WO1999041598A1

WO1999041598A1 - Gas or vapour sensor

Info

Publication number: WO1999041598A1
Application number: PCT/GB1999/000430
Authority: WO
Inventors: Bryan Stewart Hobbs; Martin John Willett; Martin Geoffrey Jones; Geraint Williams; Gary Stephen Vaughan Coles
Original assignee: City Technology Ltd
Current assignee: City Technology Ltd
Priority date: 1998-02-13
Filing date: 1999-02-11
Publication date: 1999-08-19
Anticipated expiration: 2000-08-13
Also published as: AU2532699A; GB9803158D0

Abstract

A gas or vapour sensor comprises an electrical conductor (1) in contact with material (2) whose electrical resistance varies, under working conditions, in the presence of a gas or vapour to be detected. The electrical conductor (1) carries a current in use to raise the temperature of the material (2) to an operating temperature. A filter layer (4) is supported on and surrounds the variable resistance material (2), the filter layer allowing only a gas or vapour to be dectected to pass to the variable resistance material, whereby a change in resistance of the material is determined by monitoring the voltage across the electrical conductor.

Description

1 GAS OR VAPOUR SENSOR

The invention relates to a gas or vapour sensor and also to a gas or vapour sensing assembly for detecting a gas or vapour, particularly a combustible gas or vapour.

However, the invention is not limited to the detection of combustible gases and vapours.

It is important to be able to detect leaks of gases and vapours, particularly combustible substances, so as to warn of explosive levels in the atmosphere. Examples of gases and vapours which need to be detected include hydrocarbons and other volatile organic compounds, hydrogen, ammonia etc. as well as unburnt fuel m emissions from combustion processes. A number of detectors have been developed m the past.

Pellistors (otherwise known as catalytic bead or microcalorimetπc sensors) comprise a detector element formed from a fine Pt coil resistor, coated wit a supported catalyst which forms a porous "bead" around the coil. Catalytic materials are typically precious metals such as Pd, Pt , Rh supported on refractory ceramics such as alumina, silica, etc. The coil acts as both heater and detector, maintaining the bead at a temperature of about 300-500°C depending on the gas being detected. When combustible gas, m the presence of air, contacts the bead the gas burns with the liberation of heat; the Pt coil resistance increases m proportion to the heat of combustion which is related to the concentration of combustible gas present; the change m resistance is detected and measured using a conventional Wheatstone Bridge and an electrical output in terms of combustible gas concentration is produced.

The detector bead m a pellistor arrangement is usually paired with a compensating element m the opposite arm of the Wheatstone Bridge. This compensator has a similar construction to the detector except that the bead contains no catalyst and is poisoned to prevent any gas reaction on its surface. In this way, interfering effects such aε humidity and temperature fluctuations are near equal m each element and therefore cancelled from the net output of the Bridge. The "pellistor pair" detector is used widely for the detection of combustible gases at relatively high concentrations to provide warning of explosive levels of gases, typically m the range of 0.1% (l,000ppm) to several percent. The resolution capability of these devices is not sufficiently high to enable accurate measurements much below l,000ppm which would be required for applications such as leak detection or emission measurements in flue and exhaust gases. With flue and exhaust gases where oxygen levels can be very low, air must be blendeα with the gas stream to allow combustion to occur on the catalytic bead and this necessarily gives further dilution of the gas being measured. Typical levels of gases to be measured m these applications are around 10 to 20ppm.

Pellistors are described m more detail in Chapter 2 of "Solid State Gas Sensors" published by Adam Hilger (1987) .

Semiconductor elements provide an alternative approach to pellistors.

Semiconductor elements comprise a platinum resistance heater to maintain a temperature between about 200-500°C (depending on the sensor design and gas being sensed; , a sintered semiconductor bead, or layer and two noble metal contacts to enable the conductivity of the semiconductor bead or layer to be measured. A relationship exists between the conductivity of the semiconducting material and the concentration of many combustible gases such that concentrations of these gases in the air may be determined using a simple resistance measurement of the element .

Tin oxide (Sn0₂) is the most widely used material in semiconductor gas detectors, although other materials can m principle be used, e.g. zinc oxide (ZnO) . The semiconductor may employ dopants, for example tin oxide w tn antimony oxide (Sn0₂/Sb₂0₃) and also may be mixed with precious metals such as Pt or Pd (see Chapters 4, 5 and 6 of the above referenced "Solid State Gas Sensors") .

Semiconductor sensors produce logarithmic responses to gas concentrations and therefore have the advantage compared to pellistors of being very sensitive tc low concentrations of gas. Unfortunately, they are also sensitive to many other constituents of the air which interfere with the gas measurement, m particular water vapour. Semiconducting elements are notoriously erratic and irreproducible m their behaviour and have a strong dependence on their previous history of operation and gas exposure .

More recently, a hybrid device has been developed which is effectively a combination of the semiconductor sensor and a pellistor. We will refer to this hybrid device as a "semistor". Semistors are described by S N Malchenko, I N Lychkovsky, V Baykov; Sensors and Actuators B, 2 (1992) pp 505-506, and are manufactured and marketed by New Cosmos Electric Company of Japan.

The detection element comprises a Pt coil resistance, coated with a semiconducting oxide material which forms a "bead" around the coil m a similar manner to a pellistor. The sensor function is based on shunting of the coil heater/resistance resulting from the interaction of the metai oxide semiconductor and detected gas molecules. Operational temperatures can be m the region of those used for semiconductors and pellistors, that is 200-500°C.

In a semistor, the semiconducting oxide sensing material should possess electrical properties relative to the coil such that in the presence of the gas to be detected some of the measuring current can flow through the semiconductor between the turns of the coil . Total semistor resistance is a combination of both coil and semiconductor resistance (1/R (total) = l/R(coil + 1/R (semicon) ) , therefore the lower the resistance of the semiconductor, the greater its overall effect. Attempts have been made to improve the behaviour of semiconductor devices by means of outer (shell) coatings on the tin oxide layer/bead - see H Deoeda, P Massok, C Lucat ,

F Menil, J C Aucouturier ; Meaε . Sci . Technol 8_. (1997) pp 99-110.

These coatings achieve a number of beneficial effects: (a) reduced influence of humidity, on the methane response (claims of near zero effect have been made m the range 20 to 80% R_H) (b) stabilised baseline resistance

(c) mechanical protection

(d) improved selectivity.

The coating comprised porous refractory materials such as alumina, silica, supporting noble metals such as Pt and Pd.

Debeda et al employ a reference in the form of a standard substrate and compare its heater track resistance with that of the active sensor i.e. a Pt shell coated tin oxide film on another substrate. The heater track acts as a resistance thermometer and no temperature difference is observed m the presence of methane, while propane or ethanol give a marked difference. A second measurement, i.e. the resistance of the tin dioxide thick film gives a measure of the methane concentration. Thus, two measurements are needed to give a methane specific sensor. Another membrane detector is described m "Developments of a reliable methane detector", Debeda et al , Sensors and Actuators B44 (1997) pp 248-256.

EP-A-0115953 discloses a gas sensor comprising a heat resisting electric resistor of a noble metal, and a semiconductor of sintered metal oxide covering the resistor. In this case, an outer coating of alumina, silica or aluminium silicate is used to improve the poison resistance of the semistor in contrast to the coating provided by Debeda et al which improves selectivity.

EP-A-0444753 discloses the use of two semiconductor sensors having a similar construction but operated at different temperatures wnere tney respond to a gas to _Pe detected and an interfering gas respectively. The effect of the interfering gas can then be removed from the signal obtained for the gas to be detected. However, this approach of operating at significantly different temperatures where sensitivity is optimum for the two gases does not enable compensation for other effects easily to be achieved since each sensor is likely to respond differently at the different temperatures. US-A-3864628 describes a gas sensor m which access of gas to the detector is controlled by a gas permeable membrane located m a gas entry structure which opens into a gas chamber m which the detector is located. The membrane is designed to impart a degree of selectivity by virtue of the properties of the material employed. In one example, two such sensor arrangements are used with different membranes. The system operates on the principle that one membrane is chosen so that it passes a target: gas relatively easily while interference gases are restricted while the other membrane has the opposite property. Then, during an initial phase, one sensor will be predominately responsive to the target gas while the other will be predominantly unresponsive to that target gas. This allows the effect of interference gases to be coriϊDensatec In orαer to maximise the time over wnich this difference m response occurs, the gas chambers are provided. However, this system has the drawback that after a certain period of time, both sensors will reach a plateau condition and provide similar responses so that it is no longer possible to detect the target gas separately from the interference gases .

DE 4432729 discloses a gas detection system m which a pair of gas detectors are provided. One detector is made of galium oxide sensitive to the target gas while the other is made of galium oxide but with a number of layers of zirconium oxide which renders that sensor insensitive to all gases. In this way, temperature compensation can be achieved. However, the sensor is then not able tc compensate for interfering gases such as water vapour and the like.

In accordance with a first aspect of the present invention, a gas or vapour sensor comprises an electrical ccnouctor m contact with material whose electrical resistance varies, under working conditions, in the presence of a gas or vapour to be detected, the electrical conductor carrying a current m use to raise the temperature of the material to an operating temperature; and a filter layer supported on and surrounding the variable resistance material, the filter layer allowing only a gas or vapour to be detected to pass to the variable resistance material, wnereby a change m resistance cf the material is determined by monitoring the voltage across the electrical conductor.

In this aspect of the invention, we provide a semistor which is closely surrounded by a filter layer wmch restπcts access of gases and vapours to the semiconductor material except for the gas or vapour to be sensed and avoids the need for a gas chamber. This should be contrasted with the known semistorε described above wnich are unable to obtain such gas or vapour specificity.

In accordance with a second aspect of the present invention, a gas or vapour sensing assembly comprises a first gas or vapour sensor including an electrical conductor in contact with a material whose electrical resistance varies, under working conditions, in the presence of a gas or vapour to be detected, the electrical conductor carrying a current m use to raise the temperature of the material to an operating temperature; and a second sensor having a similar construction to the first, wherein the sensors are fabricated and/or operated under suitable working conditions such that at substantially all times the sensors respond differently in the presence of the gas or vapour to be detected but m substantially the same manner to other interfering gases whereby the presence of the said gas or vapour can be detected.

In this aspect of tne invention, the gas sensors may comprise semiconductor sensors as described above in wnich the resistance cf the bead is monitored directly but preferably comprise semistors, most preferably of the type defined m accordance with the first aspect of tne invention.

The electrical conductor will typically be m the form of a coil but could also be m the form of a planar construction aε , for example, described by Debeda et al and m EP-A-0444753.

The second aspect of the invention allows not only the presence of the gas or vapour to be detected but also its concentration. Furthermore, operating the sensors so that at substantially all times they respond m substantially the same way to other interfering gases enables the effects of these to be automatically compensated without the need for additional signal processing in real time, ano w tnout the risk of the plateau effect mentioned above in connection with US-A-3864628.

In addition, it should be noted that assemblies according to the invention operate on the basis of dif erent responses from the sensors and do not necessarily require the first sensor positively to sense the said gas or vapour .

Aε mentioned previously, the sensorε and εensor assemblies according to the invention are primarily useful for sensing combustible (or at least reducing) gases and vapourε but could also be used for non-combustible gases and vapours m certain circumstances. Examples include NO,

N0₂ and S0₂.

Importantly, the sensor assembly can compensate for relative humidity effects. Conveniently, both εensorε cf the assembly are constructed m accordance with tne first aspect of the invention, the filter layers cf the sensors being different. In other cases, however, one cf tne sensorε may have no filter layer.

In a preferred approach, the second sensor is substantially insensitive to the gas or vapour tc be 5 detected and, for example, the filter layer of the second sensor may prevent all gases or vapours of a type to which the sensor woulα otherwise be sensitive from reaching the variable resistance material of that sensor.

The filter layer can comprise any inert refractory C base including, for example, alpha or gamma alumina, silica, zirconia and the like, together with a catalytic component (such aε Palladium) whicn may be applied aε a coating or incorporated m the base. This catalyst will then promote oxidation of certain gases to achieve the 5 filtering effect. The catalyst may be platinum which will react all combustible gases apart from methane at worKmg temperatures and so is suitable for the first gas sensor and palladium, indium or rhodium which will react all combustible gases including methane and which is suitable 0 for the second εensor. Of course, other materials could be used to distinguish alternative gases.

In an alternative approach, the variable resistance material of at least one of the sensors includes a dopant to control its reεistivity whereby under working conditions 5 the variable resistance material of the first εensor exhibits a significant change m reεiεtance m the preεence of the gaε or vapour to be detected while the other does not, the two materials exhibiting subεtantially the εame response to the presence of an interfering gaε. C Instead of using different filter layers, a difference between the two εenεorε is achieved by employing differential doping of the variable resistance material m the two elements or alternatively doping the variable reεistance material in only one of the sensors. Thiε 5 differential doping may be uεed with or without filter layers . The material whoεe electrical resistance varies will typically be a semiconductor material such aε tin oxiαe or zinc oxide but could be a non-εemiconductor material. In_deed, any material whoεe resistivity changes m response to the presence of gas or vapour, for example an electrically conducting polymer, could be used sucn aε polypyrrole, phthallocyanme etc.

Certain catalytic additives may be included in this material to improve its sensitivity. Each sensor of the assembly may be operated independently with different voltages being applied across the respective electrical conductors to obtain optimum performance or the sensorε may be connected in reεpect ve arms of a Wheatεtone Bridge. Some examples of sensors and sensor assemblieε according to the invention will now be described with reference to the accompanying drawings, m which: -

Figure 1 is a schematic view of a first example -cf a senεor according to the invention; Figure 2 illustrates an example of a gaε sensing assembly according to the invention;

Figure 3 is a circuit diagram of a monitoring circuit for use with the aεεembly shown m Figure 2;

Figure 4 is a second example of a gas sensing asεembly;

Figure 5 lllustrateε graphically tne effect of operating voltage on the mean response to methane of semistors doped with varying levels of antimony;

Figure 6 lllustrateε graphically tne effect of operating voltage on the mean response to water vapour of semistors doped with varying levels of antimony;

Figure 7 is a circuit diagram for use with a gas sensing assembly according to the invention;

Figure 8 illustrates graphically the response to methane of paired differentially doped semistors in which the first sensor is doped with 0.5% Sb₂0₃ and the second sensor is doped with 4.0% Sb₂0₃; 10

Figure 5 lllustrateε graphically the metnane response of Pt and Ir εhell coated semiεtorε (3% w/w composition^ operated either independently using 20f. fixed reference resistors, or aε a sensor/compensator pair using a test gaε

5 concentration of lOOOppm in dry air;

Figureε 10A and 10B illustrate grapnica_.lv tne c eaπ air case ne variation with humidity for Al₂0₃/Pt (3% w/w) or Al₂0,/Ir (3% w/w) εhell coated semiεtorε operated either with a fixed 20Ω reference resistor or as a matched pair, C with sensor behaviour shown at input powers of l37mW (Figure 10A) and 173mW (Figure 10B) respectively;

Figures 11A-11C illustrate graphically the methane response as a function of humidity for a Al₂0₃/Pt (3% w/w) shell coated semistor operated with a Al₂0₃/Ir (3. w/w, 5 shell coated compensator at an input power of 137mW (Figure 11A) and 173mW (Figure 11B) and 192mW (Figure 11C) respectively; and

Figure 12 illustrates a pair of differentially ooped semistors . 0 Figure 1 lllustrateε an example of a combustible gaε εensor for sensing methane. The device is essentially a semiεtor comprising a platinum reεiεtance wire ce l 1 embedded in a bead of semiconducting material 2, m thiε case Sn0₂ doped with 0.5% w/w Sb₂0₃. The bead 2 is coated 5 witn a layer of alumina 3 which m turn is coated with an oxidation catalyst layer 4. In an alternative approach, not shown, the oxidation catalyst could be mixed with tne alumina. More generally, the shell coating 3 could comprise any inert refractory base including alpha alumina, 0 gamma alumina, silica, zircoma and the like. The semiconductor bead 2 has a typical diameter of about 1mm or less while the coating 3,4 haε an overall thickneεε up to 0.2mm.

In use, the semiεtor εhown m Figure 1 will be used 5 with a second semistor having substantially the same construction but with the oxidation catalyst being chosen so that the pair of semistors enable a particular 11

compuεtible gaε such aε methane to be detected. For example, m the case of methane, the first senεor may be provided with a platinum oxidation catalyεt which will oxidise most combustible gaseε at the ε el_^ wniie permitting methane to paεε through the εhell so aε to interact with the bead 2. The second sensor will be provided with an indium or rnodium catalyεt m the εnell which will react all the combustible gaseε including methane. Furthermore, it has been found that these noble metalε have subεtantially the εame response to water vapour, which is the primary interfering vapour, thus both εenscrε will react m a similar way tc tne preεence c: any water vapour allowing such water vapour to be eliminated. If neceεεary, the two semiεtorε can be operated at slightly different working temperatures to reduce farther any differences between their response to water vapour. The sensorε will typically be operated at temperatures in the range 400-500°C.

In operation, if no methane is preεent , both semistors will generate substantially similar signals since all combustible gaseε present will be oxidiεed on the surfaces of the εhell coatε while humidity will have the εame effect on both εenεorε . In the presence of methane, however, that methane will be allowed to pass into the oeao 2 of tne first εensor where it will react m a conventional manner causing a variation m resistance which can be determined. The methane will not pass into the bead 2 of the second senεor and sc a differential signal will be detected indicative of the presence of methane. Figure 2 illustrates an example of a gas sensing assembly in which each εensor 10,11 lε m tne form of a layered εtructure . The εensor 10 comprises a substrate 12 made of a sintered refractory material such as alumina or silica on which is provided a platinum resistance electrode m planar form 15 which is covered by a thick film of a semiconductor material 13, in this example Sn0₂ doped witn 1% Pd. This layer 13 is then covered by a filter layer 14 12

comprising a homogenous mixture cf alumina ano 3- Ft constituting a εhell coat similar to the shell coat 3 , 4 of Figure 1.

The sensor 11 has a substrate 12' similar to the Ξ εubεtrate 12, a thick film layer 12' similar to the layer 13, a planar electrode 15', and a shell coat or filter layer 14' . The layer 14' haε an alumina base but in this case mixed with 3% Ir instead of Pt . Aε m the previous example, this difference in the εneli coat layer 14,14' 0 leadε to the methane specificity with the sensor 10 reacting to the presence of methane while methane is oxidised on the εurface of the εhell coat 14' of the εensor

11.

Figure 3 illustrates a typical circuit m which the 5 sensors of Figure 2 would be located. The two sensorε 10,11 are mounted m serieε with a 5 volt εource 16 along with an ammeter 17. The voltage V_c across the senεor 1C lε then monitored. In the absence of methane, V_£ will remain constant but m the presence of methane, the resistance of 0 the sensor 10 will vary causing a variation m the voltage

Figure 4 lllustrateε a preferred assembly using semistors with platinum resistance coils instead of planar electrodes. Thus, as shown m Figure 4, first and second 5 sensors 20,21 are provided, each sensor having a platinum coil 22,22' encased m a bead 22,23' of semiconductor material. In this case, the material is Sn0₂ doped with 0.5% w/w Sb₂0₃. The εensor 20 is provided with a shell coat or filter layer 24 comprising alumina and 3% Pt while tne 0 sensor 21 is provided with a shell coat 24' of alumina and 3% Ir. The response of these senεorε 20,21 will be similar to tnat of the εenεorε 10,11.

Each sensor 20,21 and the sensor pair together nas been tested experimentally. In this case, the sensor 20 is 5 referred to as the "sensor" and the sensor 21 as the "compensator". In each case, the sensors were tested in a pellistor-style bridge arrangement both together, and 13

individually with a fixed reεistor substituted for the other εenεor. The reεultε are shown in Figure 9. It car be εeen that the "εenεor" 20 haε a εignificant sensitivity to methane at about 150mW while the compensator 21 is Ξ εubεtantially insensitive.

Figures 10A and 10B show tne clear a r baseline responses to humidity of the detector and compensator sensorε 20,21, both separately m a fixed resistor network, and together m a "pair-bridge", at two power levels of C 137mW (approximately optimum sensitivity) anc 173mW (aoove optimum sensitivity power level) respectively. The responses are very similar so that when operated together, tne difference between the signalε lε εubεtantially zero at all applied voltageε. 5 Figureε 11A, 11B and 11C depict the humidity effect on clean air baseline and methane εensitivity (at 100 and lOOOppm) , of the pair-bridge, at tne three power ieve_s cf 137mW, 173mW and 192mW respectively. Humidity effect--? are greatly reduced in all three caseε and virtually eliminated 0 at the 192mW power level.

In the exampleε described so far, methane specificity was achieved by suitable choice cf shell coating. This approach of pair, shell -coated devices is advantageous m situations where a methane-specific response is required 5 such aε methane pipeline leak detection. In situations where a non-specific combustible gaε measurement is required, it would be posεible to operate a pair m which the detector has no shell coat thus allowing all combuεtible materialε to acceεε the semiconductor bead, and C the compensator with a shell coat containing suitable catalysts such as Pd, Ir, Rh, to prevent access of all combustible gaεes - this has yet to be confirmed m practice .

Figure 12 illustrates an alternative asεembly m whicr. 5 a pair of semistors 30,31 are provided each comprising a platinum resistance wire coil 32,32' embedded m a bead of εemiconductive material 33,33' (the coil could be replaced 14

by a planar electrode aε in Figure 2, . In this case, the two sensorε differ by virtue of their beadε 33,33' being dif erentially doped with a εuitable oopant εucr aε antimony or biεmuth. Aε will be explained below, it haε been found that by choosing suitable dopant levels, the two semistors can be arranged such that under worκmg conditions, they will exhibit substantially the same response to water vapour but a different response to a combustible gaε to be detected such as methane. Figure 5 illustrates the response of semistorε similar to the semistor 30 m Figure 12 with different αocinc levels to applied voltage across the platinum wire coil 32. Eacn semistor was located on an arm of a Wheatstone Bridge, the opposite arm including a fixed resistor. For example, a line 40 relates to a Pead with 4% w/w Sb₂0₃ doping. A line 41 corresponds to 0.5% w/w dopant and it will be seen that a peak response is achieved at about 1.4V. Each semistor comprised Sb₂0₃ doped Sn0₂ on a 9 turn, 15 micron diameter Pt wire coil. A test gaε of lOOOppm CH₄ in air (1000 cc/rain flow rate) was used at relative humidity (RH) = 0% and temperature = 20°C.

Figure 6 is similar to Figure 5 but lllustrateε the response of each of the senεorε uεed above to water vapour. In thiε case, it can be εeen that a εenεor with 0.5% w/w 5 dopant is generally insensitive to water vapour at all voltages with a general decline m sensitivity at higher voltages (line 43) while a senεor with 4% w/w oopant naε a higher sensitivity at low voltage which drops more steeply to coincide with the 0.5% w/w dopant sensor at higher C voltages (line 44) . The test gaε was BTCA 74 Air (1000 cc/mm flow rate) , RH = 96%, temperature = 20°C.

A comparison of Figures 5 and 6 shows that by using a 0.5% dopant level for the sensor 30 and a 4% dopant level for the sensor 31 while operating at 1.8V will result m a 5 combined aεεembly m which the sensor 30 is sensitive to methane while the sensor 31 is substantially insensitive to metnane and both sensors have substantially the εame (low) 15

sensitivity tc humιdιt_ . This a±iowε Jooth sensorε tc oe operated at εubεtantially the same voltage.

In general, the traditional Wheatstone Bridge aε uεed with pellistor εensorε is not suitable for differentially dopeo εemiεtorε . Thiε lε because such semistorε will nave significantly different intrinsic reεiεtanceε and the Wheatεtone Bridge circuit will inevitably result in substantially different operating voltages acroεε the elements. For this reason, a circuit of the type shown m Figure 7 is used. Figure 7 illustrates an example of a circuit for uεe with differentially doped semistor pairs εucn aε the εemiεtorε 30,31. In thiε circuit, the ceils cf tne two semistors 30,31 are electrically connected to a voltage supply level 50 set at, for example, 1.8V aε described above, the other εide of each coil 32,32' being connected to respective potentiometers 51,51', the potentiometers neing connected tc ground. In addition, each coil 32,32' lε connected to a reεpective amplifier 52,52' whose other inputs are connected to a potentiometer 52. The output of each amplifier 52,52' is applied to a differential amplifier 54 to obtain a net output εignal . A potentiometer 55 is provided to zero the output at startup.

The potentiometer 52 alters the gam of the net output signal from the amplifier 54.

Although the circuit εhown m Figure 7 haε to be used with differentially doped semistorε for the reasons given above, it also provides a number of further advantages and degrees of freedom. Firstly, the series potentiometers 51,51' can be used to introduce intentional differences between the operating voltageε of the elements. These may be uεeful to obtain fine tuning of particular compensation effectε, without accepting the large and uncontrolled offεetε which inevitably reεult when using a Wheatstone Bridge circuit with elements of substantially different resistance . 16

Secondly, it is clear that this potential advantage applies equally to elements witn intrinsically similar reεiεtanceε aε well aε those with different resistances. Thuε , we may opt to utilise the Figure 7 circuit m E preference to a Wheatεtone Bridge circuit even though it lε not strictly necessary to do so.

Thirdly, the arrangement m Figure 7 can be extended beyond the use of two elements. Thuε, if three semistors, differently sensitive to species α, β and 7, are operated C in thiε arrangement, it lε a trivial matter to use standard microprocessor or discrete electronic processing methods to deconvolve the εignalε due to each individual component. This argument can be extended to greater numbers of elementε using the usual methods employed in chemical 5 sensing arrays.

Figure 8 illustrates the response of the assembly shown in Figure 7 to varying methane concentrations for two pairs of sensorε. In this case, each sensor comprised Sb₂0₃ doped Sn0₂ on a 9 turn, 15 micron diameter Pt wire coil. 0 The detector was 0.5% Sb₂0₃/Sn₂ and the compenεator waε 4.0% Sb₂0₃/Sn₂.

Although thiε description haε concentrated on metnane reaction on antimony doped, tin oxide, semistor elementε, the general principle of differential oxide doping to 5 produce detector/compensator pairs is applicable tc the detection of other combustible gaseε, using other semiconductor and dopant materials, m both semistor and normal semiconductor configurations. The use of shell coated εtructureε, m combination with differential doping, 0 provideε an added dimenεion to forming detector/compenεator pairs with great selectivity and stability.

Claims

17 CLAIMS

1. A gaε or vapour εenεor comprising an electrical conductor m contact with material whose electrical resistance varies, under working conditionε, in the preεence of a gaε or vapour to be detected, the electrical conductor carrying a current m use to raise the temperature of the material to an operating temperature; and a filter layer supported on and surrounding the variable resiεtance material, the filter layer allowing only a gaε or vapour to be detected to pass to the variable resistance material, whereby a change in resistance of the material is determined by monitoring the voltage across the electrical conductor.

2. A gaε or vapour εenεing assembly comprising a first gas or vapour senεor including an electrical conductor in contact with a material whoεe electrical resiεtance vaπeε, under working conditionε, in the presence of a ga-s or vapour to be detected, the electrical conductor carrying a current in use to raise the temperature of the material to an operating temperature; and a second sensor having a similar construction to the first, wherein the senεors are fabricated and/or operated under suitable working conditionε εuch that at εubstantially all times the sensors respond differently in the presence of the gas or vapour to be detected but m εubstantially the same manner to other interfering gaεeε whereby the preεence of tne said gas or vapour can be detected.

3. An assembly according to claim 2, wherein at least the first gaε εensor is constructed in accordance with claim 1.

4. An asεembly according to claim 3, wherein both sensors are constructed in accordance with claim 1, the filter layers of the sensors being different.

5. An assembly according to claim 4, wherein the second sensor is εubεtantially insenεitive to the gaε or vapour to be detected. 18

6. An aεsembly according to claim 5, wherein the filter layer of the second εenεor preventε all gaεeε or vapourε of a type to which the εensor would otherwise be sensitive from reaching the variable reεiεtance material cf tnat εenεor.

7. An aεsembly according to any of claims 2 to 6 , wherein each sensor haε εubεtantially the same sensitivity to an interfering gaε, such aε water vapour.

8. An assembly according to any of claims 2 to 7 , wherein the variable resistance material of at least one of the εenεorε includes a dopant to control its reεiεtivity whereby under working conditionε the variable reεiεtance material of the firεt εensor exhibits a significant change m reεiεtance m the presence of the gas or vapour to be detected while the other does not, the two matenalε exhibiting εubstantially the same response to the presence of an interfering gaε.

9. An assembly according to claim 8, wherein the variable resistance materials of the εenεorε are differentially doped.

10. An assembly according to any of claims 2 to 9, wherein different voltages are applied across the electrical conductor extending through the sensorε .

11. An assembly according to any of claims 2 to 9, wherein the sensors are connected m respective arms of a

Wheatstone Bridge to enable changes m resistance of the variable resistance materials to be monitored.

12. A sensor according to claim 1 or an assembly according to any of claims 2 to 11, wherein the variable reεiεtance material of the or each sensor comprises a base material formed by a semiconductor oxide such as tin or zinc oxide.

13. A sensor according to claim 1 or an assembly according to any of claims 2 to 12, wherein the variable resiεtance material is doped with n or p type material.

14. A sensor assembly according to claim 13, wherein the dopant compriseε Antimony or Bismuth oxide. 19

15. A εensor according to claim 1 or an asεembiy according to any of claimε 2 tc 14, wherein the gaε or vapour to be detected iε a combustible gaε or vapour.