Abstract:
A method and device for measuring a concentration of a preselected gas in a gas sample are disclosed. The device comprises a Herriott type multipass cell ( 10 ) having a center axle ( 74 ) and a housing ( 80 A,  80 B) surrounding and spaced from the axle to provide a tubular sample cavity ( 84 ). The gas sample is pumped through the sample cavity via apertures ( 154, 156 ) provided in opposed ends of the axle. A first mirror ( 44 ) and a second mirror ( 46 ) are supported at opposed ends of the axle. A light source, e.g. a laser or LED, is provided for emitting a light beam into the sample cavity via an entry aperture ( 30 ) in the first mirror, the light beam having a wave length at which the preselected gas strongly absorbs. The beam is reflected between the mirrors for a number of times before exiting the cell via an exit aperture ( 48 ) in the second mirror and impinging on a detector ( 52 ). The device further comprises a reference detector ( 32 ) for monitoring the intensity of the unattenuated light beam and a detector for detecting the intensity of light transmitted through the second mirror after a single pass through the cell. The light source is operatively connected to a heat control assembly having a heat sink and the gas sample is passed said heat sink to augment temperature control of the light source.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     A common means of distributing energy around the world is by the transmission of gas, usually natural gas, but in some areas of the world manufactured gasses are also transmitted for use in homes and factories. Gas is typically transmitted through underground pipelines having branches that extend into homes and other buildings for use in providing energy for space and water heating. Many thousands of miles of gas pipeline exists in virtually every major city of the world. Since gas is useable only because it is highly combustible, gas leakage is a serious concern. For this reason much effort has been made to provide instrumentation for detecting small amounts of gas so that leaks can be located to permit repairs.  
         [0002]     A known and successful system for detecting small quantities of gas in the environment is by the use of absorption spectroscopy. By this technique, a light beam of a selected frequency that is highly absorbed by the particular gas for which the instrument is designed is passed through a sample of the gas. The rate of absorption of the light beam is used as an indicator of the level of concentration of the gas in the sample. A basic element of natural gas, and most manufactured gasses used for room and water heating around the world, is methane. By initiating a beam of light at a frequency that is highly absorbed by methane and passing the beam through a sample of gas the level of concentration of methane in the gas sample can be determined.  
         [0003]     In order to improve the sensitivity of detecting low levels of concentration of gas by spectral absorption, it is necessary to pass the light beam through a relatively long pathway of gas sample. Stating it another way, as the length of the light beam passing through a sample is increased, the sensitivity of the instrument to detect very small levels of gasses increase.  
         [0004]     It is easy to understand that if a beam is passed through a very long tube containing a sample of gas that the instruments requiring such a long tube would be extremely cumbersome and therefore not easily portable. To overcome this problem, others have devised systems wherein a beam of light is repeatedly reflected between opposed mirrors to thereby extend the length of exposure of the beam to a gas sample in a way that the size of the instrument can be substantially reduced. A typical absorption cell is an elongated cylinder in which mirrors are disposed at opposite ends and light is introduced into the cells through a hole in one of the mirrors. For background information relating to the use of optical devices that provide for multiple traverses of light within a test cell having opposed mirrors reference can be had to the article entitled “Long Optical Paths of Large Aperture” by J. White  J. Opt. Soc. Am . Vol 32, p. 285-288, May 1942. Another example of background information on this subject is entitled “Off-Axis Paths in Spherical Mirror Interferometers” by Herriott et al. in Applied Optics Vol. 3 pages 523-526. A further article by Herriott et al. entitled “Folded Optical Delay Lines” is found in  Applied Optics , Vol 4 p. 883-889, August 1965. Because of the early work by Herriott in the development of light absorption spectroscopy using a cell having opposed mirrors in which a light beam is repeatedly reflected, such instruments are frequently referred to as “Herriott cells.” The invention herein relates to improvements and innovations in the construction, operation and use of Herriott type cells for detecting a selected gas, such as methane. Particularly, the invention herein provides methods and systems for detecting and measuring the level of concentration of a preselected gas using an instrument that is more portable, rugged and sensitive than other instruments and systems currently available.  
         [0005]     For further background information relating to the basic subject matter of the invention herein reference maybe had to the following previously issued United States patents and other publications:  
                                       Patent or               Reference No.   Inventor   Title                   3,253,226   Herriott et al.   Optical Maser Amplifier       3,437,954   Herriott et al.   Optical Delay Line Devices       3,550,039   Herriott et al.   Optical Delay System       4,934,816   Silver et al.   Laser Absorption Detection               Enhancing Apparatus and Method       5,002,351   Wolfum et al.   Fusion Splicer for Optical               Fibers       5,121,405   Negus   Alignment Control System               for Lasers       5,291,265   Kebarbian   Off-axis Cavity Absorption               Cell       5,528,040   Lehmann   Ring-down Cavity               Spectroscopy Cell Using               Continuous Wave Excitation               for Trace Species Detection       5,550,636   Hagans et al.   Self-tuning Method for               Monitoring the Density               of a Gas Vapor Component               Using a Tunable Laser       5,637,872   Tulip   Gas Detector       5,946,095   Henningsen et al.   Natural Gas Detection               Apparatus and Method               Operable in a Moving               Vehicle       5,949,537   Inman et al.   In-line Cell for               Absorption Spectroscopy       6,064,488   Brand et al.   Method and Apparatus for               In Situ Gas Concentration               Measurement       6,157,033   Chudnovsky   Leak Detection System       US PUB   Diekmann   Infrared Optical Gas Sensor       2002/0011568       US PUB   Pilgrim et al.   Wavelength Agile External       2002/0015427       Cavity Diode Laser       US PUB   Gutin   Tunable Diode Laser System,       2002/0018496       Apparatus and Method       US PUB   Schley   Method and Device for       2002/0040590       Determining the Gas               Properties of a               Combustible Gas       US PUB   Warburton   Method and Apparatus for       2001/0045119       Determining Concentration               of a Gas       FR PUB   Ronge et al.   Precede et Dispositif de               Trace d&#39;Impuretes               dans un Echantillon de               gaz au Moyen d&#39;une               Diode Laser a Semiconducteur       FR PUB   Takeuchi et al.   Water Content Analysis       H3-260859       Device Using Semiconductor               Laser, Double Wavelength               Differential Absorption               Method       FR PUB   Takeuchi et al.   Analyseur de Teneur en eau       H5-99845       Utilisant un Laser a               Semiconducteur                  
 
 Other Publications: 
    “Folded Optical Delay Lines,” Herriott et al.,  Applied Optics  August 1965.     “Laser Beams and Resonators,” Kogelnik et al.,  Applied Optics  October 1966.     “Narrow Optical Interference Fringes for Certain Setup Conditions in Multipass Absorption Cells of the Herriott Type,” McManus et al.,  Applied Optics  Mar. 1, 1990.     “Measurement of Water Vapor Pressure and Activity Using Infrared Diode Laser Absorption Spectroscopy”, S. A. Bone, P. G. Cummins, P. B. Davies, S. A. Johnson,  Applied Spectroscopy , Vol 47, no 6, 1993.     “Diode-Laser Absorption Technique for Simultaneous Measurements of Multiple Gas Dynamic Parameters in High-speed Flows Containing Water Vapor”, M. P. Arroyo, S. Langlois, R. K. Hanson;  Applied Optics , Vol 33, no 15, 1994.     “Diode Laser Measurements of H 2 O Line Intensities And Self-Broadening Coefficients in the 1,4-μm Region”, S. Langlois, T. P. Birbeck and R. K. Hanson;  Journal of Molecular Spectroscopy , Vol 163, p 27-42, 1994.     “Absorption Measurements of Water Vapor Concentration, Temperature, and Line-shape Parameters Using a Tunable InGaAsP Diode Laser”, M. P. Arroyo and R. K. Hanson;  Applied Optics , Vol 32, no 30, 1993.     “Infrared Diode Laser Determination of Trace Moisture in Gasses”, J. A. Mucha, L. C. Barbalas,  ISA Transactions , Vol 25, no 3, 1986.     “Application of Tunable Diode Lasers in Control of High Pure Material Technologies”, G. G. Devyatykh h , V. A. Khorshev h , G. A. Maksimov h , A. I. Nadezhdinskii A , S. M. Shapin h   , Preprint.       “Laser Absorption IR Spectrometer for Molecular Analysis of High Purity Volatile Substances. Detection of Trace Water Concentrations in Oxygen Argon and Monogermane”, G. G. Devyatykh, G. A. Maksimov, A. I. Nadezhdinskii, V. A. Khorshev, S. H. Shapin;  SPIE  Vol 1724 “Turnable Diode Laser Applications”.     “Application of FM Spectroscopy in Atmospheric Trace Gas Monitoring: A Study of Some Factors Influencing the Instrument Design”, P. Werle, K. Josek and F. Slemr,  SPIE  Vol 1433 “Measurement Of Atmospheric Gases”, 1991.     “Stable Isotope Analysis using Tunable Diode Laser Spectroscopy”, Joseph F. Becker, Todd B. Sauke and Max. Loewenstein,  Applied Optics , Vol 31, no 12, 1992.     “High Sensitivity Detection of Trace Gases using Sweep Integration and Tunable Diode Lasers”, D. T. Cassidy and J. Reid,  Applied Optics , Vol 21, no 14, 1982.     “Atmospheric Pressure Monitoring of Trace Gases using Tunable Diode Lasers”, D. T. Cassidy and J. Reid,  Applied Optics , Vol 21, no 7, 1982.     “Near Infrared Diode Lasers Measure Greenhouse Gases”, A. Stanton, C. Hovde,  Laser Focus World , August 1992.     “Airborne Measurements of Humidity Using A Single Mode Pb Salt Diode Laser”, Joel A. Silver and Alan C. Stanton,  Applied Optics , Vol 26, no 13, 1987.     “Diode Laser Spectroscopy for On Line Chemical Analysis”; David S. Bomse, David C. Hovde, Daniel B. Oh, Joel A. Silver and Alan C. Stanton,  SPIE  Vol 1681, “Optically Based Method for Process Analysis”, 1992.     “Two-mirror Multipass Absorption Cell”, J. Altmann, R. Baumgart and C. Weitkamp;  Applied Optics , Vol 20, no 6, 1981.     “Long Optical Paths of Large Aperture”, J. White;  J. Opt. Soc. Am . Vol 32, p 285-288, May 1942.     “Folded Optical Delay Lines”, Herriott et al.;  Applied Optics , Vol 4 p 883-889, August 1965.     “Off Axis Paths in Spherical Mirror Interferometers”, D. Herriott, H. Kogelnik, R. Komper;  Applied Optics , Vol 3 no 4, 1964.    
 
       BRIEF SUMMARY OF THE INVENTION  
       [0027]     A method of detecting a preselected gas, such as methane, includes the steps of continuously moving a stream of sample gas through a confined testing area within a detecting instrument. A light source, such as a laser emitting diode or a light emitting diode is energized within the test instrument to emit a beam of frequency that is highly absorbed by the preselected gas. The beam is passed through the stream of gas within the confined testing area by bouncing the beam repeatedly between spaced apart mirrors in a Herriott type cell so that the length of travel of the beam within the test gas is greatly extended. Absorption of the beam is measured to provide an indication of the presence of the preselected gas.  
         [0028]     The frequency of light emitted by a typical light source, such as a laser diode or light emitting diode, is affected by the temperature of the light source and therefore the temperature must be regulated. By the provision of the invention disclosed herein, the sample gas stream, after having passed through the testing area is conducted past a heat control assembly.  
         [0029]     The heat control assembly includes a heat sink having cooling fins that are exposed to the stream of sample gas, the light source being mounted in contact with a peltier element that in turn is in heat conductive relationship with the heat sink. A thermistor senses the temperature of the heat sink and sends a control signal to a microprocessor that in turn sends temperature adjustment instructions to a power supply which then provides adjusted current to the peltier that adjusts the temperature of the light source.  
         [0030]     The Herriott type cell as used in the gas detection instrument of this invention is significantly improved by the incorporation of a central axle member that provides a confined annular shaped test area for the sample gas through which the light beam moves.  
         [0031]     A system is provided employing three photo detectors—that is: (a) a reference photo detector; (b) a multi-path photo detector; and (c) a single or direct path photo detector. By employing measurements of the three reference photo detectors the concentration of gas in the test sample can be determined with accuracy over a wider range than is typically available with existing instrumentation.  
         [0032]     The Herriott type absorption spectroscopy cell as used herein is improved in important ways including the provision wherein a light beam enters through an aperture in a first mirror and exists, after multiple passes, through an aperture in a second mirror to encounter a multi-pass photo detector. Simultaneously provision is made for measuring absorption after a single pass of the light beam. Activation of the reference photo detector is achieved by a beam splitter window.  
         [0033]     The invention herein provides a highly portable yet rugged system that it easily adaptable to mounting in a vehicle so that test samples can be continuously picked up from the environment and cycled through the test system while the vehicle is moving to enable an operator to expeditiously survey a relatively large geographical area. This highly portable system used in a vehicle provides readouts coordinated with a global positioning monitor so that gas level intensities of a geographical area can be quickly and accurately mapped.  
         [0034]     A better understanding of the invention will be obtained from the following detailed description and claims, taken in conjunction with the attached drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a diagrammatic representation of the basic elements making up the system of this invention for providing gas detection by light absorption spectroscopy. The system of this invention can be used for detecting extremely low-level concentrations of a selected gas, such as methane. The system is characterized by its portability and ruggedness. It can be either hand carried into a closed or confined environments or utilized in a moving vehicle by which maps can be generated showing the gas concentration levels of geographical areas.  
         [0036]      FIG. 2  is a partial elevational cross sectional view of the forward end portion of a Herriott type gas detection cell having improvements according to this invention. A light beam alignment system that uses hinge plates is illustrated in this figure.  
         [0037]      FIG. 3  is an elevational cross sectional view of an improved Herriott type cell for use in detecting gas concentration by light absorption spectroscopy showing improvements provided by this invention.  
         [0038]      FIG. 4  is an elevational side view of the gas detecting cell employed in this invention and shows the external appearance of the cell with cover component in place that provides an annular shaped gas cavity within the cell though which sample gas flows.  
         [0039]      FIG. 5  is a side view taken along the line  5 - 5  of  FIG. 4  showing the gas-detecting cell rotated about its axis by 90° with respect to the view of  FIG. 4 .  FIG. 5  shows the intermediate portion of the cell in cross section to reveal the annular gas sample passageway.  
         [0040]      FIG. 6  is a cross sectional view taken along the line  6 - 6  of  FIG. 5 . In this Figure the housing sleeves that are revealed in  FIGS. 4 and 5  are not shown.  
         [0041]      FIG. 7  is an exploded view of the forward portion of the cell that houses the light source such as a laser diode and illustrates the support structure that provides for alignment of the beam for entry into the sample gas cavity.  
         [0042]      FIG. 8  is a diagrammatic view of the relationship between a laser diode and associated elements by which the temperature of the diode is controlled.  
         [0043]      FIG. 9  is a diagrammatic illustration of the path the light beam takes within the annular cavity within the cell as the beam is reflected repeatedly from opposed mirrors to travel back and forth within the annular shaped sample gas cavity affording means for accurate measurement of absorption of the beam by the sample gas within the cell.  
         [0044]      FIG. 10  is a block diagram of the components employed to control the temperature of the light source employed with the Herriott type cell to provide improved accuracy of the system for detecting a selected gas.  
         [0045]      FIG. 11  is an isometric external view of a gas detection cell having a light beam alignment system that employs hinge plates.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]     Referring to the drawings and first  FIG. 1 , a block diagram of the major components of a system that can be used in practicing the methods of this invention are illustrated. The heart of the system is a cell  10  that will be described in detail subsequently, and that provides an environment in which a light beam  12  passes through a gas sample and in which absorption of the light beam is measured.  
         [0047]     The invention will be described in which a light beam is provided by a laser diode, in which case light beam  12  is a laser light beam. However, the invention can be practiced using a light source that provides a non-coherent light beam. An example of a non-coherent light source is a light emitting diode (LED). A laser diode provides a coherent light beam that is a beam of substantially uniform frequency light having the characteristic that the laser light beam does not disperse to the same extent as a non-coherent light beam. The use of a laser beam, such as produced by a laser diode, is advantageous, but the use of a laser diode is not indispensable. However, laser diodes are expensive compared to LEDs. In some applications, LEDs work satisfactorily. As used throughout this description, “laser beam” or “laser diode” are inclusive of “light beam” or “LED.” 
         [0048]     Supported to cell  10  is a structure that includes a laser diode  14  that, when energized, produces laser beam  12 . Laser diodes of the type represented by  14  are temperature sensitive. That is, the frequency of the laser light produced by diode  14  varies according to the temperature of the diode. For measurement accuracy it is important that the frequency of laser beam  12  be controlled within a fairly narrow range, which in turn means that the temperature of laser diode  14  must be controlled. For this purpose, a temperature regulating system generally indicated by the block  16  is employed and will be described in detail subsequently.  
         [0049]     The invention herein functions by moving laser beam  12  through a gas sample and determining the level of concentration of a selected gas in the gas sample by measuring absorption of the laser beam. This technology is generally referred to as “laser absorption spectroscopy.” Cell  10 , including the components secured in relation to it, provide a tunable laser diode absorption spectroscope. Flow channels are provided by which a gas sample is moved through cell  10 . Sample gas is taken in through an inlet  18  in inlet tube  20  and flows through filter  22  into the interior of cell  10 . The gas flows through cell  10  to an outlet tube  24  that connects with temperature regulating system  16 . Gas is moved through the system by means of a gas pump  26  to a discharge tube  28  by which the gas sample is returned to the environment.  
         [0050]     Laser beam  12  passes through a window (to be described subsequently). A portion of the beam passes through an aperture  30  in a first mirror  44 . The number  12 A represents the first pass of the beam internally of cell  10 . A portion of laser beam  12  is reflected by the window, the reflected beam being indicated by the numeral  12 B. A photo detector  32  is placed to receive the interception of reflected beam  12 B and provides an electrical signal that is representative of the intensity of laser beam  12 . The electrical signal from photo detector  32  is conveyed by conductor  34  to an amplifier  36  that feeds into a analog to digital converter circuit  38  that provides a referenced digital input over conductor  40  that feeds into a microprocessor  42 .  
         [0051]     Cell  10  is of a type generally known as a “Herriott” cell. This name is derived from the inventor of a cell that employs opposed mirrors that reflect a light beam back and forth between them so that a relatively long path can be obtained in a relatively shorter length instrument, and in which the path is in a circular pattern. While generally of the “Herriott” type, cell  10  of this invention has many improvements and innovations as will be described subsequently in detail.  
         [0052]     Cell  10  employs a first mirror  44  and an opposed second mirror  46 . A small aperture  30  is provided in first mirror  44  through which the laser beam passes and forms beam  12 A within the cell that first impacts second mirror  46 . Beam  12 A is reflected sequentially between mirrors  44  and  46  a number of times before exiting second mirror  46  through a small aperture  48 . The exit beam  50  impinges on a second photo detector  52  that provides a signal on conductor  54  feeding an amplifier circuit  56  that feeds a second analog to digital converter  58  that provides a digital signal on conductor  60  leading to microprocessor  42 .  
         [0053]     The methods, and the systems for practicing the methods, of this invention are used to detect selected gasses such as methane, butane, propane, ethane, oxygen, hydrogen, nitrogen, H 2 O, hydrogen fluoride, hydrogen chloride, hydrogen boride, hydrogen sulfide, ammonia, CO, CO 2 , NO, NO 2  and SF 6 . The system can be adapted to detect different selected gases by changing out the laser diode to one that produces the frequency of light most readily absorbed by the gas of interest. When a light emitting diode (rather than a laser diode) is used, the broader spectrum of light produced by it can detect more different gases but usually at higher concentrations. The system will be described as it is particularly useful in detecting methane gas since methane is the basic component of natural gas and most manufactured fuel gasses. If a leak occurs in a gas distribution system it can usually be located by detecting the presence of methane. Therefore, cell  10  employs a laser diode  14  that produces a beam characterized by a frequency that corresponds to a high degree of absorption by methane. Sample gas that is drawn in through inlet  18  and flows by way of inlet tube  20  into and through cell  10  absorbs, that is, decreases the intensity of light beam  12 A in proportion to the quantity of methane contained in the sample gas.  
         [0054]     Light beam  50  passes out aperture  48  in second mirror  46  after having been reflected many times between mirrors  44  and  46 . The way this is achieved will be described subsequently. Undergoing multiple reflections from the time beam  12 A enters cell  10  until it exits through aperture  48  means that the beam has traversed a relatively long path equal to many times the length of cell  10  which in turn means that ample provision has been made for absorption of the light beam by the presence of methane in the gas sampler.  
         [0055]     By comparing the intensity of the signal on conductor  34  with that on conductor  54  the concentration of methane in the sample gas passing through cell  10  can be ascertained. By accurate processing within microprocessor  42  the amount of methane contained in the sample gas passing through cell  10  can be determined with great accuracy and can be expressed such as in parts per million. The presence of methane can be detected at a sensitivity down to a few parts per million or even, ideally, to a sensitivity of one or less than one part per million.  
         [0056]     As previously stated, beam  12  emanating from laser  14  passes through a first aperture  30  in first mirror  44  to provide beam  12 A within the cell. When the initial passage of laser beam  12 A within cell  10  encounters second mirror  46  most of the beam intensity is reflected back towards first mirror  44  and subsequently repeatedly reflected between first mirror  44  and second mirror  46  to finally pass out through second window  48  to form exit beam  50 . However, when beam  12 A strikes second mirror  46  a small portion of the intensity of the beam passes through the mirror even though no aperture or window is provided since most mirrored surfaces are not 100% reflective. The portion of light beam  12 A that passes through second mirror  46  provides a second exit beam  62  that engages a third photo detector  64 . This produces an electrical signal on conductor  66  passing to a third amplifier  68  that feeds an analog to digital converter  70  sending a digital signal by way of conductor  72  to microprocessor  42 . The employment of two separate exit beams  50  and  62  emanating from cell  10  to activate photo detectors  52  and  64  is an important attribute of the invention herein. It is apparent that only signals appearing on conductors  40  and  60  feeding microprocessor  42  are required to measure low levels of concentration of methane in the gas passing through cell  10 . It is important to detect very small levels of methane in the sample gas, which is accomplished by employing a long light path for the laser beam before the beam exits through window  48 , however, this arrangement fails if a broader scale of methane detection is required. If methane is present at a relatively high level in the sample gas passing through the cell the laser beam will be substantially completely absorbed before it exists through window  48  so that insufficient intensity of the beam remains for use in computing the percentage of methane of greater concentration in the gas sample. This problem is overcome by the use of third photo detector  64 . Second exit beam  62  travels a relatively short distance through the gas sample, therefor the attenuation of beam  62  occurs at a rate that can provide a measurement even when the percentage of methane in the test gas is many times that which is detectable by photo detector  52 . In other words, the employment of two separate exit beams  50  and  62 , one having a short length light path in the test gas and the other having a long length light path provides a system wherein the range of concentration of methane that can be measured is greatly expanded.  
         [0057]     In the preferred practice of the invention, laser light beam  12  is not energized by a steady state voltage to produce a steady state beam but, in the contrary, laser  14  is pulsed with a saw tooth wave shaped current. Each pulsation of laser diode  14  generates a pulsed laser beam  12  that varies in frequency over a selected bandwidth. Each current pulse produces light that varies in frequency above and below the frequency that undergoes the greatest absorption of the specific gas the instrument is designed to detect.  
         [0058]     Since laser  14  is energized by a particular pulsed current waveform, the resultant signals generated by photo detectors  32 ,  52  and  64  (see  FIG. 1 ) are characterized by that particular waveform. Therefore, within microprocessor  42  absorption is detected by electronically dividing the signal of photo detectors  52  and  64  by the signal of photo detector  32 .  
         [0059]      FIGS. 2 through 7  illustrate details of a preferred embodiment of cell  10 . As previously stated cell  10  is, generally speaking, of the Herriott type, however with significant and important changes, innovations, and improvements.  
         [0060]      FIG. 3  is a cross-sectional view showing the basic construction of cell  10 . This figure shows a structure providing first mirror  44  and second mirror  46 . A unique feature of cell  10  as seen in  FIGS. 2, 3 ,  5  and  6  is the employment of a central axle member  74  having a first end  76  that engages first mirror  44  and a second end  77  that engages second mirror  46 . The exterior surface  78  of central axle  74  is contoured, that is—of less diameter in the middle than at the ends  76  and  77  for purposes, which will be described subsequently.  
         [0061]     Surrounding central axle  74  is a housing  80  that, in the illustrated arrangement, as shown in  FIG. 4 , is formed of two matching portions  80 A and  80 B that fit together. Housing  80 A,  80 B provides an interior surface  82  spaced from the central axle external surface  78  providing an elongated annular area  84 . It is this annular area  84  through which sample gas moves that absorption of light from the laser diode takes place and by which the concentration of methane in the gas sample is detected.  
         [0062]     There is formed in first mirror  44  a first aperture  30  as seen in  FIG. 2 . Light beam  12  from laser diode  14  first passes through a lens  86  mounted in an opening in a lens support plate  89  and encounters an inclined window  88 . At window  88  a portion of beam  12  is reflected to provide the laser beam  12 B that was discussed with reference to  FIG. 1 . Beam  12 B passing through a lens  90  and encounters first photo detector  32 . Beam  12  passes through inclined window  88  and through first aperture  30  in first mirror  44 .  
         [0063]     As seen in the left hand end of  FIG. 3  second mirror  46  has aperture  48  as previously mentioned that is in alignment with a passageway  92  in a cap member  94  by which the beam exits from the cell. In alignment with passageway  92  is photo detector  52  described with respect to  FIG. 1 . In  FIG. 3  aperture  30  in first mirror  44  and apertures  48  in second mirror  46  are shown as if they are in a vertical plane taken through a longitudinal axis (not shown) of axle  76 . This is for purposes of illustration only as there is no requirement that they be in the same plane and as seen in  FIG. 9 , which will now be discussed, apertures  30  and  48  are typically not in the same plane.  
         [0064]     The pattern of light travel of the beam-entering cell  10  is illustrated in  FIG. 9  wherein the first mirror  44  and second mirror  46  are represented by circles with smaller circles indicating first aperture  30  and second aperture  48 . Laser diode indicated at  14  produces laser beam  12 A as previously described that passes through first aperture  30  into the cell. Beam  12 A strikes second mirror  46 . As previously stated, a portion of beam  12 A passes through second mirror  46  producing beam  62  that strikes third photo detector  64 . The major portion of beam  12 A is reflected as indicated by arrow  96 . The beam travels back and forth between mirrors  44  and  46  a large number of times and finally exits through aperture  48  in second mirror  46 , the exiting beam being indicated by numeral  50  described with respect to  FIG. 1 . Beam  50  strikes second photo detector  52 .  
         [0065]      FIG. 9  graphically illustrates the unique beam path that travels a plurality of times back and fourth between mirrors  44  and  46  within the annular area that surrounds the central axle. This arrangement provides a very rigid cell structure having an extremely long path through which the laser beam travels through the annular area that is continuously supplied with sample gas. This long path, achieved by multiple reflections of the light beam, provides for a high level of sensitivity of absorption of the laser beam while at the same time affords a compact, sturdy and easily portable system for detecting the presence of a selected gas, such as a methane.  
         [0066]     As seen in  FIG. 9 , light beam  12 A enters the cell at an oblique angle relative to the imaginary longitudinal axis of axle  74 . This causes the point of incidence of the light beam striking opposed mirrors  44  and  46  to progress radially around the mirrors. The cross-sectional area of the annular absorption area  84  through which the light beam traverses between opposed mirrors  44  and  46  is greatest at the mirror surfaces and least at the midpoint between the mirrors. For this reason the diameter of axle  74  is least at the midpoint between the axle opposed ends  76  and  77  as shown in  FIG. 3 .  
         [0067]     Laser diode  14  is supported by a laser mount structure generally indicated by the numeral  98  as seen in  FIGS. 4, 5 ,  6  and  7 . A different laser mount structure  98 A is shown in  FIGS. 2 and 11 .  FIG. 7  is an exploded view of important portions of laser mounting structure  98  that is supported to one end of cell  10  by means of structural brackets  100  shown in  FIGS. 4 and 5 . As best seen in  FIG. 7  the laser mounting structure  98  includes a support base  102  having integrally formed parallel portions  104  and  106 . That is, the support base includes integrally formed portion  104  that is hinged to base  102  about a vertical axis while portion  106  is integrally hinged  15  to portion  104  about a horizontal axis. This unique double axis arrangement permits the alignment of the beam from the laser diode to be very accurately adjusted to provide the critical paths as illustrated in  FIG. 9  so that the multiple paths can be accomplished and so the beam properly exits the cell to intersect photo diodes  52  and  64 . An alternate embodiment of the double axis beam alignment system of this invention will be described subsequently with reference to  FIGS. 2 and 11 .  
         [0068]     As seen in  FIG. 7 , an exchanger piece  108  has an integral forwardly extending plug portion  110 . Exchanger piece  108  is secured to support base  102  by a spatial ring  112 . Positioned on the forward surface of exchanger piece  108  is a non-metallic isolator  114 . As will be described in more detail subsequently, mounted on the forward surface  116  of the exchanger piece plug portion  110  is a peltier device  118 , a substrate  120 , a thermistor  122  and laser diode  14 . The structure relationships between metallic drain  110 , peltier  118 , substrate  120 , thermistor  122  and diode  14  are diagrammatically illustrated in  FIG. 8 .  
         [0069]     As shown in  FIG. 7 , the rearward end of exchange piece  108  includes an integral tubular portion  124  that contains therein heat exchange fins  126  (See  FIGS. 7 and 8 ). In a heat conductive relationship with plug portion  110 , which functions as a metallic heat drain, is a resister  128  as shown in  FIG. 8 .  
         [0070]     An important aspect of the invention is the method and system for controlling the temperature of laser diode  14 . For effective measurement of gas concentration by spectrographic absorption of a light beam it is important that the frequency of the light beam be controlled within a narrow range. A laser diode can be designed to provide the frequency of light that is most readily absorbed by methane molecules. However, if the frequency varies from the critical absorption frequency the accuracy of the system is reduced. Further, the frequency of light emitted by a laser diode is affected by the temperature of the diode. The system of this invention for controlling the temperature of laser diode  14  is best illustrated in  FIGS. 8 and 10 .  FIG. 10  diagrammatically shows the relationship between laser diode  14  and its heat control components. Laser diode  14  is secured to a substrate  120  such as by the application of thermo-transmitting adhesive. Also secured to substrate  120  is a thermistor  122 . Substrate  120  is bonded to a peltier element  118  that functions as a thermoelectric cooling element. Peltier  118  is thermally bonded such as by soldering, to a metallic drain  110  that is the plug portion of exchanger piece  108  as seen in  FIG. 7 . The metallic drain in turn is in thermal contact with one or more heat exchanger fins  126  as seen in  FIGS. 2, 7  and  8 . Also, in thermal contact with metallic drain  110  is a resistor  128  as seen in  FIG. 8  and in the electrical diagram of  FIG. 10 .  
         [0071]     Referring to  FIG. 10  the electrical interrelationships of the heat control components are illustrated. Thermistor  122  provides a voltage signal proportional to the temperature of substrate  120 , which in turn is linked to the temperature of laser diode  14 . A signal from thermistor  122  is fed to current generator circuit  130  and the signal from generator  130  is fed to an analog to digital converter  132 , the output of which is passed to a microprocessor  138 . A temperature selector circuit  136  provides a voltage output directly related to the desired temperature of the substrate  120  and therefore of laser diode  14 , the output of which is fed to a microprocessor  138 . In microprocessor  138  the signal from the temperature selector circuit  136  is compared with the digitally encoded temperature detected by thermistor  122  to provide an output control signal on conductor  140 . The signal on conductor  140  is fed to a heat mode on/off switch  142 . When switch  142  is “on” this signal is fed to resistor  128  whose function is to supply heat as required to the heat exchanger plug portion  110 .  
         [0072]     The output from microprocessor  138  is fed to a peltier control signal generator  144  that in turn is fed to a digital to analog converter  146 , the output of which is fed to a peltier current generator  148  which in turn supplies a control current to peltier element  118 .  
         [0073]     Under most operating conditions the function of the heat exchanger system of the invention is to cool laser diode  14 . That is, laser diodes typically generate significant heat and therefore normally it is necessary to remove heat from the laser diode to keep it within the desired operating range. For this reason under normal operating conditions resistor  128  is not employed since its only function is to supply heat when necessary to the plug portion heat exchanger  110  so that by means of peltier  118  the heat is transferred to substrate  120  and thereby functions to provide a warm environment for laser diode  14 .  
         [0074]     Since it is normally necessary, under typical ambient temperatures to cool the laser, an important aspect of this invention is the concept of utilizing the test gas as a cooling medium. As shown in  FIG. 1  the test gas, after having passed through cell  10  is conducted to thermo regulating system  16  that is illustrated in  FIGS. 8 and 10  and the elements of which are also illustrated in  FIG. 7 . Test gas that is moved through cell  10  in which the concentration of methane is determined, also advantageously is used as a cooling medium. The test gas passes out of cell  10  through the cooling system and past cooling fins  126  contained within housing tubular portion  124  as illustrated in  FIG. 7  and diagrammatically in  FIG. 8 .  
         [0075]     The current consumed by a laser diode at selected voltages can be used as an indicator of the diode&#39;s temperature. That is, as the temperature of a laser diode increases the resistance to current flow increases. This characteristic can be used as a means of regulating the diode&#39;s temperature. By noting the current consumed by a diode the temperature control system disclosed in  FIGS. 8 and 10  can be employed. Thus, in place of thermistor  122  an ammeter in series with laser diode  14  can be used to supply a control signal to measurement current generator  130  to produce an appropriate signal that is fed to A/D converter  132  and thence to microprocessor  138 . By use of proper software the microprocessor directs temperature corrective action by peltier device  118  or resistor  128 . While this system has good theoretical possibilities, as a practical matter, achieving the required accuracy of diode temperature control using diode voltage and current has been difficult.  
         [0076]     The route taken by sample gas entering and exiting cell  10  is illustrated best by reference to  FIG. 3  in conjunction with  FIG. 1 . After sample gas is drawn into inlet tube  20  and through filter  22 , the gas enters cell  10  through a passageway  150  shown in dotted outline in cap member  94 . From passage  150 , gas enters into an axial recess  152  in the second end  77  of central axle  74 . A plurality of spaced apart small diameter passageways  154  extend in radial planes from axial recess  152  to the surface  78  of axle  74  to connect with annular absorption area  84 . The plurality of openings  154  serve to equally distribute the sample gas entry into one end of absorption area  84 .  
         [0077]     The gas traverses within absorption area  84  from the second end  77  to the first end  76  of axle  74 . Adjacent the first end  76  of axle  74  are a plurality of passageways  156  in radial planes that communicate with an axial recess  158 .  
         [0078]     Communicating with axial recess  158  is an exit passageway  160  by which sample gas flows from cell  10 . The plurality of small passageways  156  that connect axial recess  158  to annular absorption area  84  are seen in  FIGS. 2 and 3 . Sample gas after passing out of cell  10  through exit passageway  160  flows through a length of flexible tubing  162  (see  FIG. 5 ). Closing the rearward end of exchanger piece  108  is an end cap  164  that has a radial passageway  166  that receives an end of flexible tube  162  as shown in  FIG. 5 . A second passageway  168  in cap  164  (see  FIG. 7 ) provides for communication with gas pump  26  as seen in  FIG. 1  which can be accomplished such as by a flexible tube  170  illustrated in  FIG. 1 , but not shown on the other figures. Sample gas is drawn by the action of pump  26  to move into annular absorption area  84  in a way that the gas is equally distributed around the entire annular passageway so that as a light beam is bounced repeatedly between opposed mirrors  44  and  46 , the opportunity for absorption of the light beam is equally distributed and in an arrangement wherein gas passageways are most effectively formed within the end portions of axle  74 .  
         [0079]     The cell for measuring the concentration of a preselected gas, such as methane as illustrated and described herein is generally configured to operate at or near atmospheric pressures however the system functions successfully in the range of about 0.1 to about 2.0 atmospheres. Basically, the system as illustrated is not intended to test high-pressure gas samples.  
         [0080]     The gas detecting system of this invention is particularly adaptable for use in conducting a survey of a geographical area to find where gas leakage may be occurring. The gas detection system herein is particularly adapted for use as a survey instrument due to the confined annular absorption area that is achieved by the use of a center axle  74  surrounded by housing components  80 A and  80 B resulting in a relatively small volume sample gas test area having a relatively long path for a light beam to pass. Sample gas within the test area is quickly changed by the continuous action of gas pump  26 . Taking advantage of these unique features adapts the gas detection system of this invention to be moved at a relatively fast rate (compared to existing gas detection systems) over a geographical area. Specifically, the gas detection instrument as described herein can be moved about by a moving vehicle at a speed that enables a relatively large geographical area to be checked for possible gas leakage in a relatively short time.  
         [0081]     When the system of this invention is transported by a vehicle it is important that the sample gas inlet  18  as seen in  FIG. 1  be extended exteriorly of the vehicle so that gas is constantly drawn into the system from the local environment as the vehicle moved from one part of a geographical area to another.  
         [0082]     In order to provide accurate information as to areas of gas concentration that may be significant, the system of this invention is particularly adaptable for use with a global positioning system  172  as indicated in  FIG. 1 . Global positioning system  172  is in communication with microprocessor  42 . Further, by use of a printer  174  coupled to microprocessor  42  and global positioning system  172 , a map can be generated with a printout of detected gas concentration levels. A user can survey an area, such as a plant site, a village, an industrial park, a portion of a larger city, or any area of interest, and obtain a map with indicated levels concentration of a specific gas, such as methane. In this way, a user can quickly determine the areas where increased gas concentration exists and the instrument of this invention can then be returned to that area where it can be carried about (as opposed to being transported in a vehicle) for more detailed inspection to determine where gas leaks are occurring.  
         [0083]      FIGS. 2 and 11  show an alternate embodiment of a laser mount structure  98 A that has a different double axis arrangement for alignment of the beam from the laser diode. A structural bracket  176  is secured to the forward end of cell  10 . Bracket  176  supports a base plate  178  (see  FIG. 11 ). An adjustable plate  180  is hinged to base plate  178  by a thin, flexible, first hinge sheet  182 . A second thin, flexible, hinge sheet  184  (see  FIGS. 2 and 11 ) supports exchange piece  186  having a tubular portion  188  that functions as a heat exchange housing, the outer end of which is closed by an end cover  190 . Tubular portion  188  houses heat exchange fins  126 . Tubular portion  188  has passageways  166 A and  168 A for the flow of sample gas corresponding to passageways  166  and  168  as described with reference to  FIG. 7 .  
         [0084]     The hinge sheets  182  and  184  bend within their elastic limits as the laser beam is aligned with cell  10  and therefore serve the same function as the integrally formed hinges of parallel portions  104  and  106  of support plate  102  as described with reference to  FIG. 7 . Both structures permit the laser diode to pivot about axii that are in perpendicular planes. The laser mount structure  98 A of  FIGS. 2 and 11  is preferred to the laser mount structure of  FIG. 7  due to its economy of manufacture. Otherwise, the two embodiments function the same way to do the same thing and achieve the same results.  
         [0085]     While the invention has been described with a certain degree of particularity, it is manifest that many changes may be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification, but is to be limited only by the scope of the attached claims, including the full range of equivalency to which each element thereof is entitled.