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METHOD AND APPARATUS FOR MEASUREMENT OF TRANSMITTANCE AND SCATTER OF LIGHT IN WATER
This invention relates to methods and apparatus for measuring the effects of suspended particulates upon the transmission of light through a liquid medium, at times referred to in the art as turbidimeters.
As one example, the presence of particulates in a 10 natural stream of water affects the degree of penetration of sunlight into the stream hence affects the growth of marine flora and fauna in the stream. In ecological studies, for example, it thus becomes important to know the precise effect of existing particulates upon the passage 15 of light through the medium, either at a given time and over an extended period of time.
Possible particulates suspended in a liquid medium may vary greatly in their nature, such as the soil particulates that are carried into a water stream through ero- 20 sion wherein the particulates not only vary in size and shape but also vary in their chemical nature. All such particulates, however, exhibit some influence upon a beam of light which is directed into the particulate-containing medium, thereby giving rise to the term "turbid- 25 ity" which has come to be applied loosely as a measure of the lack of clarity of a liquid medium that contains suspended particulates. This term is less than adequate for such purpose in that it is not susceptible to precise quantification. 30
In conventional turbidimetry the Jackson Candle Turbidimeter is generally employed. A column of liquid is increased in length until the image of the flame from a "standard" candle viewed through the column loses contrast and becomes diffused. The length of the col- 35 umn at which the image is degraded into a uniform disc of light is read against a scale of units designated Jackson Turbidity Units (JTU). This method has become generally accepted, even though the quantity it measures is not well-defined and is based on the subjective 40 judgment of an observer.
While the Jackson measurement is still in wide use today, most electronic instruments, including laboratory and field monitors, employ designs measuring light scatter perpendicular to the incident beam. Other elec- 45 tronic instruments employ measurements of the attenuation of the incident beam. In general, neither measurement correlates well with JTU values, nor do they correlate well with the amount of particulate matter present. Most especially, the methods themselves are 50 not standardized. Among the problems which have plagued the prior art are light-source instability, detect tor instability, variations in particulate deposits on sight windows, etc. Further, many of these prior art measurements have attempted to measure light transmission 55 through the medium in exact conventional units of light transmittance, thereby apparently unknowingly failing to take into consideration extraneous factors such as background light, reflected light and/or other similar factors. Nonuniformity of results has been characteristic 60 of such prior art measurements.
More specifically, in conventional turbidimeters where light transmission is measured, it may be said mathematically that there is measured the intensity of light I, transmitted through a fixed path of length 1, 65 from a source of light of intensity I0. As a practical matter, however, I0 commonly is neither well known nor is it stable over the period of time during which a
measurement takes place. In addition, the measurement of I, is affected by reduction in light intensity by the accumulation of particulates on the light source, detector, and/or on windows which protect them, and also by variations in detector response or source strength. Consequently, such measurements are both inaccurate and imprecise.
In accordance with the present disclosure, the inventor has found that precise quantification of turbidity is obtainable through multiple determinations of the light attenuation coefficient of the particulate-bearing medium at a plurality of path lengths in the attenuating medium, and through multiple determinations of the scattering coefficient of the particulate-bearing medium at a selected angle of incidence and at a plurality of path lengths in the attenuating medium. In- the preferred apparatus, these determinations employ a common light source and a common detector. In the present system, there are eliminated such adverse factors as light-source instability, detector instability, variations in particulate deposits on windows, and other problems, through the use of the ratio of the intensity measurements at different locations in the medium.
It is therefore an object of the present invention to provide an improved method for measuring the effect of particulates in a liquid medium upon the transmission of light through such medium. It is another object to provide a method for substantially simultaneously measuring the transmittance and scatter of light in a liquid medium. It is another object to provide apparatus for measuring transmittance and/or scatter of light in a liquid medium.
Other objects and advantages will be recognized from the following description including the claims and drawings in which:
FIG. 1 is a representation of an instrument for measuring transmittance and scatter of light in a liquid medium and embodying various features of the invention;
FIG. 2 is a fragmentary representation of the light source portion of the instrument depicted in FIG. 1 but rotated 90 degrees;
FIG. 3 is a fragmentary representation showing the reverse side of the light source depicted in FIG. 2;
FIG. 4 is a schematic depicting certain concepts of light transmittance associated with the present invention;
FIG. 5 is a schematic depicting one embodiment of multiple-path-length detection of transmittance of light employing a common light source and a common detector;
FIG. 6 is a schematic illustrative of certain concepts relating to the effects of window coatings on the measurement of transmittance or scatter of light in a liquid medium;
FIG. 7 is a schematic depicting certain concepts associated with determination of light scatter associated with the present invention;
FIG. 8 is a schematic depicting one embodiment of a control system employed in conjunction with the device depicted in FIG. 1.
Generally stated, in a sample volume of a particulatebearing liquid medium, the penetration of the medium by a light beam of an intensity l0 at the surface of the liquid is influenced by several mechanisms. In most cases, the effects of particulate matter in the medium dominate. The present inventor has determined that quantification of such dominating mechanism is possible through a plurality of determinations of the light attenuation coefficient a of the sample and, preferably, a like plurality of determinations of the scattering coefficient of the sample, both such types of determinations being made at a plurality of path lengths, li, and I2, within the sample and over a relatively short period of time, e.g., less than one minute, for a given series of determinations.
For short distances, the amount of light lost along the direction of a beam of light in a liquid medium by scattering and absorption is simply proportional to the incident intensity and to the amount of liquid in the light path; that is, the decrease in intensity is proportional to the length of the light path. Thus, referring to FIG. 4:
Where la and lb are the intensities at distances xfl and X&
respectively; and a is the proportionality constant giv-
ing the fraction of light lost per unit distance along the
light path, that is, .the light attenuation coefficient.
Equation (2) follows from Eq. (1); here AI is the small 25
change in intensity corresponding to the small distance
Ax. The negative sign in Eq. (2) is necessary because I
decreases as x increases, that is, (lb—la) is a negative
quantity when (xb—xa) is a positive quantity.
Writing Eq. (2) in differential form, and integrating 30 from x=0, where I=I0, to x=li, where I —li, provides /2/^i=e~a('2_/" the following:
water bodies, and also in industrial waste water discharges.
Even where particulate absorption and scattering dominate, a is not expected necessarily to correlate directly with the amount of particulate matter present. The attenuation coefficient a is determined not only by the amount of particulate matter present but also by its size, distribution, and its index of refraction. A consequence of this, for example, is that two liquid samples containing the same concentrations of particulate matter may have different values of a, by virtue of different particle size distributions and/or different particle compositions (e.g., indices of refraction).
In accordance with the disclosed method, there is made a measurement of light intensity at two different path lengths li and I2 in the attenuating medium. In practice, this means only that the source or detector must be movable and that the distance moved must be measured or known from preset conditions.
To understand this measurement in terms of the mathematics set forth above, reference is directed to FIG. 5 and rewriting of Eq. (5) for each of the distances li and I2.
Dividing Eq. (7) by Eq. (6);
Equation (5) characterizes the transmission of any radiation through an absorbing and/or scattering medium.
As disclosed herein, the light attenuation coefficient a, is one quantity measured. This coefficient is charac- 45 teristic of the light-attenuating medium, i.e., liquid plus suspended particulates, and is the fraction of light lost per unit distance along the path. (Light loss described by may occur by absorption, scattering, or any combination of mechanism.) Notably, the coefficient a does 50 not depend mathematically on the intensity of the light source, path length through the liquid, etc.
As stated above, the light attenuation coefficient a and its value is the fractional light intensity lost per unit path length in the liquid. Its units might be cm-1, m-1, or 55 percent per centimeter, percent per meter, etc.
As noted, the coefficient of attenuation a describes light attenuation or loss, regardless of the mechanisms by which the light is lost. The mechanisms by which light may be lost or attenuated include absorption by 60 the liquid itself, absorption by particulates in the liquid, scattering by molecules of the liquid and scattering by particulates in the liquid, where the scattering angle is outside the acceptance angle of the detector, and scattering and absorption which may occur at liquid-liquid 65 interfaces when chemically separated liquids co-exist in a sample. As a practical matter, the effects of absorption and scattering by particulates are important in natural
where h=h—li, the change in path length.
Equation (9) is the basic equation of the present attenuation measurement. The quantity measured is a. To determine a uniquely and unambiguously, it is required only to measure li and I2 and to know the distance between the points at which these measurements are made. Once a has been determined, Equation (9) may be solved to obtain the percent of light transmitted (I2/I1) over path length h. This proposed method is simple in principle and in practice. It is more precise and more accurate than conventional fixed-distance attenuation measurements, and the quantity measured, a, is well-defined and can be understood in physical terms.
The method avoids conventional problems associated with light source and detector stability, fouling of windows, etc.; requiring stability only for a fraction of a minute.
Commonly, over a period of time, particulate matter will deposit onto the windows protecting various elements such as the source and detector elements of an instrument employed in making light intensity determinations. To understand the effect of these window coatings on the measurement, reference is invited to FIG. 6, which shows that the light intensity incident on the first window is Is, the intensity exiting from the first window is I0(the incident light intensity l0 referred to above), li is the intensity of light transmitted through the liquid and incident on the second window (the same as li above), and Id\ is the intensity of the light exiting from the second window and incident on the detector at the first location. Defining wi as the fraction of light lost in the first window and its coatings, and W2 as the fraction