Patent Publication Number: US-7592595-B1

Title: Wide concentration range gas detection

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of Provisional U.S. Patent Application Ser. No. 60/786,499 filed 28 Mar. 2006. 

   BACKGROUND OF THE INVENTION 
   This invention relates to a method and apparatus for sensitive high speed spectroscopic gas detection over a wide range of gas concentrations from less than one (1) part per million (ppm) up to 100%. 
   In general, the transmission or absorption of radiation passing through a radiation absorbing medium, such as a gas, may be graphed as a function of wavelength or frequency as an optical absorption spectrum. The absorption spectrum consists of a set of discrete absorption lines characteristic of the gas of interest. In general, absorption measurements within a narrow frequency interval and comparable or smaller than the line width near an absorption peak can be used as a sensitive measure of the number or concentration of absorbing gas molecules present. At very large concentrations, all of the radiation within a small frequency interval near an absorption peak is heavily absorbed. This is called absorption line saturation. However, when measurements are made away from the peak absorption frequency or over a wider spectrum where the frequency interval of measurement is larger, the absorption measurement includes contributions from frequencies which are not saturated. These off-line absorption measurements are highly dependent on the shape of the absorption lines. Line shape dependent measurements include the integrated absorption over an entire line or multiple absorption lines grouped into bands which are characteristic of the absorbing species. 
   A typical method of measuring gas concentration relies upon absorption of electromagnetic radiation by the target gas atoms or molecules when electromagnetic radiation is passed through a sample containing a gas which absorbs some of the radiation. However, such known methods are limited with respect to the range of concentrations over which the target gas atoms or molecules can be detected. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is one object of this invention to provide a method for measuring the concentration of a gas from very low concentrations to very high concentrations, that is, from less than 1 part-per-million (ppm) up to 100% gas. 
   This object is addressed by a method in which a first absorption measurement is made over a first frequency interval or set of frequency intervals in which is one or more absorptions from a spectral band of interest. The first absorption measurement is ideally selected to be near the frequency of a peak in the absorption spectrum. This provides high sensitivity at low gas concentrations. A second absorption measurement is made over a second frequency interval or set of frequency intervals. The second absorption measurement is made to include a contribution from the broadening of the absorption spectrum at higher concentrations. The second absorption measurement provides sensitivity at higher concentrations. The two absorption measurements are then combined to deduce the target sample concentration as the absorption line spectrum changes shape with concentration. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein: 
       FIG. 1  is a diagram showing absorption measurements made over two frequency intervals for a single Lorentz absorption line in accordance with one embodiment of this invention; 
       FIG. 2  is a diagram showing the effect of line saturation and broadening on the absorption line for two concentrations, ×1 and ×2, in which ×2 is greater than ×1; 
       FIG. 3  is a diagram showing the progressive increases associated with increases in concentration in the absorption of a frequency interval at low concentrations; 
       FIG. 4  is a diagram showing the saturation of absorption over the interval of  FIG. 3  and progressive absorption due to line broadening within a second interval at higher gas concentrations; 
       FIG. 5  is a diagram showing a single optical element line broadening gas detector for use in the method of this invention; 
       FIG. 6  is a diagram showing a two optical element line broadening gas detector for use in the method of this invention; 
       FIG. 7  is a diagram showing a representation of absorption signals for wide dynamic range gas concentration measurement in accordance with the method of this invention; 
       FIG. 8  is a diagram showing representative absorption signals over the two frequency intervals employed in the method of this invention; and 
       FIG. 9  is a diagram showing data obtained from application of the method of this invention to a methane gas sample. 
   

   DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
   The invention claimed herein is a method for measuring the concentration of a gas in which a measurement of a first electromagnetic absorption (A 1 ) by the gas over a first frequency interval (I- 1 ) is combined with a measurement of a second electromagnetic radiation absorption (A 2 ) over a second frequency interval (I- 2 ).  FIG. 1  shows a diagram of exemplary absorptions and frequency intervals. The first measurement A 1  is made over a wavelength interval I- 1 , which is a subsection of the absorption line  10 . The second measurement A 2  shown in  FIG. 1  is made over a second measurement interval I- 2 . In general, the intervals I- 1  and I- 2  may be any convenient interval for the measurement. The intervals may overlap and may include multiple absorption lines although the measurement intervals in  FIG. 1  are shown as distinct intervals over a single absorption line for the sake of clarity. 
   As previously indicated, off-line absorption measurements are highly dependent on the shape of the absorption lines. Line shape dependent measurements include the integrated absorption over an entire line or multiple absorption lines grouped into bands which are characteristic of the absorbing species.  FIG. 1  shows a narrow band absorption section near the peak and another absorption section away from the peak on a single absorption line having a Lorentz line shape. Those skilled in the art will appreciate that this description is readily extended to multiple lines or different line shapes. Clearly the two absorption measurements are affected by the shape of the line or lines within the frequency measurement interval. At low concentrations, sensitive measurements may be obtained by measuring changes in absorption at or near the peak absorption frequency. These peak absorption measurements quantify the magnitude or height of the absorption line. The net absorption within the frequency interval is represented by the areas A 1  and A 2  under the curve in  FIG. 1 . At higher concentrations, the absorption line saturates and the principle change in absorption comes from broadening of the width of the line. This saturation effect is shown in  FIG. 2 . 
   The detailed variation of shape of absorption lines with concentration is influenced by a number of factors. The principal physical cause of line broadening is collisions between gas atoms or molecules. These collisions are affected primarily by the temperature and pressure or concentration of the gas. A key feature of the line shape is the variation of line width with pressure or concentration. This line broadening is sometimes called collisional broadening, pressure broadening or Lorentz broadening of the line. 
   The effect of line broadening on an absorption measurement can be discussed using diagrams. At low concentrations, the situation is shown in  FIG. 3  for progressively higher concentrations of gas. The two absorption measurements are shown over frequency intervals I- 1  and I- 2 . The associated absorptions A 1  and A 2  are represented by the areas under the curves in these two intervals. The interval I- 1  is set at or near the absorption peak. The second interval I- 2  is away from the peak.  FIG. 3  shows how the absorption measurement A 1  at low concentrations varies with the changing magnitude (height of the line).  FIG. 4  shows the situation at high concentrations where the line is saturated in the interval I- 1  but the absorption A 2  increases due to line broadening. 
   Here we discuss the broadening of a Lorentz line shape, but those skilled in the art will appreciate that the method applies to any line shape which varies with concentration of the absorbing species. The pressure variation of linewidth is well known for a Lorentz line shape and is given by 
           Δν   =         (   Δν   )     0     ⁢     P     P   0       ⁢       (     T     T   0       )       -     1   2                 
and may be more generally represented as a power law in both normalized pressure and temperature for other line shapes:
 
           Δν   =         (   Δν   )     0     ⁢       (     P     P   0       )     m     ⁢       (     T     T   0       )     n             
where (Δv) 0  is the linewidth at the standard pressure P 0  and temperature T 0 , P is the effective pressure reflecting the partial pressure or concentration of the gas, T the temperature, and m and n are the pressure broadening and temperature broadening exponents, respectively. At constant temperature or near the standard temperature, the temperature effect is either constant or negligible. At low concentrations where the effective pressure is near the standard pressure, the ratio of P/P 0  is nearly 1 and the pressure induced broadening also can be neglected. At high concentrations corresponding to high partial pressures of the target gas, a major contribution to the absorption comes about due to the increase in the line width.
 
   There are several ways in which the two frequency intervals I- 1  and I- 2  may be selected and absorption measurements A 1  and A 2  may be made. These include, but are not limited to 1) I- 1  and I- 2  selected with fixed bandpass filters; 2) I- 1  selected with a tunable filter and I- 2  selected with a fixed bandpass filter; 3) I- 1  selected with a fixed bandpass filter and I- 2  selected with a tunable filter; 4) I- 1  and I- 2  selected with a tunable filter; 5) I- 1  selected by a tunable filter and I- 2  selected by the light source spectrum such as with a laser or LED; and 6) I- 1  selected by a fixed bandpass filter and I- 2  selected by the light source spectrum such as with a laser or LED. 
   It is to be understood that I- 1  and I- 2  may be distinct non-overlapping frequency or wavelength intervals or, alternatively, the intervals I- 1  and I- 2  may partially or completely overlap. Also, it is to be understood that there are many ways to select the intervals I- 1  and I- 2  other than the few listed above. 
   The preferred technique for making the measurement is where I- 1  is selected by a tunable filter and I- 2  is selected by a fixed bandpass filter.  FIG. 5  and  FIG. 6  show schematic diagrams of specific implementations of the technique. Both implementations comprise a source (S) of electromagnetic radiation, i.e. light,  20  and an optical element  11  to direct the light towards a detector  15 . Optical element  11  may be either a transmitting element such as a lens or a reflecting element such as a mirror. The light path is defined schematically by the dashed lines  16  and the absorbing species to be detected would be present in the light path. Both implementations further comprise a first filter  17  to define the wavelength interval I- 2  for the second absorption measurement and a second filter  18  which is used to make a measurement over a wavelength subinterval I- 1 . Filter  18  in accordance with one embodiment of this invention is an electro-optical modulator (EOM) where the interval I- 1  is defined by the electrical tuning range of the EOM. A simple example of an appropriate EOM is a transverse electro-optic modulator. Electro-optic modulators are described extensively in standard textbooks. The radiation is then incident on a detector  15  as in  FIG. 5  or the radiation is directed using a second optical element  21  onto the detector  15  as shown in  FIG. 6 .  FIG. 6  is similar to the gas detector shown in  FIG. 5  with the addition of the second optical element  21  which may again be a transmitting or reflecting element to assist in directing the light signal onto the detector  15 . The detector may be any type appropriate for the radiation to be detected. One example of a candidate detector is a photodiode. The detection instrument includes electronics to control the temperature of the instrument, tune the transmission of EOM, and produce signals from the detector which represent the absorptions due to any absorbing species present in the light path. Although this technique describes the use of an electro-optic modulator, other types of optical tunable filters could be used including acousto-optic tunable filters, spacing or angle tuned etalons, or other means for obtaining a tunable filter response over the interval of interest. 
   The absorption signals produced by this arrangement are illustrated in  FIG. 7 . The filter  18  is modulated electronically to produce a time varying signal as the filter transmission is modulated over the interval I- 1 . The tunable filter is modulated over I- 1  between absorption values a 1  and a 2  as shown in  FIG. 7 . The signal amplitude A 1  which serves as a measure of the absorption over interval I- 1  is proportional to the change in absorption values a 1  and a 2 , A 1 =b*(a 1 −a 2 ) where b is a proportionality constant for modulations which are small relative to the line width. The absorption signal A 1  is a measure of the absorption change or derivative of the line over the interval I- 1 . A 2  is the area under the curve over interval I- 2 . The filter  17  selects a measurement window I- 2  and the absorption A 2  is represented by the area under the curve defined by the combined filtering effect of both filters  17  and  18 . The sample concentration is derived as a function of the two absorption signals C=f (A 1 , A 2 ). The signals A 1  and A 2  are plotted schematically in  FIG. 8  as a function of gas concentration from zero concentration to 100% gas for a fixed measurement path length. The signal A 1  is seen to rise linearly at low concentrations, peak when the slope of the line is highest, and then decrease as the concentration becomes large enough to produce absorption line saturation and the linewidth broadens further. Since the absorption signal A 1  is not monotonic with concentration, there is an ambiguity in determining the concentration from the signal A 1  alone. The absorption A 2  may be explained using Beer&#39;s law for the change in transmission intensity I of the radiation:
 
 I ( v )= I   0 ( v ) e   −ax  
 
where
         a=k(v)L   k=gas absorption coefficient   L=optical pathlength   x=gas concentration   v=frequency       

   The transmission is expressed as T=I/Io which falls exponentially with increasing concentration (x) and the absorption is A=1−T. Note that the absorption coefficients are frequency or wavelength dependent and, thus, the appropriate absorption may be selected by choosing the measurement frequency or wavelength interval carefully and summing or integrating the response over the measurement interval. The signal A 2  from interval I- 2  also rises linearly, but it is monotonic and does not saturate at the higher concentrations so that the additional measurement of A 2  removes the ambiguity. There is only one concentration which produces a given pair of absorption signals A 1  and A 2 .  FIG. 9  shows the results obtained from application of the method of this invention to a methane gas sample. 
   Although the discussion above relates to a single absorption line, the argument may be used for multiple absorption lines each having an associated absorption A 1 , over a first measurement interval I- 1 , and second absorption A 2 , measurement interval I- 2 , and summing the absorptions of the intervals. 
   
     
       
         
           
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   It is also to be understood that a filter having a periodic filter response may optionally be designed to match a set of absorption lines of a target absorbing species if desired and the contributions from a set of lines be summed using this technique. 
   While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.