New applications of tin oxide gas sensors II. Intelligent sensor system for reliable monitoring of ammonia leakages

Cooling plants have to be monitored due to coolant leakages for well-timed signalling if predefined concentration levels are exceeded. Available monitors mostly suffer on long-time stability or insufficient gas identification capability and both give reasons for numerous false alarms. A prototype of an intelligent field suitable ammonia leakage monitor was developed which may overcome this drawback in future. The microprocessor supported hardware operates a sensor head with three tin oxide-based gas sensors simultaneously. Capability of gas identification and concentration determination of TGS-type gas sensors could be considerably improved by periodic variation of the working temperature of the sensor elements, simultaneous sampling of the sensitive layer conductance and application of a rather simple new algorithm for analysis of the resulting conductance-time profiles (CTP). Test results of gas analysis experiments at various ammonia concentrations and humidities based on a (4 x 3) matrix-calibration field are presented. The remarkably good accuracy of ammonia monitoring at various concentrations and humidities and the excellent capability of ammonia identification without recalibration was demonstrated by long-term test experiments over several weeks. (C) 2002 Elsevier Science B.V. All rights reserved.