reading is used in subsequent calibration calculations, as described below in connection with FIGS. 3-5. Basically, a prior "measure"
reading and a "calibration" reading, both taken with the same fluid sample present, are effectively compared to remove the effect of the fluid sample from the "calibration"
reading. Then the residual value is effectively compared to the known, i.e. previously determined, actual absorbance of the calibration filter. From this, or a series of such measurements, a calibration adjustment can be determined, or equivalently, a report may be generated as to the degree to which the instrument has drifted from the initial state.
The values given in Figs. 3-5 are defined as follows:
2û~3~98 A F - FILrER obaorbanca valua ua-d In calculatlona A FO - Inltlal abaorbanca volu- ~or FILTER
A F(n) - Calcualtod abaorbonc- valu- ot FILTER
wlth nth aampla In ~Itu A Fmln - Mtnlmum llmlt tor FILTER obaorbonc- volua A Fmax - Ma~lmum ll~lt ~or FILTER ab~orbanCO valv-A r~M(n)- Abaorbanco voluo o~ nth aampla wlth CALIeRATION mlttar A M - ~b~orbanco m o~ur~d A ~ax - Maxlmumn llmlt tor mcaaurod abaorbanc-a A S(n) - Abaorbanca valuo wlth nth aampl- In ~Itu A z - ZERO obaorbanca valua uaod In colculatlon-A zo - Inltlal abaorbanc- valu- tor ZERO concantratlan A Z(n) - Colculatod ab~orbanco valuo ~or ZERO
wlth nth Jam41- In altu A Z(n) - ~Inlmum llm~t ~or ZERO abaorbanc- valu~
A Z(n) - M~xlmum llmlt ~or ZERO abaorbanc- valuo C S(n) - Calculat-d nth aampla concantratlon C STD - Concontrotlon volua corroaponalna to FIETER aol-cted (Valu- ~ntoraa by oparator) When the instrument is first turned on, the central processing unit performs an initialization routine, as shown in FIG. 3. This initialization routine is performed 30 on an empty cell 12 or a cell filled with "zero" sample (sample containing "zero" concentration of the component of interest, e.g., for color in liquids, distilled water is used as a "zero" sample) in order to obtain an initial zero value for the instrument. Thus an initial "measure"
operation is performed by turning the measure lamp on 32, calculating and storing the resulting absorbance value for 2D~3398 zero sample present (Azo) 34 and turning the measure lamp off 36.
Likewise, an initial "calibration" operation for the filter is performed by turning the calibration lamp on 36 and performing a further measure operation with the circuitry to obtain an absorbance measurement, AM. The initial measured absorbance value of the calibration filter AFO, is then obtained by subtracting the absorbance value for zero Azo from the measured absorbance AM' this value AFO
then being stored.
Now, with the calibration lamp off 40, and the measure emitter on, a further measurement 40 is performed, the zero values AFO and Azo are respectively stored as the filter and absorbance values AF and A~ to be used in further calculations, and using these values the zero value of the scale reading instrument is set. Here the known absorption value of the calibration filter, i.e. the concentration level of the compound of interest that it represents, is used to standardize the readings of the instrument. This completes the initialization routine 44.
Once the initialization routine is complete, the instrument may be used to perform absorbance measurements, and to display these values on the display 24 or otherwise present them to the user or another piece of equipment.
These values may be expressed to the user as concentration values. After a certain period, which is user-definable, the instrument performs a self calibration.
The self calibration routine, which is &hown in flow diagram in FIGS. 4 and 5, includes a sensitivity correction routine that is used to adjust the sensitivity of the instrument and a zero correction routine that adjusts the zero value of the instrument. As may be seen in FIG. 6, a 20~3398 sensitivity change corresponds to a change in the gain of the instrument or ~y/~x, while a change in zero value corresponds to a change in the zero intercept b of the instrument.
S The sensitivity correction routine (FIG. 4) begins with a measure operation 50 that reads the absorbance A~
for sample n. The central processing unit calculates a corresponding sample concentration value using the formula shown at step 52 of FIG. 4 and displays it.
The self calibration routine then determines whether the calibration is within range by determining if the absorbance value measured for the current sample A~(n) added to the current filter value used in calculations Ap is within the range of measurement of the system 54. If it is not, it is probable that the operating range for the automatic calibration has been exceeded. In this case, calibration can not take place, and the central processor unit flags this condition to the user.
If the unit can be calibrated, the central processing unit turns off the measure lamp and turns on the calibration lamp 56 to obtain and store 58 an absorbance value for the sample and the filter together. From this value, the central processing unit computes a calculated absorbance value for the filter AF(n) 60. If this absorbance value is the same AF that currently is used 62, the system is properly calibrated, and the calibration routine is complete 76.
If the measured absorbance value of the calibration filter is different, then it is known that the instrument has drifted since the actual absorbance of the calibration - -filter itself is a known, fixed quantity. To correct for this instrument drift, the central processing unit generates 20~33~8 -a new current filter absorbance value AFI 64, taking into consideration past trends of the value. The new value will be determined by the average of a set of most recent values, unless there is an appreciable trend in one direction or the other, which might be caused by a system malfunction, for example, component failure.
The central processing unit then tests the new filter absorbance value to see if it is within the allowed range 60. If it is outside the range, the instrument displays an error indication 74 and ceases its attempt at calibration 76. If the new, adjusted, filter absorbance value AF is withln the proper range, it is stored for further use.
The central processing unit then reports to the user a sensitivity value ~S that indicates to the user that the calibration has shifted. This value is obtained from the equation in block 70 of FIG. 4. By comparison of the value of new and previous determinations of AF~ the performance history of the sensitivity of the instrument can be determined and reported. Execution of the "calibration"
operation proceeds 72 to the zero correction routine.
The zero correction routine, which is shown in flow diagram in FIG. 5, generates a zero value for use in further measurements. This routine begins with a calculation of a zero value computed by subtracting the current filter absorption value AF and the current measured sample absorbance A~n) from the measured absorbance of the filter and sample together A~(n) for the current sample 80. If this value is determined to be within an acceptable range of zero absorbance values, the current zero value is updated'''~'''''''''''' 84, the zero shift is reported to the operator 86, and the calibration routine is completed 90. If the value is outside of the acceptable range, an error condition is flagged 88.
Once the calibration routines are completed, the instrument is ready to perform further measurements, until it is recalibrated again. The above method may thus repetitively recalibrate the instrument by simply performing successive updates of the Ap variable.
It will be observed that one assumption inherent in the above discussion in respect of in-line operation, is that the flow of fluid must have an absorbance that is substantially constant during a measure operation and the following calibration operation (i.e., its characteristic maximum rate of change is small), or the calibration may be performed incorrectly. This is illustrated in FIG. 7, where lS it can be seen that between the measurement points To and T
when the detector measures unabsorbed light, the concentration of the solution must remain stable. It is found in practice that the time interval for making such measurements is sufficiently short that concentrations in many processes to be monitored do not vary significantly, so that valid readings in-line can be obtained.
For on-line analysis, i.e. when a sample is captured and retained in the sample cell, the same sample remains in the cell for both "measure" and "calibration" operations so that this is of no concern.
In some cases, constituents of the sample may cause the baseline absorption around the wavelength of interest to be skewed by an amount X, as shown in FIG. 8. This can cause the reference value to depend on the concentration of constituents that are not important to the measurement and thus affect the accuracy of measurements. In this situation, two or more reference detectors may be used, and 20~3398 their average value used to determine the effective reference value, as illustrated in FIG. 9.
By use of the instrument as described, the calibration of the instrument of Fig. 1 can be automatically checked and corrected during continuing operations without evacuating the sample or using any test samples. The system also contains no moving parts which contributes to compactness as well as to reliability and less need for maintenance. This instrument, therefore, is particularly well suited for use in rugged field applications such as waste water monitoring where dependable operation without the need for regular support by highly qualified technicians is important.
Furthermore, this optical apparatus and associated method are equally useful for measurements in the visible, infrared and ultra-violet regions of the spectrum, when supplied with the appropriate emitters, filters, and detectors.
In certain instances, it may be useful to replace the two lamps with a single source and a shutter mechanism or its equivalent thereby allowing the same source to illuminate the sample via the sample and filter paths. This requires the addition of a moving part that can be less reliable or more expensive, but may be warranted in certain situations. For example, when using an arc source or other embodiment in which the emitted radiant energy from the emitter source is optimized at its maximum, a single emitter and suitable means for switching between from an optical path with and without the calibration filter present can be employed.
Numerous other instruments may be manufactured to measure absorbance or other physical parameters using the calibration method of the invention.
20~3398 For example, FIG. lo shows a probe-based configuration suitable for use as a titrometer or a colorimeter. This instrument operates in much the same way as the spectrophotometer embodiment, with the exception that the light passes through the sample fluid 100 in the probe 102 and is reflected off of a reflecting surface 104 back through the probe before the resulting absorbance is measured. The sampling technique of the above-mentioned patent may be employed to especial advantage, or other techniques may be used.
An instrument for measuring concentration by detecting scattered radiation is shown in FIG. 11. This instrument includes an emitter, such as an ultraviolet arc source 110 that may emit light into a sample cell 118 and excite the sample 116 and cause a certain degree of scattering. Detectors directly opposite the incident path 122 of the exciting radiation and at right angles thereto 120 measure transmitted and scattered radiation, respectively. The ratio of the amplitude of detected scattered radiation and the transmitted scattered radiation is proportional to the concentration of the material being measured.
In this instrument, the calibration filter 112 of known absorbance that is translated into and out of the optical path of the detector provides the ability to perform alternate "measure" operations and "calibration" operations.
This instrument may be calibrated in essentially the same way as the dual lamp spectrophotometer of Fig. 1. The basis structure of this embodiment is thus applicable to scattering effects such as Raleigh scattering and to fluorescence measurements.
Referring to FIG. 12, a turbidimeter embodiment includes a source 120 that provides optical radiation along -two optical paths 122, 124. The sample path brings a portion of the radiation through a sample cell 126 and a sample filter 128 to a sample detector 130. The reference path brings a portion of the radiation through a reference filter 132 to a reference detector. A calibration filter driving mechanism 138 may translate a neutral density calibration filter 136 of known absorbance into and out of the optical path in series with the sample cell.
The ratio of the received radiation energy at the sample detector to that received at the reference detector is related to the transmission through the sample. Higher values of transmission are indicated as lower values of turbidity and vice-versa. The neutral density calibration filter, of known absorbance, provides the ability to perform measure operations and calibration operations. This instrument may therefore be calibrated in much the same way as the dual lamp spectrophotometer.
In the embodiments shown, stored measurements are used to effectively adjust the instrument performance. In other embodiments, calibration errors kept in memory without updating working values, and the instrument performance may readily be revised and used, e.g. by computer using the raw instrument data for measure and calibration readings together with stored values representing an initialized condition of the instrument.
Other embodiments are within the scope of the following claims.
What is claimed is:
Claims (43)
1. An optical analytical instrument for quantitative measurements capable of producing amplitude readings based upon the effect of a fluid upon a selected fixed wavelength in a beam of optical radiation to determine a quantitative physical parameter of the fluid, said instrument comprising optical source means of suitable optical radiation, detector means responsive to said selected fixed wavelength within a predetermined amplitude measurement range of the instrument, a fluid zone for a sample fluid, optical path means from said optical source means through said fluid zone to said detector means, reading and control circuitry for taking from said detector means at least two amplitude readings of optical energy of said selected fixed wavelength that result from the influence of the fluid on said optical radiation without there having occurred substantial change to the fluid, means during one of said readings for including in said optical path a filter of known absorbance that absorbs a predetermined portion of the energy at said fixed wavelength, said portion representing a substantial fraction of said amplitude measurement range of the instrument so that the respective reading represents an amplitude calibration reading within said measurement range of the instrument, calibration-value-determining means operable while a sample of said fluid remains unchanged in said fluid zone, first to compare, effectively, said two amplitude readings at said selected fixed wavelength to remove said effect of said fluid from the value of said calibration reading, second, to compare, effectively the residual value of said calibration reading to a known value based on the known absorbance of said filter, and, third, on the basis of said second comparison, to determine an amplitude calibration value for said instrument based on amplitude values derived while said fluid sample remains unchanged in said fluid zone.
2. An optical analytical instrument capable of producing readings based upon the effect of a fluid upon a beam of optical energy to determine a physical parameter of the fluid, said instrument comprising optical source means of suitable optical radiation, detector means, a fluid zone for a sample fluid, optical path means from said optical source means through said fluid zone to said detector means, reading and control circuitry for taking from said detector means at least two readings of optical energy that has been influenced by the fluid without there having occurred substantial change to the fluid, means during one of said readings for including in said optical path a filter of known absorbance so that the respective reading represents a calibration reading, calibration-value-determining means constructed first to compare, effectively, said two readings to remove said effect of said fluid from the value of said calibration reading, second, to compare, effectively the residual value of said calibration reading to a known value based on the known absorbance of said filter, and, third, on the basis of said second comparison, to make a calibration adjustment based on values derived while said fluid sample remained unchanged in said fluid zone, wherein said optical source means comprise two separately energizable radiation emitters, said optical path means from said source having two branches, one defining a respective optical path from each radiation emitter, with both branches transmitting via a substantially common path through said sample, one of said emitters, denoted the "measure" emitter, energized by said control means during the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energized by said control means during the taking of a calibration reading, said filter of known absorbance being included in the optical path from said calibration emitter to said fluid zone.
3. An optical analytical instrument capable of producing readings based upon the effect of a fluid upon a beam of optical energy to determine a physical parameter of the fluid, said instrument comprising optical source means of suitable optical radiation, detector means, a fluid zone for a sample fluid, optical path means from said radiation source through said fluid zone to said detector means, reading and control circuitry for taking from said detector means at least two readings of optical energy that has been influenced by the fluid without there having occurred substantial change to the fluid, means during one of said readings for including in said optical path a filter of known absorbance so that the respective reading represents a calibration reading, calibration-value-determining means constructed first to compare, effectively, said two readings to remove said effect of said fluid from the value of said calibration reading, second, to compare, effectively the residual value of said calibration reading to a known value based on the known absorbance of said filter, and, third, on the basis of said second comparison, to make a calibration adjustment based on values derived while said fluid sample remained unchanged in said fluid zone, wherein said optical source means comprises two separately energizable radiation emitters, said optical path from said source having two branches, one defining a respective optical path from each radiation emitter, with both branches transmitting via a substantially common path through said sample, one of said emitters, denoted the "measure"
emitter, energized by said control means during the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energized by said control means during the taking of a calibration reading, said filter of known absorbance being included in the optical path from said calibration emitter to said fluid zone, and wherein said instrument has no moving parts for accomplishing calibration, said control means constructed and arranged such that normal and calibration readings for use by said instrument in establishing said calibration adjustment are effected by selective energization of said emitters during operation.
emitter, energized by said control means during the taking of a normal measurement reading and the other emitter, denoted the "calibration" emitter, energized by said control means during the taking of a calibration reading, said filter of known absorbance being included in the optical path from said calibration emitter to said fluid zone, and wherein said instrument has no moving parts for accomplishing calibration, said control means constructed and arranged such that normal and calibration readings for use by said instrument in establishing said calibration adjustment are effected by selective energization of said emitters during operation.
4. The optical analytical instrument of claim 1, 2 or 3 in the form of an in-line analytical instrument in which said fluid zone comprises a conduit for at least part of a process stream through which process fluid flows, said process stream having a characteristic maximum rate at which said parameter can change, and said reading and control means constructed, in relation to said process stream, to take said two readings within such a short time interval that there is insufficient passage of time to allow substantial change to occur in said parameter of said fluid in said process stream.
5. The optical analytical instrument of claim 1, 2 or 3 with said fluid zone in the form of a titration chamber constructed to be filled by a liquid process stream and, after titration, flushed only by the process stream.
6. The optical analytical instrument of claim 1, 2 or 3 with said fluid zone in the form of a titration chamber constructed to receive a known repeatable small sample of process stream liquid and after titration said chamber is constructed to be flushed with a suitable liquid.
7. The optical analytical instrument of claim 5 in the form of a colorimeter.
8. The optical analytical instrument of claim 1, 2 or 3 in the form of a colorimeter, the portion of said optical path in said fluid zone being defined by a probe which includes a reflector whereby energy which enters said fluid zone is reflected to pass back through said fluid zone to said detector means.
9. The optical analytical instrument of claim 1, 2 or 3 in the form of an absorption spectrophotometer, said detector means comprising at least two detectors sensitized to respectively different wavelengths, one wavelength corresponding to a characteristic absorption of a component of interest in said fluid zone, the detected signal varying with the concentration of said component, and the other wavelength being a reference wave-length for which said component of interest has relatively low absorbance.
10. The optical analytical instrument of claim 1, 2 or 3 wherein said detector means is positioned to receive scattered or fluorescent radiant energy from said sample.
11. The optical analytical instrument of claim 10 wherein said filter is positioned to filter said scattered or fluorescent radiant energy.
12. The optical analytical instrument of claim 10 wherein said detector is positioned to receive Raleigh scattering or fluorescence.
13. The optical analytical instrument of claim 1 in the form of a turbidimeter.
14. The optical analytical instrument of claim 13 further including an optical reference path bypassing said sample.
15. The optical analytical instrument of claim 1, 2 or 3 wherein said means defining said optical path from said radiation source to said fluid zone is comprised of optical fibers.
16. The optical analytical instrument of claim 1, 2 or 3 wherein said means to make a calibration adjustment is constructed to calibrate the instrument.
17. The optical analytical instrument of claim 16 wherein said means responsive to make said calibration adjustment comprises means to adjust the sensitivity of the instrument.
18. The optical analytical instrument of claim 16 wherein said means to make said calibration adjustment comprises means to adjust the zero reference of said instrument.
19. The optical analytical instrument of claim 1, 2 or 3 including means to provide said known value in the form of means to store a reading of the detector means taken with said filter of known absorbance in place and with said fluid zone empty.
20. A method of calibrating an optical analytical instrument capable of producing amplitude readings based upon the effect of a fluid upon a selected fixed wavelength in a beam of optical radiation to determine a quantitative physical parameter of the fluid, comprising providing optical source means, optical path means, and optical detector means responsive to said selected fixed wavelength, taking from said detector means at least two amplitude readings at said selected fixed wavelength and within a predetermined amplitude measurement range of the instrument, of optical energy that result from the influence of the fluid on the optical radiation in a fluid zone without there having occurred substantial change to the fluid, including in said optical path means a filter of known absorbance that absorbs a predetermined portion of the energy at said fixed wavelength that represents a substantial fraction of said amplitude measurement range of the instrument so that one of said amplitude readings represents a calibration reading within said measurement range of the instrument, first, comparing, effectively, said two amplitude readings to remove said effect of said fluid from the value of said calibration reading, second, comparing, effectively, the residual value of said calibration reading to a known value based on the known absorbance of said filter, and, on the basis of said second comparison, determining an amplitude calibration value for the instrument based on values derived while said fluid sample remains unchanged in said fluid zone.
21. A method of calibrating an optical analytical instrument capable of producing readings based upon the effect of a fluid on a beam of optical energy to determine a physical parameter of the fluid, comprising providing optical radiation source means that includes a "measure" emitter and a "calibration" emitter separately energizable by control means, optical path means, and optical detector means, taking from said detector means at least two readings of optical energy, that has been influenced by the fluid in a fluid zone without there having occurred substantial change to the fluid, by energizing said "measure" emitter by said control means during the taking of a normal measurement, and by energizing said "calibration" emitter by said control means during the taking of a calibration reading while a filter of known absorbance is included in said optical path means from said calibration emitter to said fluid, first, comparing, effectively, said two readings to remove said effect of said fluid from the value of said calibration reading, second, comparing, effectively, the residual value of said calibration reading to a known value based on the known absorbance of said filter, and, on the basis of said second comparison, making a calibration adjustment based on values derived while said fluid sample remained unchanged in said fluid zone.
22. A method of calibrating an optical analytical instrument capable of producing readings based upon the effect of a fluid on a beam of optical energy to determine a physical parameter of the fluid, comprising providing optical radiation source means that includes a "measure" emitter and a "calibration" emitter separately energizable by control means, optical path means, and optical detector means, taking from said detector means at least two readings of optical energy, that has been influenced by the fluid in a fluid zone without there having occurred substantial change to the fluid, by energizing said "measure" emitter by said control means during the taking of a normal measurement, and by energizing a "calibration" emitter by said control means during the taking of a calibration reading while a filter of known absorbance is included in the optical path from said calibration emitter to said fluid, first, comparing, effectively, said two readings to remove said effect of said fluid from the value of said calibration reading, second, comparing, effectively, the residual value of said calibration reading to a known value based on the known absorbance of said filter, and, on the basis of said second comparison, making a calibration adjustment based on values derived while said fluid sample remained unchanged in said fluid zone, wherein said calibration adjustment is effected by selective energization of said emitters during operation without moving any part of said instrument.
23. The method of claim 20, 21 or 22 wherein said fluid flows in a process stream and said method further comprising taking two readings by said control means within such a short time interval that there is insufficient passage of time to allow substantial change to occur in said parameter of said fluid in said process stream.
24. The method of claim 20, 21 or 22 further including the steps of filling a titration chamber by a liquid process stream and, after titration, flushing said titration chamber only by the process stream.
25. The method of claim 20, 21 or 22 further including the steps of filling a titration chamber by a liquid process stream and, after titration, flushing said titration chamber by a suitable liquid.
26. The method of claim 24 wherein the calibration is performed for a colorimeter.
27. The method of claim 25 wherein the calibration is performed for a colorimeter.
28. The method of claim 20, 21 or 22 wherein the calibration is performed for an absorption spectrophotometer.
29. The method of claim 20, 21 or 22 wherein the calibration is performed for a turbidimeter.
30. The method of claim 20, 21 or 22 wherein said step of making a calibration adjustment calibrates the instrument.
31. The method of claim 30 wherein said calibration adjusts the sensitivity of the instrument.
32. The method of claim 30 wherein said calibration adjusts the zero reference of the instrument.
33. The method of claim 20, 21 or 22 further including the steps of storing a reading of the detector means taken with said filter of known absorbance in place and with said fluid zone empty.
34. The method of claim 20, 21 or 22 wherein said step of making a calibration adjustment includes updating a current filter absorbance value.
35. The method of claim 34 wherein said calibration adjustment step is periodically repeated and said current filter value is successively updated during these repetitions.
36. The method of claim 34 wherein past trends in said current absorbance value are taken into consideration in updating said current absorbance value.
37. The apparatus of claim 1 or the method of claim 20 wherein, respectively, the instrument is constructed or the method is conducted to measure concentration or the like, C, according substantially to the relationshipin which the values are substantially as defined in the foregoing specification.
38. The method of claim 25 wherein the calibration is performed for a colorimeter.
39. The optical analytical instrument of claim 1 wherein said substantial fraction is of the order of 30 percent.
40. The method of claim 20, 21 or 22 wherein said substantial fraction is of the order of 30 percent.
41. The optical analytical instrument of claim 1 wherein said instrument has no moving parts for accomplishing calibration.
42. The method of claim 20 wherein said calibration adjustment is effected without moving any part of said instrument.
43. The optical analytical instrument of claim 6 in the form of a colorimeter.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/596,513 | 1990-10-12 | ||
| US07/596,513 US5125747A (en) | 1990-10-12 | 1990-10-12 | Optical analytical instrument and method having improved calibration |
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| Publication Number | Publication Date |
|---|---|
| CA2053398A1 CA2053398A1 (en) | 1992-04-13 |
| CA2053398C true CA2053398C (en) | 1996-06-25 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002053398A Expired - Lifetime CA2053398C (en) | 1990-10-12 | 1991-10-11 | Optical analytical instrument and method |
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|---|---|
| US (1) | US5125747A (en) |
| EP (1) | EP0480753B1 (en) |
| JP (1) | JP3306079B2 (en) |
| AT (1) | ATE158409T1 (en) |
| CA (1) | CA2053398C (en) |
| DE (1) | DE69127670T2 (en) |
| HK (1) | HK1003343A1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5357336A (en) * | 1991-08-08 | 1994-10-18 | The Dow Chemical Company | Method and apparatus for multivariate characterization of optical instrument response |
| US5370842A (en) * | 1991-11-29 | 1994-12-06 | Canon Kabushiki Kaisha | Sample measuring device and sample measuring system |
| US5298978A (en) * | 1992-02-28 | 1994-03-29 | Artel, Inc. | Pipette calibration system |
| US5492673A (en) * | 1992-02-28 | 1996-02-20 | Artel, Inc. | Reagent system for calibration of pipettes and other volumetric measuring devices |
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- 1991-10-11 CA CA002053398A patent/CA2053398C/en not_active Expired - Lifetime
- 1991-10-11 EP EP91309385A patent/EP0480753B1/en not_active Expired - Lifetime
- 1991-10-14 JP JP26504591A patent/JP3306079B2/en not_active Expired - Fee Related
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| US5125747A (en) | 1992-06-30 |
| EP0480753B1 (en) | 1997-09-17 |
| EP0480753A2 (en) | 1992-04-15 |
| HK1003343A1 (en) | 1998-10-23 |
| JPH04259847A (en) | 1992-09-16 |
| DE69127670D1 (en) | 1997-10-23 |
| ATE158409T1 (en) | 1997-10-15 |
| DE69127670T2 (en) | 1998-04-09 |
| JP3306079B2 (en) | 2002-07-24 |
| EP0480753A3 (en) | 1992-12-30 |
| CA2053398A1 (en) | 1992-04-13 |
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