Sensors and Actuators B 44 (1997) 512–516 An intelligent gas sensing system M.E. Hassan Amrani, Richard M. Dowdeswell, Peter A. Payne *, Krishna C. Persaud Department of Instrumentation and Analytical Science, UMIST, PO Box 88, Manchester M60 1QD, UK Received 6 June 1997; accepted 10 June 1997 Abstract Electrically conducting organic polymers are widely used as a means of gas, odour or aroma analysis using multi-element array techniques coupled with direct current (d.c.) interrogation techniques. Recently it has been established that the use of alternating current (a.c.) interrogation gives rise to improved performance. In addition, the need to use multi-element arrays is much reduced since a single sensor can be interrogated at a wide range of frequencies. This gives rise to much increased information content for the measurements. This paper describes the use of alternating current (a.c.) interrogated conducting organic polymers coupled with neural network pattern recognition techniques for a system to determine the compositional fraction of volatile vapour mixtures. Experiments have been conducted on binary, tertiary and quaternary mixtures of vapours and compositional fractions have been determined to within 5%. © 1997 Elsevier Science S.A. Keywords: Conducting polymers; Gas sensors; Multi-frequency a.c. measurements; Compositional fraction determination 1. Introduction Electrically conducting organic polymers are known to have gas, odour and aroma sensing properties. Ad- sorption and desorption of volatile species causes a measurable change in the conducting polymer direct current (d.c.) resistance and by employing an array of different conducting polymer materials, patterns of re- sistance change can be used to characterise a volatile species [1]. The choice of polymer materials used in an array is based on the concept that each element should have a broad but overlapping specificity to a range of volatile species of interest to the experimenter [2]. Al- though in some cases work on such a measurement approach is still at the research stage, there are a number of commercial instruments now available, one of which uses 32 elements in an array to achieve the necessary instrument performance [3]. We have recently described interrogation techniques based on alternating currents (a.c.) in which a single sensor element can be employed to attain the same degree of performance as that from a multiplicity of elements connected as an array [4,5]. The advantages bestowed by a.c. interrogation derive mainly from the increase in information content that a.c. interrogation delivers. Over a wide range of frequencies, we can elect to measure conductance, capacitance and inductance and we can process these data to obtain derived mea- surands such as the dissipation factor which is defined as: Resistance Absolute value of reactance (capacitive or inductive) The reactance value passes through zero at the sensor circuit resonant frequency and for the structure that we have selected, this will happen twice, once as the reac- tance goes from negative to positive, and then again at a higher frequency going from positive to negative. The slope of the reactance curve with frequency defines the shape of the dissipation factor plotted against fre- quency and this slope is seen to change in quite a marked sense with adsorption and desorption of volatile components on the sensor. Although in theory due to division by zero, the maximum value of dissipa- tion factor in all cases is infinity, in practice, by using simple moving average computation, the difficulties of division by zero are avoided and information concern- ing resonant frequencies and numerical value of the peak of the dissipation factor have been found to give * Corresponding author. Tel.: +44 161 2004883; fax: +44 161 2004879; e-mail: p.a.payne@umist.ac.uk 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S09 2 5 -4 005(97)00 24 0 -2