GAS DETECTION DEVICES

Abstract: A device (19), for measuring changes in the intensity of a beam (52) of ultra-violet light, includes a gas tight container (10) having a window (11) translucent to ultra-violet light and containing detector means (12a, 20) sensitive to changes in the amount of ultra-violet light entering the container. The detector means (12a) might be a gas, ionisable by ultra-violet light, which fills the container (10), or a plate (20) having photo-electric sensitivity to ultra-violet light. The device includes means (13, 14, 15, 16, 17; 25, 26, 27, 28) for measuring the ultra-violet light. The device can be used as a gas detector, measuring the loss in ultra-violet light in a beam (52) from a lamp (51) crossing a passage (50) through which flows an airflow containing a contaminant gas of the type which absorbs or is ionised by ultra-violet light.

Description: Gas Detection Devices. The present invention relates to gas detection devices, particularly those of the type which use the ionisation of gases by ultra- violet light. One use of such devices is in the detection and investigation of gas flows, when a tracer gas is introduced into a main gas flow. Another is in the detection of the presence of an extraneous gas in a flow of a particular gas. The main gas flow with which these devices is used is of air, and this specification will therefore, for convenience, refer to the main gas flow as airflow. Similarly, for convenience, any other gas in the airflow, whether a tracer gas or an extraneous gas, will be referred to as a test gas. Devices of this type operate by passing the airflow through a beam of ultra-violet light from a lamp which is preferably tuned for maximum ionisation effect on the test gas, and measuring the current flow between two electrodes caused by ionisation of the gas as the airflow is passed between the electrodes. Such devices are described in, for example. GB 1576^74 and PCT/GB92/01313- In the latter of these an ionising ultra-violet light is passed through a gas-carrying airstream which then passes between two co-axial potentially biassed annular electrodes, ionisation being substantially completed before the airflow enters the gap between the electrodes. The device is calibrated so that the current between the electrodes provides a measure of the concentration of the test gas in the airstream. It is pointed out that, comparing the ionisation potential of various compounds, {for example water (12.59eV), ethane (11.65eV), ammonia (10.15eV), nitrobenzene (9.82eV) and styrene (8.47eV)}. it is possible to monitor certain compounds against a background of others by selection of UV source wavelength. Such capability clearly has application in a variety of monitoring areas such as in environmental pollution monitoring for release of gases and vapours or monitoring of laboratory processes where gases and vapours are emitted. It has been discovered, however, that in devices of this type the response in terms of current flowing as the presence of the test gas is increased is linear up to a certain concentration, then rises less rapidly, and then actually declines. For example, in a particular instrument when propylene was used as a tracer gas in an airflow the response was linear up to a concentration of 100 parts per million (ppm) , then increased non-linearly up to a maximum concentration of 1000 ppm, after which it decreased. The concentrations for which these devices can be used is therefore limited. Also devices of this type are useful only in respect of gases which are ionised by ultra-violet light. In practice, many gases affect a beam of ultra-violet light passing therethrough by absorbing light rather than by being ionised. According to the present invention a device, for measuring changes in the intensity of a beam of ultra-violet light, includes a gas tight container having a window translucent to ultra-violet light and containing detector means sensitive to changes in the amount of ultra-violet light entering the container. In one embodiment of the present invention the detector means is a gas, ionisable by ultra-violet light, which fills the container. In this embodiment measurement means might conveniently include a pair of spaced electrodes with means for applying a potential across the space therebetween and means for measuring the current across the space. In another embodiment the container might contain a plate having photo-electric sensitivity to ultra-violet light. In this embodiment measurement means might include an anode shielded from direct radiation by the ultra-violet light and an electrical connection between the anode and the plate. With this embodiment the container might be evacuated, or might be filled with a selectively light-absorbing gas, so rendering the container opaque to particular energy levels of ultra-violet light and transparent to others. The device will in use be associated with a measurement circuit including driving means, such as a battery, and current indicator means, such as an electrometer. In use, the device might be positioned with the window forming part of the surface of one side of a passage at an opposite side of which is an ultra-violet lamp which shines a beam of ultra-violet light across the passage, through the window and into the container. When an airflow through the passage contains no test gas the beam of ultra-violet light will cross into the container somewhat diminished by absorption in the normal components of the airflow, giving a maximum reading on the current indicator. With a test gas of a type which causes a further diminution of the ultra-violet light, either by becoming ionised, by absorption, or both, the consequential reduction in light entering the container will be measured by the measurement circuit and can be related, by prior calibration, with the concentration of test gas in the airstream. In one use of the invention involving a passage and ultra-violet lamp the device might be used in conjunction with a gas detector of the type described in PCT/GB92/01313, with annular electrodes extending downstream from the passage: with this detector the ionisation of a test gas in an airstream passing through the detector is measured by the electrodes and calibration allows this measurement to be related to the concentration of test gas in the airstream. Thus the ratio of the total loss to the loss due to ionisation loss can be determined, so increasing the information that can be obtained about the test gas (for example, whether or not there is more than one test gas in the airstream) . According to another aspect of the present invention a method of detecting the concentration of a test gas in an airflow includes the step of passing at least part of the airflow through a passage between an ultra violet lamp and a container, the container having one surface translucent to ultra-violet light and containing detector means sensitive to ultra-violet light and measurement means whereby the amount of ultra-violet light entering the container can be quantified, and shining a beam from the ultra-violet lamp across the gap into the container. Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, of which: Figure 1 is an elevation, in section, of one embodiment of the present invention, Figure 2 is an elevation, in section, of another embodiment of the present invention, Figure 3 is an elevation, in section, of the embodiment of Figure 1 in use . Figure 4 is an elevation, in section, of the embodiment of Figure 1 in use in conjunction with a gas detector of the type described in PCT/GB92/01313, Figure 5 is a graph showing the relationship between the concentrations of an ionisable gas in an air flow and the current across two electrodes between which the gas flow passes, at three levels of ultra-violet flux impinging on the gas stream, Figure 6 is a graph showing the response of an instrument according to the invention when used with an airstream carrying krypton, and Figure 7 is a graph from which the total photoabsorption cross-section, of a gas in a gap between a device according to the invention and an ultra-violet lamp, can be calculated. A device 19 according to the invention (Figures 1 and 2) has a gas-tight sealed container 10 of material opaque to ultra-violet light but having a window 11 translucent to ultra-violet light. In a first embodiment (Figure 1) space 12 within the container is filled with a gas, indicated by 12a, which is ionisable by ultra-violet light. Two electrodes 13 , 14 are connected by a measuring circuit 17 including a potential applying means (such as a battery) 1 and a current indicator in the form of an electrometer 16 In use of this embodiment of the device ultra-violet light entering the container 10 through the window 11 ionises the gas within the space 12 resulting in flow of an electric current in the circuit 17 which is measured by the electrometer 16. The size of the current is dependent on the level of ionisation of the gas which is dependent on the amount of ultra-violet light entering the container. In a second embodiment (Figure 2) a plate 20 of a material photo-electrically sensitive to ultra-violet light is positioned in the space 12 such that ultra-violet light entering through the window 11 will impinge thereon. The plate 20 is electrically connected by circuitry 27 to an annular anode 28 which surrounds the window 11 but is optically shielded therefrom by a shield 29- The circuitry 27 contains a potential biasing means, such as a battery, 2 and an electrometer 39- When in use, this embodiment of the invention operates in a similar fashion to that of the first embodiment as described above with reference to Figure 1, with the photo-electricity caused on the plate 20 varying according to the intensity of the ultra-violet light falling thereon. The space 12 might be evacuated, thus reducing the danger of contamination to the material of the plate 5 20, or might be filled with a selectively light-absorbing gas. Prepared for use (Figure 3. where an instrument 49 is shown using an embodiment of the type shown in Figure 1) , the device 19 is installed at one side of a channel 0 with an ultra-violet lamp 51 so positioned at the other side of the channel as to direct a beam 52 of 10 ultra-violet light across the channel through the window 11 into the space 12 within the container 10. The channel 50 forms part of an air passage 55 defined by structure 56 in which guide members 57 lead into the channel. A fan 58 i positioned in the passage 55 to draw air through the channel 50. 15 The arrangement of Figure ~ is calibrated by applying power to the measuring circuit 17. operating the ultra-violet lamp 11 passing an airflow containing known concentrations of a test gas through the passage 50, and noting the measurements recorded by the electrometer 16. In the absence of test gas there will be little or no loss of 20 ultra-violet light as the beam crosses the gap 0 and the electrometer 16 will give a maximum reading. As the concentration of test gas increases there will be an increasing loss of ultra-violet light, as a result of ionisation of the test gas, of adsorption, or of a combination of these. Consequently less light will enter the space 2 12; this will result, in the embodiment of Figure 1, in a reduction in the ionisation of the ionisable gas and, in the embodiment of Figure 2, in a reduction in the photo-electric effect on the plate 20. There will therefore be a reduction in the reading of the electrometer l6. The resultant calibration can be stored in any of the 30 conventional ways - for example on calibration sheets - and will usually be stored in at least one form in a microcomputer 59 which forms part of the instrument 49. In use, the arrangement of Figure deployed in a position where it is desired to determine the concentration of a test gas, and 35 the measuring circuit 17 and ultra-violet lamp 51 switched on. Air suspected to contain the test gas is drawn through the channel 50 and beam 2 and the reading on the electrometer 16 compared with the calibration to give the concentration of test gas in the airstream. Figure 6 shows the results of measurements made on a prototype version of a device 19. having space 12 filled with gas ionisable by ultra-violet light, with krypton gas as the test gas in air at the concentrations (C ppm) indicated, in the range 20 to 2000 ppm. The response of the device 19 is in terms of the change in the standing current (£i nA) produced by the presence of the absorbing gas. Curve A is for an initial standing current of 100 nA and curve B for 10 nA. The initial standing current is obtained with zero concentration of krypton in the detector gap. Where it is suspected that an airstream might be contaminated with more than one trace gas, of which the concentration of only one is of interest, the use of the embodiment of Figure 2, with the space 12 filled with a selectively light-absorbing gas, may be used to render the device 19 translucent to energy levels of ultra-violet light associated only with the trace gas of interest and opaque to other energy levels. As an alternative way of fulfilling this objective a layer of selectively light-absorbing gas might be interposed between the ultra-violet lamp 5 and the channel 50, for example by using a gas filled hollow window (shown in dotted lines at 80 in Figure 3) . In another instrument 70 using the invention the device 19 is positioned opposite an ultra-violet lamp 51 in the air passage 50 of a gas detector of the type described in PCT/GB92/01313, in which concentric electrodes 71t 72 are positioned downstream of the passage. The gas detector as described in PCT/GB92/01313 operates by ionising gas passing through the ultra-violet light beam 52 in the passage 50. the degree of ionisation being measured by a circuit 73 containing a power source 74 and electrometer 75. and comparing the reading of the electrometer with a calibration. Calibration is as described above, with reference to the present invention. When an airstream carrying a tracer gas is passed through a gas detector such as that described in PCT/GB92/01313. the response is as illustrated in Graphs A, B, C in Figure 5. where the tracer gas was propylene, and A, B and C have progressively decreasing ultra-violet fluxes impinging on the gas stream. It will be seen that the current across the electrodes increases substantially linearly with increasing tracer concentration up to a level A', B', C where it becomes non¬ linear and eventually starts to actually decrease. It can be seen how the response can be maintained in the linear form up to higher levels of concentration by changing the ultra-violet flux from the lamp. The recovery of linearity and the shifting of the maximum signal to higher values at higher concentrations on altering the advection speed and the distance to the electrodes from the lamp is also shown on the graph. For example the lines labelled B L and A s show the recovery of linearity achieved as compared with the line A L by virtue of changing these two variables. Devices according to the present invention give a response which is substantially linear up to a higher concentration of trace gas than does the gas detector of PCT/GB92/01313 (see, for example. Figure 4). Devices 19 according to the present invention can therefore be used to extend the range of the gas detector. Alternatively devices 19 can be used in conjunction with the gas detector to give greater information about a trace gas or a mixture of trace gases, using the property that, whilst the electrometer 75 of the gas detector 70 gives a reading which is indicative of the ionisation of tracer gas, and therefore of a loss of ultra-violet light in the beam 52 attributable solely to ionisation of the test gas, the electrometer 16 of the device 19 gives a reading indicative of total ultra-violet light loss. By suitable calibration of the circuits 17, 73 the ratio between ultra-violet light loss due to absorption by a trace gas and loss due to ionisation of the trace gas can be determined. A number of devices 19 according to the present invention, for example one with an ionisable gas in the space 12, one with the space 12 evacuated, and some with various selectively light-absorbing gases in the space 12, might be associated with a gas detector according to PCT/GB92/01313. The gas detector can therefore be used by itself, or with one of the devices 19 installed, thus providing a combination capable of determining the concentrations of a large number of trace gases over a large range of concentrations. A number of hollow windows 80 filled with selectively light-absorbing gases might also be associated with the gas detector for installation between the ultra-violet lamp a d the channel 50. In a separate mode of operation (Figure 7) a device 19 can be used to measure the total photoabsorption cross section of a gas in the detector gap. Figure 7 shows the detector cell standing current (i nA) measured for different separations (x cm) between the UV source and the detector cell. Minus 1 times the slope of this line is equal to the product of the gap x, the absorbing gas concentration, and the total photoabsorption cross section of the gas under test. In this case the photoabsorbing species is the oxygen component of air, for which the measurement yields the value of 0.34 x 10 " 1 cm 2 . The device provides a convenient means of measuring this quantity without the need to employ the methods of vacuum ultraviolet spectrometry. In the second embodiment of the device 19. as described with reference to figure 2, suitable materials for the photo-electric responsive plate 20 will be apparent to those skilled in the art, and might be chosen for particular requirements. In one test instrument 3l6 Stainless Steel provided adequate results. It will, of course, be realised that many variations of the above described items and methods of using the items are possible within the scope of the invention. For example, rather than inducing flow with a fan 8 downstream of the passage 0 a driving fan may be used or even, in outdoor environments, fans dispensed with and natural airflow used as an impelling force. Each of the various instruments might also be used with a probe, to allow close access to, for example, possible sources of pollution.

Claims: CLAIMS What is claimed is: 1. A device (19). for measuring changes in the intensity of a beam (52) of ultra-violet light, including a gas tight container (10) characterised in having a window (11) translucent to ultra-violet light and containing detector means (12a, 20) sensitive to changes in the amount of ultra-violet light entering the container. 2. A device as claimed in Claim 1 characterised in that the detector means (12a) is a gas, ionisable by ultra-violet light, which fills the container (10) . 3- A device as claimed in Claim 2 characterised in that the container contains a pair of spaced electrodes (13. 14). 4. A device as claimed in Claim 3 characterised in including a measurement circuit (17) connected to the electrodes (13. 14). 5- A device as claimed in Claim 1 characterised in that the detection means is a plate (20) having photo-electric sensitivity to ultra-violet light. 6. A device as claimed in Claim 5 characterised in that it includes an anode (28) shielded from direct radiation by ultra-violet light entering the container (10) through the window (11). 7. A device as claimed in Claim 5 or in Claim 6 characterised in that the container (10) is evacuated. 8. A device as claimed in Claim 5 or in Claim 6 characterised in that the container (10) is filled with a selectively light-absorbing gas. 9. A device as claimed in any one of Claims 6 to 8 characterised in that it includes a measurement circuit (27) connected between the plate (20) and the anode (28). 10. A device as claimed in Claim 4 or in Claim 9 characterised in that the measurement circuit (17. 27) includes power means (15, 25) and current indicator means (16, 26). 11. An instrument (49) characterised in that a device (10) as claimed in any one of Claims 1 to 10 is positioned with the window (11) forming part of the surface of one side of a passage (50) at an opposite side of which is an ultra-violet lamp (51) which shines a beam of ultra-violet light (52) across the passage, through the window (11) and into the container (10). 12. An instrument (49) characterised in that a device (10) as claimed in any one of Claims 1 to 7 is positioned with the window (11) forming part of the surface of one side of a passage (50) at an opposite side of which is an ultra-violet lamp (51) which shines a beam of ultra-violet light (52) across the passage, through the window (11) and into the container (10), there being a layer of selectively light-absorbing gas between the ultra-violet lamp (51) and the passage 50. 13. An instrument (49) as claimed in Claim 12 characterised in that the selectively light-absorbing gas is contained in a hollow window (80). 14. An instrument (49) as claimed in any one of Claims 11 to 13 characterised in that it includes means (58) for inducing flow through the passage (50). 15. An instrument (70) characterised in that it includes an instrument as claimed in any one of Claims to 14 with the addition of a pair of concentric electrodes (71t 72) leading from the passage 50. 16. A method of detecting the concentration of a test gas in an airflow characterised in that it includes the step of passing at least part of the airflow through a passage (50) between an ultra violet lamp (51) and a container (10), the container (10) having a window (11) translucent to ultra-violet light and containing detector means (12a, 20) sensitive to ultra-violet light and measurement means (13. 14, 15, 16, 17; 25, 26, 27, 28) whereby the amount of ultra-violet light entering the container (10) can be quantified, and shining a beam (52) from the ultra-violet lamp (51) across the passage (50) into the container (10). 17. A method of detecting the concentration of a test gas in an airflow as claimed in Claim 14 characterised in that there are a pair of concentric electrodes (71. 72) downstream of the passage (50) and means (73. 74, 75) for detecting and measuring ionisation of the test gas.

Inventors: GRIFFITHS RICHARD FREDERICK, JONES CHRISTOPHER DAVID, GRIFFITHS, RICHARD, FREDERICK, JONES, CHRISTOPHER, DAVID
Applicants: GRIFFITHS RICHARD FREDERICK, JONES CHRISTOPHER DAVID, SECR DEFENCE BRIT, GRIFFITHS, RICHARD, FREDERICK, JONES, CHRISTOPHER, DAVID, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1993
Main classification: G01N21
IPC codes: G01J1, G01N21, G01N27