Frequency counting interrogation techniques applied to gas sensor arrays

This paper presents a simple, elegant interrogation technique for conducting polymer gas sensor arrays which takes advantage of their resistive and capacitive properties. To accomplish this, a four-channel Wien bridge oscillator system has been designed and constructed in which the conducting polymer sensor forms one of the arms of the bridge circuit. As gas or odour molecules adsorb and desorb from the sensor surface, the resistance and capacitance changes cause a consequent change in the Wien bridge oscillator frequency which is readily measured. Results of experiments using several volatile species are reported herein. Methods of improving the sensitivity of the oscillator-based approach have also been investigated and preliminary results of these investigations are also reported. The approach that has been adopted is applicable to any other form of sensor where the electrical properties change in response to a stimulus. Many other forms of phase shift oscillator can also be employed. (C) 1999 Elsevier Science S.A. All rights reserved.