Measuring system and process using zero shift compensation circuit

Abstract

A zero compensation circuit particularly is used in an oxygen sensing apparatus and method employing a sensing cell including two pairs thermistors and a source of an inhomogenous magnetic field. One thermistor of each pair is electrically heated and is positioned inside the magnetic field, and the second thermistor of each pair is positioned adjacent to the respective first thermistor of each pair, substantially outside the magnetic field. When oxygen is present in the sensing cell, the thermistors inside the magnetic field generate a gas flow in the direction of the adjacent thermistors of each pair positioned outside the magnetic field, thus tending to reduce the temperature of the thermistors inside the field and increase the temperature of the thermistors outside the field. The thermistors are connected in a measuring bridge circuit including elements for maintaining the temperature of the thermistors at a substantially constant level, thereby providing a signal that can be used to compensate for changes in the thermal characteristics of the background gases. The zero compensation circuit is formed of an inner bridge used to null the effects of background gases on the oxygen measuring circuit. The bridge uses non-linear voltage limiting elements to allow for compensation of more than one background gas.