GAS DETECTION DEVICE AND METHOD

Abstract: A gas detection device for distinguishing at a single point, two simultaneously emitted gases and, and in particular, for measuring the individual concentration of each gas at the point, comprising two flow passages into which a sample comprising the two gases is input, gas induction means for drawing the sample into and through the flow passages and means for irradiating each of the flow passages with ultraviolet radiation of a different energy. The device may comprise two sources of ultraviolet radiation, wherein each of the sources (e.g. a krypton and an argon lamp) emits radiation into one of the flow passages. Upon irradiation of the gases (e.g. propane and propylene) within the sample the gases are ionised, one of them being ionised by radiation in one of the flow passages and the other being ionised by radiation in both of the flow passages. Each flow passage has an associated bias electrode, and a collector electrode which are arranged downstream of the irradiated flow passages. The currents at the collector electrodes give an indication of the ionised gas (or gases) in each flow passage. The invention also relates to a method of measuring, at a single point, the individual concentrations of two simultaneously emitted gases. This measurement may be used in the development of models and other tools for determining, for example, gas hazard or nuisance. The device and method have particular application in environmental pollution monitoring, atmospheric tracer gas detection and in the monitoring of gas or vapour emitting processes.

Description: GAS DETECTION DEVICE AND METHOD The invention relates to a gas detection device, and method, which is suitable for environmental pollution monitoring, atmospheric tracer detection and monitoring gas or vapour emitting processes. In particular, the device is capable of detecting and distinguishing between two simultaneously emitted gases so that the individual contributions of each gas to the total concentration may be determined. The detection and measurement of airborne pollutants is an ongoing and increasing requirement in a wide range of activities such as environmental pollution and meteorological studies. In particular, there is a need for fast response point sensors which are capable of providing information on the short term fluctuations in concentration which occur in the vicinity of all emission sources. For example, this is useful way of measuring the concentration of hazardous materials in the atmosphere. Furthermore, in addition to the need for a measurement capability for hazardous materials per se, a useful technique in the development of control methods (e.g. models) is to study the behaviour of a surrogate material, or tracer material, under similar conditions. This may be done by releasing a tracer compound into the atmosphere and detecting it at a distant point by suitable detection means. Many tracer materials have been used in the past, for example fluorescent particles, sulphur dioxide and radioactive isotopes of gases such as krypton. Sulphur hexafluoride has also been used widely as a tracer although it is expensive. In particular, as it can be detected down to very low levels (typically 1 part in lθ'°) it is useful for long range studies, up to several hundred kilometres. For detection in the region of 1-2 km downwind oi ' a source, tracer gases such as propylene have been used successfully. Any suitable detection system for measuring such tracer materials at a range of several kilometres from source must be capable of operating in field conditions. A suitable detector with this capability is the Ultra Violet Ionisation Chamber detector, or the UVIC® detector, as described in US patent 5 572 137. The UVIC® detector is an ultra violet exciter device comprising an ultra violet lamp for emitting ultra violet radiation into an inlet tube into which the tracer gas is introduced. The energy of the ultra violet radiation is such that the tracer gas, for example propylene, is ionised and electrons are collected further downstream to give a measure of the tracer gas concentration. The UVIC detector is capable of measuring concentration fluctuations arising from a single source. The measurement of concentration fluctuations arising from a single source is an important element in quantifying the hazard or nuisance of a particular substance (that is a reference to the toxicity or flammability of a substance or a reference to the malodour). However, in practice it is not unusual for there to be several closely spaced sources emitting simultaneously and this needs to be accounted for when developing models and other tools to describe the hazard. In the past this has been done by summing the concentrations measured from each source at the point in question. However, analysis has shown that this approach is only acceptable if the process associated with the hazard is completely linear in its response to concentration such as, for example, exposure to airborne radionuclides. This is not the case, for example, for the toxicity, flammability or malodour of a substance as non linear effects are a significant part of the process and this technique tends to be inaccurate. There is therefore a need to be able to examine, at a single point, the behaviour of concentrations in air of a pair of simultaneously emitted tracer gases. The statistics of the individual contributions to the total concentration can then be determined and an accurate estimate of the hazard developed. One known system for achieving this includes a UVIC detector and a Flame Ionisation Detector (FID) in co-location. The FID uses a controlled hydrogen flame as an ionising agent as opposed to the ultra violet lamp in the UVIC detector. The ions produced by the flame are advected both electrically and mechanically towards a small biased electrode where they are collected. Although this system is capable of providing useful data on two tracer gases emitted simultaneously, the FID is not suited for operation in field conditions. For example, the logistics requirements posed by the ~ need for a hydrogen supply and the heavy electrical power demands of the FID are inconvenient for field use. Furthermore, the water vapour generated by the combustion of hydrogen tends to condense on some of the inner surfaces of the instrument. This compromises the integrity of the electrical insulation, leading to noisy and unstable signals and therefore low data quality. The present invention overcomes the problems associated with the UVIC/FID system in that it removes the need for a hydrogen supply and does not have high electrical power demands. Furthermore, the device is convenient for use in field conditions, and performs reliably even under adverse conditions, for example, blowing dust, rain and mist. The device is capable of measuring the behaviour of a pair of simultaneously emitted tracer gases, at a single point, so that the individual contributions to the total concentration can be determined and the concentration fluctuations of the two gases may be measured. This is an important aspect of the development of models and other tools for describing a hazard or nuisance of a substance. The accuracy with which this measurement can be made provides a significant improvement over conventional single source summing techniques which do not allow for non-linear processes. The system has particular application in the field of environmental pollution monitoring, atmospheric tracer detection and in the monitoring of gas or vapour emitting processes. According to one aspect of the present invention, a gas detection device for distinguishing two different gases within a sample comprises; two flow passages into which the sample is input, each flow passage having at least one inlet and at least one outlet and an exciter zone and each having a longitudinally extending axis, gas induction means for drawing the sample into and through the flow passages, means for emitting ultra violet radiation into the two flow passages, wherein the radiation emitted into one of the flow passages is of sufficient energy to be capable of ionising at least one of the gases and the radiation emitted into the other flow passage is of sufficient energy to be capable of ionising both of the gases, such that upon irradiation by one or more of the sources the gases may be ionised and generate ions, two electrode arrangements, each comprising at least one bias electrode and at least one collector electrode, having voltage supply means for supplying a voltage to the one or more bias electrode such that they may be differently charged to the collector electrode or electrodes, wherein each of the electrode arrangements is mounted within a different one of the flow passages such that the one or more collector electrode in each collects the ions produced in a different one of the flow passages, and current measuring means, sensitive to the effects of the ions being neutralised on the collector electrodes, for providing an output from each of the electrode arrangements dependent upon the amount of gas or gases ionised within each of the flow passages. In a preferred embodiment, the gas detection device comprises an input passage, having an inner wall, and means for segregating at least part of the input passage so as to provide two flow passages, each having an outer wall. The input passage and the flow passages may have any one of a substantially circular, elliptical, rectangular or hexagonal cross-section. The device may also comprise means for substantially preventing the flow of the sample along the longitudinally extending axis, between the inner wall of the input passage and the outer wall of the flow passages. The gas detection device may comprise a single source of ultra violet radiation having means for selectively transmitting radiation of selected energy into each of the flow passages such that the sample in each of the flow-passages is irradiated with radiation of different energy. Alternatively, the device may comprise two sources of ultra violet radiation, each for emitting radiation into a different one of the flow passages, such that the sample in each of the flow passages is irradiated with radiation of different energy. The two sources of ultra violet radiation may have different emission spectra. Alternatively, they may have substantially the same emission spectra and each source may comprise filtering means for selectively transmitting radiation of the required energy into the flow passages. For example, the device may comprise any of a krypton lamp and a xenon lamp, an argon lamp and a deuterium lamp or a xenon lamp and a krypton lamp. The device may comprise a krypton lamp for emitting radiation having energy of less than 10.95 eV into one of the flow passages and an argon lamp for emitting radiation having energy of greater than 10.95 eV into the other flow passage. The argon lamp may comprise a LiF window through which ultra violet radiation is transmitted into one of the flow passages and the krypton lamp may comprise a MgF 2 window through which ultra violet radiation is transmitted into the other flow passage. In this embodiment, the device is capable of distinguishing propane and propylene gases. The distance between the exciter region in each of the flow passages and the corresponding electrode arrangement may be varied. The gas flow induction means may be a fan which may be operated at a variable speed such that the rate of flow of gas through the input passage may be varied. Each of the electrode arrangements may comprise a substantially tubular outer electrode extending substantially along the longitudinal axis of at least part of the length of the input passage and a rod inner electrode, wherein the outer electrode is mounted concentrically around the rod inner electrode. Any one of the outer electrode and the rod electrode in each of the electrode arrangements may comprise one or more electrode sections. The outer electrode in each electrode arrangement may be biased by the voltage supply means such that it is differently charged to the corresponding rod electrode such that ions generated as a result of the ionisation of the tracer gas or gases are collected at the inner electrode. Alternatively, the rod electrode in each electrode arrangement may be biased such that it is differently charged to the corresponding outer electrode and ions generated as a result of the ionisation of the tracer gas or gases are collected at the outer electrode. Typically, the voltage applied to the bias electrode in each electrode arrangement is substantially the same. According to a second aspect of the invention, a method for distinguishing two different gases in a sample comprises the steps of; (i) inputting the sample into two separated flow passages, (ii) irradiating the sample in each of the two flow passages with radiation of a different energy, wherein the radiation emitted into both of the flow passages is of sufficient energy to be capable of ionising at least one of the gases and radiation emitted into just one of the flow passages is of sufficient energy to be capable of ionising both of the gases, such that one of the gases is ionised in both flow passages and the other gas is ionised in just one flow passages and ions are generated as a result of the ionisation process, (iii) passing the irradiated sample in each flow passage through a different electrode arrangement comprising at least one collector electrode and at least one bias electrode , (iv) applying a voltage to the one or more bias electrode in each electrode arrangement such that the bias electrode or electrodes may be differently charged to the collector electrode or electrodes and the ions generated by the ionisation process may be collected at the one or more collector electrode in each electrode arrangement, and (v) measuring the current at each of the collector electrodes. In a preferred embodiment of this aspect of the invention, the method comprises the further step of; (vi) deducing the individual concentrations of the gases in the sample from the measured currents. The invention will now be described by example only with reference to the following figures in which; Figure 1 shows a diagram of a conventional UVIC device which may be used to measure concentration fluctuations of a gas arising from a single source, Figure 2 shows a diagram of the gas detection device of the present invention, Figure 3 shows an enlarged view of the electrode arrangement included in the gas detection device shown in Figure 2, Figure 4 shows a black diagram of the circuitry which may be used in the gas detection device shown in Figure 2, Figure 5 shows a diagram of the exterior of the gas detection device shown in Figure 2 and Figure 6 shows test results obtained using the gas detection device of the present invention. Figure 1 shows a diagrammatic cross sectional view of a conventional Ultra Violet Ionisation Chamber (UVIC) detector 1. The UVIC detector 1 comprises an input tube 2 having an open inlet 3 at one end and a fan unit 4 at the opposite end so that, in operation, the device draws air through the inlet 3 and along the length of the tube 2. An ultra violet lamp 5 is mounted externally of the tube so that ultra violet radiation 6 is emitted into the tube 2 into an exciter zone 7 through an aperture 8 in the tube wall. An electrode unit 9 is situated downstream of the exciter zone comprising an outer electrode 10 coaxial along the length of the tube and a rod inner electrode 11. The outer electrode 10 is biased by means of a DC power unit (not shown) by a wire 12 passing through as insulating plug 13 in the tube wall. The rod inner electrode 11 lies with its longitudinal axis along the longitudinal axis of both the tube 2 and the outer electrode 10 and is connected to the other leg of the DC power supply unit via a wire 14. The ultra violet lamp is a krypton lamp which provides radiation 6 at 10.03 eV and 10.65 eV and is capable of ionising propylene (having an ionisation potential of 9.73 eV). If, for example, a propylene tracer gas enters through the inlet 3 to the tube it is irradiated with ultra violet radiation 6 and is ionised. The electrons generated as a result of the ionisation process pass downstream and are collected at the electrode unit 9. By measuring the current collected by the electrode, the concentration of propylene in the tube 2 may be determined. Thus, the device enables the concentrations fluctuations rising from a single source of propylene tracer gas to be measured. The UVIC detector 1 also has a strong response to ammonia (NH 3 ) since its ionisation potential (10.16 eV) is close to and just below that of one of the two principle emission bands of the ultra violet krypton lamp 5. Whilst the UVIC detector 1 is capable of measuring gas concentration fluctuations arising from a single source, in practice it is common for there to be several closely spaced gas sources emitting simultaneously. The measurement of concentration fluctuations of two simultaneously emitted tracer gases at a single point has been achieved using a gas detection system comprising the UVIC device, as shown in Figure 1, in co-location with a Flame Ionisation Detector (FID), as described previously. However, a problem with the FID detector is that its behaviour in field conditions can be unreliable. Also, it is not particularly convenient for use in the field due to the need for a hydrogen supply and the FID also requires a high power supply. Referring to Figure 2, the gas detection device 15 of the present invention comprises an input passage 16, or tube, having an open inlet 17 at one end and a fan unit 18 at the opposite end having a power supply (not shown). The fan unit 18 may be any means for inducing gas flow in the tube 16 such that in operation air is drawn into the tube 1 at the inlet 17, is drawn along a length of the tube 16 and is output from an outlet 18a. In operation a combination of any of air, gas or vapour may be drawn through the tube 16. For the purpose of this specification, the phrase "gas" shall be taken to mean a gas or a vapour. The tube 16 may typically be a length of tubing which may have more than one inlet and outlet at the respective ends. The tube may be rigid or, in some application, it may be useful if the end portion is flexible to aid probing. The device 15 also comprises a flow separator 19 having its longitudinal axis substantially parallel with the longitudinal axis of the tube 16. The flow separator 19 is situated substantially at the centre of the tube 16 such that it divides the tube into two separate flow passages 20a.20b. The device also comprises two lamps 21,22, each lamp having an associated power supply 23,24, for emitting ultra violet radiation 23. The lamps 21,22 are mounted on substantially opposite sides of the tube 16 such that each emits radiation 23 into a different one of the flow passages 20a,20b. As shown Figure 2, the mounting of the lamps 21,22 on the side of the tube 16 may be such that the radiation 23 is emitted into the flow passages 20a,20b via apertures 25 in the tube wall. Alternatively, the mounting of the lamps 21,22 may be such that the surface of the ultra violet lamp through which radiation is emitted forms part of the wall of the tube 16. It may be preferable to mount the lamps 21,22 at a slight recess from the tube wall (i.e. not protruding into the flow passages 20a,20b) so as to maximise the flow of air and/or gas through the respective flow passages 20a,20b. Each lamp has a dedicated coaxial ion collection electrode arrangement which is mounted within the tube and is situated downstream of the flow passages. An enlarged diagram of the electrode arrangements is shown in Figure 3 but, for clarity, is not drawn in full detail in Figure 2. Referring to Figure 3, the electrode arrangements 26a,26b are separated by a small gap, the size of the gap being dictated primarily by mechanical considerations (e.g. the need for means for supporting the electrode arrangements 26a,26b). Each electrode arrangement 26a,26b may comprise a tubular outer electrode 27a,27b, which may be biased by means of a bias supply 28 (i.e. a "bias" electrode 27a,27b), and an inner electrode 29a,29b (i.e. a "collector" electrode). The tubular outer electrode, 27a and 27b, may be concentrically mounted around the inner electrode, 29a and 29b respectively, such that the inner electrode extends longitudinally along the length of the outer electrode 27a,27b. Each of the outer electrodes 27a.27b and the inner electrodes 29a,29b may be a single electrode or, alternatively, each may comprise a plurality of electrode sections. The longitudinal axes of the electrode arrangements 26a,26b are preferably substantially parallel to the longitudinal axis of the tube 16 so as to maintain the passage of flow between the inlet 17 and the fan unit 18. The bias supply 28 provides a suitable voltage to the outer electrodes 27a,27b such that each may be differently charged from the associated inner electrode 29a,29b respectively. Each inner electrode 29a,29b is connected to subsequent electronic components 30a (not shown in detail in Figure 2), for example for reducing the drift and noise and for measuring the electrode currents at outputs S. and S . Referring back to Figure 2, in a preferred embodiment, the tube 16 may be of substantially circular cross-section, although it may also take other cross-sectional forms, such as rectangular, hexagonal or elliptical. Typically, the flow passages 20a,20b may have substantially equal diameters. For convenience, the outer electrodes 27a,27b may have substantially the same cross- sectional shape as the flow passages 20a,20b. The inner wall of the tube 16 may conveniently be fabricated from a metal of relatively high electrical resistance and, more preferably, of reasonably high workfunction with regard to the emission of electrons upon irradiation with ultra violet radiation. Furthermore, it is preferable if the inner wall material does not retain traces of gases used upon its surface. Suitable materials from which the tube 16 may be made are, for example, aluminium and steel. If the tube inner wall is of a non-insulating material, e.g. aluminium or steel, the outer electrodes 27a,27b must be insulated from the inner wall of the tube wall. The insulation of the outer electrodes 27a,27b from the tube 16 may be achieved by mounting the outer electrodes 27a,27b on insulating rings or spacers 50 (as shown in Figure 3) which also serve as support means for the outer electrodes 27a,27b. The insulating rings 50 may be placed between the outer electrodes 27a,27b and the inner sidewall of the tube 16 such that they circle the circumference of the outer electrodes 27a,27b. The insulating rings also prevent air and/or gas from flowing around the outside of the outer electrodes 29a,29b. This is important as it is desirable for as much of the gas input to the tube to pass between the electrode arrangements 26a,26b. This may be achieved using alternative means other than the insulating rings but it is convenient to use the rings 50 for both purposes. The inner electrodes 29a,29b may conveniently be rod electrodes which preferably extend along substantially the full length of the corresponding outer electrode, except that a portion of each of the inner electrodes 29a,29b may be bent to penetrate the wall of the tube 16 through an insulating plug (not shown) for subsequent connection to the electronic circuitry 30a. Connecting the bent portion of the inner electrodes 27a,27b to the tube wall at the insulating plug provides a suitable means of supporting the inner electrodes 27a,27b. The object of the invention is to provide a system which is capable of detecting two tracer gases emitted simultaneously from a single point. Therefore, typically in operation, a sample of gas is drawn into the tube 16, via the inlet 17, by the gas induction means 18 wherein the gas sample will typically comprise two such tracer gases in air. The tracer gases are irradiated in the flow passage regions 20a.20b by the ultra violet radiation 23 emitted from the lamps 22,21 respectively. If upon irradiation the tracer gas becomes ionised, oppositely charged ion pairs, or an ion and an electron, are produced which are then drawn into the electrode arrangements 26a,26b, by the gas flow induction means. In each electrode arrangement 26a,26b, the outer electrode and the inner electrode are oppositely charged such that the inner electrode in each case is a "collector" electrode for positively charged ions produced by the ultra violet ionisation process. The choice of the particular lamps and tracer gases used is determined by the need for both of the tracer gases to be ionisable by radiation emitted from one of the lamps and for just one of the tracer gases to be ionisable by radiation emitted from both the lamps. That is, the lamp emitting the higher energy radiation must be able to ionise both gases and the lamp emitting the low energy radiation must only be able to ionise one gas (i.e. the lower ionisation potential gas). In a preferred embodiment of the invention, the device 15 may comprise an argon lamp (21) and a krypton lamp (22). In operation, the krypton lamp 22 is emits radiation at 10.03 eV and 10.65 eV and the argon lamp 21 emits more energetic radiation at 1 1.60 eV and 1 1.80 eV. The argon lamp comprises a LiF (lithium fluoride) coated window (not shown) which permits radiation at 11.60 eV and 11.80 eV to be transmitted. The krypton lamp may typically comprise a MgF 2 (magnesium fluoride) coated window (not shown) which permits radiation at 10.03 eV and 10.65 eV to be transmitted. The device comprising a krypton lamp and an argon lamp may conveniently be used to simultaneously measure propane and propylene tracer gas concentrations. The ionisation potential of propylene (C 3 H 6 ) is 9.73 eV and therefore propylene gas will be ionised by radiation 23 emitted from both the krypton lamp 22 (at 10.03 eV and 10.65 eV) and the argon 21 lamp (at 11.60 eV and 11.80 eV). Propane (C 3 H g ) has an ionisation potential of 10.95 eV and therefore a propane tracer gas will only be ionised by the higher energy radiation emitted from the argon lamp 21. The current collected at the inner electrode 29a associated with the krypton lamp 22 will therefore be in proportion to the concentration of propylene gas in the flow passage 20a. However, the current collected at the inner electrode 29b associated with the argon lamp 21 arises as a result of the ionisation of both the propylene and propane tracer gases in the flow passage 20b and therefore this current is proportional to the total concentration of both gases. For example, the output signals, at S j and S 2 , generated from the argon lamp and krypton lamp electrode arrangements 26b and 26a respectively, may be related to the individual propylene (C 3 H 6 ) and propane (C 3 H g ) tracer concentrations in the following way; S, = A(7H 6 )+ B(7H S ) Equation 1 and S 2 = C(C 3 H 6 ) Equation 2 where A,B and C are linear calibration constants. The linear calibration constants may be determined in an exposure chamber prior to measurement. Knowing these values, it is then possible to extract the propane and propylene concentrations from the signals output from the device at Sj and S 2 . The calibration process may conveniently be carried out in a sealed air-filled chamber. The chamber would typically comprise means of injecting the required quantities of tracer gas into the chamber and a mixing fan which would be required to ensure a uniform concentration of the gas was achieved quickly following injection into the chamber. Typically, for the purposes of calibration, the device 15 would first be exposed to a range of concentrations of the individual tracer gases and then to a mixture of the two. The concentrations of the tracer gas in air would typically be in the range of between 0-2000 ppm by volume for this purpose. As well as using krypton and argon lamps in the device 15, several other lamp and tracer gas combinations may also be envisaged which satisfy the requirements for dual-gas measurement. For example, a krypton lamp may be used in combination with a xenon lamp, or an argon lamp may be used in combination with a deuterium lamp. As well as propane and propylene tracer gases, other tracer gas combinations may be used, such as ammonia and propane, or ethylene and propane. Alternatively, a xenon and krypton lamp combination may be used with a benzene and propylene tracer gas combination. In practice, environmental and economic factors are likely to limit the choice of tracer gas which may be used. It may also be possible to use two sources of ultra violet radiation having the same emission spectra to irradiate both flow passages 20a,20b, for example two krypton sources. The two sources will have the same emission spectra (i.e. emit radiation comprising the same wavelength components and energies) but the window of each source may be fitted with a suitable filter, so that radiation of the required energy or energies may be selectively passed into the flow passages 20a,20b. In any practical device, such as that shown in Figure 2, ionic recombination is likely to occur in the region between the portion of the separate passages 20a,20b the ultra violet radiation irradiates and the point at which ions are collected further downstream in the electrode arrangements 26a,26b. This is an undesirable effect as it prejudices the accuracy of the measurement. These recombination processes may be minimised by reducing the distance through which the ionised gas passes prior to collection at the inner electrodes 27a,27b and also be increasing the flow speed. The use of the flow separator 19 ensures that two separate regions are created in the tube 16 (passages 20a,20b). This is essential to the operation of the device as the ionisation products resulting from argon lamp ' induced ionisation and the krypton lamp induced ionisation processes must not mix prior to ion collection further downstream at the inner electrodes 27a.27b. Furthermore, as the lamps are preferably mounted in substantially opposing positions, the illumination of the lamps with ultra violet radiation from the other may interfere with lamp performance. The flow separator 19 also prevents this occurring. In a preferred embodiment of the invention, the flow passages 20a,20b may be formed within the tube 16 by the presence of the flow separator 19, as shown in Figure 2). Alternatively, the separate flow passages 20a,20b may be formed from two separate tube sections, within each of which the associated electrode arrangement 26a,26b is situated downstream of the ultra violet lamp positions. In this embodiment, the common input section of the tube (i.e. length / on Figure 2) is absent. Whilst this arrangement may be suitable for some applications, a slight uncertainty is introduced in that the two gases ionised in the separate flow passages are not sampled at a precise single point. It may be preferable for the region of the separate flow passages which is irradiated with the radiation 23 to be as close to the electrodes as possible, so that the ionised gas or gases have as short a distance as possible to travel before they are collected. For some applications, however, it may be useful if this distance is variable. In any case, it is preferable to avoid significant ionisation of the gas or gases in the space between the electrodes. The amount of radiation impinging on the electrodes 27a,27b,29a,29b should be kept to a minimum or avoided altogether. If very high flow rates are to be used, the irradiation zone within the separated flow passages 20a,20b may be further upstream than shown in Figure 2. It may be preferable if the tube 16 is moveable along its longitudinal axis with respect to the electrode arrangements 26a,26b such that the position of the irradiated regions in the separated flow passages 20a,20b may be varied by an operator depending on the intended use of the device. This may be achieved if the windows or apertures on both sides of the tube, through which the radiation 23 enters the tube 16, extend along a continuous length of the tube 16. Alternatively, the tube 16 may comprise several apertures on each side of the tube 16. By uncovering selected apertures on each side of the tube and moving the tube along its longitudinal axis, the selected apertures may be aligned with the lamps 21,22, providing a variable distance between the regions irradiated by the lamps and the electrode arrangements 26a,26b. In either case, slidable covers would be required for placement over the apertures when not in use or the portion of the aperture not in use. Figure 4 shows a block diagram of one circuit which may be used to control the gas detection device 15 shown in Figure 2. The operation of the circuit shown in the figure would be conventional to one familiar with the art. The features of the circuit are as follows; EHTG; EHT generator, MPS: main power supply, EM; electrometer, OS; off-set control, FSC; fan speed controller, SGA; switched gain amplifier, RS; range switch, LPF; low pass filter, INIA; inverting/non-inverting amplifier, 1(1); current corresponding to output S,, 1(2); current corresponding to output S 2 . The currents 1(1) and 1(2) measured at the outputs S, , S 2 may be displayed on a visual display 31. The operation of the circuit shown in Figure 4 would be conventional to one familiar with electrical circuits and the circuit is one of many circuits which may be used to control the device and measure the charge collected at the electrodes 29a,29b. The current sensing part of the circuitry is sensitive to the effects of ions being neutralised upon the surfaces of the electrodes collecting the ions, such that an output dependent upon the amount of each tracer gas (or gases) ionised within the flow passage is provided and the amount of each individual tracer gas at the measurement point may be determined. The lamp power supply units 23,24 and the bias supply 28 are connected to main power supplies. The fan unit 18 is also connected to a main power supply via a fan speed controller (FSC) which enables the rate of flow of " gas through the tube to be adjusted. Preferably, the gas flow induction means 18 may be in the form of an electric fan. Flow rates of around 4 x 10 "4 m s " may be 3 - 1 ■ conveniently achieved using a radial fan, but for increased flow rates e.g. at least 4 x 10 " m s it may be convenient to use a centrifugal fan. A schematic diagram of an example of the exterior of the complete gas detection device 15 is shown in Figure 5. The tube 16, preferably having the interior configuration shown in Figures 2 and 3, is preferably mounted on a support 35 such that it may be moved along its longitudinal axis with respect to the support 35. This support 35 may also form an ultra violet source box for housing the ultra violet lamps 21,22. The ultra violet support box may be mounted on separate housing means 36 for housing the bias supply 28 and the circuitry for measuring the electrode currents and for reducing the drift and noise, as shown in Figure 4. Alternatively, the housing means 36 may be provided separately from the ultra violet source box 35 and tube 16. The gas flow induction means 18 are situated at the outlet of the tube 16 and means for supplying power to this unit 18 may be held within the ultra violet source box 35 or the housing means 36. The device may be a hand held device or a fixed device and a carrying strap or handle may also be provided (not shown). The device 15 comprises a control 37 for activating the ultra violet lamps (or controls for activating both lamps independently) which may have an associated visual display 38 for indicating lamp operation. The device also comprises controls 39 for varying the sensitivity of the current sensing part of the circuitry, a control for backing off the zero reading (i.e. the off-set controls) 40 and a control for activating the electrodes bias voltage 41. The device 15 may also comprise a visual indicator 31 (as indicated in Figure 4) to give an indication of the current measured at each electrode, from which the individual concentrations of the tracer gases may be deduced. Figure 6 shows the performance that can be achieved with the device of the present invention when tested under field conditions. The device under test comprised an argon and a krypton lamp. In this test continuous sources of propylene (0.3 L min " ) and propane (3 L min " ) were located 1 m above ground level at a crosswind separation of 5 m. The device under test was positioned 10 m downwind of the pair of sources. The ionisation currents, following amplification, were logged at a 10 Hz sampling rate on a digital computer. The results obtained were not atypical. The results were obtained using Equations 1 and 2, as described earlier i.e. the concentrations of propane and propylene were determined from the output signals, Sj and S 2 . of the argon and the krypton lamps respectively. The results show excellent time resolution and also indicate the ability of the device to distinguish between two chemically similar species. For a hand held device, the typical dimensions of the tube may be between 1 -5 cm. Fixed devices may have larger diameter tubes in which case proportionally larger electrodes will be required. Typically, each of the outer electrodes 27a,27b has a length of several centimetres, e.g. 3-5 cm and the radial spacing of the inner and outer electrodes, 27a,29a and 27b,29b, in each arrangement 26a.26b may be between 0.2 and 1.0 cm. The voltage across each of the outer and inner electrodes, 27a,29a and 27b,29b, may typically be between 20-1000V with currents of the order of 10 nA generated by near-maximal ionised tracer gas levels contacting the electrodes. The voltage should be selected such that the device operates in the saturation region, that is a further increase in the bias voltage does not result in an increased collection of ions. The bias voltage applied to the outer electrodes 29a,29b may be either positive or negative with respect to the virtually earthed inner electrode. Typically, the current display 31 will have a sensitivity of between 300 pA to 30-100 nA. Both of the outer electrodes 27a,27b may be biased by means of the same bias supply, as shown in Figure 2, and may be electrically at the same potential. However, it may also be possible to bias the outer electrodes 27a,27b with different voltages depending on the intended application of the device 15. If the bias voltages applied are substantially different, it may be preferable to mechanically re-arrange the two electrode arrangements 26a.26b. In an alternative embodiment to that shown in Figure 2, the inner electrodes 27a,27b may be biased with respect to the outer electrodes 29a,29b such that the latter operate as the "collector" electrodes from which fhe ' output signals may be taken. The electrodes may be made from any suitably conductive metal, such as stainless steel or copper and, preferably, may be made from a relatively inert material such as gold plated brass. The ring seals or spacers for providing the insulation of the electrodes 27a,27b from the wall of the tube 16 may be made of a suitably insulating material, such as polytetra-fluoroethylene (PTFE) and Darvic (IBM RTM). The gas detection device is particularly suitable for operation in field conditions, including conditions of blowing dust, rain and mist. Furthermore, it may conveniently take the form of a hand held device. It may be used to examine the behaviour, at a single point, of the concentrations in air of simultaneously emitted tracer gases and is compatible for operation with gases are which are convenient and suitable for release into the open air.

Claims: Claims 1. A gas detection device for distinguishing two different gases within a sample comprising; two flow passages into which the sample is input, each flow passage having at least one inlet and at least one outlet and an exciter zone and each having a longitudinally extending axis, gas induction means for drawing the sample into and through the flow passages, means for emitting ultra violet radiation into the two flow passages, wherein the radiation emitted into one of the flow passages is of sufficient energy to be capable of ionising at least one of the gases and the radiation emitted into the other flow passage is of sufficient energy to be capable of ionising both of the gases, such that upon irradiation by one or more of the sources the gases may be ionised and generate ions, two electrode arrangements, each comprising at least one bias electrode and at least one collector electrode, having voltage supply means for supplying a voltage to the one or more bias electrode such that they may be differently charged to the collector electrode or electrodes, wherein each of the electrode arrangements is mounted within a different one of the flow passages such that the one or more collector electrode in each collects the ions produced in a different one of the flow passages, and current measuring means, sensitive to the effects of the ions being neutralised on the collector electrodes, for providing an output from each of the electrode arrangements dependent upon the amount of gas or gases ionised within each of the flow passages. 2. The gas detection device of claim 1, comprising an input passage, having an inner wall, and means for segregating at least part of the input passage so as to provide two flow passages, each having an outer wall. 3. The gas detection device of claim 2 wherein the input passage has any one of a substantially circular, elliptical, rectangular or hexagonal cross-section. 4. The gas detection device of any of claims 1-3, wherein the flow passages have any one of a substantially circular, elliptical, rectangular or hexagonal cross-section. 5. The gas detection device of claim 2, and further comprising means for substantially preventing the flow of the sample along the longitudinally extending axis between the inner wall of the input passage and the outer wall of the flow passages. 6. The gas detection device of claim 2 comprising a single source of radiation having filtering means for selectively transmitting radiation of selected energy into each of the flow passages such that the sample in each of the flow passages is irradiated with radiation of different energy. 7. The gas detection device of claim 2 comprising; two sources of ultra violet radiation, each for emitting radiation into a different one of the flow passages such that the sample in each of the flow passages is irradiated with radiation of different energy. 8. The gas detection device of claim 7 wherein the two sources of ultra violet radiation have different emission spectra. 9. The gas detection device of claim 7 wherein the two sources of ultra violet radiation have substantially the same emission spectra and wherein each source comprises filtering means for selectively transmitting radiation of selected energy into the flow passages. 10. The gas detection device of claim 9 comprising any of a krypton lamp and a xenon lamp, an argon lamp and a deuterium lamp or a xenon lamp and a krypton lamp. 11. The gas detection device of claim 10 comprising a krypton lamp for emitting radiation having energy of less than 10.95 eV into one of the flow passages and an argon lamp for emitting radiation having energy of greater than 10.95 eV into the other flow passage. 12. The gas detection device of claim 11 wherein the argon lamp comprises a LiF window through which ultra violet radiation is transmitted into the flow passage. 13. The gas detection device of claim 12 wherein the krypton lamp comprises a MgF 2 window through which ultra violet radiation is transmitted into the flow passage. 14. The gas detection device of any of claims 11,12 or 13 wherein the device is capable of distinguishing propane and propylene gases. 15. The gas detection device of claim 2 wherein the distance between the exciter region in each of the flow passages and the corresponding electrode arrangement may be varied. 16. The gas detection device of claim 2 wherein the gas flow induction means is a fan. 17. The gas detection device of claim 16 wherein the gas flow induction means may be operated at a variable speed such that the rate of flow of the gases through the input passage may be varied. 18. The gas detection device of claim 2 wherein each of the electrode arrangements comprises; a substantially tubular outer electrode extending substantially along the longitudinal axis of at least part of the length of the input passage and a rod inner electrode, wherein the outer electrode is mounted concentrically around the rod inner electrode. 19. The gas detection device of claim 18, where any one of the outer electrode and the rod electrode in each of the electrode arrangements comprises two or more electrode sections. 20. The gas detection device of claim 17 or 18 wherein the outer electrode in each electrode arrangement is biased by the voltage supply means such that it is differently charged to the corresponding rod electrode such that ions generated as a result of the ionisation of the gas or gases are collected at the inner electrode. 21. The gas detection device of claim 17 or 18 wherein the rod electrode in each electrode arrangement is biased by the voltage supply means such that it is differently charged to the corresponding outer electrode such that ions generated as a result of the ionisation of the gas or gases are collected at the outer electrode. 22. The gas detection device of claim 19 or 20 wherein the voltages applied to the bias electrode in each electrode arrangement is substantially the same. 23. The gas detection device of claim 19 or 20 wherein the voltages applied to the bias electrode in each electrode arrangement are different. 24. A method for distinguishing between two different gases in a sample comprising the steps of; (i) inputting the sample into two separated flow passages, (ii) irradiating the sample in each of the two flow passages with radiation of a different energy, wherein the radiation emitted into both of the flow passages is of sufficient energy to be capable of ionising at least one of the gases and radiation emitted into just one of the flow passages is of sufficient energy to be capable of ionising both of the gases, such that one of the gases is ionised in both flow passages and the other gas is ionised in just one flow passages and ions are generated as a result of the ionisation process, (iii) passing the irradiated sample in each flow passage through a different electrode arrangement comprising at least one collector electrode and at least one bias electrode , (iv) applying a voltage to the one or more bias electrode in each electrode arrangement such that the bias electrode or electrodes may be differently charged to the collector electrode or electrodes and the ions generated by the ionisation process may be collected at the one or more collector electrode in each electrode arrangement, and (v) measuring the current at each of the collector electrodes. 25. The method of claim 24, and further comprising the step of; (vi) deducing the individual concentrations of the gases in the sample from the measured currents.

Inventors: JONES CHRISTOPHER DAVID, JONES, CHRISTOPHER, DAVID
Applicants: JONES CHRISTOPHER DAVID, SECR DEFENCE, JONES, CHRISTOPHER, DAVID, THE SECRETARY OF STATE FOR DEFENCE
Application year: 1998
Main classification: G01N27
IPC codes: G01N27, G01N33