GB2127537A - Gas detection apparatus - Google Patents
Gas detection apparatus Download PDFInfo
- Publication number
- GB2127537A GB2127537A GB08225672A GB8225672A GB2127537A GB 2127537 A GB2127537 A GB 2127537A GB 08225672 A GB08225672 A GB 08225672A GB 8225672 A GB8225672 A GB 8225672A GB 2127537 A GB2127537 A GB 2127537A
- Authority
- GB
- United Kingdom
- Prior art keywords
- laser
- gas
- detector
- detected
- signal
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 238000001514 detection method Methods 0.000 title claims abstract description 17
- 238000010521 absorption reaction Methods 0.000 claims abstract description 14
- 239000007789 gas Substances 0.000 claims description 50
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical group O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 claims description 8
- 230000003287 optical effect Effects 0.000 claims description 7
- 230000004048 modification Effects 0.000 claims description 5
- 238000012986 modification Methods 0.000 claims description 5
- 229910002092 carbon dioxide Inorganic materials 0.000 claims description 4
- 239000001569 carbon dioxide Substances 0.000 claims description 4
- 230000002238 attenuated effect Effects 0.000 claims description 3
- 238000012360 testing method Methods 0.000 claims description 2
- 238000011156 evaluation Methods 0.000 claims 1
- 238000006243 chemical reaction Methods 0.000 abstract description 3
- 238000011896 sensitive detection Methods 0.000 abstract description 3
- 238000000034 method Methods 0.000 description 12
- 238000012544 monitoring process Methods 0.000 description 7
- 230000005855 radiation Effects 0.000 description 5
- 238000012545 processing Methods 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- VGGSQFUCUMXWEO-UHFFFAOYSA-N Ethene Chemical compound C=C VGGSQFUCUMXWEO-UHFFFAOYSA-N 0.000 description 2
- 239000005977 Ethylene Substances 0.000 description 2
- 238000000862 absorption spectrum Methods 0.000 description 2
- 239000000443 aerosol Substances 0.000 description 2
- 239000013626 chemical specie Substances 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 238000003909 pattern recognition Methods 0.000 description 2
- 230000007704 transition Effects 0.000 description 2
- 238000000576 coating method Methods 0.000 description 1
- 238000002485 combustion reaction Methods 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000005553 drilling Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000003344 environmental pollutant Substances 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 230000007257 malfunction Effects 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 230000001473 noxious effect Effects 0.000 description 1
- 231100000719 pollutant Toxicity 0.000 description 1
- 238000004321 preservation Methods 0.000 description 1
- 230000000644 propagated effect Effects 0.000 description 1
- 230000001902 propagating effect Effects 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 230000003595 spectral effect Effects 0.000 description 1
- 238000010183 spectrum analysis Methods 0.000 description 1
- 230000009466 transformation Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/04—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by beating two waves of a same source but of different frequency and measuring the phase shift of the lower frequency obtained
Landscapes
- Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Analytical Chemistry (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Chemical & Material Sciences (AREA)
- Optics & Photonics (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
Abstract
In contrast to known differential absorption gas detection systems which employ two lasers, a single laser 10 is sequentially tuned through a range of wavelengths, including one at which a trace gas to be detected absorbs and one at which it does not. Part of the laser output is passed through an area to be monitored to a detector 26, e.g. by backscatter, and part is passed through a reference cell 20 containing a sample of the gas to be detected to a detector 22. The presence or absence of the gas in said area is determined by electronic comparison of the detector signals by a microprocessor 36 after A/D conversion. A further part of the laser output may be deflected to a detector 18 to provide a reference signal. I.R. light going to the monitored area may be chopped, at 24, to enable a phase sensitive detection system 28 to be used. <IMAGE>
Description
SPECIFICATION
Gas detection apparatus
This invention relates to the detection of gases in the atmosphere by the use of differential absorption technique. In this technique, a gas may be detected at a position remote from the observer, by virtue of the fact that it is capable of absorbing light at some wavelengths but not at others. Thus, if laser radiation at two wavelengths (one near resonance with an absorption line of the gas and one off resonance with his absorption line) passes through the region of space containing the gas, there will be a differential absorption between the two radiation signals. This differential absorption may be observed by well known arrangements of optical detectors and phase sensitive detection electronic circuitry.The presence or absence of the gas in the path of the radiation signals may be inferred from the ratio of the optical signals at each of the two wavelengths.
Since the invention of the laser, there has been considerable interest in the possibility of applying its well known advantages to the problem of remote monitoring and detecting of gaseous chemical species in the atmosphere. These gaseous chemical species may take the form of trace concentrations of well known atmospheric pollutants such as SO2 and H2S whose presence and concentration is a matter of interest and concern for agencies charged with the resposibility for preservation of the quality of air, particularly in urban and industrial areas. A second class of conditions where remote monitoring is useful or even required by law relates to safety procedures on sites where dangerous, noxious chemicals are, or may be, produced and which are, or may be, vented to the atmosphere either as a matter of course or as the result of a malfunction in the production process.Thus, it is required to provide monitoring equipment at chemical plants producing dangerous chemicals, for example ethylene, which may be vented to the atmosphere as a result of an accidental leak either on the plant itself or aiong a pipeline. Such equipment is also required to detect combustion products in areas where fire would result in serious accident. In some activities e.g. drilling for oil or gas, dangerous gases such as H2S may sometimes be encountered which, if not detected, may result in serious consequences. One known technique for the detection of such trace gases uses the principle of differential absorption.To realise this technique in the prior art, it has been necessary to use two separate laser devices, one tuned to frequency near resonance with an absorption line of the gas it is required to monitor, and the second laser tuned to a frequency where the gas absorbs very weakly or not at all. These two laser signals suitably processed are propagated through the relevant region of space and detected remotely after a single transit, or else near the transmitting laser after reflection from a topographic target or from aerosols. The ratio of the detected signals at the two frequencies is monitored as a measure of the concentration of the trace gas in the path of the two colinear laser beams.
It is an object of the present invention to provide a system which has all the advantages of the differential absorption technique, while using only a single laser rather than the two lasers required previously.
In a differential absorption gas detection system according to the present invention, a single laser is sequentially tuned through a range of wavelengths, including one at which a trace gas to be detected absorbs and one at which it does not, part of the laser output being passed through an area to be monitored and part being passed through a reference cell containing a sample of the gas to be detected, the presence or absence of the gas in said area being determined by electronic comparison of the signals detected downstream of the area under test and of the reference cell.
Such a single laser system results in several advantages in terms of reduced cost and increased simplicity of the optical system design.
In one embodiment, the tuning of the laser can be achieved, for example, by means of a piezoelectric length transducer suitably designed into the laser structure to enable modulation of the distance between the mirrors forming the optical cavity of the laser. The amplitude of the signal from the sequentially and repetitively tuned laser may be monitored by splitting off a small fraction of the laser output and directing it into a suitable detector. This signal constitutes what is often described as the "signature" of the laser.
Another small fraction of the transmitted signal is also separated from the main beam and caused to propagate through a small "callibration" cell containing a suitable concentration, appropriately buffered, of the gas or gases it is designed to detect. The transmitted signal is detected by a suitable detector. Each of these two signals is suitably processed and electronically stored in a microcomputer. The main laser beam is caused to propagate across the intended monitoring area by whatever arrangement of optical and scanning system is necessary. The return signal is collected and focussed onto a sensitive detector. This signal now represents the laser signature uniformly attenuated due to its transit except in the event that the trace gas is present, when the presence of the latter will be shown by a modification of the signature.This modification will be spectrally identical to the modification produced in the calibration gas cell.
The signal propagating through the monitoring zone can require electronic processing by well known electronic techniques such as phase sensitive detection. The microcomputer may now be used to compare the signature of the return signal with that which transmits through the reference cell. It is assumed that suitable processing is carried out on these signals such as
A/D conversion, fast Fourier transform and other well known pattern recognition techniques to allow the trace gas-induced signature modiciation to be detected as an indication of the presence of the gas.
The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:
Fig. 1 is a schematic diagram showing an example of a laser "signature"; and
Fig. 2 is a block diagram illustrating one embodiment of a laser apparatus in accordance with the present invention.
In contrast to the known system which requires the use of two lasers, each of which must be designed to have a highly dispersive resonator (usually achieved by the use of a diffraction grating), the embodiment of Fig. 2 in accordance with the present invention needs only a single laser 10 which, in this particular example, is a carbon dioxide laser. Furthermore, the present system does not require the use of a diffraction grating as one of its resonator elements, but instead uses a pair of suitably selected laser mirrors (not shown).The laser 10 has a resonator (not shown) whose optical length may be modulated, for example by the application of a suitable voltage waveform to a piezoelectric transducer supporting one of the mirrors, As indicated somewhat schematically in Fig. 1 , the "signature" so produced consists of a plurality of discrete components, each of which represents radiation on a single laser transition within the band of lines defined by the laser mirror coatings.
As shown in Fig. 2, a small fraction of the output of the laser 10 is split off by a first beam splitting device 14 and is guided towards a second such beam splitting device 1 6 where a further fraction is reflected into a detector 1 8. The detector 18, which might for example be a pyroelectric detector, thus monitors the laser output "signature" produced by applying the appropriate voltage waveform to the laser cavity length transducer 12. The signal whic'h is transmitted by the beam splitting device 1 6 is caused to pass through a cell 20 which contains a suitable concentration of the gas it is desired to detect. It is assumed that the laser "signature" is so selected as to include a laser line which is significantly attenuated by the gas which is to be deflected.For example, the presence of ethylene causes significant absorption in the P(14) rotational-vibrations line of the 001-100 transition of carbon dioxide. The signal detected by a further detector 22 is a laser signature modified by the absorption spectrum of the trace gas in a very specific manner. Thus, the signal at the detector 22 (considered in relation to the signal at the detector 18) is a characteristic "template" of the trace gas.
The main fraction of the laser output which is transmitted by the beam splitting device 14 can be chopped at a frequency f by a chopper 24 and then, using appropriate optics, is transmitted across the zone where trace gas monitoring and detection is required. The signal returning either from a topographic target or scattered by aerosols is detected by a sensitive infrared detector 25, such as a cooled photodetector, and phase sensitive detector circuitry 28.
In the event that the trace gas is present in the path of the beam, the return signal at the output of the detector 26 will manifest the characteristic "spectral footprint" of the trace gas which has been electronically stored from detectors 1 8 and 22. Thus, a comparison of the signals at the detectors 18, 22 and 26 will enable the recognition of the trace gas whose presence it is required to detect. This can be accomplished following A/D conversion in respective analogue to digitial converters 30, 32, 34 and suitable digital processing techniques of pattern recognition (which are well known in themselves), such as fast
Fourier transformation and spectral analysis. This analysis can be carried out in a microprocessor 36 whose output can be either a digital readout of the concentration of the trace gas, and/or an alarm.
By suitably selecting the range of laser lines comprising the laser signature, the aforegoing technique will allow the simultaneous detection/monitoring of several trace gases which are present in the path of the beam. This can be accomplished by an appropriate combination of the use of the calibration gas cell 20 and data stored in the computer memory of the absorption spectrum characteristics of the gases it is desired to detect.
Other embodiments may use a laser other than a carbon dioxide laser, provided that the wavelength of the emitted beam can be changed by some suitable mechanism over a range of wavelengths such that the gas to be detected absorbs one possible wavelength and dces not absorb another possible wavelength within that range.
In still further embodiments, instead of using the technique of direct detection wherein the return signal is detected conventionally at the detector 26, heterodyne detection can be employed wherein the return signal is mixed with a local oscillator signal before being detected.
Heterodyne detection typically gives an improvement in sensitivity by several orders of magnitude over direct detection. The technique requires, however, a source of radiation close to the frequency of the incoming backscattered laser frequency. This may be derived either from the original laser, or from a separate laser with a closely controlled output frequency.
Claims (7)
1. A differential absorption gas detection system, wherein a single laser is sequentially tuned through a range of wavelengths, including one at which a trace gas to be detected absorbs and one at which it does not, part of the laser output being passed through an area to be monitored and part being passed through a reference cell containing a sample of the gas to be detected, the presence or absence of the gas in said area being determined by electronic comparison of the signals detected downstream of the area under test and of the reference cell.
2. A gas detection system as claimed in claim 1 wherein the tuning of the laser is achieved by means of a piezoelectric length transducer suitably designed into the laser structure to enable modulation of the distance between the mirrors forming the optical cavity of the laser.
3. A differential absorption gas detection system wherein the signal from a sequentially and repetitively tuned laser is monitored by splitting off a small fraction of the laser output and directing it into a first detector, this signal constituting the "signature" of the laser; another small fraction of the transmitted signal is also separated from the main beam and caused to propagate to a second detector through a calibration cell containing a suitable concentration of the gas or gases it is required to detect; each of the signals obtained from the first and second detectors is processed and electronically stored; the main laser beam is caused to propagate across an area to be monitored, the return signal being collected by a third detector, this latter signal representing the laser signature uniformly attenuated due to its transit, except in the event that the trace gas is present, when the presence of the latter will be shown by a modification of the signature which is spectrally identical to the modification produced in the calibration gas cell; and wherein the signature of the return signal is compared with that which transmits through the reference cell to enable the presence or absence of the trace gas to be evaluated.
4. A gas detector system as claimed in claim 3 wherein the third detector employs heterodyne detection, said return signal being mixed with a local oscillator signal before being detected.
5. A gas detector system as claimed in claim 3 or 4 wherein the storage and evaluation of the detected signals is performed by a microcomputer.
6. A gas detector system as claimed in claim 3 or 4 wherein the laser is a carbon dioxide laser.
7. A differential absorption gas detection system, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08225672A GB2127537B (en) | 1982-09-09 | 1982-09-09 | Gas detection apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08225672A GB2127537B (en) | 1982-09-09 | 1982-09-09 | Gas detection apparatus |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2127537A true GB2127537A (en) | 1984-04-11 |
| GB2127537B GB2127537B (en) | 1986-09-10 |
Family
ID=10532780
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08225672A Expired GB2127537B (en) | 1982-09-09 | 1982-09-09 | Gas detection apparatus |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2127537B (en) |
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0318752A2 (en) * | 1987-12-03 | 1989-06-07 | Techform Engineering Ag | Device for analysing traces of gas |
| US5028129A (en) * | 1990-01-05 | 1991-07-02 | Ball Corporation | Differential absorption ranging method and apparatus |
| WO1992016828A2 (en) * | 1991-03-19 | 1992-10-01 | Welsh Water Enterprises Ltd. | Organic pollutant monitor |
| EP0592015A2 (en) * | 1988-06-08 | 1994-04-13 | Karl Stefan Riener | Device for spectroscopically determining concentrations of gases in gaseous mixtures |
| US5544186A (en) * | 1994-01-25 | 1996-08-06 | Carl-Zeiss-Stiftung | Gas laser for emitting light modulated at two different wavelengths and an arrangement incorporating the gas laser to detect a gaseous substance |
| US10458904B2 (en) | 2015-09-28 | 2019-10-29 | Ball Aerospace & Technologies Corp. | Differential absorption lidar |
| US10921245B2 (en) | 2018-06-08 | 2021-02-16 | Ball Aerospace & Technologies Corp. | Method and systems for remote emission detection and rate determination |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1523605A (en) * | 1974-08-09 | 1978-09-06 | Licentia Gmbh | Method and arrangement for measuring small gas concentrations |
| EP0016303A1 (en) * | 1979-03-14 | 1980-10-01 | Allied Corporation | Photoacoustic rotational Raman spectroscopy |
-
1982
- 1982-09-09 GB GB08225672A patent/GB2127537B/en not_active Expired
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1523605A (en) * | 1974-08-09 | 1978-09-06 | Licentia Gmbh | Method and arrangement for measuring small gas concentrations |
| EP0016303A1 (en) * | 1979-03-14 | 1980-10-01 | Allied Corporation | Photoacoustic rotational Raman spectroscopy |
Cited By (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0318752A3 (en) * | 1987-12-03 | 1990-07-18 | Mutek-Laser Und Opto-Elektronische Gerate Gmbh | Method and device for analysing traces of gas |
| US5002391A (en) * | 1987-12-03 | 1991-03-26 | Mutek-Laser Und Optoelectronische Gerate Gmbh | Method and system for (trace) gas analysis |
| EP0318752A2 (en) * | 1987-12-03 | 1989-06-07 | Techform Engineering Ag | Device for analysing traces of gas |
| EP0592015A2 (en) * | 1988-06-08 | 1994-04-13 | Karl Stefan Riener | Device for spectroscopically determining concentrations of gases in gaseous mixtures |
| EP0592015A3 (en) * | 1988-06-08 | 1994-08-10 | Reiner Szepan | Device for spectroscopically determining concentrations of gases in gaseous mixtures |
| US5028129A (en) * | 1990-01-05 | 1991-07-02 | Ball Corporation | Differential absorption ranging method and apparatus |
| WO1992016828A3 (en) * | 1991-03-19 | 1993-02-04 | Welsh Water Enterprises Ltd | Organic pollutant monitor |
| WO1992016828A2 (en) * | 1991-03-19 | 1992-10-01 | Welsh Water Enterprises Ltd. | Organic pollutant monitor |
| US5420432A (en) * | 1991-03-19 | 1995-05-30 | Welsh Water Enterprises Limited | Organic pollutant monitor |
| AU660305B2 (en) * | 1991-03-19 | 1995-06-22 | Acer Consultants Limited | Organic pollutant monitor |
| US5544186A (en) * | 1994-01-25 | 1996-08-06 | Carl-Zeiss-Stiftung | Gas laser for emitting light modulated at two different wavelengths and an arrangement incorporating the gas laser to detect a gaseous substance |
| US10458904B2 (en) | 2015-09-28 | 2019-10-29 | Ball Aerospace & Technologies Corp. | Differential absorption lidar |
| US10921245B2 (en) | 2018-06-08 | 2021-02-16 | Ball Aerospace & Technologies Corp. | Method and systems for remote emission detection and rate determination |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2127537B (en) | 1986-09-10 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19950909 |