IPC분류정보
국가/구분 |
United States(US) Patent
등록
|
국제특허분류(IPC7판) |
|
출원번호 |
UP-0592473
(2005-03-08)
|
등록번호 |
US-7705988
(2010-05-20)
|
우선권정보 |
GB-0405252.8(2004-03-09); GB-0406559.5(2004-03-24); GB-0407533.9(2004-04-02); GB-0421319.5(2004-09-27) |
국제출원번호 |
PCT/GB2005/000876
(2005-03-08)
|
§371/§102 date |
20060911
(20060911)
|
국제공개번호 |
WO05/088275
(2005-09-22)
|
발명자
/ 주소 |
|
출원인 / 주소 |
|
대리인 / 주소 |
|
인용정보 |
피인용 횟수 :
8 인용 특허 :
17 |
초록
▼
Apparatus for detecting a target gas in a monitored space includes two laser diodes driven by drive circuits at electrical frequencies f and f′ that are not harmonically related. The lasers operate at mean wavelengths Λ1 and Λ2 respectively close to two different absorption lines
Apparatus for detecting a target gas in a monitored space includes two laser diodes driven by drive circuits at electrical frequencies f and f′ that are not harmonically related. The lasers operate at mean wavelengths Λ1 and Λ2 respectively close to two different absorption lines of the target gas and are scanned over wavelength ranges ∂Λ1 and ∂Λ2 respectively. The outputs from the lasers are collimated by an optical element and delivered to a receiver element after passing through the space. The receiver element focuses the radiation from both lasers onto a detector where the optical signals are combined into a single electrical signal with principal frequency components f and f′. A quantity of target gas Q1 is calculated from the amplitude of frequency component f1 for measurements made around wavelength Λ1 and a quantity of target gas Q2 is calculated from the amplitude of frequency component f2 for measurements made around wavelength Λ2.
대표청구항
▼
What is claimed is: 1. A method of detecting a target gas in a monitored space comprising applying an electrical control current to a laser diode so as to generate optical radiation of a wavelength defined by the control current, transmitting the optical radiation across the monitored space and det
What is claimed is: 1. A method of detecting a target gas in a monitored space comprising applying an electrical control current to a laser diode so as to generate optical radiation of a wavelength defined by the control current, transmitting the optical radiation across the monitored space and determining the optical absorption thereof, wherein: the control current defines two mean wavelengths Λ1 and Λ2 for the optical radiation and includes electrical modulation at two frequencies f and f′ respectively; and wherein Λ1 and Λ2 are respectively close to two separate optical absorption lines of the target gas and f and f′ are not harmonically related. 2. A method of detecting a target gas in a monitored space as claimed in claim 1 wherein the optical radiation is generated from a single laser diode and the control current comprises a bias component which is alternated between two values respectively defining Λ1 and Λ2. 3. A method of detecting a target gas in a monitored space as claimed in claim 1 wherein the optical radiation is generated from two laser diodes of which one has a said control current comprising a bias component of value defining Λ1 and the other has a said control current comprising a bias component defining Λ2. 4. A method of detecting a target gas in a monitored space as claimed in claim 2 wherein the electrical modulation is sinusoidal. 5. A method of detecting a target gas in a monitored space as claimed in claim 3 wherein the electrical modulation is sinusoidal. 6. Apparatus for detecting a target gas in a monitored space, which apparatus comprises a laser diode operable to transmit radiation across the monitored space and a first optical receiver operable to receive the transmitted radiation and determine optical absorption thereof, wherein: a control current is applied to the laser diode to define two mean wavelengths Λ1 and Λ2 for the optical radiation and is electrically modulated at two frequencies f and f′ respectively; and Λ1 and Λ2 are respectively close to two separate optical absorption lines of the target gas and f and f′ are not harmonically related. 7. Apparatus for detecting a target gas in a monitored space as claimed in claim 6 wherein the apparatus comprises a single laser diode and the control current applied thereto comprises a bias component alternated between two values respectively defining Λ1 and Λ2. 8. Apparatus for detecting a target gas in a monitored space as claimed in claim 7, which apparatus is arranged for the detection of methane, ethane, propane or ethylene, wherein: the bias component of the laser diode control current is varied in a manner determined to operate the laser diode at wavelengths suitable for scanning either of methane's absorption lines at 1684 nm and 1687.3 nm and one or more of the other gases' absorption lines or features at 1684.3 nm, 1686.4 nm and 1687.0 nm; the scanning component repetitively scans the laser diode's wavelength over the chosen absorption lines or features; the optical radiation from the laser diode is collected and transmitted through the monitored space and subsequently illuminates an optical detector; and an electrical signal from this optical detector is processed to determine the gas or gases present in the monitored space and the amounts of each gas present. 9. Apparatus for detecting a target gas in a monitored space as claimed in claim 8 wherein the amount of methane gas present in the monitored space is determined and then the amount of hydrogen sulphide present is estimated using a coefficient relating the amount of methane to the amount of hydrogen sulphide for the solution gas of a particular field or facility. 10. Apparatus for detecting a target gas in a monitored space as claimed in claim 9 wherein said apparatus includes means to update said coefficient. 11. Apparatus for detecting a target gas in a monitored space as claimed in claim 9, which apparatus is arranged to deliver outputs representing the concentrations or quantities of gases calculated or estimated present in the monitored space or sample measurement chamber, wherein said outputs include: analogue electrical signals proportional to the concentration or quantity of each gas; a digital electronic signal conforming to a defined protocol and containing numerical information conveying the concentration or quantity of each gas; and a numerical representation of the concentration or quantity of each gas. 12. Apparatus for detecting a target gas in a monitored space as claimed in claim 9 wherein said apparatus comprises two laser diodes, one operated at wavelengths to scan absorption lines of flammable gases including methane, ethane and propane, the other operated at a wavelength to scan an absorption line of hydrogen sulphide. 13. Apparatus for detecting a target gas in a monitored space as claimed in claim 12 comprising an alarm actuated when hazardous gases are detected by the apparatus, wherein the alarm is actuated only when the apparatus detects both hydrogen sulphide and methane. 14. Apparatus for detecting a target gas in a monitored space as claimed in claim 13 wherein the alarm is actuated only when the detection apparatus detects methane above a predetermined threshold level. 15. Apparatus for detecting a target gas in a monitored space as claimed in claim 14 wherein said threshold level is determined from records of the sourness of petrochemicals handled at the facility. 16. Apparatus for detecting a target gas in a monitored space as claimed in claim 7, which apparatus is arranged for the detection of hydrogen sulphide, wherein: the bias component of the laser diode control current is varied in a manner determined to operate the laser diode at one or more wavelengths suitable for scanning any of methane's optical absorption lines or features; the scanning component repetitively scans the laser diodes wavelength over the chosen absorption line(s) or feature(s); the optical radiation from the laser diode is collected and transmitted through the monitored space and subsequently illuminates an optical detector; and an electrical signal from this optical detector is processed to determine the amount of methane gas present in the monitored space; whereafter the amount of hydrogen sulphide present is estimated using a coefficient relating the amount of methane to the amount of hydrogen suiphide for the solution gas of a particular field or facility. 17. Apparatus for detecting a target gas in a monitored space as claimed in claim 16 wherein said apparatus includes means to update said coefficient. 18. Apparatus for detecting a target gas in a monitored space as claimed in claim 16, which apparatus is arranged to deliver outputs representing the concentrations or quantities of gases calculated or estimated present in the monitored space or sample measurement chamber, wherein said outputs include: analogue electrical signals proportional to the concentration or quantity of each gas; a digital electronic signal conforming to a defined protocol and containing numerical information conveying the concentration or quantity of each gas; and a numerical representation of the concentration or quantity of each gas. 19. Apparatus for detecting a target gas in a monitored space as claimed in claim 16 wherein said apparatus comprises two laser diodes, one operated at wavelengths to scan absorption lines of flammable gases including methane, ethane and propane, the other operated at a wavelength to scan an absorption line of hydrogen sulphide. 20. Apparatus for detecting a target gas in a monitored space as claimed in claim 19 comprising an alarm actuated when hazardous gases are detected by the apparatus, wherein the alarm is actuated only when the apparatus detects both hydrogen sulphide and methane. 21. Apparatus for detecting a target gas in a monitored space as claimed in claim 19 wherein the alarm is actuated only when the detection apparatus detects methane above a predetermined threshold level. 22. Apparatus for detecting a target gas in a monitored space as claimed in claim 21 wherein said threshold level is determined from records of the sourness of petrochemicals handled at the facility. 23. Apparatus for detecting a target gas in a monitored space as claimed in claim 7 wherein: the optical radiation from the laser diode is split into two fractions; one said fraction is passed through a retained sample of the target gas and illuminates an optical detector, the signal from this optical detector being used by the transmitter to maintain the position and width of the target gas absorption line with respect to the scanning component waveform; the second said fraction is transmitted through a monitored space to illuminate an optical detector in a receiver, the signal from this optical detector being processed to calculate the quantity of target gas present in the monitored space, this quantity being output by the receiver; and wherein the transmitter includes means of electronically introducing a replica absorption feature into the intensity of the optical output of the laser diode, the position, width and shape of the replicated absorption feature corresponding to that known to be produced by the absorption line of the target gas and being actively maintained by the transmitter, and the size of the replicated absorption feature being a controlled variable, calculated to simulate the presence of a nominated quantity of target gas in the monitored space. 24. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein: the drive current to the laser diode is produced by a digital synthesizer which uses a Digital-to-Analogue Converter (DAC) to output a sequence of voltages which are turned into a current by a voltage to current (V-I) converter; the sequence of voltages is calculated to produce the desired current waveforms, the waveforms including those necessary to bias and scan the laser diode; and when required a replica absorption feature is introduced into the output of the laser diode simulating the presence of a nominated quantity of target gas in the monitored space. 25. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein: the laser diode in the transmitter scans a total of two or more chosen absorption lines of one or more target gases; the retained gas sample includes a quantity of each of the one or more target gases and is used to maintain the position and width of each chosen absorption line of the one or more target gases with respect to the scanning component waveform; the transmitter includes means of electronically introducing replica absorption features into the intensity of the optical output of the laser diode; and wherein the position, width and shape of these replica absorption features correspond to that known to be produced by the one or more target gases' absorption lines and being actively maintained by the transmitter, and the sizes of the replica absorption features are controlled variables, calculated to simulate the presence of nominated quantities of the one or more target gases in the monitored space. 26. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein said apparatus comprises two or more laser diodes arranged to detect or measure one or more target gases and wherein: the output from each laser diode is split into two fractions, one fraction used to illuminate a retained sample of the one or more target gases to maintain the position and width of the one or more absorption lines with respect to the scanning component waveform, and the other fraction transmitted through a monitored space to a receiver; the receiver is capable of detecting and processing the optical signals from the two or more laser diodes to calculate the quantities of the one or more target gases in the monitored space; the transmitter is provided with means of electronically introducing replica absorption features into the intensity of the optical output of each of the laser diodes, the position, width and shape of the replicated absorption features corresponding to that known to be produced by the target gases' absorption lines and being actively maintained by the transmitter, and the sizes of the replicated absorption features being controlled variables, calculated to simulate the presence of nominated quantities of the one or more gases in the monitored space. 27. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein means are provided for an operator or control system to instruct the transmitter to simulate the presence of nominated quantities of one or more target gases in the monitored space, the transmitter subsequently electronically simulating the presence of the nominated quantities of the one or more target gases. 28. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein in the event that the transmitter diagnoses a failure to maintain the position and width of the one or more target gas absorption lines with respect to the scanning component waveform that it cannot correct, the transmitter either stops transmitting or modulates its laser diode with a signal indicating to the receiver that the transmitter has a problem which requires attention. 29. Apparatus for detecting a target gas in a monitored space as claimed in claim 28 wherein, in the event that the receiver does not receive a signal from the transmitter, or receives a signal from the transmitter indicating that the transmitter has a problem which requires attention, the receiver changes its output to indicate the loss of transmitter signal or the presence of a transmitter problem. 30. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein the transmitter and the receiver are physically part of a single gas detection or measurement apparatus, and wherein gas is drawn or diffuses into a sample measurement chamber in order to be illuminated by laser diode radiation and measured. 31. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein instead of illuminating a retained sample of the target gas for wavelength registration purposes, a fraction of the output from the transmitter's laser diode illuminates an optical component possessing transmissive or reflective properties determined to provide the wavelength registration function, having illuminated said component, the transmitted or reflected illumination being concentrated onto an optical detector in the transmitter. 32. Apparatus for detecting a target gas in a monitored space as claimed in claim 31 wherein said optical component includes a narrow-band interference filter, a diffraction grating, a holographic optical element, an etalon or a fiber Bragg-grating. 33. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein: the position and width of the target gas absorption line with respect to the scanning component waveform is not actively maintained by the transmitter, the signal from the optical detector in the transmitter instead being used solely to monitor the position, width and shape of the target gas absorption line with respect to the scanning component waveform; means of communication is provided between the transmitter and the receiver, such means being used to provide the receiver with data relaying the position, width and shape of the target gas absorption line; a signal from the receiver's detector is processed using the available target gas absorption line position, width and shape data to calculate the quantity of target gas present in the monitored space, this quantity being output by the receiver; the transmitter includes means of electronically introducing a replica absorption feature into the intensity of the optical output of the laser diode, the position, width and shape of the replicated absorption feature corresponding to that known to be produced by the target gas' absorption line, and the size of the replicated absorption feature being a controlled variable, calculated to simulate the presence of a nominated quantity of target gas in the monitored space. 34. Apparatus for detecting a target gas in a monitored space as claimed in claim 23 wherein the means of collecting the laser diode radiation and transmitting it through the monitored space includes combinations of free-space optical elements and/or fiber-optics. 35. Apparatus for detecting a target gas in a monitored space as claimed in claim 6 wherein the apparatus comprises two laser diodes of which one has a control current comprising a bias component defining Λ1 and the other has a control current comprising a bias component defining Λ2. 36. Apparatus for detecting a target gas in a monitored space as claimed in claim 35 wherein the electrical modulation applied to each laser diode is sinusoidal. 37. Apparatus for detecting a target gas in a monitored space as claimed in claim 36 wherein: said sinusoidal component is synchronously alternated between the two non-harmonically related electrical frequencies f and f′ at which the laser's wavelength is scanned across one or the other of the chosen absorption lines for a prescribed interval; the optical radiation from the laser diode is collected and transmitted through the monitored space and subsequently illuminates an optical detector; and an electrical signal from this optical detector is amplified, digitized and processed to determine the magnitudes of frequency components f, f′, f1 and f2, where frequencies f1 and f2 are similar order harmonics of the non-harmonically related electrical frequencies f and f′, normalization of the magnitudes of f1 and f2 with respect to their fundamentals. 38. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein said apparatus includes means operative: to calculate quantities Q1 and Q2, separate estimates of the amount of target gas in the monitored space based upon the normalized magnitude of frequency components f1 and f2; to compare quantities Q1 and Q2, to determine the quality of their agreement with each other and previous results for measurements made through the monitored space; and to apply rules dependent upon this quality, and use Q1 and Q2 in combination with previous results to calculate the quantity of target gas present in the monitored space. 39. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein the wavelength scanning ranges for each laser diode are non-harmonically related and have significantly different characteristic distances with respect to the formation of coherent interference fringes. 40. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein: each target gas absorption line is scanned at two, non-harmonically related electrical frequencies and measurements of any absorption by such lines are made by determining the magnitude of the two, similar order harmonics of the non-harmonically related scanning frequencies; and wherein this process is carried out for each absorption line being scanned; and wherein this process is performed simultaneously, all electrical scanning frequencies being chosen to be non-harmonically related. 41. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein that the two mean wavelengths close to two separate optical absorption lines of the same target gas are chosen such that: (a) both are in regions of low absorption by atmospheric gases; or (b) one is in a region of low absorption by atmospheric gases while the other is in a region of higher absorption by atmospheric gases; or (c) one is in a region affected by absorption by one particular atmospheric gas species while the other is in a region affected by absorption by another atmospheric gas species. 42. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein the rules governing the use of two estimated gas quantities Q1 and Q2 in combination with results for previous measurements made through the monitored space to calculate the quantity of gas present in the monitored space are such that: (a) if Q1 and Q2 are in close agreement, a large fraction of the average of Q1 and Q2 is added to a balancing fraction of the running average of previous results; while (b) if Q1 and Q2 are in reasonable but not close agreement, a lesser fraction of the average of Q1 and Q2 is added to a larger balancing fraction of the running average of previous results; while (c) if only Q1 or only Q2 is in close or reasonable agreement with the running average of previous results, the quantity which is not in close or reasonable agreement is rejected while a lesser fraction of the close or reasonably agreeing quantity is added to a larger balancing fraction of the running average of previous results; while (d) if Q1 and Q2 are in poor agreement with each other and the running average of previous results, both Q1 and Q2 are rejected and only the running average of previous results is used. 43. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein the results of measurements performed upon target gas lines in regions of known low absorption by atmospheric gases are used to discriminate the effects of absorption by atmospheric gases in regions of more significant absorption by atmospheric gases from genuine changes in target gas concentration, thereby enabling any offsets arising from such absorption to be compensated for. 44. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein the diode lasers are VCSELs. 45. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein the means of collecting the laser radiation and transmitting it through the monitored space includes combinations of free-space optical elements and fiber-optics. 46. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein instead of amplifying, digitizing and digitally processing the detector signal(s) to determine the magnitudes of the various frequency components, the frequency component magnitudes are determined by amplifying the detector signal(s) and synchronously detecting the various frequency components using multiple synchronous detectors operating in parallel upon the signal(s). 47. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein said gas is drawn into a sample measurement chamber in which it is illuminated by the laser diode radiation. 48. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein said apparatus is arranged to detect hydrogen sulphide by measurement of any combination of two or more of the hydrogen sulphide absorption lines at 1582.13 nm, 1589.24 nm, 1589.42 nm, 1589.54 nm, 1589.97 nm and 1593.05 nm. 49. Apparatus for detecting a target gas in a monitored space as claimed in claim 37 wherein the means of output for the concentration or quantity of gas calculated present in the monitored path or sample measurement chamber includes: an analogue electrical signal proportional to the concentration or quantity of gas; a digital electronic signal conforming to a defined protocol and containing numerical information conveying the concentration or quantity of gas; and a numerical representation of the concentration or quantity of gas upon a display which is associated with or forms part of the apparatus, or the opening or closing of relays at prescribed concentrations or quantities of gas, such relays and the necessary control circuitry either being associated with or forming part of the apparatus. 50. Apparatus for detecting a target gas in a monitored space as claimed in claim 6 wherein: said apparatus includes an optical splitter operative to split the radiation into two fractions of which one fraction is transmitted across the monitored space to said first optical receiver and the other is passed through a retained sample of the target gas to a second optical receiver; the control current applied to the laser diode is controlled by a feedback signal from said second optical receiver so that absorption of the radiation has a distortion pattern specific to the target gas, characterized in that said distortion pattern includes two harmonics of the modulation frequency, each of substantial magnitude. 51. Apparatus for detecting a target gas in a monitored space as claimed in claim 50 wherein the distortion pattern includes an even harmonic and an odd harmonic. 52. Apparatus for detecting a target gas in a monitored space as claimed in claim 50 wherein the distortion pattern includes three harmonics of substantial magnitude. 53. Apparatus for detecting a target gas in a monitored space as claimed in claim 50 wherein said harmonics have a predetermined relationship in both magnitude and phase angle. 54. Apparatus for detecting a target gas in a monitored space as claimed in claim 53 wherein the quantity of target gas present in the monitored space is calculated from the signal from the first optical receiver and the specific distortion pattern. 55. Apparatus for detecting a target gas in a monitored space as claimed in claim 50 wherein the transmitter includes an optical detector to which a further fraction of the optical radiation is directed by the optical splitter, and said detector produces a signal representing the magnitude and phase of any component of said distortion pattern present in the radiation from the laser diode in the absence of absorption by target gas and said signal is subtracted from the output of the first optical receiver. 56. Apparatus for detecting a target gas in a monitored space as claimed in claim 50 wherein the control current is produced by a digital synthesizer including a digital-to-analogue converter (DAC) to output a sequence of voltages and a voltage-to-current (V-I) converter to convert said voltages into a current. 57. Apparatus for detecting a target gas in a monitored space as claimed in claim 50 wherein the laser diode sequentially scans a plurality of selected absorption lines of a target gas. 58. Apparatus for detecting a target gas in a monitored space as claimed in claim 57, which apparatus is operable to scan selected absorption lines of a plurality of target gases, wherein said retained sample includes a quantity of each said gas. 59. Apparatus for detecting a target gas in a monitored space as claimed in claim 57 wherein said apparatus comprises a plurality of said optical transmitters and one said optical receiver operative to receive transmitted radiation from each of said plurality. 60. Apparatus for detecting a target gas in a monitored space as claimed in claim 50 wherein the apparatus includes measurement means operative to calculate the quantity of target gas in the monitored space. 61. Apparatus for detecting a target gas in a monitored space as claimed in claim 60 wherein the monitored space is defined by a measurement chamber into which in use the target gas is admitted and illuminated by said radiation. 62. Apparatus for detecting a target gas in a monitored space as claimed in claim 60 wherein the measurement means provides an output indicating the calculated quantity of target gas, and said output comprises an analogue representation of said quantity, a digital representation of said quantity and a numerical display of said quantity signal. 63. Apparatus for detecting a target gas in a monitored space as claimed in claim 62 wherein the apparatus includes an alarm operative automatically to signal measurement of a quantity of gas above a predetermined threshold. 64. Apparatus for detecting a target gas in a monitored space as claimed in claim 50 including means to collect the optical radiation and transmit it across the monitored space, wherein said means includes combinations of free-space optical elements and/or fiber-optics. 65. Apparatus for detecting a target gas in a monitored space as claimed in claim 6 wherein: the laser diode control current has bias and wavelength scanning components so arranged that absorption of optical radiation from the laser diode by target gas produces a specific distortion ‘fingerprint’ including at least two harmonics of the wavelength scanning component frequency each of substantial magnitude and known, fixed magnitude ratio(s) and phase angles; said optical radiation is split into two fractions; one said fraction is passed through a retained sample of the target gas and illuminates a first optical detector; the second said fraction is transmitted through said monitored space to illuminate a second optical detector in a receiver; a signal from the first optical detector is sent to said receiver as representative of the target gas. 66. Apparatus for detecting a target gas in a monitored space as claimed in claim 65 wherein: the signal from the first optical detector is used by the transmitter to maintain the conditions necessary for generation of the specific distortion ‘fingerprint’; and the signal from said second optical detector is processed in relation to said specific distortion ‘fingerprint’ to calculate the quantity of target gas present in the monitored space and the receiver providing an output signal representative of the calculated quantity. 67. Apparatus for detecting a target gas in a monitored space as claimed in claim 66 wherein: an additional fraction of the optical radiation from the laser diode is sampled, this fraction directly illuminating an optical detector inside the transmitter; the signal from this detector is used to measure the magnitudes and phases of any ‘fingerprint’ components present in the waveform output by the laser diode in the absence of absorption by target gas; and this information is continuously communicated to the receiver to be subtracted from its measurements of the waveform of optical radiation that has been transmitted through the monitored space. 68. Apparatus for detecting a target gas in a monitored space as claimed in claim 66 wherein: the laser diode in the transmitter sequentially scans a total of two or more chosen absorption lines of one or more target gases and the retained gas sample includes a quantity of each of the one or more target gases and is used to maintain the conditions necessary for generation of specific distortion ‘fingerprints’ for each chosen absorption line of the one or more target gases. 69. Apparatus for detecting a target gas in a monitored space as claimed in claim 66 wherein: two or more laser diodes are used to detect or measure one or more target gases; the output from each laser diode is split into two fractions, one fraction used to illuminate a retained sample of the one or more target gases to maintain the conditions necessary for generation of specific distortion ‘fingerprints’ for each chosen absorption line of the one or more target gases, and the second fraction being transmitted through a monitored space to a receiver; the receiver is capable of detecting and processing the optical signals from the two or more laser diodes to calculate the quantities of the one or more target gases in the monitored space, this processing making use of the known, specific distortion ‘fingerprints’ which are being actively maintained by the transmitter. 70. Apparatus for detecting a target gas in a monitored space as claimed in claim 69 wherein: additional fractions of the optical radiation outputs from the laser diodes are sampled, which fractions directly illuminate an optical detector inside the transmitter; a signal from this detector is used to measure the magnitudes and phases of any ‘fingerprint’ components present in the waveforms output by the laser diodes in the absence of absorption by target gas; and this information is continuously communicated to the receiver to be subtracted from its measurements of the waveforms of optical radiation that has been transmitted through the monitored space. 71. Apparatus for detecting a target gas in a monitored space as claimed in claim 66 wherein the transmitter and the receiver are physically part of a single gas detection or measurement apparatus in which gas is drawn or diffuses into a sample measurement chamber in order to be illuminated by laser diode radiation and measured, the calculated gas quantity being output by the apparatus. 72. Apparatus for detecting a target gas in a monitored space as claimed in claim 66 wherein the means of output for the quantity of gas calculated present in the monitored space or sample measurement chamber includes: an analogue electrical signal proportional to the quantity of gas; a digital electronic signal conforming to a defined protocol and containing numerical information conveying the quantity of gas; a numerical representation of the quantity of gas upon a display which is associated with or forms part of the apparatus, or the opening or closing of relays at prescribed concentrations or quantities of gas, such relays and the necessary control circuitry either being associated with or forming part of the apparatus. 73. Apparatus for detecting a target gas in a monitored space, which apparatus comprises two or more laser diodes, wherein: each laser diode is being driven by a bias current which causes it to operate at a mean wavelength close to a different optical absorption line of the same target gas and is scanned across this line by a sinusoidal current component at a frequency which is non-harmonically related to any other scanning frequency used; the optical radiation from all said laser diodes being collected and transmitted through the monitored space and subsequently illuminating one or more optical detectors; an electrical signal from the detector or detectors is amplified, digitized and processed to determine the magnitude of components at the fundamental scanning frequencies and similar order harmonics of these fundamental frequencies; each harmonic is normalized with respect to the magnitude of its fundamental; separate estimates of the quantity of target gas present in the monitored space are calculated based upon each normalized harmonic; these quantity estimates are compared with each other and previous results for measurements made through the monitored space; and rules are applied dependent upon this quality, use of these quantities in combination with previous results to calculate the quantity of target gas present in the monitored space, the calculated quantity of gas being output by the apparatus. 74. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the laser diodes are located in positions calculated to minimize formation of coherent interference fringes with common phase, amplitude or frequency. 75. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the radiation from each laser diode is collected and collimated by separate optical elements with different, non-harmonically related effective focal lengths and thicknesses. 76. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein three mean wavelengths close to three distinct optical absorption lines of the same target gas are chosen such that: (a) all are in regions of low absorption by atmospheric gases; or (b) two are in regions of low absorption by atmospheric gases while the other is in a region of higher absorption by atmospheric gases; or (c) one is in a region of low absorption by atmospheric gases while the others are in regions of higher absorption by atmospheric gases; or (d) all are in regions affected by absorption by different atmospheric gas species or combinations thereof. 77. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the rules governing the use of three estimated gas quantities Q1, Q2 and Q3 in combination with results for previous measurements made through the monitored space to calculate the quantity of gas present in the monitored space are such that: (a) if Q1, Q2 and Q3 are in close agreement, a large fraction of the average of Q1, Q2 and Q3 is added to a balancing fraction of the running average of previous results; while (b) if either Q1 and Q2, or Q2 and Q3, or Q1 and Q3 are in close agreement with each other and the running average of previous results, the quantity which is not in close agreement is rejected while a large fraction of the average of the remaining quantities is added to a balancing fraction of the running average of previous results; while (c) if Q1, Q2 and Q3 are in reasonable but not close agreement with each other and the running average of previous results, a lesser fraction of the average of Q1, Q2 and Q3 is added to a larger balancing fraction of the running average of previous results; while (d) if Q1, Q2 and Q3 are in reasonable but not close agreement with each other but not in close or reasonable agreement with the running average of previous results, a still lesser fraction of the average of Q1, Q2 and Q3 is added to a still larger balancing fraction of the running average of previous results; while (e) if only one of the quantities Q1, Q2 or Q3 is in close agreement with the running average of previous results, the other quantities are rejected and a fraction of the remaining quantity is added to a larger balancing fraction of the running average of previous results; while (f) if Q1, Q2 and Q3 are in poor agreement with each other and the running average of previous results, Q1, Q2 and Q3 are rejected and only the running average of previous results is used. 78. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein all the laser diodes are located closely together on a common temperature stabilized mount and have their outputs collimated by a common optical element. 79. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the laser diodes and the laser diode bias currents are such that all the laser diodes are simultaneously operating at their correct mean wavelengths with near optimal output power while at a common temperature. 80. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the wavelength scanning ranges for each laser diode are non-harmonically related and have significantly different characteristic distances with respect to the formation of coherent interference fringes. 81. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein: each target gas absorption line is scanned at two, non-harmonically related electrical frequencies and measurements of any absorption by such lines are made by determining the magnitude of the two, similar order harmonics of the non-harmonically related scanning frequencies; and wherein this process is carried out for each absorption line being scanned; and wherein this process is performed simultaneously, all electrical scanning frequencies being chosen to be non-harmonically related. 82. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein that the two mean wavelengths close to two separate optical absorption lines of the same target gas are chosen such that: (a) both are in regions of low absorption by atmospheric gases; or (b) one is in a region of low absorption by atmospheric gases while the other is in a region of higher absorption by atmospheric gases; or (c) one is in a region affected by absorption by one particular atmospheric gas species while the other is in a region affected by absorption by another atmospheric gas species. 83. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the rules governing the use of two estimated gas quantities Q1 and Q2 in combination with results for previous measurements made through the monitored space to calculate the quantity of gas present in the monitored space are such that: (a) if Q1 and Q2 are in close agreement, a large fraction of the average of Q1 and Q2 is added to a balancing fraction of the running average of previous results; while (b) if Q1 and Q2 are in reasonable but not close agreement, a lesser fraction of the average of Q1 and Q2 is added to a larger balancing fraction of the running average of previous results; while (c) if only Q1 or only Q2 is in close or reasonable agreement with the running average of previous results, the quantity which is not in close or reasonable agreement is rejected while a lesser fraction of the close or reasonably agreeing quantity is added to a larger balancing fraction of the running average of previous results; while (d) if Q1 and Q2 are in poor agreement with each other and the running average of previous results, both Q1 and Q2 are rejected and only the running average of previous results is used. 84. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the results of measurements performed upon target gas lines in regions of known low absorption by atmospheric gases are used to discriminate the effects of absorption by atmospheric gases in regions of more significant absorption by atmospheric gases from genuine changes in target gas concentration, thereby enabling any offsets arising from such absorption to be compensated for. 85. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the diode lasers are VCSELs. 86. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the means of collecting the laser radiation and transmitting it through the monitored space includes combinations of free-space optical elements and fiber-optics. 87. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein instead of amplifying, digitizing and digitally processing the detector signal(s) to determine the magnitudes of the various frequency components, the frequency component magnitudes are determined by amplifying the detector signal(s) and synchronously detecting the various frequency components using multiple synchronous detectors operating in parallel upon the signal(s). 88. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein said gas is drawn into a sample measurement chamber in which it is illuminated by the laser diode radiation. 89. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein said apparatus is arranged to detect hydrogen sulphide by measurement of any combination of two or more of the hydrogen sulphide absorption lines at 1582.13 nm, 1589.24 nm, 1589.42 nm, 1589.54 nm, 1589.97 nm and 1593.05 nm. 90. Apparatus for detecting a target gas in a monitored space as claimed in claim 73 wherein the means of output for the concentration or quantity of gas calculated present in the monitored path or sample measurement chamber includes: an analogue electrical signal proportional to the concentration or quantity of gas; a digital electronic signal conforming to a defined protocol and containing numerical information conveying the concentration or quantity of gas; and a numerical representation of the concentration or quantity of gas upon a display which is associated with or forms part of the apparatus, or the opening or closing of relays at prescribed concentrations or quantities of gas, such relays and the necessary control circuitry either being associated with or forming part of the apparatus. 91. A method of detecting a target gas in a monitored space comprising providing a tunable source of optical radiation, tuning said source so as to generate optical radiation of defined wavelength, transmitting the optical radiation of defined wavelength across the monitored space and determining the optical absorption thereof, wherein: said source is tuned to define two mean wavelengths Λ1 and Λ2 for the optical radiation with modulation at two frequencies f and f′ respectively, where Λ1 and Λ2 are respectively close to two separate optical absorption lines of the target gas and f and f′ are not harmonically related. 92. Apparatus for detecting a target gas in a monitored space, which apparatus comprises: a source of optical radiation tunable to generate optical radiation of defined wavelength and configured and arranged to transmit the optical radiation of defined wavelength radiation across the monitored space; and an optical receiver operable to receive the transmitted radiation and determine optical absorption thereof; wherein: said optical source is tuned to define two mean wavelengths Λ1 and Λ2 for the optical radiation with modulation at two frequencies f and f′ respectively, where Λ1 and Λ2 are respectively close to two separate optical absorption lines of the target gas and f and f′ are not harmonically related.
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