WO2007064370A2 - Ultra low cost ndir gas sensors - Google Patents

Ultra low cost ndir gas sensors Download PDF

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Publication number
WO2007064370A2
WO2007064370A2 PCT/US2006/030440 US2006030440W WO2007064370A2 WO 2007064370 A2 WO2007064370 A2 WO 2007064370A2 US 2006030440 W US2006030440 W US 2006030440W WO 2007064370 A2 WO2007064370 A2 WO 2007064370A2
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Prior art keywords
source
detector
sensor
temperature
gas species
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PCT/US2006/030440
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French (fr)
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WO2007064370A3 (en
Inventor
Jacob Y. Wong
Chi Wai Tse
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Airware Inc.
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Priority claimed from US11/197,790 external-priority patent/US7358489B2/en
Priority claimed from US11/198,106 external-priority patent/US7329870B2/en
Application filed by Airware Inc. filed Critical Airware Inc.
Publication of WO2007064370A2 publication Critical patent/WO2007064370A2/en
Publication of WO2007064370A3 publication Critical patent/WO2007064370A3/en

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    • 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
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • 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
    • G01N21/3151—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 using two sources of radiation of different 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
    • G01N21/25—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
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/359—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using near infrared light
    • 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
    • G01N21/59—Transmissivity
    • G01N21/61—Non-dispersive gas analysers

Abstract

The concentration of a single gas species can be detected using a single beam NDIR gas sensor having a differential infrared source element producing radiation having a first spectrum at a first high temperature and a second spectrum at a second lower temperature, a detector and a dual pass band filter. The gas sensor further includes a driver driving the source at either the fjrst or second temperatures; a feed back loop; a differential gain amplifier creating a high cycle amplified output during a high cycle and a low cycle amplified output during a low cycle; a controller for synchronizing the driver and driving the source at the first temperature and applying high cycle amplification during the high cycle and driving the source at the second temperature and applying a low cycle amplification during the low cycle; and a signal processor for determining the concentration of he gas species.

Description

ULTRA LOW COST NDIR GAS SENSORS
Field of the Invention
The present invention generally relates to the field of gas sensing devices and, more particularly, to NDIR gas analyzers.
Background of the Invention
Non-Dispersive infrared (NDIR) gas analyzers have been used for detecting the presence and concentration of various gases for over four decades. The NDIR technique has long been considered as one of the best methods for gas measurement. In addition to being highly specific, NDIR gas analyzers are also very sensitive, stable and easy to operate and maintain.
In contrast to NDIR gas sensors, the majority of other types of gas sensors today are in principle interactive. Interactive gas sensors are less reliable, generally nonspecific, and in some cases can be poisoned or saturated into a nonfunctional or irrecoverable state.
Despite the fact that interactive gas sensors are mostly unreliable and that the NDIR gas measurement technique is one the of best there is, NDIR gas analyzers have still not enjoyed widespread usage to date mainly because of the fact that their cost is still not low enough as compared to other inferior gas sensors for many applications.
In the past, NDIR gas analyzers typically included an infrared source, a motor-driven mechanical chopper to modulate the source, a pump to push or pull gas through a sample chamber, a narrow bandpass interference filter, a sensitive infrared detector plus expensive infrared optics and windows to focus the infrared energy from the source to the detector. In an attempt to reduce the cost and simplify the implementation of the NDIR methodology, a low-cost NDIR gas sensor technique was earlier developed. This low-cost NDIR technique employs a diffusion-type gas sample chamber of the type disclosed in U.S. Pat. No. 5,163,332, issued on Nov. 17, 1992 to Wong, the present applicant. This diffusion-type gas sample chamber eliminates the need for expensive optics, mechanical choppers and a pump for pushing or pulling the gas into the sample chamber. As a result, a number of applications using NDIR gas sampling technique, which were previously considered impractical because of cost and complexity, have been rendered viable ever since.
In the ensuing years since U.S. Pat. No. 5,163,332 was issued, Wong, the present applicant, has continued to refine and improve low-cost NDIR gas sampling techniques as evidenced by the issuance of U.S. Pat. Nos. 5,222,389 (Jun. 1993), 5,341 ,214 (Aug. 1994), 5,347,474 (Sep. 1994), 5,453,621 (Sep. 1995), 5,502,308 (Mar. 1996), 5,747,808 (May 1998), 5,834777 (Nov. 1998) and 6,237,575 (May 2001 ) to same. Until recently, efforts to reduce the cost of an NDIR gas sensor have been concentrated mainly in the areas of developing lower cost infrared components, improving sensor structural and optical designs and forging innovations and simplifications in electronic signal processing circuits. Hardly any significant effort has been devoted to sensor cost reduction via new NDIR sensor methodology. Up until now, the most prevalent NDIR gas sensor today is a dual beam device having a signal and a reference beam implemented with a single infrared source and two separate infrared detectors, each having a different interference filter. The signal filter contains a narrow spectral passband that allows radiation relevant to the absorption of the gas to be detected to pass. Thus the presence of the gas of interest will modulate the signal beam. The reference filter contains a narrow spectral passband that is irrelevant to the gas in question and also to all the common gases present in the atmosphere. Therefore the reference beam will stay constant and act as a reference for the detection of the designed gas species over time. Although the dual beam technique works well for a host of applications, especially with the detection of relatively low concentration of Carbon Dioxide (CO2) gas (400 - 2,000 ppm) for HVAC (Heating, Ventilation and Air Conditioning) and IAQ (Indoor Air Quality) applications, the cost of the sensor is limited by the expensive detector package which contains two detectors each equipped with a different interference filter. Furthermore, the dual beam NDIR gas sensor still has a number of shortcomings that require special treatments in order to render the sensor adequately reliable and stable for use over time. These shortcomings include the aging of the infrared source which might cause the spatial distribution of infrared radiation reaching the detectors to change; the same applies to the non-uniform aging of the inner reflective surfaces of the sample chamber affecting the spatial distribution of the impinging radiation at the detector assembly, the different aging characteristics for the two interference filters each being manufactured via different deposition processing steps and optical materials and finally the potential different aging characteristics for the two detectors.
Logic would dictate that in order to improve the performance and to lower the cost of the ever more popular dual beam NDIR gas sensor, one has to resort to system structural or optical simplification and/or system components reduction. Taking the case of the dual beam NDIR gas sensor as an example, there are two ways that one can accomplish these objectives. First, one can reduce the number of detectors from two to one which automatically implies that the number of interference filters would also be reduced from two to one. In other words, one can attempt to convert the dual beam sensor methodology to a single beam one. Alternatively, one can increase the measurement capability of the dual beam sensor from being able to detect just one gas into one that can simultaneously detect two or more gases. If either of these two cases is successful, it would be equivalent to being able to reduce the unit cost for the dual beam NDIR sensors. It is of interest to note that back in 1991 and prior to the issuance of U.S.
Pat. No. 5,163,332 (1992) to Wong for the advent of the so-called "waveguide sample chamber," the same inventor has earlier advanced the concept of a single beam NDIR sensor methodology using a spectral ratioing technique with a differential temperature source in U.S. Pat. No. 5,026,992 (1991 ). After almost 15 years, this concept has to date neither been proven to be viable in theory nor has it been experimentally demonstrated in order to illustrate its practicality. It was found out only very recently by Wong, the current applicant and the original author of U.S. Pat. No. 5,026,992 (1991 ), that although the concept advanced in said patent was sound, the method did not work when the prescribed steps were followed exactly according to the teaching of the patent.
There is still a long felt need in a variety of industries and applications to use lower cost NDIR gas sensors, and so far this desire has gone unanswered. It is this need that the current application seeks to address and bring about a new and novel technique for the design and implementation for ultra low cost NDIR gas sensors.
SUMMARY OF THE INVENTION
The present invention relies upon a single beam NDIR gas sensor for detecting the concentration of a gas species in a sample chamber with a differential infrared source element that can produce radiation having a first spectrum when its temperature is at a first high temperature and a second spectrum when its temperature is at a second lower temperature, a detector for generating a detector output and a dual pass band filter located between the source element and the detector. The present invention is generally directed to such an NDIR gas sensor which also includes a driver for driving the source at either the first or the second temperature, a feed back loop to sense an operation voltage of the source, a differential gain amplifier for creating a high cycle amplified output during a high cycle and a low cycle amplified output during a low cycle, and a controller for synchronizing the driver so that the source is driven at the first temperature and a high cycle amplification is applied to the detector output during the high cycle and the source is driven at the second temperature and a low cycle amplification is applied to the detector output during the low cycle while a signal processor determines the concentration of the gas species through use of the high cycle amplified output and the low cycle amplified output.
In a separate aspect of the present invention, the concentration of a gas species is determined by such an improved NDIR gas sensor by the steps of driving the source element at a first high temperature and then applying a high cycle amplification to the detector output to create a high cycle amplified output, driving the source element at a second low temperature and than applying a low cycle amplification to the detector output to create a low cycle amplified output and determining the concentration of the gas species through use of the high cycle amplified output and the low cycle amplified output.
In another separate aspect of the present invention, a single beam NDlR gas sensor uses a thermally insulated tube sample chamber, an incandescent miniature light bulb with a filament surrounded by a glass envelope secured at a first end of the sample chamber, a single infrared detector secured at a second end of the sample chamber, a dual bandpass filter (having a neutral passband and an absorption passband for the gas species) mounted at the single infrared detector between the bulb and the detector, a controlled heater secured to the tube for maintaining the sample chamber at a preselected temperature greater than an ambient temperature when the sensor is turned on, a driver for the bulb with a high input power level and a low input power level so that the bulb will emit radiation at first and second voltage outputs characterized by two corresponding Planck curves dependent upon temperatures, a feed back loop to sense an operation voltage of the bulb, a differential gain amplifier for creating a high cycle amplified output during a high cycle and a low cycle amplified output during a low cycle, a controller for synchronizing the driver so that the bulb is driven at the high input power level and a high cycle amplified gain is applied to the detector output during the high cycle and the bulb is driven at the low input power level and a low cycle amplified gain is applied to the detector output during the low cycle and a signal processor for determining the concentration of the gas species through use of the high cycle amplified output and the low cycle amplified output.
In a related but still separate aspect of the present invention, a single beam NDIR gas sensor such as was just described is used to detect the concentration of a gas species by heating the sample chamber to a preselected temperature greater than an ambient temperature and maintaining the sample chamber at the preselected temperature, driving the bulb at a first high voltage input and then applying a high cycle amplification to the detector output to create a high cycle amplified output, driving the bulb at a second low voltage input and than applying a low cycle amplification to the detector output to create a low cycle amplified output and then determining the concentration of the gas species through use of the high cycle amplified output and the low cycle amplified output. In addition, a feed back loop can be used to sense the operation voltage of the bulb while the bulb is synchronized so that it is driven at the first high voltage input and the high cycle amplified output is applied to the detector output during a high cycle and the bulb is driven at the second low voltage input and the low cycle amplified output is applied to the detector output during a low cycle.
In still a further group of aspects of the present invention, the glass envelope of the incandescent miniature light bulb used in the single beam NDIR gas sensor is maintained at an equilibrium temperature (such as approximately 30 degrees Celsius) during the low cycle operation state by the controlled heater, the equilibrium temperature is a constant temperature that varies by less than two degrees Celsius while the ambient temperature is 22 degrees Celsius, and the glass envelope of the incandescent miniature light bulb is the primary radiation emitter during the low cycle.
In yet a further group of aspects of the present invention, the single beam NDIR gas sensor sample chamber is secured to a first side of a printed circuit board, the signal processing circuit components are mounted on a second side of the printed circuit board, an insulated aluminum tube sample chamber is configured with at least one substantial U-bend and a casing surrounds the printed circuit board.
The present invention is also generally directed to a method for detecting the concentrations of N gas species from a single beam NDIR gas sensor having a differential infrared source and an (N+1 ) - passband filter (having a neutral passband and N absorption passbands for N gases) mounted at a single infrared detector by driving the infrared source with N input power levels to render the source into emitting at N distinct temperatures whose radiation outputs are characterized by N corresponding Planck curves which are dependent only upon the respective source temperatures and which link a Spectral Radiant Emittance MsubLamba with wavelength, measuring N detector outputs at the single infrared detector and detecting the concentrations of N different gas species, each of the N gas species having its own unique infrared absorption passband, by (a) setting up N causality relationship equations linking outputs of the detector respectively for N different source temperatures and a set of relevant parameters of the sensor components, (b) determining the values of all of the parameters for the N equations utilizing appropriate boundary conditions except the N concentrations for the respective N gas species, and (c) solving for the N gas concentrations with the measured N detector outputs, there being N equations and N unknowns, when N is an integer of 2 or more.
In an additional, separate group of aspects of the present invention, a single beam NDIR gas sensor for detecting the concentrations of N gas species according to the method of the present invention is disclosed which includes a differential infrared source (which may be a genuine or a non-genuine blackbody source), a single infrared detector, a multiple-passband filter mounted at the single infrared detector having a neutral passband and N absorption passbands for N gases species incorporated into the multiple-passband filter, each of the N gas species having its own unique infrared absorption passband, a driver for the infrared source with N input power levels so as to render said source into emitting at N distinct temperatures whose radiation outputs are characterized by N corresponding Planck curves which are dependent only upon the respective source temperatures and which link a Spectral Radiant Emittance MsubLamba with wavelength, and electronics for detecting the concentrations of N different gas species by solving N causality relationship equations with N unknowns linking outputs of the detector respectively for N different source temperatures and a set of relevant parameters of the sensor components that have been determined utilizing appropriate boundary conditions except the N concentrations for the respective N gas species, wherein N is an integer of 2 or more.
In still other, separate aspects of the present invention, each of the N absorption passbands for N gases is specific to passing a particular spectral radiation for one of the N gases to be detected, the values of all of the parameters of the N causality relationship equations except for the N concentrations for the respective N gas species are performed as part of an initialization process and then the N concentrations can be carried out repeatedly as part of a real time process to detect the concentrations of N different gas species through use of N calibration curves.
Accordingly, it is a primary object of the present invention to develop a new and novel sensor concept for the realization of the long sought after ultra low cost NDIR gas sensors. This and further objects and advantages will be apparent to those skilled in the art in connection with the drawings and the detailed description of the preferred embodiment set forth below.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1. The spectral transmission curve for an actually fabricated dual passband filter (2.2 μ and 4.26 μ) for use with a single beam CO2 NDIR sensor utilizing the currently invented methodology.
Figure 2. The large amplitude difference in the detector outputs as observed experimentally (upper trace) between the TH and the TL states. The same outputs for the two states (lower trace) after the currently invented real time programmable infrared source control is applied to the single detector circuit.
Figure 3. A schematic circuit illustrating the real time programmable infrared source control. Figure 4. A schematic circuit illustrating the control of the radiation source via the synchronization of the detected high and low signals by the microprocessor in one 'AC cycle using the multi-channel Analog-to-Digital converter chip.
Figure 5. A schematic diagram illustrating the currently invented sample chamber configuration for controlling and regulating the temperature of the single beam sample chamber.
Figure 6. Three blackbody Planck curves depicting respectively infrared source temperatures of T1 = 9000K, T2 = 7000K and T3 = 5000K. Also shown are the Center Wavelengths (CWL's) of absorption bands for gases G1 , G2, G3 and the neutral reference gas GN.
Figure 7. The transmittance curve for the custom 4-passband interference filter depicting the four respective Center Wavelengths (CWL's) of the absorption bands for the gases G1 , G2, G3 and the neutral reference.
DETAILED DESCRIPTION OF THE INVENTION
The most prevalent NDIR gas sensor today is a dual beam device having a signal and a reference beam implemented with a single infrared source and two separate infrared detectors, each having a different interference filter. The signal filter contains a narrow spectral passband that allows radiation relevant to the absorption of the gas to be detected to pass. Thus the presence of the gas of interest will modulate the signal beam. The reference filter contains a narrow spectral passband that is irrelevant to the gas in question and also to all the common gases present in the atmosphere. Therefore the reference beam will stay constant and act as a reference for the detection of the designed gas species over time. Although the dual beam technique works well for a host of applications, especially with the detection of relatively low concentrations of carbon dioxide (CO2) gas (400 - 2,000 ppm) for HVAC (Heating, Ventilation and Air Conditioning) and IAQ (Indoor Air Quality) applications, the cost of the sensor is limited by the expensive detector package which contains two detectors each equipped with a different interference filter. Furthermore, the dual beam NDIR gas sensor still has a number of shortcomings that require special treatments in order to render the sensor adequately reliable and stable over time. These shortcomings include the aging of the infrared source which might cause the spatial distribution of infrared radiation reaching the detectors to change; the same applies to the non-uniform aging of the inner reflective surfaces of the sample chamber affecting the spatial distribution of the impinging radiation at the detector assembly, the different aging characteristics for the two interference filters each being manufactured via different deposition processing steps and materials and finally the potential aging characteristics for the two detectors to change differently.
In order to improve the performance and cost of the ever more popular dual beam NDIR gas sensor, one has to seek favorable opportunities in the detector assembly end of this class of sensors. Needless to say, if one can reduce the number of detectors from two to one which in effect reduces the dual beam technique into a single beam one, while at the same time render this new technique adequately workable for an accurate, reliable and stable NDIR gas sensor, then the goal of achieving an ultra low cost sensor might become viable.
The first task at hand is to find out how to create spectrally and functionally a dual beam situation with only a single infrared source and a single detector. One conclusion is that we will be able to do very little with the infrared detectors and the interference filters which the dual beam sensor carries because they are passive components. Therefore, one approach is to do something with the infrared source which is an active component. As observed in U.S. Pat. No. 5,026,992 (1991 ) issued to Wong, one can change the spectral characteristic output of the source according to the Planck's radiation curves by driving it at different power levels so as to assume different blackbody temperatures at different times. This can in fact be readily achieved since one has to pulse the infrared source anyway as in the dual beam technique. By so doing it is possible to create two beams with different spectral characteristics for the source. However, how would the detector respond differently to these two beams? Again by observing U.S. Pat. No. 5,026,992 (1991 ) mentioned earlier, one can resort to the use of a dual passband interference filter with one of the spectral bands relevant to the gas to be detected and the other simply a reference or neutral band. Thus, at least in concept, as advanced in the earlier cited patent, with the use of a single detector carrying a dual passband filter and by driving the infrared source at two different power levels, one should theoretically be able to derive information about the gas in question by calibrating the ratio of the outputs for the two beams with the concentration levels for the gas, very much like the way for a conventional dual beam NDIR gas sensor.
While this thinking approach to achieve our goal at hand appears sound in concept, it is a totally different story when it comes to carrying it out in practice. This method simply does not work even when using a genuine blackbody source which assumes only one single temperature when driven at one particular power level and radiates according to one unique Planck curve. It was recently found out experimentally that in order for this method to be able to detect low to medium concentrations of gas, e.g. 400-2,000 ppm of CO2 with a resolution of +/- 50 ppm, and even with a sample pathlength of 8 inches (longer than what is usually needed for a conventional dual beam sensor), the power levels needed to drive the infrared source in order to create two adequate and spectrally different beams have to differ by more than a factor of 20. In other words, the low power driven beam has such a small generated signal at the detector that it cannot be readily processed by any reasonably designed processing electronics.
One potential solution to this problem is to increase the gain of the amplifier stages following the detector. However, this is not feasible as we have created in reality just a single beam with a single set of processing electronics. Since the two spectral beams created with the use of only one source and one detector will in essence be processed by the same signal electronics, arbitrarily increasing the gain of the amplifier stages will no doubt render the low power beam signal more amenable to processing but will also send the high power beam signal to the rail (exceeding voltage supply limits). Unless some novel signal processing approach is advanced as by the present invention, such an approach will simply not work.
Another potential solution to this problem is to shift the driving power ratio to higher blackbody temperatures in order to increase the low power beam signal at the detector. Unfortunately, unlike the tungsten filament of a miniature light bulb, which can be driven to temperatures in excess of 2,0000C, genuine blackbody sources with temperatures above 75O0C simply are not readily available unless they are custom fabricated and consequently carry a very high unit cost. Therefore the approach of operating the infrared source at a higher temperature ratio using a genuine blackbody source is inconsistent with the goal of trying to develop an ultra low cost NDIR gas sensor and thus is not cost-wise logical.
Even if we could somehow make the conventional genuine blackbody source work with the currently proposed single detector approach, the cost for a genuine blackbody source might still be prohibitively high so as to render the proposed approach impractical. Since the goal of the present invention is to bring forth a novel approach of using just one infrared source and one infrared detector for achieving the goal of implementing an ultra low cost NDIR gas sensor, such a technique must also be made to work with a non-genuine blackbody source, such as a much lower cost miniature incandescent light bulb.
The present invention advances a novel single beam methodology, with the use of a low cost non-genuine blackbody source such as an incandescent light bulb, and an infrared detector equipped with a dual passband interference filter. In order to overcome the signal processing difficulty in the handling of two beam signals with a vast amplitude discrepancy, a novel real time programmable infrared source control technique is advanced. Such a technique enables the common signal processing electronics for the detector to attain a synchronized multiple amplifier gain capability for two or more output power states from the infrared source. The present invention further advances a novel sample chamber configuration for the sensor in order to render the use of a non-genuine blackbody source, in lieu of a genuine blackbody source, for successfully using a single beam methodology for the implementation of an ultra low cost NDIR gas sensor.
As mentioned earlier, the concept advanced in U.S. Pat. No. 5,026,992 calls for operating a genuine blackbody source alternately at a high emission temperature state, TH, and then at a low emission temperature state, TL, in order to shift the spectral content of the source. The theoretical example for the detection of methane using this methodology as cited in said patent uses TH and TL equal to 7230K and 5230K respectively which provides a 1.0% change in the calculated output signal (Rs or the voltage ratio for the TH and TL states) for detecting a level of 10,000 ppm of methane. Even in this theoretical example, the output for state TH can be calculated using Planck's curves to be more than 11 times the output for the state TL. In this simulated calculation, a genuine blackbody source having an area of ~2 mm x 2mm is used together with the characteristics for the dual passband filter (2.20 μ and 3.40μ) as suggested in the patent mentioned above and a standard thermopile detector having a typical responsibility of -200 V/W. In order to demonstrate experimentally an even more difficult disposition for the detection of CO2 gas, an actual dual bandpass filter having Center Wavelengths (CWL's) at ~ 2.20μ and ~4.23 μ respectively was procured as depicted in Figure 1. Using a 1.5 mm x 1.5 mm thick film resistor fabricated on an 10 mils thick alumina substrate as a genuine blackbody source and the dual passband filter as shown in Figure 1 mounted on a 1mm x 1 mm thermopile detector can (TO-18), the voltage outputs at the detector for driving the genuine infrared source at 75O0C (TH state) and 3000C (TL state) respectively are shown in Figure 2. It can be seen from Figure 2 (upper trace) that the voltage amplitude for the TH state 1 is almost an order of magnitude greater than the voltage amplitude for the TL state 2, thus practically demonstrating the difficulty in the implementation of the single beam NDIR gas sensor concept as advanced in U.S. Pat. 5,026,992 (1991 ). For a methane single beam NDIR gas sensor requiring a much higher resolution such as 100 ppm, e.g., the source temperature for the low emission state has to be much lower than 5230K in order for it to work properly. The resultant discrepancy between the output voltages for states TH = 7230K and TL = 3230K is estimated to be 50 times or more. This creates an extraordinarily difficult situation for the design of a single signal processing circuit serving both the TH and TL output states from one and the same infrared detector. An adequate amplifier gain for the TL output state would easily increase the output level for the TH state to exceed the voltage supply limit thus rendering the signal processing circuitry effectively nonfunctional. In order to overcome this difficulty in the design of a suitable signal processing circuit for this differential source temperature single beam NDIR sensor concept, the present inventors advance the methodology of a real time programmable infrared source control for attaining a synchronized multiple amplifier gain capability for two or more output states from a single detector. Such a control is shown schematically in Figure 3.
As shown in Figure 3, one can see that as many as three feedback loops are operating simultaneously between the microprocessor 3 and the Radiation control - current source 4. At a particular point in time, the digital data stream from microprocessor 3 is routed through a Digital to Analog conversion chip 5 in order to generate a programmed DC voltage to drive the Radiation control - current source 4 with the help of the Emitter Follower 6 and Voltage Supply 7. The correct adjustment of the programmed voltage for the source is determined by the use of a feedback loop to sense the operation voltage of the source which is then converted using Analog to Digital converter 8 before returning back to the microprocessor 3. Meanwhile it is the microprocessor 3 that generates a Radiation ON/OFF control signal 9 for synchronizing (or alternating) the correct programmed voltages for operating both the TH and the TL source emission states. In summary, the High and Low signals detected in one "AC" cycle are synchronized by the microprocessor 3 to control the radiation source 4 and the multi-channel ADC 10 simultaneously as shown in more detail in Figure 4.
As shown in Figure 4, the microprocessor 3 detects the High and Low signals from the Multi-Channel ADC 10 fed by both the Hi cycle amplifier 11 and Low cycle amplifier 12 from the front end amplifier 13 generated by the single source detector 14. By processing these signals every AC cycle, the microprocessor 3 is able to synchronize the two different voltage levels applied to just one single radiation source. Furthermore, the different gain factors applied to the Hi and Low cycle amplifiers are also correctly applied to the signals detected during the High and Low cycles thereby eliminating the possibility that the voltage level for the High cycle (or TH) may exceed the supply voltage limit. This operational feature is illustrated in Figure 2 (lower trace) when applied to the experimentally implemented single beam CO2 sensor using an actual dual passband filter. As one can see in Figure 2 (lower trace), the amplified voltage for the TH state 15 and the amplified voltage for the TL state 16 which correspond respectively to the non-amplified voltages 1 and 2 (upper trace) are both in range despite their great discrepancy in the pre-amplified signal levels.
The differential temperature source concept for implementing a single beam NDIR gas sensor as disclosed in U.S. Pat. No. 5,026,992 (1991 ) calls for the use of a genuine blackbody source. In other words, the suggested infrared source to be used must behave precisely like a blackbody with its output or spectral radiant emittance, Mλ, uniquely determined by a single source temperature as prescribed by the well-known Planck's Law. As alluded to earlier, the use of genuine blackbody sources that are available today might still be too cost limiting contrary to the ultra low cost goal that the current applicants are trying to achieve. For the past two decades, the use of very low cost miniature incandescent light bulbs as non-genuine but practical infrared sources for NDIR gas sensors, including the dual beam sensor types, has gained worldwide acceptance. The cost advantage for the ultra low cost single beam NDIR sensor could be significant if a non-genuine blackbody source like the incandescent light bulb could be utilized in lieu of a genuine blackbody one. The reason why incandescent light bulbs are considered as non-genuine blackbody sources can be explained as follows. Typically an incandescent miniature light bulb has a tungsten filament packaged in vacuum surrounded by a glass envelope. When the light bulb is used as a pulsating infrared source, the tungsten filament will be turned alternately on and off. The tungsten filament taken alone is a genuine blackbody source emitting radiation in all wavelengths long and short dependent upon its operating temperature. Meanwhile the spectral transmission characteristic of the glass envelope has a sharp cutoff somewhere between 3 and 4.5 microns. Thus some of the long wavelength radiation emitted by the tungsten filament will be absorbed by the envelope resulting in a rapid rise in temperature when the tungsten filament is turned on. After some operation time has elapsed, the tungsten filament and the bulb envelope will come to a thermal equilibrium. The net result is that in addition to the tungsten filament acting as a high temperature infrared source (a genuine blackbody) for the incandescent light bulb, the bulb envelope also behaves as a second infrared source albeit at a much lower temperature. But since the effective area of the bulb envelope is very much larger than that for the tungsten filament, its contribution to the total radiation output for the light bulb as an infrared source could be comparable to that of the tungsten filament itself. The resultant spectral output of an incandescent light bulb is therefore a spectral convolution of the two separate sources, namely the tungsten filament and the bulb envelope. For this reason the incandescent light bulb is technically considered as a non-genuine blackbody source since it is not uniquely characterized by just one single source temperature. No teaching or suggestion can be found in U.S. Patent No. 5,026,992
(1991 ) as to how the spectral ratioing differential source temperature concept might work or not work for a single beam NDIR sensor if a non-genuine blackbody is used in lieu of a genuine one as the infrared source. However, in order to achieve the goal of being able to manufacture an ultra low cost single beam NDIR sensor using this method, the present authors advance a novel sample chamber configuration for the sensor in order that a non-genuine blackbody source, in this case an incandescent miniature light bulb, can work successfully. As discussed earlier, in order to make the source differential temperature concept work one must create enough spectral contrast between the TH and the TL states. An efficacious way to accomplish this, like in the case for using a genuine blackbody as the infrared source, is to operate the TL state at as low a temperature as possible.
When a miniature incandescent light bulb is used as the infrared source, the temperature of the light bulb envelope becomes the primary radiation emitter for the TL state. This is due to the fact that the temperature of the filament during TL is very low (typically 300 - 4000K) and the area of the filament is also very small when compared with the effective area of the light bulb envelope (~100 times less). Furthermore, the light bulb envelope, being made out of glass, is absorbing a lot of long-wavelength radiated energy from the hot filament when it is in the TH state. Some of the absorbed heat persists to the immediately following TL state. Unfortunately this situation creates a serious problem for the sensor operation. The reason is that when the sensor is operating at or above room temperature, no problem arises because in the TL state, the light bulb envelope does not lose much heat to the environment and continues to retain its relatively high temperature as a radiation emitter. However, when the operating temperature of the sensor is below room temperature, the envelope starts to lose its efficacy as an efficient radiation emitter due to the rapid loss of heat from its emitting surface to the environment. When the operating temperature of the sensor approaches O0C or below, the light bulb envelope as an infrared source is virtually shut down because of the fact that its temperature will approach O0C or lower and therefore cease to be an effective infrared source for the single beam sensor.
The current invention advances a simple sample chamber configuration for the single beam sensor in order to cope with this potential problem by first designing the sample chamber in the form of an insulated U-bend shape tube 17 (insulation not shown) about 6 inches long and made out of aluminum, which is a good thermal conductor, as illustrated in Figure 5. An aluminum strut or beam 23 which houses a 3-watt wire-wound resistor 22 as a heater and a thermistor 24 for monitoring its temperature thermally connects the middle sections of the two ends of the U-tube as shown in Figure 5. The entire insulated sample chamber configuration including the U-tube sample chamber 17, the heater strut 23, the miniature incandescent light bulb 20 mounted at one end of the U-tube and the infrared detector 21 mounted at the other end is secured with hardware to one side of a printed circuit board (PCB) 18. The signal processing circuit components are mounted on the other side of the PCB 18.
The heater strut 23 serves to regulate the temperature of the entire insulated aluminum sample chamber configuration 17 to an elevated temperature above ambient at all times when the sensor is first turned on. This sample chamber configuration 17 with the strategically located heater strut 23 prevents the loss of heat from the light bulb envelope in the TL state to the ambient, even when the temperature of the latter falls below O0C. This novel configuration enables the single beam NDIR sensor to operate properly at all ambient temperatures. The sample chamber configuration 17 works both for the diffusion sampling mode and for the flow through sampling mode. In the former case, small holes located diagonally in pairs are drilled along the insulated U-bend tube approximately one half inch apart for the sampled air to freely diffuse through the sample chamber for detection. In the latter case two miniature hose fittings are secured one at each end of the U-tube sample chamber so that air can be pushed through or pulled through the sample chamber for detection as desired. Finally the entire PCB 18 housing the single beam NDIR gas sensor can be fitted into any plastic casing with appropriate dimensions as desired.
Thus, there has been described a methodology using a real time programmable blackbody radiation source control circuit for successfully implementing a differential source temperature single beam NDIR gas sensor. Furthermore, a novel sample chamber configuration is advanced in order to enable the use of a non-genuine blackbody source for successfully implementing a differential source temperature single beam NDIR gas sensor. So far, the present invention has been directed to a one-channel application or a single gas detection. The rest of the present invention will now address a multi-channel application for simultaneously detecting two or more gases, while still remaining as a single beam NDIR gas sensor.
In order to accomplish a single beam NDIR gas sensor capable of simultaneously detecting two or more gases, a new mathematical model based upon a new conceptual framework covering the analytical procedures for achieving said results has to be developed anew. The thinking of this conceptual framework is distinctly different from the ad hoc approach taken in U. S Pat. No. 5,026,992 (1991) for exploiting the fact that by operating a genuine blackbody source at two different input power levels, one is capable of shifting the spectral content of the source in two distinct ways according to the well-known Planck's Law of radiation. The current invention takes a much broader view in realizing that by operating the infrared source at N distinct power levels, it is equivalent to having N+1 detectors each equipped with a unique bandpass filter of its own. With the exception of one single detector among the N+1 ones, which is equipped with a reference or neutral filter, namely radiation that passes such a filter is irrelevant to the N gas species of interest for detection and also to all common gases that are present in the atmosphere, each of the N remaining detectors has a unique bandpass filter of its own for detecting a specific gas species. Such an equivalence is elegantly expressed as a set of simultaneous equations encompassing all the characteristic parameters for the blackbody source, the single infrared detector, the (N+1)-passband filter and last but not least, all the absorption properties of the gases to be detected. In order to be able to quantitatively formulate such a mathematical model, one must have a full comprehension of the physics of NDIR gas detection and all the relevant gas laws that govern the behavior of all the pertinent gases to be detected. The logistical thinking for the currently invented functional formulation for a single beam NDIR gas sensor capable of detecting N gas species can be briefly summarized as follows. First, the interference filter equipped at the single detector must have N+1 passbands each of which is specific to passing a particular spectral radiation. As alluded to earlier, N is also the number of input power levels used to drive the source creating in effect N distinct emitting blackbody temperatures for the source. Second, among the N+1 passbands, one is designated as the reference or neutral and it passes radiation that is irrelevant to the N gas species to be detected and also to all the common gas species that are present in the atmosphere. Third, each of the remaining N passbands passes the radiation that is relevant to or will be absorbed by one specific gas species to be detected. Fourth, for the detection of N gas species by the single beam NDIR gas detector, there will be N distinct input power levels sequentially driving the blackbody source (genuine or non-genuine type). Fifth, for each of the N input power levels driving the source, a causality relationship is set up linking the output of the detector with the other pertinent parameters including the presence or absence of the gases to be detected and their respective concentration levels. Sixth, once the N such equations are set up, system phenomenological conditions are defined in such a way as to identify or permit the calculations of all the constant parameters appearing in the set of N equations. Finally, there remain only N unknowns in the set of N equations representing respectively the concentrations of the gas species to be detected. As such these N unknowns can be readily determined to yield the concentrations of the gas species that are present in the single beam gas chamber. The advantage in a single beam NDIR sensor design and implementation using a differential source temperature concept so as to be able to simultaneously detect two or more gases is many-fold. First and foremost, having fewer infrared detectors is a simpler approach both from the sensor design and cost standpoints. The cost of N+1 detectors with N+1 interference filters is many times that of a single detector with just one (N+1 )-passband filter. It is certainly true that present interference filter design and fabrication technologies might limit the value of the number N to less than 5. ' However, once technology permits, the cost of an N+1 passbands filter should not be much more expensive than a single passband filter. Thus, from the cost standpoint, a single beam NDIR gas sensor capable of detecting 2 or more gases simultaneously with just one filter could be very significant. Second, since there is in essence only one optical beam in this method design, one of the major disadvantages for the current widely used dual beam NDIR sensor, namely the non-uniform radiation distribution at the multi- detector assembly caused principally by the aging of sensor components, is virtually eliminated, and, for this reason the single beam NDIR gas sensor is inherently more reliable and stable over time. Third, because of the fact that there is only one infrared source and one infrared detector and the output signals are taken to be the ratio of the detector outputs for any two different source temperature emission states, everything common to the two temperature states, such as dirt in the windows or inside the sample chamber, aging for the detector and the source etc., are further minimized as compared to the traditional dual beam NDIR sensor.
The logistical thinking for the currently invented functional formulation will now be described in more detail. In theory, such a formulation can be generalized to any value of N where N is the number of gas species that can be simultaneously detected by the single beam NDIR sensor and N+1 is the number of passbands possessed by the single filter equipped at the single detector. For simplicity and efficacy of explanation, and without sacrificing any substance of the invention, we will arbitrarily set N = 3. In other words, we will describe the formulation for a single beam NDIR detector capable of detecting simultaneously three different gas species using a custom interference filter having N+1 =4 passbands at the detector. Since N is also the number of input power levels used to drive the source in order to produce three distinct blackbody temperatures, we will have in this case three individual Planck's curves for representing the three different spectral radiant emittances, Mλ's, available from the source. Figure 6 shows schematically three blackbody Planck curves (N = 3) for the mathematical formulation of the present invention for a three-channel NDIR gas sensor, namely a sensor capable of simultaneously detecting the concentration of gases G1 , G2 and G3 in the sample chamber. For a single beam NDIR sensor capable of detecting three gas species using the present invention, an "N+1=4"-passbands filter would have to be used.
Figure 6 shows three blackbody Planck curves for temperatures at T1 , T2 and T3 (in deg K), respectively. The blackbody Planck curve 1 is shown for source temperature T1 = 9000K or 6270C. The blackbody Planck curve 2 is shown for source temperature T2 = 7000K or 4270C. The blackbody Planck curve 3 is shown for source temperature T3 = 5000K or 2270C. Let the three gas species to be detected be G1 , G2 and G3 with the respective CWL of their absorption bands, 4, 5 and 6 respectively located at λ1 , λ2 and λ3 as shown in Figure 6. Also shown in Figure 6 is the neutral reference band 7, located at λN. The corresponding spectral transmittance characteristics for the custom A- passband filter to be used in the current formulation are shown in Figure 7. As alluded to earlier above, we have implicitly implied in the present formulation that a full comprehension of the physics and the state-of-the-art technology for NDIR gas detection, together with all the relevant gas laws governing the behavior of all pertinent gases to be detected, are assumed to be well understood. Such is illustrated for the four passbands depicted in Figure 7 which have to be spectrally well separated from one another so as not to overstress the present technology limit for the design and fabrication of multi-passband filters. Once we have defined the general framework for the current formulation as presented above, we are now ready to set up the causality relationship linking the outputs of the detector with the other pertinent sensor component parameters including the presence or absence of the gases to be detected and their respective concentrations. Let M 1 (T), M2(T) and M3(T) be the spectral radiant emittances, M% for the three optical channels respectively for gas species G1 , G2 and G3 at blackbody temperature T. Similarly, let MN(T) be the Mλ for the reference optical channel at source temperature T. In other words, M1(T) is the spectral radiant emittance, Mλ, of the blackbody source at temperature T impinging on the detector equipped with a custom spectral filter Fλ1 (having only the passband at CWL = λ1 ). In this case the detector voltage signal output, VMI,T, can be quantitatively expressed as follows: VMI,T = e x M1 (T) x η(λ1 ) x Δλ1 x η(OS) x SR X G(COM) volts [1]
Where
VM 1, T = Detector signal output for infrared source operating at temperature T for optical channel M1 e = Blackbody source emissivity assumed to be independent of infrared source temperature T and λ M1 (T) = Spectral radiant emittance of blackbody source at T0K for filter
Fλ1 η(λ1 ) = Transmittance efficiency of filter Fλ1 at CWL
Δλ1 = Full Width Half Maximum (FWHM) of filter Fλ1 η(OS) = Overall Optical System efficiency for single beam sensor SR - Detector Responsivity (V/W) which is independent of λ for thermopile detectors G(COM) = Common first stage amplifier gain for signal processing circuit, same for all three optical channels
Since the blackbody Planck or Mλ(T) curves are uniquely determined once the temperature of the blackbody source is known, one can establish the relationships between MN(T), M 1 (T), M2(T) and M3(T) at temperature T0K respectively as follows:
MN(T)/M1 (T) = rN1(T) ; MN(T)/M2(T) = rN2(T) ; MN(T)/M3(T) = rN3(T) [2]
where ΓNI(T), ΓN2(T) and rN3(T) are constants and can be theoretically calculated from the blackbody Planck curves for any temperature T0K. Thus substituting T1 , T2 and T3 for T in Equation [2], one has rN1(T1 ), rNi(T2), rNi(T3), rN2(T1 ), rN2(T2), ΓN2(T3), ΓN3(T1 ), ΓN3(T2) and rN3(T3) are all constants and can be calculated from the Planck blackbody curves like those illustrated in Figure 1. For example, the ratio rNi is simply the value of Mλ at λN (CWL of neutral filter) divided by the Mλ value at λ1 (CWL of absorption filter for gas G1 ). Thus substituting source temperatures T1 , T2 and T3 for T in Equation [2], one has rNi(T1), rNi(T2), rNi(T3), ΓN2(T1 ), ΓN2(T2), rN2(T3), rN3(T1 ), ΓN3(T2) and ΓN3(T3) and they are all constants and can be calculated from the Planck blackbody curves for source temperatures T1 , T2 and T3 respectively like those illustrated in Figure 6. For a given sample chamber design for the sensor, let the absorption of the gases G1 , G2 and G3 be α, β and γ respectively. Note that in general, the absorption α, β and γ are very mildly dependent upon the gas temperature but is independent of the blackbody source temperatures T1 , T2 or T3. Assuming that there are no scattering losses like in most gas detection or measurement scenarios, the respective transmittances tG for gases G1 , G2 and G3 respectively are given as follows:
tGi = 1.- α; tG2 = 1 - β; tG3 = 1 - γ
Thus when gas species G1 , G2 and G3 are absent in the sample chamber, α = β
Figure imgf000025_0001
•
The detector output a, b and c respectively for the three optical channels when the 4-passband filter is in place at the detector can now be expressed as:
a = K(T1 ) + A [ ai X tGi x M1 (T1 ) + a2 x tG2 x M2(T1 ) + a3 x tG3 x M3(T1) ] [3]
b = K(T2) + A [ a! x tG1 x M1 (T2) + a2 x tG2 x M2(T2) + a3 x tG3 x M3(T2) ] [4]
c = K(T3) + A [ a! x tG1 x M1 (T3) + a2 x tG2 x M2(T3) + a3 x tG3 x M3(T3) ] [5]
where:
K(T1 ), K(T2) and K(T3) are constants, namely independent of α, β and γ of the gases to be detected when MN(Tj) [i=1 ,2,3] are known and are given as follows:
K(T1 ) = e x MN(TI) x η(λN) x ΔλN x η(OS) x SR x G(COM) = k x MN(TI);
K(T2) = e x MN(T2) x η(λN) x ΔλN x η(OS) X SR x G(COM)
= k x MN(T2);
K(T3) = e x MN(T3) x η(λN) X ΔλN X η(OS) X iR x G(COM)
A = e x η(OS) x SR x G(COM);
ai = η(λ1) x Δλ1
a2 = η(λ2) x Δλ2
a3= η(λ3) x Δλ3
Using Equation [2] and substituting the various temperatures of T1 , T2 and T3 for T, we have:
a = K(T1 ) + A x MN(TI ) [ai x tG1/ rN1(T1 ) + a2 x tG2/rN2(T1 ) + a3 x tG3/rN3(T1 )] [6]
b= K(T2)+ AxMN(T2)[a1xtG1/rN1(T2) + a2xtG2/rN2(T2) + a3 x tG3/rN3(T2)] [7]
c= K (T3) + Ax MN(T3) [a-, x tG1/rN1(T3) + a2 x tG3/rN2(T3) + a3 x tG3/rN3(T3)] [8]
Substituting the values of K(TI)1 K(T2) and K(T3) into Equations [6], [7] and [8] above, we have:
a = MN(TI ) [ k + Ax[^x tG1/rN1(T1 ) + a2 x tG2/rN2(T1 ) + a3 x WrN3(TI )]] [9]
b = MN(T2) [ k + A x [ ai x WrNi(T2) + a2 x tG2/rN2(T2) + a3 x tG3/rN3(T3)]] [10] c = MN(T3) [ k + A x [ an x tG1/rN1(T3) + a3 x tG2/rN2(T3) + a3 x tG3/rN3(T3)]] [11]
We have now successfully established the causality relationships between the outputs of the detector with all the relevant sensor parameters for the three optical channels related respectively to the three blackbody temperatures of the source as expressed in Equations [9], [10] and [11] above. The next step of our formulation is to define the necessary system phenomenological conditions for the sensor in order to permit the numerical evaluation of all the non-constant parameters in Equations [9], [10] and [11].
By flowing only nitrogen gas through the sample chamber and creating a situation where none of gas species G1 , G2 nor G3 are present, we have .GI = tG2 = tG3 = 1.0 and Equations [9], [10] and [11] can now be rewritten with new constants as follows:
a = MN(TI ) [k + d1]; where d1 = A x [ai/rN1(T1 ) + a2/rN2(T1 ) + a3/rN3(T1 )] [12]
b = MN(T2) [k + d2]; where d2 = A x [a-|/rNi(T2) + a2/rN2(T2) + a3/rN3(T2)] [13]
c = MN(T3) [k + <J3]; where d3 = A x Ia1ZrN1(TS) + a2/rN2(T3) + a3/rN3(T3)] [14]
Note that k, d1 , d2 and d3 are constants that can be calculated. By experimentally measuring the values of a, b and c, with only nitrogen in the sample chamber, one can determine the values of MN(TI ), MN(T2) and MN(T3) by using Equations [12], [13] and [14] respectively as follows:
MN(TI ) = a / (k + d1 ); MN(T2) = b / (k + d2); MN(T3) = c / (k + d3)
When the values of MN(TI ), MN(T2) and MN(T3) are determined via the use of just nitrogen gas present in the sample chamber, Equations [9], [10] and [11] contain only the unknowns tG1, tG2, tG3 and the measured detector outputs for the three optical channels, namely a, b and c. The rest of the parameters are system constants that can be a priori calculated. Thus by measuring the values of a, b and c in Equations [9], [10] and [11], the concentrations of the gas species G1 , G2 and G3 can be determined simultaneously as expressed respectively by the value of tGi, t,G2 and tG3. Finally, let us determine how to establish the individual calibration curves for gas species G1 , G2 and G3. The calibration curve for G1 can be determined via Equation [9] with the use of only nitrogen gas (tGi = tG2 = tG3 = 1.0 or α = β = γ = 0) and a number of samples (e.g. 6) with known concentration of G1 gas. With tc32 = to3 = 1.0, Equation [9] can be rewritten as:
a = m + n x tci or tG-i = (a - m) / n [15]
where the constants m and n are given as:
m = MN(TI ) x k + A x [a2/rN2(T1) + a3/rN3(T1 )] and
n = A x a-i / rNi(T1)
By using Equation [15], for a particular concentration of the G1 gas, we can determine the corresponding tG-ι. In other words, we can now determine the concentration curve for the G1 gas as follows:
G1 gas concentration (ppm) |GI
0 0 p pppmm V1
200 ppm V2
500 ppm V3
1 ,000 ppm V4
1 ,200 ppm V5 1 1 ,,550000 p pppmm V6
2,000 ppm V7 By determining the set of "Vi" values (i = 1 through 7) via putting into the sample chamber only nitrogen and known concentration of G1 gas, one can obtain the calibration curve for G1 since subsequently it is the .GI value that is being measured by the single beam NDIR gas sensor designed using the differential source temperature technique.
Similarly, the calibration curves for gas species G2 and G3 can also be determined. After such calibrations for all the three gases are inputted to the sensor, subsequent measured values of tc-i, tβ2 and tβ3 will provide simultaneously the to-be-determined concentration values for the gases species G1 , G2 and G3 present in the sample chamber of the sensor.
Thus it has been described above the mathematical formulation of the present invention for a multiple differential source emission temperature technique encompassing, as an example, the simultaneous detection of three gases using a four-passband interference filter. Such a mathematical formulation is not limited to the simultaneous detection of only three gases. It works for the simultaneous detection of N gases with a custom "N+1"-passband filter. The limitation, however, lies in the state-of-the-art for the design and fabrication of these multi- passband filters. It is also limited by the spectral location of the gases to be detected, their spectral separation and also the availability of appropriate neutral reference bands to be used with such a technique.
While the invention has been described herein with reference to certain examples, those examples have been presented for illustration and explanation only, and not to limit the scope of the invention. Additional modifications and examples thereof will be obvious to those skilled in the art having the benefit of this detailed description. Further modifications are also possible in alternative embodiments without departing from the inventive concept. For example, the present invention is especially well suited to development of a simple multichannel NDIR gas sensor for detecting both water vapor and carbon dioxide, and such a sensor would be especially well suited to HVAC and IAQ applications and represents a tremendous potential advance in the field, not to mention the possibility of tremendous energy savings from use of a such a sensor having a much lower cost than sensors presently available for use in such situations. In this regard, it would be especially desirable to construct such a sensor using a custom 3-passband filter encompassing the absorption band of CO2 at 4.26 microns, the absorption band of water vapor at 2.60 microns and a neutral reference band at 3.91 microns, and two appropriate driving temperatures for the infrared source.
Accordingly, it will be apparent to those skilled in the art that still further changes and modifications in the actual concepts described herein can readily be made without departing from the spirit and scope of the disclosed inventions as defined by the following claims.

Claims

What is claimed is:
1. In a single beam NDIR gas sensor for detecting the concentration of a gas species in a sample chamber with a differential infrared source element that can produce radiation having a first spectrum when its temperature is at a first high temperature and a second spectrum when its temperature is at a second lower temperature, a detector for generating a detector output and a dual pass band filter located between the source element and the detector, the improvement of which comprises: a driver for driving the source at either the first or the second temperature; a feed back loop to sense an operation voltage of the source; a differential gain amplifier for creating a high cycle amplified output during a high cycle and a low cycle amplified output during a low cycle; a controller for synchronizing the driver so that the source is driven at the first temperature and a high cycle amplification is applied to the detector output during the high cycle and the source is driven at the second temperature and a low cycle amplification is applied to the detector output during the low cycle; and a signal processor for determining the concentration of the gas species through use of the high cycle amplified output and the low cycle amplified output.
2. A method for detecting the concentration of a gas species from a single beam NDIR gas sensor having a differential infrared source element that can produce radiation having a first spectrum when its temperature is driven by a driver at a first low temperature and a second spectrum when its temperature is driven by the driver at a second higher temperature, a detector for generating a detector output and a dual pass band filter located between the source element and the detector, comprising the steps of: driving the source element at a first high temperature and then applying a high cycle amplification to the detector output to create a high cycle amplified output; driving the source element at a second low temperature and than applying a low cycle amplification to the detector output to create a low cycle amplified output; and determining the concentration of the gas species through use of the high cycle amplified output and the low cycle amplified output.
3. A single beam NDIR gas sensor for detecting the concentration of a gas species, comprising: a thermally insulated tube sample chamber; an incandescent miniature light bulb with a filament surrounded by a glass envelope secured at a first end of the sample chamber; a single infrared detector secured at a second end of the sample chamber; a dual bandpass filter mounted at the single infrared detector between the bulb and the detector, said dual bandpass filter having a neutral passband and an absorption passband for the gas species; a controlled heater secured to the tube for maintaining the sample chamber at a preselected temperature greater than an ambient temperature when the sensor is turned on; a driver for the bulb with a high input power level and a low input power level so as to render said bulb into emitting at a first voltage output and a second voltage output whose radiation outputs are characterized by two corresponding Planck curves dependent upon temperatures; a feed back loop to sense an operation voltage of the bulb; a differential gain amplifier for creating a high cycle amplified output during a high cycle and a low cycle amplified output during a low cycle; a controller for synchronizing the driver so that the bulb is driven at the high input power level and a high cycle amplified gain is applied to the detector output during the high cycle and the bulb is driven at the low input power level and a low cycle amplified gain is applied to the detector output during the low cycle; and a signal processor for determining the concentration of the gas species through use of the high cycle amplified output and the low cycle amplified output.
4. The sensor of claim 3, wherein the glass envelope is maintained at an equilibrium temperature during the low cycle operation state by the controlled heater.
5. The sensor of claim 4, wherein the ambient temperature is 22 degrees Celsius.
6. The sensor of claim 3, wherein the equilibrium temperature is a constant temperature that varies by less than two degrees Celsius.
7. The sensor of claim 3, wherein the sample chamber is secured to a first side of a printed circuit board.
8. The sensor of claim 7, wherein the signal processing circuit components are mounted on a second side of the printed circuit board.
9. The sensor of claim 3, wherein the preselected temperature is approximately 30 degrees Celsius.
10. The sensor of claim 9, wherein the glass envelope is the primary radiation emitter during the low cycle.
11. The sensor of claim 3, wherein the insulated tube sample chamber is comprised of aluminum.
12. The sensor of claim 11, wherein the insulated tube sample chamber is comprised with at least one substantial U-bend.
13. The sensor of claim 12, further comprising a casing which surrounds the printed circuit board.
14. A method for detecting the concentration of a gas species from a single beam NDIR gas sensor having a thermally insulated tube sample chamber, an incandescent miniature light bulb with a filament surrounded by a glass envelope secured at a first end of the sample chamber, a single infrared detector secured at a second end of the sample chamber, a dual bandpass filter mounted at the single infrared, detector between the bulb and the detector, said dual bandpass filter having a neutral passband and an absorption passband for the gas species, and a controlled heater secured to the tube, comprising the steps of: heating the sample chamber to a preselected temperature greater than an ambient temperature and maintaining the sample chamber at the preselected temperature; driving the bulb at a first high voltage input and then applying a high cycle amplification to the detector output to create a high cycle amplified output; driving the bulb at a second low voltage input and than applying a low cycle amplification to the detector output to create a low cycle amplified output; and determining the concentration of the gas species through use of the high cycle amplified output and the low cycle amplified output.
15. The method of claim 14, comprising the further step of using a feed back loop to sense an operation voltage of the bulb and synchronizing the bulb so that it is driven at the first high voltage input and the high cycle amplified output is applied to the detector output during a high cycle and the bulb is driven at the second low voltage input and the low cycle amplified output is applied to the detector output during a low cycle.
16. The method of claim 14, wherein the ambient temperature is 22 degrees Celsius.
17. The method of claim 16, wherein the preselected temperature is approximately 30 degrees Celsius.
18. The method of claim 14, wherein the glass envelope is the primary radiation emitter at the second low voltage input.
19. A method for detecting the concentrations of N gas species from a single beam NDIR gas sensor having a differential infrared source and a (N+1)- passband filter mounted at a single infrared detector, comprising the steps of:
(1) driving the infrared source with N input power levels so as to render said source into emitting at N distinct temperatures whose radiation outputs are characterized by N corresponding Planck curves which are dependent only upon the respective source temperatures and which link a Spectral Radiant Emittance MsubLamba with wavelength;
(2) measuring N detector outputs at the single infrared detector; and
(3) detecting the concentrations of N different gas species, each of the N gas species having its own unique infrared absorption passband, by (a) setting up N causality relationship equations linking outputs of the detector respectively for N different source temperatures and a set of relevant parameters of the sensor components, (b) determining the values of all of the parameters for the N equations utilizing appropriate boundary conditions except the N concentrations for the respective N gas species, and (c) solving for the N gas concentrations with the measured N detector outputs, there being N equations and N unknowns; wherein a neutral passband and N absorption passbands for N gases are incorporated into the (N+1 )-passband filter; and wherein N is an integer of 2 or more.
20. The method of claim 19, wherein the infrared source is a non- genuine blackbody source.
21. The method of claim 20, wherein the infrared source is a genuine blackbody source.
22. The method of claim 19, wherein each of the N absorption passbands for N gases is specific to passing a particular spectral radiation for one of the N gases to be detected.
23. The method of claim 19, wherein steps (3)(a) and (3)(b) are performed as part of an initialization process.
24. The method of claim 23, wherein step (3)(c) is repeated along with steps (1) and (2) to repeatedly determine the N gas concentrations of the N gas species.
25. The method of claim 19, comprising the further step in step (3) of using N calibration curves to detect the concentrations of N different gas species.
26. A method for detecting the concentrations of N gas species from a single beam NDIR gas sensor having a differential infrared source and a multiple- passband filter mounted at a single infrared detector wherein a neutral passband and N absorption passbands for N gases species are incorporated into the multiple-passband filter, each of the N gas species having its own unique infrared absorption passband, comprising the steps of:
(1) setting up N causality relationship equations linking outputs of the detector respectively for N different source temperatures and a set of relevant parameters of the sensor components; (2) determining the values of all of the parameters for the N equations utilizing appropriate boundary conditions except the N concentrations for the respective N gas species;
(3) driving the infrared source with N input power levels so as to render said source into emitting at N distinct temperatures whose radiation outputs are characterized by N corresponding Planck curves which are dependent only upon the respective source temperatures and which link a Spectral Radiant Emittance
MsubLamba with wavelength; (4) measuring N detector outputs at the single infrared detector; and
(5) detecting the concentrations of N different gas species by solving the N causality relationship equations for the N gas concentrations with the measured N detector outputs, there being N equations and N unknowns, wherein N is an integer of 2 or more.
27. A single beam NDIR gas sensor for detecting the concentrations of N gas species, comprising: a differential infrared source; a single infrared detector; a multiple-passband filter mounted at the single infrared detector, said multiple-passband filter having a neutral passband and N absorption passbands for N gases species incorporated into the multiple-passband filter, each of the N gas species having its own unique infrared absorption passband; a driver for the infrared source with N input power levels so as to render said source into emitting at N distinct temperatures whose radiation outputs are characterized by N corresponding Planck curves which are dependent only upon the respective source temperatures and which link a Spectral Radiant Emittance MsubLamba with wavelength; and electronics for detecting the concentrations of N different gas species by solving N causality relationship equations with N unknowns linking outputs of the detector respectively for N different source temperatures and a set of relevant parameters of the sensor components that have been determined utilizing appropriate boundary conditions except the N concentrations for the respective N gas species, wherein N is an integer of 2 or more.
28. The sensor of claim 27, wherein the infrared source is a non- genuine blackbody source.
29. The sensor of claim 27, wherein the infrared source is a genuine blackbody source.
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