The development of a gas detection system utilizing the piezoelectric crystal sorption detector
The development of a dual cell, piezoelectric crystal sorption detector system for the detection of atmospheric ammonia is discussed together with a review of recent studies on other systems utilizing piezoelectric crystal devices. The development of a data handling facility on an Intel 8080 type microprocessor is also described. The microprocessor records the frequency output from both crystals, from which it generates a value for the actual concentration of ammonia. A brief review indicates that the use of microprocessors in such systems is becoming more common. The system has been examined for reproducibility of response in a variety of conditions in order to build up a pattern of performance characteristics with respect to a number of variable factors. Detailed examination of the response of the system to ammonia in conditions of variable humidity, temperature, coating mass, coating area and gas flow rate have been made. As a result of these studies it has been possible to perform in situ data analysis upon responses by computerised correction routines for deviations from "normal" conditions, i.e. dry, humidity ≤ 30% R.H., 25°C and for a constant mass of coating material. In addition to this work, tests have been carried out on samples taken from the effluent gas mixture containing variable amounts of ammonia. Analysis of these samples has given an accurate guide to the actual concentration of the analyte gas mixture. This work has been necessary in order to calibrate the system and provide a comparison for the precision of the piezoelectric crystal sensor system. These data have also indicated the length of time required for the attainment of equilibrium conditions after alteration of the composition of the analyte gas. The detection of atmospheric ammonia down to p.p.b. levels of NH 3 is described together with the variation of the ammonia response in conditions where moisture levels of up to 75% R.H. are experienced. Limited studies of the effects of temperature other than at 25°C have been made and from this it has been shown that both moisture and ammonia responses on both crystals are affected by the alteration in temperature.