GB2169715A

GB2169715A - Gas sensors

Info

Publication number: GB2169715A
Application number: GB08600907A
Authority: GB
Inventors: Trevor Kim Gibbs; Howard Alfred Buckenham; Paul Gotley
Original assignee: Neotronics Ltd
Current assignee: Neotronics Ltd
Priority date: 1985-01-16
Filing date: 1986-01-15
Publication date: 1986-07-16
Also published as: GB2169715B; GB8501039D0; GB8600907D0

Abstract

A gas sensor having a gas diffusion barrier (12), a sensing electrode (16), a sensing circuit giving an output signal in accordance with the amount of gas sensed by the sensing electrode (12) and a thermistor (18) that is connected to the sensing circuit to compensate the output signal for variations in ambient temperature. The thermistor (18) is located in the space between the gas diffusion barrier (12) and the sensing electrode (16) and preferably in contact with the latter. Such an arrangement improves the temperature compensation to the sensor output signal provided by the thermistor. One terminal 23 of the thermistor is arranged outside the sensor so that it can be connected to the sensing circuit manually when the sensor is ready for use and not during storage. <IMAGE>

Description

SPECIFICATION Gas sensors The present invention relates to gas sensors, e.g. gaseous oxygen sensors and especially to gas sensors having a gas diffusion barrier limiting the amount of gas reaching the sensing electrode.

A well-known problem of gas sensors of this type is that the electrical output of the sensor vanes with changes in the ambient temperature. To solve this problem, it is known to connect a thermistor permanently into the sensor circuit to vary the output signal of the sensor in accordance with the prevailing temperature.

Hitherto, when thermistors have been used, they have been embedded in the housing wall of the sensor or they have been located underneath the sensing electrode in the cell electrolyte. However, in the course of our researches, we have found that there is a timelag between a change in temperature and the resulting thermistor compensation. Thus there is a hysteresis in the output of the sensor when there is a temperature change since the change in the output signal from the sensing electrode due to the temperature change either precedes or lags behind the compensation brought about by the thermistor.

It is an object of the present invention to improve the temperature compensation of the thermistor.

We have found that the variation in electrical output of a sensor with temperature can be greatly reduced if the thermistor is located in the space between the sensing electrode and the diffusion barrier. This is surprising because until now it was believed according to one school of thought (see for example British Patent Specification No. 1571282) that the temperature-dependent nature of the sensor output was due to the variation of the rate of transport of gas through the diffusion barrier and hence dependant on ambient atmospheric temperature; it was for this reason that the thermistor has been located on the sensor housing near to the diffusion barrier.According to a second school of thought, it was believed that the transformation of the gas into a dissolved ionic species at the catalyst of the sensing electrode was the rate-determining step and thus the step that was most affected by temperature changes; the thermistor was thus located in contact with the catalyst on the electrolyte side of the sensing electrode.

In accordance with a first aspect of the present invention, there is provided a gas sensor comprising a diffusion barrer, a sensing electrode located below the diffusion barrier and separated therefrom by a space, a sensing circuit which provides an output signal in accordance with the amount of gas sensed by the sensing electrode, and a thermistor connected or connectible to modify the output signal in accordance with the temperature it senses, which themistor is mounted inside the sensor in the space between the diffusion barrier and the sensing electrode.

More specifically, the thermistor is preferably located in contact with the gas phase side of the sensing electrode. The sensing electrode is preferably of the type comprising a layer of catalyst supported on a porous carrier. The diffusion barrier may be one or more porous layers or bodies or one or more capillary passageways. For ease of assembly, the thermistor is preferably mounted on the interior surface of the top cap. The sensor may be of the galvanic- or the polarographic-type.

The incorporation of a thermistor gives rise to further problems in the case of galvanic oxygen sensors because they must be stored in an oxygen-free atmosphere before they are inserted into the gas monitor. This is because the thermistor, which is connected between the two output terminals of the sensor, allows a leak current to pass between the terminals in the presence of oxygen, which consumes the galvanic material. The usual storage atmosphere is nitrogen but it will be appreciated that it is an expensive procedure to package the sensor in a gas-tight container in a nitrogen atmosphere and that it would be must more desirable to store the sensor in air.

It is an object of the present invention to overcome this problem.

According to a second aspect of the present invention this problem is solved by a gas sensor, preferably an oxygen sensor, comprising: a sensing circuit that includes a sensing electrode and first and second output terminals, the sensing circuit providing a signal at the two output terminals in accordance with the amount of the gas sensed by the sensing electrode, and a thermistor having first and second terminals, wherein:: said first sensing circuit terminal is electrically connected to said first thermistor terminal and said second sensing circuit terminal and said second thermistor terminal are capable of being electrically connected together by manual manipulation from outside of the sensor so that the sensor can be stored in a state in which the two said second terminals are not electrically connected but allowing the said second terminals to be electrically connected together when it is desired to operate the sensor.

When being stored, the second sensor circuit output terminal is not electrically connected to the second thermistor terminal and it is thus not possible for the sensor to be short-circuited through the thermistor, therefore making it possible to store the sensor in air instead of in a nitrogen atmosphere with out deterioration of the sensor. When it is desired to use the sensor, the two second terminals are simply electrically connected together and the sensor becomes operational.

Means may be provided for connecting together the two second terminals, for example the connecting means may be a screw, a clip, a switch or a nut on a threaded post. Alternatively, the two second terminals can be electrically connected together by connecting them both to the same connector in a gas monitor.

Gas monitors, which process the output signals of the gas sensors to give a read-out of the amount of gas present, presently have two connectors that, on installation of the sensor in a monitor, are attached to the two output terminals of the sensor. So that it is not necessary to modify the position of the monitor connectors, it is preferred that the second thermistor contact is a flying lead so that it can be connected to the same monitor connector as the second sensor circuit terminal.

The invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of the top of a sensor according to the invention; Figure la is an enlargement of the part of the sensor top shown in the dotted circle in Fig. 1.

Referring to Figs. 1 and la, the sensor has a top cap 10 including a capillary passageway 12. Below the capillary, there is a gas dispersing mat 14 and a sensing electrode 16. The mat 16, which is highly porous, spreads out gas that has passed through the capillary 12 over the whole area of the sensing electrode 16. A thermistor 18 is attached to the underneath of cap 10 and has leads 20 and 22; lead 22 passes through a seal 24 in the cap and a tag 23 is attached to the end of the lead 22 while output lead 20 is attached directly to the cap 10.

In use, the sensor is installed in a gas monitor which processes the sensor output. As is known, the sensor has two output terminals, one being in electrical contact with the can (not shown) in which the sensor is housed and the other being in electrical contact with the cap 10. These terminals are attached to corresponding monitor connectors. Lead 22 is connected to the same monitor connector as the can terminal, while lead 20, because it is connected to the cap 10, which is electrically conductive, is connected to the cap terminal.

In this way, the thermistor is connected across the output terminals of the sensor.

Claims

1. A gas sensor comprising a diffusion barrier, a sensing electrode located below the diffusion barrier and separated therefrom by a space, a sensing circuit which provides an output signal in accordance with the amount of gas sensed by the sensing electrode, and a thermistor connected or connectible to modify the output signal in accordance with the temperature it senses, which themistor is mounted inside the sensor in the space between the diffusion barrier and the sensing electrode.

2. A gas sensor as claimed in claim 1, wherein the thermistor is in contact with gas phase side of the sensing electrode.

3. A gas sensor as claimed in claim 1 or claim 2, which includes 8 top cap and wherein the thermistor is located on the interior surface of the top cap.

4. A gas sensor as claimed in in any one of claims 1 to 3, which includes a gas dispersing layer located in the space between the diffusion barrier and the sensing electrode.

5. A gas sensor as claimed in any one of claims 1 to 4, wherein: the sensing circuit has first and second output terminals, the thermistor has first and second terminals, said first sensing circuit terminal is electrically connected to said first thermistor terminal and said second sensing circuit terminal and said second thermistor terminal are capable of being electrically connected together by manual manipulation from outside of the sensor so that the sensor can be stored in a state in which the two said second terminals are not electrically connected but allowing the said second terminals to be electrically connected together when it is desired to operate the sensor.

6. A gas sensor as claimed in claim 5, wherein the second sensor circuit terminal and the second thermistor terminal are both located on the outside of the sensor.

7. A sensor as claimed in claim 6, wherein the second sensor circuit terminal and the second thermistor terminal are both capable of being attached to the same connector in a gas monitor.

8. A gas sensor as claimed in claim 5, which includes connecting means for electrically connecting the second thermistor terminal to the second sensing circuit terminal.

9. A sensor as claimed in claim 8, wherein the connecting means is selected from the group consisting of a screw, a clip, a switch and a nut on a threaded post.

10. A sensor as claimed in any one of claims 1 to 9, which is an oxygen sensor.

11. A gas sensor comprising: a sensing circuit that includes a sensing electrode and first and second output terminals, the sensing circuit providing a signal at the two output terminals in accordance with the amount of the gas sensed by the sensing electrode, and a thermistor having first and second terminal, wherein: said first sensing circuit terminal is electri cally connected to said first thermistor terminal and said second sensing circuit terminal and said second thermistor terminal are capable of being electrically connected together by manual manipulation from outside of the sensor so that the sensor can be stored in a state in which the two said second terminals are not electrically connected but allowing the said second terminals to be electrically connected together when it is desired to operate the sensor.

12. A gas sensor as claimed in claim 11, wherein the second sensor circuit terminal and the second thermistor terminal are both located on the outside of the sensor.

13. A sensor as claimed in claim 11, wherein the second sensor circuit terminal and the second thermistor terminal are both capable of being attached to the same connector in a gas monitor.

14. A gas sensor as claimed in claim 11, which includes connecting means for electrically connecting the second thermistor terminal to the second sensing circuit terminal.

15. A sensor as claimed in claim 14, wherein the connecting means is selected from the group consisting of a screw, a clip, a switch and a nut on a threaded post.

16. A sensor substantially as hereinbefore described with reference to and as shown in Fig. 1 or Fig. la of the accompanying drawings.