A Microprocessor-Controlled Nitrogen-Dioxide Sensing System

The electrical conductivity of semiconducting phthalocyanine films shows excellent sensitivity to electron-acceptor gases and in principle provides the basis for a low-cost, low power-requirement, compact sensing system for nitrogen dioxide. However, such films suffer a number of disadvantages, including slow response and reversal, reduced response in humid conditions and, in some cases, chemical reactivity following exposure to high NO2 concentrations or for long periods. We now report the development of a microprocessor-controlled sensing system which has been shown to minimize the influence of these disadvantages. The instrument operates on the principle of controlling the sampling and sensor operating conditions so as to maintain reproducible kinetics, optimum conditions for minimizing humidity effects and minimal risk of reaction with NO2. The results of experiments designed to determine these optimum operating conditions will be summarized and the construction and evaluation of a prototype instrument incorporating these features will be described.