GB2043913A

GB2043913A - Gas Detector

Info

Publication number: GB2043913A
Application number: GB8002818A
Authority: GB
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1979-02-15
Filing date: 1980-01-28
Publication date: 1980-10-08
Also published as: GB2043913B; DE3005528A1

Abstract

An element whose room temperature electrical resistance is dependent upon the concentration of a reducing gas to which it is exposed. It comprises a major proportion of indium oxide and a minor proportion of platinum black. Gas sensors incorporating the element are also described. Preferably it contains between 2% and 121 DIVIDED 2% of platinum black. It is made by grinding a mixture of indium oxide or an indium salt decomposable to it and platinum black or a platinum compound decomposable to it, baking and sintering.

Description

SPECIFICATION Gas Detectors This invention relates to gas detectors and especially, although not exclusively to gas sensitive resistive elements. The applications for gas sensitive detectors (CO and H2 detectors in particular) are many and varied and range from the monitoring of controlled atmospheres found, for example, in mines or air conditioned buildings, to the monitoring of traffic pollution or the detection of fires. Detectors, sensitive to gas concentrations between 10 and 105 ppm, are / particularly useful since they provide early warning of failure in a system or of a smouldering fire well before visible signs become apparent.

It is known (see for example BF 1526751 or BP 1,257,155) that the electrical resistance of some metal oxides decreases on exposure to a chemical reducing gas (CO, H2,CH4, for example) or to vapours of ethanol or methanol, but this effect is only sufficiently large to be of practical use if the oxide is maintained at a relatively high temperature, typically between 2500C and 5000C. It is known, (see for example BP 1,464,415) that the electricai resistance of an element formed by adding platinum black to stannic oxide (SnO2) is sensitive to carbon monoxide even at room temperature, although for a practically convenient size such an element has an inconveniently high resistance, of the order of megaohms, and moreover is rather sensitive to changes of ambient temperature or humidity.

It is an object of the present invention to provide an improved form of gas sensitive resistive element suitable for use in a gas detector at, or slightly above, the ambient temperature (typically 200C).

According to one aspect of the invention there is provided a gas sensitive resistive element comprising a major proportion of indium oxide In2 03, and a minor proportion of platinum black.

Preferably the element should contain sufficient platinum black to catalyse substantially all the indium oxide with which it is mixed but preferably should not contain substantially more than this amount. It has been found that the element should preferably contain between 2% and 1221% of platinum black expressed as a percentage of the combined weights of indium oxide and platinum black, and between 421% and 621% of platinum black is found to be particularly suitable.

The element may include an encapsulating film of a fluorethylene polymer (e.g. FEP-TEFLON (RTM)) having a thickness of between 0.0005 inches and 0.01 inches, a thickness of about 0.001 inches being particularly suitable. Such films are substantially impermeable to carbon monoxide, methane and the vapors of ethanol and methanol so that an encapsulated element serves as hydrogen sensor.

According to a further aspect of the invention there is provided a gas detector comprising a gas sensitive resistive element of the kind described above and a circuit capable of generating a signal indicative of the resistance of the element. Such a sensor may comprise means for passing current through the element, and means for detecting the voltage drop across the element, and whilst for some practical applications this may be adequate it may be advantageous to provide a circuit adapted to compensate for a temperature dependence of the resistance of the element.

The detector may therefore comprise, in the opposite arms of a bridge circuit, two resistive elements having substantially the same resistance in the absence of a reducing gas, and having substantially the same temperature coefficient, wherein one of the elements is a gas sensitive resistive element in accordance with the present invention and is relatively more sensitive td a detected gas than the other element and wherein a signal detected across the bridge circuit is indicative of a detected gas and is substantially insensitive to change of ambient temperature.

The said other element may be a thermistor or alternatively it may be an element comprised solely of indium oxide or a major proportion of indium oxide and a minor proportion of an indium salt. Such a sensor is responsive to CO, H2,CH4, ethanol and methanol, for example. The said other element may alternatively comprise a major proportion of indium oxide and a minor proportion of platinum black and include an encapsulating film of a fluorethylene polymer having a thickness between 0.0005 inches and 0.01 inches. Since the encapsulating film permits the passage of hydrogen gas only, this combination of elements in the bridge provides a detector suitable for the detection of carbon monoxide, methane and an alcohol, but not hydrogen gas.

It may be advantageous to provide a detector capable of distinguishing a number of different gases and vapours. Accordingly there is provided a selective gas detector comprising a first, encapsulated, gas sensitive resistive element of the kind described in accordance with the present invention, a second gas sensitive resistive element formed of a major proportion of stannic oxide and a minor proportion of platinum black and having an encapsulating film of fluorethylene polymer of a thickness between 0.0005 inches and 0.01 inches, and a circuit associated with each element capable of generating an output signal when the resistance of the associated element falls below a predetermined level, and an output means for receiving output signals and providing a first indication if output signals are received from both elements, and a second indication if an output signal is received only from the second element.

The selective gas detector may also comprise a third gas sensitive resistive element formed of a mixture of indium or stannic oxide and a minor proportion of platinum black, and a further circuit capable of providing an output signal when the resistance of the third element falls below a predetermined level, the said output means being adapted to receive output signals from the three circuits and for providing a first indication if a signal is received from all three elements, a second indication if a signal is received from the second and third elements only, and a third indication if a signal is received from the third element only.

According to a yet further aspect of the invention there is provided a process for making a gas sensitive resistive element of the kind described above comprising the steps of grinding a mixture of a major proportion of indium oxide, or of an indium salt decomposable to said oxide, and a minor proportion of platinum black or a platinic compound decomposable to said minor proportion, and baking said material whereby any indium salt or platinic compound is decomposed and the mixture is sintered.

Preferably the said minor proportion lies in the range 2% to 1 2T% of platinum black expressed as a percentage of the combined weights of indium oxide and platinum black, and between 4T% and 6+% is found to be particularly useful.

In order that the invention may be more fully understood specific embodiments thereof are now described, by way of example only, with reference to the accompanying drawings of which, Figures 1 a and 1 b show two alternative forms of gas sensitive resistive element in accordance with the present invention, Figure 2 shows a typical variation in the resistance of the element an exposure to a chemical reducing gas, Figures 3a, 3b, and 3c show three different gas detectors incorporating the gas sensitive resistive element of the present invention, and Figure 4 shows a gas detector which also incorporates the gas sensitive resistive element of the present invention and is suitable for distinguishing a number of different reducing gases.

The resistive element of one example of the present ivnention is typically prepared by grinding in acetone a mixture of 85.5% In203, 4.5% platinum black, and to aid sintering, 10% clay (kieselguhr, for example). When the acetone has evaporated, the resulting powder is typically mixed with water to form a paste which is then shaped to form a block, rod or disc, and then baked to sinter the material. Preferably the mixture should contain between 2 and 123% of platinum black (expressed as a percentage of the combined weights of platinum black and indium oxide) but between 4T% and 6T% of platinum black is found to be particularly suitable.The mixture may also contain compounds of indium (e.g. indium sulphate, for example) or compounds of platinum (e.g. platinum sulphate) which on baking respectively decompose to indium oxide or platinum.

It has been found that the room temperature (typically 200 C) resistance of an element constituted in this way varies with the concentration of a chemical reducing gas (CO, H2, CH4, ethanol or methanol, for example) to which it is exposed.

A disc shaped element is illustrated in Figure 1 a and in a typical example is about 1 cm in diameter and between 0.4 mm and 0.5 mm thick.

Aluminium or silver contacts, 2 are usually evaporated onto the opposite faces 1 of the disc and suitable leads 3 attached. Alternatively contacts could be provided by painting silver paste onto the surface of the oxide.

In an alternative arrangement the paste, comprised of indium oxide, platinum black, clay and water is coated onto an alumina or quartz substrate and then baked. Figure 1 b shows a tubular substrate 4 upon which the paste 1 is coated. Suitable electrodes 2 and leads 3 are applied as described above. To enhance the sensitivity of the element it is advantageous to provide as large a surface area as possible for exposure to the reducing gas.

An element formed as described above may also be coated with a film of a fluorethylene polymer (e.g. FEP-TEFLON (RTM)) which provides an essentially inpenetrable barrier to carbon monoxide whilst permitting hydrogen gas to pass therethrough relatively unhindered. Such a coated element is also impervious to water vapour, alcohol vapour or CH4,a and so provides a selective hydrogen gas sensor, which is relatively insensitive to changes of humidity. The encapsulating film should preferably have a thickness of between 0.0005 inches and 0.01 inches, although a thickness of about 0.001 inches is found to be particularly suitable.

The resistance of the element may be monitored continuously by passing current therethrough and detecting the voltage drop across it. Figure 2 shows examples of a few typical results produced by monitoring the resistance of an element. These results were produced by introducing a shot of gas (CO gas in the case of curve 2, and H2 gas in the case of curves 1 and 3) into an enclosure accommodating the element.

The resistance of the element was monitored from the instant the gas entered the enclosure.

Initially the concentration of gas in air was about 1% O/o and the resistance of the element was observed to fall steadily. As the concentration of gas descreased, however, due to its diffusion away from the vicinity of the element, the resistance was observed to rise, although the minimum resistance was only reached after a delay, due to the finite response time of the element. Although the initial concentration of gas shown in the examples was about 1% in air, the element is also responsive to concentrations as low as 102 ppm.

Typicaliy an element of the present invention has a resistance of about 1 K whereas an element formed of stannic oxide, Sino2, for example, having the same shape and size has a resistance in excess of 1 O9Q. Curve 1 in Figure 2 was produced by monitoring the resistance of an element of indium oxide and platinum black encapsulated in a fluorethylene polymer (FEP TEFLON (RTM)) and although the sensitivity to hydrogen gas is reduced there is no detectable change of resistance on exposure to carbon monoxide gas.

Although the temperature dependence of the element of the present invention is considerably reduced (approximately half that of an element of SnO2 and platinum biack) it may be desirable to provide compensation for a change in its resistance due to a change of ambient temperature, and examples of suitable circuits are illustrated in Figure 3.

Referring firstly to Figure 3a, the bridge circuit B includes in opposite arms resistive elements, 1 and 2, which have substantially the same resistance in the absence of a reducing gas.

Further resistors, 3 and 4, (each of about 1 OK) are placed in the remaining arms of the bridge and a variable resistor 5 may also be included in one of the arms to finely adjust the balance of the bridge or alternatively to offset the balance, by a known amount to create a threshold level. In one embodiment both elements, 1 and 2 are formed of indium oxide and one of the elements also contains between 42% and 63% by weight of platinum black and is thereby rendered sensitive to a reducing gas (H2, CO, CH4 or an alcohol vapour, for example) at room temperature (typically 200 C). Since both elements are equally sensitive to changes of temperature and/or humidity, however, these effects are substantially canceiled by the bridge circuit and do not contribute to the output signal therefrom.The output signal is almost entirely due to the response of the platinum doped element to a detected reducing gas and is fed to a micropower operational amplifier 6.

In an alternative emebodiment both elements 1 and 2 contain indium oxide and a minor proportion of platinum black, although one of the elements is encapsulated in a film typically 0.001 inches thick, of a fluorethylene polymer (FEP TEFLON (RTM)). Since only the encapsulated element is rendered substantially insensitive to CO gas (an alcohol vapour if present) the bridge circuit generates an output signal only when CO (and/or alcohol) is detected by the unencapsulated element, and so this arrangement serves as a selective carbon monoxide (and alcohol) sensor.

In yet another example, element 1 is comprised of a gas sensitive resistive element of the present invention formed of indium oxide and a minor proportion of platinum black, whilst element 2, in the opposite arm of the bridge, is a thermistor having substantially the same temperature coefficient as element 1. The temperature coefficient is defined as the change of resistance of the element for a unit change of temperature (in OC) and for a typical gas sensitive element is about -2OC. If the resistance of element 1 changes due to a change in the ambient temperature, therefore, this change is substantially cancelled by a corresponding change in the resistance of the thermistor in the opposite arm of the bridge.Any change in the gas concentration, however, is detected only by the gas sensitive element 1 and the resulting unbalance in the bridge is detected by the amplifier 6, as before.

In each of the above described embodiments an alarm circuit may also be provided which is triggered whenever the bridge circuit deviates from the balance condition by more than a predetermined amount. The circuit, indicated in block 10, provides such an alarm and includes a Zener diode, 7. If the input to circuit 10 from the amplifier 6 exceeds the breakdown voltage of the Zener diode the alarm 8 is triggered. Clearly the level at which the alarm is triggered is controlled by suitable adjustment of the variable resistor 6 and by a suitable choice of the diode 7.

If the ambient temperature (and/or humidity) is known to vary more slowly than the concentration of gas then these effects may be substantially eliminated by suitable high pass filtering. Figure 3b illustrates an arrangement to achieve such filtering, the gas sensitive element of the present invention being shown at S, and the components 101 and 102 of the high pass filter 100 being chosen to have a suitably long time constant, typically in excess of 200 seconds, which is sufficient to eliminate expected variations of resistance due to changes of ambient temperature and/or humidity. The output from the amplifier 103 may also be fed to an alarm circuit similar to that shown at 10 in Figure 3a.

The circuit shown in Figure 3c provides a digital method of compensation in which switch, S1, is repeatedly closed for a relatively short period (typically for about 10 mS in every second) so that the signal level at A, which is indicative of the resistance of the gas sensitive resistive element S, is periodically transferred to a capacitor C1 and appears on the side "1" of capacitor C2. The level at C2 drifts slowly up or down as the temperature and/or humidity changes but because a switch S2 is closed, typically every 30 mins, the level is referenced to ground so that the overall drift is small. The signals which pass C2 are compared with a reference voltage Vref to provide an alarm signal after amplification.

It may be desirable to provide a gas detector which is capable of distinguishing each of a number of gases to which it may be exposed.

This is achieved in one embodiment by providing a first element formed of a mixture of indium oxide and a minor proportion of platinum black encapsulated in a film of FEP-TEFLON (RTM), and using this element in conjunction with a second, similarly encapsulated, element formed of stannic oxide and a minor proportion of platinum black. Whilst the encapsulated element of stannic oxide is sensitive to both hydrogen gas and carbon monoxide gas, the encapsulated element of indium oxide is sensitive only to hydrogen gas, so that an output from both elements indicates the presence of hydrogen gas whereas an output from the second element only, indicates the presence of carbon monoxide gas.

In another embodiment a third, unencapsulated, element formed of indium oxide (or alternatively stannic oxide) and a minor proportion of platinum black is provided, and the arrangement may then be used to distinguish more gases. Since the third element is sensitive to H2 gas, CO gas and alcohol vapour, then if all three elements produce an output signal, hydrogen gas has been detected, if an output is produced only by the second and third elements, carbon monoxide gas has been detected, and if only the third element produces an output alcohol vapour has been detected, alcohol vapour being incapable of penetrating the FEP-TEFLON (RTM) film which encapsulates the first and second elements.Each element may be incorporated within a bridge circuit, of the kind described earlier for a single detector, and each bridge circuit may include means for setting a threshold level so that an output is only produced if the detected gas concentration exceeds a predetermined level. The output from each element, or each bridge circuit incorporating the element, is fed to a logic circuit, shown in Figure 4, which is capable of distinguishing possible combinations of signals produced. The numbers 1 to 3 shown at the left of Figure 4 correspond to the respective inputs from the first, second and third elements, the symbols used in the Figure being standard to logic circuits. The circuit is arranged to indicate a particular gas by means an appropriate LED which is only energised if the correct combination of elements produces an output.

The present invention provides a gas sensitive resistive element which is operable at room temperature and moreover has a relatively low resistivity. A range of gas sensors, incorporating the resistive element are also described.

Claims

Claims

1. A gas sensitive resistive element comprising a major proportion of indium oxide, In203, and a minor proportion of platinum black.

2. An element according to Claim 1 comprising between 2% and 123% of platinum black expressed as a percentage of the combined weights of indium oxide and platinum black.

3. An element according to Claim 2 comprising between 43% and 63% of platinum black expressed as a percentage of the combined weights of indium oxide and platinum black.

4. An element according to Claims 1 to 3 including a clay.

5. An element according to Claims 1 to 4, including an encapsulating film of fluorethylene polymer having a thickness of between 0.0005 and 0.01 inches.

6. A gas detector comprising a gas sensitive resistive element according to Claims 1 to 4 and a circuit capable of generating a signal indicative of the resistance of said element.

7. A gas detector according to Claim 6 comprising in opposite arms of a bridge circuit two resistive elements having substantially the same resistance in the absence of a reducing gas and having substantially the same temperature coefficient, wherein one of said elements is an element according to any one of Claims 1 to 4 and is relatively more sensitive to a reducing gas than the other element, and wherein a signal detected across said bridge circuit is indicative of a detected gas and is substantially independent of a change in the ambient temperature.

8. A gas detector according to Claim 7 wherein said other element is a thermistor or an element comprised solely of indium oxide or indium oxide and an indium salt.

9. A gas detector according to Claim 7 wherein said other element is an element according to Claim 5.

10. A gas detector according to Claim 6 comprising a first gas sensitive resistive element according to Claim 5, a second gas sensitive resistive element formed of a major proportion of stannic oxide, SnO2, and a minor proportion platinum black and having an encapsulating film of a fluorethylene polymer of a thickness between 0.0005 inches and 0.01 inches, a circuit associated with each element capable of generating an output signal when the resistance of the associated element falls below a predetermined level, and an output means for receiving said output signals and providing a first indication if output signals are received from both elements, and a second indication if an output signals is received only from the second element.

1 A gas detector according to Claim 10 comprising a third gas sensitive resistive element formed of a mixture of indium or stannic oxide and a minor proportion of platinum black, and a further circuit capable of generating an output signal when the resistance of the third element falls below a predetermined level, the said output means being adapted to receive output signals from the three circuits and for providing a first indication if signals are received from all three elements, a second indication if signals are received from the second and third elements only, and a third indication if a signal is received from the third element only.

1 2. A process for making a gas sensitive resistive element according to Claim 1 comprising the steps of grinding a mixture of a major proportion of indium oxide, or an indium salt decomposable to said major proportion of indium oxide, and a minor proportion of platinum black or a platinic compound decomposable to said minor proportion of platinum black and baking said material, whereby an indium salt or platinic compound is decomposed and the mixture is sintered.

13. A process according to Claim 12 including adding a clay before baking.

14. A process according to Claims 12 or 13 wherein said minor proportion of platinum black lies in the range 2% to 122% expressed as a percentage of the combined weights of indium oxide and platinum black.

15. A process according to Claim 14 wherein said minor proportion of platinum black lies in the range 43% to 6T% expressed as a percentage of the combined weights of indium oxide and platinum black.

1 6. A gas sensitive resistive element substantially as hereinbefore described.

1 7. A process for making a gas sensitive resistive element substantially as hereinbefore described.

18. A gas detector substantially as hereinbefore described by reference to and as illustrated in any one of Figures 3 and 4 of the drawings.