Solid-state chemical sensor apparatus and methods

Abstract

In solid-state chemical sensors, such as the highly sensitive and selective sensor for the detection of halogenated compounds comprises a bead of sodium lanthanum fluoride silicate, having the molecular formula NaLa.sub.4 (SiO.sub.4).sub.3 F, the performance and lifetime are adversely affected by a build-up of insulating reaction product between two electrodes. To prolong the lifetime, intermittent sensor operation is achieved by applying a voltage between the sensor electrodes during only a small fraction of each measurement cycle and/or by reversing the voltage in successive measurement intervals and/or by switching the connections between additional reserve electrodes and/or by periodically exposing the sensor to chemically filtered air at regular time intervals during a major fraction of each measurement cycle. The response speed and sensitivity of the NaLa.sub.4 (SiO.sub.4).sub.3 F sensor are improved when platinum black is interspersed with or otherwise added to the bead material during the preparation of the sensor. Improved reproducibility and additional sensing capabilities are also achieved by means of a feedback circuit that assures automated temperature constancy of the sensor bead and, at the same time, permits simultaneous estimation of the concentration of both halogen-containing and halogen-free combustible compounds. Provisions for a jump in the sensor temperature and for exposing the sensor to halogen-depleted air at the end of each measurement cycle minimize the effects of baseline drift and thereby yield higher sensitivity and measurement accuracy.