WO2003087786A1

WO2003087786A1 - Method and apparatus for gas detection

Info

Publication number: WO2003087786A1
Application number: PCT/GB2003/001497
Authority: WO
Inventors: Mark Clifford Wilson; Andrew Patrick Ashcroft
Original assignee: Bae Systems Plc
Priority date: 2002-04-08
Filing date: 2003-04-07
Publication date: 2003-10-23
Also published as: AU2003229892A1; GB0208037D0

Abstract

Apparatus and method for identifying a the presence of a gas (7) is described. The gas detector includes an infra red source (2) emitting radiation into a chamber (5) containing the gas to be analysed. The radiation (8) is absorbed by the gas and the resulting radiation (8') is detected by a suitable infra red detector (4). The spectrum produced by the detector (4) will be characteristic of the gas (7) in the chamber (5), hence allowing the detection of a given gas from within a mixture of gases (7).

Description

METHOD AND APPARATUS FOR GAS DETECTION

This invention relates to methods and apparatus for detecting and quantifying gases. More particularly, but not exclusively, the invention relates to methods and apparatus for detecting and quantifying gases using gas spectrometry and having radiation sources following non-linear paths within a gas cell.

Gases absorb radiation passing through them, and the absorption spectra produced can be monitored and recorded, enabling identification and detection of specific or target gases from within a mixture of gases. These absorption features can be interpreted and allow the gas concerned to be identified and/or detected from within a mixture of gases. For example, carbon monoxide may be generated by central heating boilers not operating correctly. This carbon monoxide needs to be detected in domestic situations to prevent illness and the possibility of death.

This method of gas analysis is traditionally performed using apparatus having a long linear optical path within a ceil containing the gas under test. The radiation is passed into a cell, normally having the gas flowing continuously therethrough, and then focussed onto an IR detector at a position remote from the radiation source but nevertheless in the radiation path. In this way, the radiation detected by the detector has been absorbed by the gas in the cell. Therefore, the spectrum of the radiation detected will be characteristic of the gas in the cell. At some stage, the radiation may be optically filtered to restrict the spectral content to the wavelengths where characteristic absorption occurs.

It is a disadvantage of this technique that the optical path needs to be long enough for a characteristic spectrum of the gas to be produced, which can lead to large and clumsy equipment. Furthermore, the length and the complexity of the optics requires that the apparatus used in a benign and static environment, therefore making it unsuitable for domestic or industrial locations. ln accordance with the invention there is provided a gas detector comprising a radiation source, a cavity and a detector, the cavity containing a gas including a gas to be detected, in which the radiation is reflected by the surfaces of the cavity a plurality of times such that path length of the radiation from the source to the detector is greater than the linear distance from the source to the detector.

Furthermore, there also is provided a method of detecting the presence of a gas comprising the steps of passing the gas through a cavity, transmitting radiation into said cavity and detecting the radiation absorbed by the gas in the cavity, said radiation being reflected a plurality of times by the surfaces of the cavity.

In addition, there is further provided a tuneable filter for gas detector apparatus, comprising a radiation source and a cavity interposed between the source and the gas to be detected, said cavity defined by two plates and further being variable in size by respective movement of the plates, in which movement of the plates causes the wavelength of the radiation transmitted toward the gas to be varied according to the size of the gap defined by the plates, thereby causing the gas to interact with multiple wavelengths and hence produce signals characteristic of absorption spectra for multiple gases if present.

Accordingly, the problems described above are overcome as the apparatus can be greatly reduced in size without compromising the detection capability. Furthermore, the manufacture of a small rugged and potentially cost effective gas spectrometer is envisaged.

The invention will now be described with reference to the following drawings in which;

Figure 1 shows a schematic representation of a gas spectrometer in accordance with the invention, in which the spectrometer includes an integrating sphere, Figure 2 shows a schematic representation of a gas spectrometer in accordance with a second embodiment of the invention, in which the spectrometer includes an etalon, namely an air film enclosed between two moveable, half-silvered, plane parallel plates,

Figure 1 shows a gas spectrometer 1 comprising an IR source 2, an integrating sphere 3 and a detector 4. The integrating sphere 3 comprises a cavity 5 having reflective inner surfaces 6. A gas 7 which may, or may not, contain the gas to be detected and quantified (the 'target gas') is passed through the cavity 5. The IR radiation 8 generated by the IR source 2 passes into the cavity 5 of the integrating sphere 3, interacts with the gas 7 and the resulting radiation 8' is detected by the detector 4. The signal generated by the detector 4 is processed in a standard manner to produce a spectrum characteristic of the gas in the cavity 5. The spectrum can then be analysed to establish the presence and quantity of target gas in the gas 7.

The integrating sphere 3 replaces the usual linear path length for known gas spectrometers, whilst increasing the path length travelled by the radiation 8. As the radiation 8 is passed into the cavity 5 of the integrating sphere 3, it is internally reflected by the inner surface of the integrating sphere 3. In this way, the IR radiation 8 has a much longer path length from the IR source 2 to the detector 4. Furthermore, it is likely to interact with the gas 7, and hence the target gas if present, to a greater extent than a gas spectrometer having a linear path from source to detector.

The wavelength of the radiation 8 incident on the detector 4 is spectrally restricted by narrowband optical filters in a window placed between the detector 4 and the radiation 8'. In this way, the detector 4 can be restricted to seeing only radiation of wavelengths that are heavily absorbed by the target gas. When the target gas is present in the sphere, some of the radiation 8 at this wavelength is absorbed and the signal generated by the detector 4 drops. ln a single-channel configuration (where only one gas is to be detected, and only one characteristic spectrum generated by the detector is being output) it would be possible for sudden spurious drop in the IR source 2 output to mimic the presence of the target gas. The solution to this is to add a second sensitive detector element 4' to use as a reference channel optically isolated from the first detector 4. This channel is optically tuned to a different region of the spectrum where the target gas (or any other gas that is likely to be present) has no absorption features. By referencing the output channels of. the detector 4 against this channel 4', confidence in the output signal can be assured. It is not necessary to have the sensitive elements of the detector channel discrete and separate from the reference. There is significant potential benefit from having the output channel and reference channel, in series and indeed on the same monolithic slab of sensitive material. If this arrangement is designed such that the signal polarity of the reference is opposite to that of the active channel, then the net will be close to zero (assuming that the radiation intensity is equal at both wavelengths). Only when the active signal changes will a significant net signal be seen. Additional benefits of this arrangement are that the active and reference elements are well matched (because they come from the same material and processing) and that thermal drift and microphonic interference are self-compensated.

It is also possible to detect multiple gases. This can be done by adding further detectors 4" as output channels to be referenced against the first.

In the apparatus described above, the source 2 is a broadband IR source. In the simplest case this could be a small low-power incandescent tungsten- filament bulb. However, such a source is optimised for operation with a filament temperature of approximately 3000K, which places the peak emission at near 1 μm wavelength. An ideal source would have peak output at the target wavelengths. It is estimated that by reducing the optimum filament temperature to 1000K, an increase in signal by a factor of three would be achieved for a 'typical' target absorption feature.

The proposed detector 4 is a LiTaO₃ pyroelectric device. The response of such a device is proportional to the rate of change of element temperature rather than to the absolute level of photon flux. For this reason, the radiation must be modulated to produce a signal.

It may be desirable for the apparatus to be free from moving parts, in order to keep the unit simple and compact. Therefore, the source can be electrically modulated as the only alternative to mechanical chopping of the beam. To achieve a good modulation in output intensity of the source, it is preferable that the temperature modulation depth be as large as possible. This imposes a requirement on the source filament that it be extremely low thermal mass, high area and high emissivity.

Preferably, the encapsulation for the filament should be chosen to transmit appropriately to the target wavelength. A quartz bulb is sufficient to 4.5μm wavelengths, a can with a sapphire window would be required for wavelengths to 6μm and Germanium window for longer wavelengths.

Preferably, there should be no direct radiation path from the source 2 to the detector 4.

As described above, the purpose of the integrating sphere 3 is twofold. Firstly, it acts as a low loss radiation collector that maximises the amount of radiation that falls upon the sensitive elements of the detector 4. The detector 4 is seeing the IR source 2 through a very wide field of view, limited only by the design of the detector 4. Using an integrating sphere, this is achieved without the use of bulky, delicate and expensive optics., thereby reducing the size and complexity of the apparatus. Secondly, the integrating sphere 3 is a diffuse reflector that provides an extremely long path length for radiation passing between the source and detector. This path length is entirely within a cavity that contains the gas under test. The longer the path length, the greater the absorption by a target gas and the better the signal produced at the detector 4. Radiation entering the integrating sphere 3 will undergo reflection until it is either absorbed at the sphere surfaces, or at the detector 4. By maximising the reflectivity of the sphere wall and reducing the radiation sink area of the detector 4, the mean path length for radiation passing through a 25mm sphere could realistically be raised to several metres.

It will be appreciated that in order to detect multiple gases, multiple channels or multiple detectors 4, 4', 4" must be used. This concept is limited as eventually, all the output channels will overlap optically. Although this may result in total spectral coverage over a given waveband, it may increase the complexity and size of the apparatus. This is essentially producing a simple gas spectrometer.

The advantages of such a gas spectrometer apparatus are clear, any gas with a sufficiently strong absorption feature within the waveband could be detected without the need to alter the detector 4. The apparatus could also be much less susceptible to false readings generated by gases with similar spectral features because these gases would have to have absorption features resembling those of the target over a much larger spectral band.

There are, however, considerations which limit the number of channels. As more and more channels are added the sink area of the integrating sphere 3 increases. This has the effect of decreasing the path length. Furthermore, the cost of the detector 4 will increase with the number of channels.

In a second embodiment of the invention, a functional gas spectrometer of reasonable resolution is proposed where the source radiation is filtered by a tuneable filter and furthermore, the tuned wavelength is scanned across the waveband of interest. In this way, the radiation source no longer has to be modulated as the radiation intensity will be time-varying as the filter scans across a spectral feature, the resolution is only limited by the resolution of the tuneable filter (which in some cases can be very high), the reference channel can be removed, reducing cost and detector area and the frequency of operation is no longer limited by the source.

Preferably, the tuneable filter is placed over the source rather than the detector 4 because the ray-paths are much more predictable here than at the detector 4 (radiation will radiate out from the filament). This makes collimation of the beam easier (preferable for many tuneable filter types). However, it will be appreciated that the tuneable filter may be placed between the cavity and the detector (i.e. over the detector) and the same result will be achieved.

The second embodiment of the invention is shown in Figure 2, where the tuneable filter used is a Fabry-Perot etalon 10. A Fabry-Perot etalon is essentially an air gap bounded by two half silvered plane parallel plates. In

Figure 2, the plates 11 12 are shown enlarged and can be moved by a piezo gap driver 13. A broadband IR source 14 is used and radiation generated by the source 14 is emitted, reflected by a parabolic retroreflector 16 and incident on the plates 11 ,12. A ray of light from the source 14 enters the gap 15 through the first plate 11 and is multiply reflected within the gap 15. The radiation is then transmitted through the second plate 12 and into the cavity of the integrating sphere 3. The size of the gap 15 between the plates 11 ,12 determines the wavelength of the radiation transmitted through the second plate 12 and into the cavity of the integrating sphere 3.

If the plates 11 , 12 are moveable with reference to one another, then radiation of varying wavelengths can be generated and transmitted into the integrating sphere 3. In this way, the characteristic absorption spectra of multiple target gases can be detected by the detector 4, as different wavelengths of radiation will interact differently with different gases to produce the relevant characteristic spectra. Although a Fabry-Perot etalon has been described above, any suitable tuneable filter may be used. Furthermore, the filter may be placed between the source 2, 14 and the cavity or the detector 4 and the cavity.

The apparatus described above uses an IR source. However, any suitable source may be used capable of interacting with the target gas 7 and producing a spectrum having the characteristic absorption features of the gas to be detected and quantified. The detector 4 is a pyroelectric detector, such as a lithium tantalate detector. However, any suitable detector capable of detecting the radiation emitted may be used.

It will be appreciated that the integrating sphere need not be absolutely spherical. An ovoid cavity will act in a similar manner. Indeed, an ovoid cavity can be shown to provide increased path lengths for radiation entering the cavity compared to a spherical cavity.

Furthermore, although the surfaces of the cavity should be smooth, a similar effect can be achieved with roughened surfaces. Additionally, the structure of integrating sphere may be formed by the inner surfaces of a generally spherical body. However, the body need not be generally spherical, and any body having generally spherical or ovoid inner surfaces capable of reflecting radiation in the desired manner may be used.

Claims

1. A gas detector comprising a radiation source, a cavity and a radiation detector, the cavity containing a gas including a gas to be detected, in which the radiation is reflected by the surfaces of the cavity a plurality of times such that path length of the radiation from the source to the detector is greater than the linear distance from the source to the detector.

2. A gas detector according to claim 1 in which the cavity is generally spherical and is defined by the inner surfaces of a body.

3. A gas detector according to claim 1 or 2 further comprising a tuneable filter capable of tuning the radiation such that the wavelength of the radiation is varied across a waveband of interest.

4. A gas detector according to claim 3 in which the tuneable filter is a Fabry-Perot etalon.

5. A gas detector according to claim 3 or 4 in which the tuneable filter is placed between the source and the cavity or between the detector and the cavity.

6. A gas detector according to any preceding claim in which the source is a broadband infra red source.

7. A gas detector according to any preceding claim in which the detector is a pyroelectric detector capable of detecting radiation absorbed by the gas to be detected in the cavity.

8. A method of detecting the presence of a gas comprising the steps of passing the gas through a cavity, transmitting radiation into said cavity and detecting the radiation absorbed by the gas in the cavity, said radiation being reflected a plurality of times by the surfaces of the cavity.

9. A tuneable filter for gas detector apparatus, comprising a radiation source and a cavity interposed between the source and the gas to be detected, said cavity defined by two plates and further being variable in size by respective movement of the plates, in which movement of the plates causes the wavelength of the radiation transmitted toward the gas to be varied according to the size of the gap defined by the plates, thereby causing the gas to interact with multiple wavelengths and hence produce signals characteristic of absorption spectra for multiple gases if present.

10 A gas detector as hereinbefore described with reference to the accompanying diagrammatic drawings.

11. A method of detecting a target gas from a mixture of gases as hereinbefore described with reference to the accompanying diagrammatic drawings.

12. A tuneable filter for a gas detection system as hereinbefore described with reference to the accompanying diagrammatic drawings.