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Abstract

A zirconium oxide shell forms a chamber of fixed volume, and a controller monitors voltage across the shell indicative of difference between inside and outside oxygen levels, applying a signal to pump oxygen. Pumping is effected to empty the chamber, to develop an internal reference, for example by comparison to ambient air, for calibration, and thereafter to substantially continuously null the difference, i.e., to track the oxygen level of a sample environment. Integrated pump current yields a measure of the oxygen added to or removed from the chamber. A suitable material for forming the sensor is a mixture of about 91% zirconium oxide and 9% yttrium oxide, which is formed to shape as a paste or slurry and sintered at about 1400.degree. C., and then platinum coated, for example by a CVD process, to form porous electrodes on each surface. The controller may apply a sequence of current pulses, and check the Nernst cell output indicative of .DELTA.P across the wall, until .DELTA.P=0. The controller integrates the applied charge, e.g. number and width of pulses. By operating to maintain .DELTA.P=0, leakage drift is avoided, and required relaxation time between readings becomes negligible. Positive and negative pulses may be tallied in a running sum to determine the present reading, and Nernst cell output may be applied as a feed forward signal to the controller, which determines and then applies the number of pulses or amount of charge needed to again null the output, while the processor adds the present differential as a correction to the last stored reading to provide the oxygen level as an immediate output.

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