By varying the control voltages of commercially available zirconia-based automotive exhaust gas oxygen sensors, the concentration of other oxygen-containing molecules such as H_2O and CO_2 can be measured. Typical water concentration sensors that measure relative humidity via swelling of certain polymers have a narrow temperature range of operation (typically < 120°C) and limited accuracy of about ± 0.4% absolute. Initial measurements with modified zirconia oxygen sensors, both in the laboratory and on engine dynamometers, have shown that water vapor concentration in a gas stream can be measured accurately to ± 0.1% over the range from 0-20 vol% H_2O at temperatures up to 800°C.
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