GB2256055A

GB2256055A - Gas sensor incorporating malfunction testing

Info

Publication number: GB2256055A
Application number: GB9210333A
Authority: GB
Inventors: David Edward Williams
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1991-05-18
Filing date: 1992-05-14
Publication date: 1992-11-25
Anticipated expiration: 2012-05-14
Also published as: DE69209837D1; EP0591240B1; EP0591240A1; GB9210333D0; DE69209837T2; GB2256055B; JPH07500412A; CA2109619C; JP3155275B2; WO1992021018A1; GB9110797D0; CA2109619A1

Abstract

A gas sensor, for detecting the presence of a gas in a gas mixture, comprises two pairs of electrodes (2, 3; 2, 4) mounted on a sensing element (5) either with the electrodes of each pair spaced apart by a different amount, or with the relationship between the electrodes of one pair and the active surface of the sensing element being different in the case of each set. In use, the ratio of the electrical resistances across the two pairs of electrodes is measured, and the measured values of this ratio are compared with a calibration curve indicating how the ratio varies with concentration of the gas to be detected.

Description

Gas Sensors The present invention relates to the detection of: on or more gaseous media in the presence of other gaseous media, and more specifically to sensors for such a purpose which include a sensing elenent made of a semiconducting metal oxide material.

It is well known that the electrical conductivities of metal oxide semiconducting materials are sensitive to the presence of various gases or vapours and can be used in sensors to detect their presence, see for example, GE specification 2,149,120A; 2,19,121A; 2,149,122A; 2,149,123A; 2,1,264A and 2,218,532A and papers "he Tin Oxide Gas Sensor and its applications" J. t7atson, Sensors and Actuators 5(1984) 29-42; "The Detection and measurement of CO using ZnC Single Crystals" T,3. Bott et.al., Sensors and Actuators 5(1984) 65-73; "The role of catalysis in Solid State Gas Sensors" S.J.Gentry et.al., Sensors and Actuators 10(19686) 141-163 and "Selectivity in Semiconductor Gas Sensors" S.R. Morrison, Sensors and Actuators 12(19R7) 425-440; and Electrical Conduction in Solid State gas 5 Sensors J.W. Gardner, Sensors and Actuators 18(1989) 373-387.

All such gas sensors rely on the gaseous medium under observation impinging on a surface of a body of the semiconducting metal oxide material and then undergoing some reaction with it which affects the conductance of the semiconducting metal oxide material, which is detected by means of at least one pair of electrodes which are formed upon the body of semiconducting metal oxide material.

Hence their performance can be affected by the presence of substances, gaseous or otherwise, other than the materials to be detected which affect the surface chemistry of the body of semiconducting metal oxide material. It is important to be able to detect when such spurious effects are occurring.

The present invention is based upon the fact that in a perfectly operating detector, at a given temperature the ratio of the resistance between a first pair of electrodes which form part of the sensor and that between a second pair of electrodes, which also form part of the sensor, but which have a different separation than that between the first pair of electrodes, should vary in a consistent way as a function of the concentration of one gaseous medium, to which the sensor material is adapted to respond, in the presence of another.

If, however, any changes occur in the surface chemistry of the semi conducting metal oxide material such as may be caused by a poisoning material, then the ratio of the two resistances corresponding to various compositions of the gaseous mixture will no longer vary in the same way as before. Hence by making continuous measurements of the ratio of the resistances between the two sets of electrodes and comparing the changes in the ratio of the resistances with a calibration curve, one can distinguish between changes due to real changes in the composition of the gaseous medium under observation and spurious changes due to changes in the performance of the sensor.

According to the present invention there is provided a method for determining the presence of a first gaseous medium in a second gaseous medium, comprising the operations of: 1) Exposing to the second gaseous medium an active surface of a body of electrically conducting material the properties of which are sensitive to the presence of the first gaseous medium in the second gaseous medium and which forms part of a gas sensor, 2) measuring as a function of time the ratio of the electrical resistances between a first set of electrodes and a second set of electrodes formed on the body of electrically conductive material, the spacings between the sets of electrodes or their position in relation to the said active surface of the body being different 3) comparing the measured values of the said ratio of the resistances beteen the two sets of electrodes with a calibration curve indicative of the variation of the said ratio with the concentration of the first gaseous medium in the second gaseous medium thereby to determine both t composition of the gaseous medium and to detect any malfunctioning of the sensor.

Also according to the present invention there is provided a gas sensor device for determining the presence of a first gaseous medium in a second gaseous medium, comprising, a body of an electrically conducting material the conductivity of which is sensitive to the presence of the first gaseous medium in the second gaseous medium, a first set of electrodes formed on the body of electrically conducting material, a second set of electrodes formed on the body of electrically conducting material the distances between the electrodes of the first and second sets of electrodes or the relationship between the electrodes of the first set of electrodes and an active surface of the body of electrically conducting material being different from that of the electrodes of the second set of electrodes and means for determining the ratio of the resistances between a pair of electrodes of the first set and that between a pair of electrodes of the second set.

In a preferred embodiment of the invention, the pair of electrodes used are constituted by a common electrode situated assymetrically bewteen two outer electrodes. This arrangement is applicable to both planar and cylindrical geometries for the device.

In another arrangement, the electrically conducting body is in the form of a porous disk with a common electrode formed over one planar surface, a central disk electrode on the other planar surface of the disk and an annular electrode surrounding the central disk electrode and concentric with it.

A preferred material for use in the present invention comprises a semiconducting metal oxide ceraric material, which may be in the form of a single such oxide or a mixture of such oxides. Examples of such oxides are tin (Iv) oxide, zinc oxide, tungsten (5JI) oxide and the oxides described in UK Patent Specifications Nos: 2,1A9,120A; 2,149,121A; 2,149,122A; 2,149,123A; 2,166,244A. The above oxides can be made to be catalytic for a combustion reaction for use in the performance of the present invention by providing a thin surface coating of particles of one of the well-known catalytic metals such as Pt or Pd; alternatively, they can be made to be catalytic for a decomposition reaction by providing a coating of a suitable material. A decomposition catalyst may be chosen that is specific to a selected gas. Also, the semiconducting metal oxide material can be chosen to be sensitive to a decomposition product of a selected gas.

Examples of gases the presence of which may be detected by the present invention are hydrocarbons such as methane, ethane, propane, butane, ethylene, benzene and toluene; carbon monoxide; hydrogen; ammonia; hydrogen sulphide; nitrogen dioxide; sulphur dioxide; alcohol vapours such as those of methanol and ethanol; and aldehyde and ketone vapours such as those of formaldehyde, acetone and methyl ethyl ketone.

The invention will now be described, by way of example, with reference to the accompanying drawings in which, Figure l(a) and (b) are sectional and plan views of one embodiment of the invention; Figure 2 (a) and (b) are sectional and plan views of a second embodiment of the invention; Figure 3 is a representation of a third embodiment of the invention; Figure t shows, for the sensor of Figure 1, the variation of the ratio of the resistance measured between widely spaced electrodes to that measured between closely spaced electrodes as a function of gas concentration for reactive and unreactive gases;; Figure 5 shows for the sensor of Figure 2, a three-diensional plot of the ratio of the resistance measured between a common electrode and an inner disk electrode and that measured between the common electrode and an outer ring electrode for the embodiment of Figure 2 as a function both of the concentration of gas and temperature; and Figure 6 is a plot for a planar sensor as shown in Figure 1 of the resistance relative to that in the absence of the gas to be detected, for widely spaced electrodes against that for narrowly spaced electrodes for a sensor which is working perfectly and one which is not.

Peferring to the drawings, a first sensor erbocyin-l t'le invention consists of a gas-impermeable substrate such as a piece of alumina upon which are deposited three electrodes 2, 3 and 4. The central electrode 3 is closer to the electrode 2 than to the electrode 4. A body of semi conducting metal oxide material covers the electrodes 2, 3 and t in part to form a sensing element 5. The sensing element 5 is arranged to be porous and to have a conductivity which is sensitive to a gas to be detected by the sensor. If necessary a catalytic layer can be deposited upon the sensing element 5 to ensure that the said gas either burns or is decomposed so as to cause a change in the conductivity of the material of the sensing element 5.

Figure 2 shows another embodiment of the invention.

Referring to the figure, the sensing element is in the form of a disk 21 of porous semiconducting metal oxide material.

A metal electrode 22 covers one plane face of the sensing element 21. On the other plane face of the sensing element 21 is a central disk electrode 23 and an outer annular electrode 24.

The sensing element 21 together with its electrodes 22, 23 and 24 are sandwiched between two impervious insulating tiles 25 and 26. Contacts 27 and 28 are attached to the edges of the electrodes 22 and 24, respectively, and a contact 29 is attached to the electrode 23 via a hole 30 in the appropriate tile, 25.

Figure 3 shows a sensor of tubular geometry, but in fact it is similar to the sensor of figure 1. In the figures, corresponding elements have corresponding reference numerals. Contact with the electrodes 2, 3 and 4 is made via wires 31,.32 a nd 33 which run inside the tubular substrate 34.

In describing the operation of such devices, it is necessary to introduce to parameters Kp and ; p is measure of the sensitivity of the material of the sensor layer to a given gas and hence its concentration in a gaseous medium under test. KT is a measure of the rate of combustion on the sensor surface of the gas and of diffusion through the sensing element of the gas, or products of its combustion or decomposition. KT is a function of the operating temperature of the device and this gives the opportunity to use one sensor for the detection of different gases in a mixture by varying the operating temperature of the sensor.

Referring to figures 4 and 5, figure 4 shows, or the sensor of figure 1, the variation of the ratio of the resistance measured between the electrodes ? and ss to that measured between the electrodes 2 and 3 as a function (Kp) of the gas concentration; for a reactive gas, such as carbon monoxide, or hydrogen and an unreactive gas, such as methane. Both curves were obtained at the same temperature, hence XT is constant in both cases.

Figure 5, on the other hand, shows for the sensor of figure 2 a three-dimensional plot of the ratio of the resistances between the inner disk 23 and the electrode 22 and the outer annular electrode 24 and the electrode 22 when both Kp, the gas concentration parameter, and T, the gas decomposition rate parameter, vary.

Both figures show that, for unreactive gases (which have a low decomposition parameter KT) the ratio of resistances between the different pairs of electrodes is independent of the gas concentration, whereas for reactive gases (which have a high decomposition parameter KT) the ratio of the resistances between the different pairs of electrodes varies considerably with the concentration of the gas. Any zero drift of the sensor is, of course, cancelled out in the taking of the respective resistance ratios.

If a further number of electrodes of different spacings are used in a planar sensor, or different radial positions in a disk sensor, then taking appropriate ratios will allow the measurement of multiple gases in mixtures to be made, since usually it will be possible, especially if use is made also of the possibility of varying the temperature, that one particular gas of the mixture has a composition gradient extending across the gas sensitive part of the sensor whereas the other gases in the mixture are either uniform in concentration throughout the gas sensitive part of the sensor or have a concentration which falls rapidly to zero at the outer surface of the sensing element of the sensor.

Referring to figure 6, which shows a plot for a planar sensor such as that of figure 1, of the resistance measured between the wider spaced electrodes 3 and 4 against the resistance measured between the closer electrodes 2 and 3 with, as parameter Kp (the concentration of the reactive gas). The value of KT is fixed as the figure refers to a single gas at a fixed sensor temperature. If the sensor is operating correctly, then the resistance between the two pairs of electrodes, as the concentration of reactive gas changes, will move along the line shown. This line may be referred to as "the operating line". If something other than the concentration of the reactive gas changes, then the measured operating point will move off the expected operating line and the operating line of the sensor as measured will change also. Also shown in figure 6 is a measured operating line where the sensing element 21 of the sensor has become poisoned such that in the outer part of the sensing element 21 extending from the surface some fraction of the thickness of the sensing element 21 inwards, the reactive gas does not burn and the conductivity of the material of the sensing element 21 does not respond to the presence of the reactive gas.

Thus should a point defined b measured resistances be found to be off an operating line obtained under perfect conditions as a calibration curve, then there is indicated a change in conditions other than a change in the concentration of the reactive gas. Factors other than the poisoning of the sensitive element which cause such a change include drifts in the zero resistance of the sensor and the presence of reactive gases other than a specific reactive gas.The use of sensors according to the invention and the taking of repeated measurements to derive a measured operating line enables such changes to be distinguished from changes in the concentration of the specific reactive gas for the detection of which the sensor is to be used, hence possibly avoiding false alarms if a sensor is being used to monitor the composition of the given mixture, or indicating when a sensor has become faulty and needs to be changed. The value of the gas concentration would be obtained from the value of one of the measured resistances, or from the ratio of the measured resistances when compared with calibration values.

In circunstances where it is known that progressive sensor poisoning takes place a more elaborate arrangement (not illustrated) enables the progression of the poisioning of the sensor to be followed and a warning given of when it is no longer performing usefully. Instead of two pairs of electrodes being used, there are three pairs of electrodes, of narrow, intermediate and wide spacings. There are now two operating lines, defined by narrow vs intermediate and narrow vs wide electrode spacing resistances. Poisoning of the sensitive element will affect the narrow/wide operating line first and the changes of this operating line will chart the progress of the poisoning. Eventually the narrow/intermediate operating line will begin to be affected. The onset of this change can be used to trigger a warning device.

If three pairs of electrodes are used, then the three resistances define an operating surface instead of an operating line and a measured operating point would move off this surface in the event of poisoning of the sensor.

It is evident that the above statements apply equally to the disk geometry where the inner disk 33 corresponds to the closer electrodes 2 and 3 of the planar geometry and concentric ring electrodes of increasing radius to progressively wider spaced electrodes of the planar geometry.

So far as the material of the sensitive layer is concerned, any of the materials listed above can he used.

A particular material is chosen in relation to a specific reactive gas to be detected. For example, if it is desired to detect methane in air, then tin dioxide is a suitable material for the sensitive layer. Carbon oxide in air may be detected also using tin dioxide for the sensitive layer.

Claims

Claims

1. A method for determining the presence of a first gaseous medium in a second gaseous medium, comprising the operations of: 1) Exposing to the second gaseous medium an active surface of a body of electrically conducting material the properties of which are sensitive to the presence of the first gaseous medium in the second gaseous medium and which forms part of a gas sensor, 2) measuring as a function of time the ratio of the electrical resistances between a first set cf electrodes and a second set of electrodes formed on the body of electrically conductive naterial, the spacings between the sets of electrodes or their position in relation to the said active surface of the body being different 3) comparing the measured values of the said ratio of the resistances beteen the two sets of electrodes with a calibration curve indicative of the variation of the said ratio with the concentration of the first gaseous medium in the second gaseous medium thereby to determine both the composition of the gaseous medium and to detect any malfunctioning of the sensor.

2. A method according to Claim 1 including the operation of measuring as a function of time the ratio of the electrical resistances between the first set of electrodes and at least a third set of electrodes the separation between which are different from those between the first and second sets of electrodes and comparing the measured values of the ratio of the resistances between the first and third or other sets of electrodes with a calibration curve as before so as to determine differences in the composition of the gaseous medium as indicated by the measurements made using different sets of electrodes, thereby to detect any progressive failure of the sensor due to detereoration of the electrical properties of the electrically conducting medium.

3. a gas sensor device for determining the presence of a first gaseous medium in a second gaseous medium, comprising a body of an electrically conducting material the conductivity of which is sensitive to the presence of the first gaseous medium in the second gaseous medium, a first set of electrodes formed on the body of electrically conducting material, a second set of electrodes formed on the body of electrically conducting material the distances between the electrodes of the first and second sets of electrodes or the relationship between the electrodes of the first set of electrodes and an active surface of the body of electrically conducting material being different from that of the electrodes of the second set of electrodes and means for determining the ratio of the resistances between a pair of electrodes of the first set and that between a pair of electrodes of the second set.

4. A gas sensor according to claim 3 wherein the sets of electrodes are formed by a common electrode and a plurality of electrodes situated at different distances from the common electrode.

5. A gas sensor according to claim 3 or clain 4 wherein the electrodes are deposited upon a body of insulating material and are covered by a layer of the electrically conducting material, the said electrically material being permeable by the gaseous mediun.

6. A gas sensor according to any of claims 3 to 5 wherein the body of electrically conducting material is in the form of a lanina.

7. A gas sensor according to any of claims 3 to 5 wherein the body of electrically conducting material is in the form of a cylinder.

8. A gas sensor according to claim 3 wherein the electrically conducting material is in the form of a disk which is permeable to the gaseous medium and there is provided an electrode deposited upon one planar surface of the disk, a central electrode on the other planar surface of the disk and at least one other annular electrode surrounding the central electrode, and both planar surfaces of the disk are covered with an impervious insulating layer.

9. A gas sensor according to any of claims 3 to 8 wherein the electrically conducting material has associated therewith a material which is adapted to catalyse reactions between the gaseous medium under test and the electrically conducting medium so as to enhance changes in the electrical conductivity of the electrically conducting medium due to the presence of the gaseous medium to be detected.

10. A gas sensor according to claim 9 wherein the catalyst is adapted to catalyse a combustion reaction of a constituent of the gaseous medium the presence of which is to be detected.

11. A gas sensor according to clain 9 wherein the catalyst is adapted to catalyse a decomposition reaction of the constituent of the gaseous medium the presence of which is to be detected.

12. A gas sensor according to any of claims 3 to 11 wherein the electrically conducting material is a semi conducting oxide ceramic material.

13. gas sensor according to any of claims 3 to 12 including means for heating the body of electrically conducting material to an appropriate operating temperature.

1. A method for determining the presence of a first gaseous medium in a second gaseous medium substantially as hereinbefore described and with reference to the accompanying drawings.

15. 4 gas sensor for determining the presence of a first gaseous medium in a second gaseous medium substantially as hereinbefore described and with reference to the accompanying drawings.