A chemiresistor sensing device generally contemplates the use of a power supply transmitting current through a sensor which contains a semiconductor material, such as a metal oxide. The semiconductor material behaves as a chemiresistor. A chemical influence can be caused by an ambient gas interacting with the semiconductor material and can be monitored by a change in the resistance or conductance of the material by the use of electrodes which transmit the change in conductance to a monitor or detector, such as a voltmeter or ohmmeter.
A typical chemiresistor sensor has a resistor layer, such as a heater resistor, an electrical connection to the heater, a support layer, such as an alumina substrate, a conductor layer (often composed of interdigitated electrodes) and a deposited chemical sensing layer, for example tungsten oxide is used in U.S. Pat. No. 4,822,465 to detect sulfide. The thickness and manner in which the metal oxide semiconductor material is applied to the sensor electrodes are of importance to the functioning of the sensor, because the depth and microstructure of the metal oxide layer can affect both the selectivity and sensitivity of the tungsten oxide layer to sensed gas. The term "sensed gas" is used herein to designate one or more gases which will cause a change in resistivity of a chemiresistor gas sensor.
Thin film chemiresistor gas sensors have good response times, but are relatively expensive to produce and are subject to relatively rapid failure when used in an aggressive environment in which the sensor material itself is subject to degradation. Thin film sensors using semiconductor materials comprised of oxides, such as WO.sub.3-x, SnO.sub.-x, KNb.sub.3 O.sub.8-x and ZnO.sub.1-x, where x is from 0 to 0.5, have shown good sensitivity for detecting reducing gases, such as hydrogen, anhydrous ammonia, hydrazine, propane, butane, methyl alcohol, ethyl alcohol and hydrogen sulfide. Willis et al., U.S. Pat. No. 4,197,089, discloses an example of a thin film hydrogen sulfide gas sensor. U.S. patent application Ser. No. 677,729, filed Mar. 29, 1991, by Royster et al, U.S. patent application Ser. No. 934,920, filed Aug. 25, 1992, by Royster et al, and U.S. patent application Ser. No. 934,937, filed Aug. 25, 1991, by Marrese et al disclose the preparation of a sensor layer and a hydrogen sulfide gas sensor. Zuckerman, U.S. Pat. No. 4,423,407, describes a gas sensor which can use a thin film or a thick porous layer. Gas sensors having a porous sensor layer have the shortcoming of slower resistivity response times, but are resistant to degradation, because they are thicker than comparable thin films.
Chemiresistor sensors have a further shortcoming that many sensor materials are subject to saturation with sensed gas. Partial saturation may permit continued, but possibly degraded, use; however, irreversible saturation of the sensor material necessitates replacement of the entire gas sensor.
It would therefore be desirable to provide a chemiresistor sensor which can use a thin film, is relatively rugged and can be renewed without replacement of the entire gas sensor and a sensor base and a method for renewing a gas sensor.