HUMIDITY SENSING CHARACTERISTICS OF A LIMITING CURRENT-TYPE PLANAR OXYGEN SENSOR FOR HIGH-TEMPERATURES

A limiting current type planar oxygen sensor with a unique structure has been developed. This new sensor has a cathode and an anode of Pt on the same plane as the ZrO2 electrolyte, and the cathode also serves as a hole to limit gas diffusion. Studies was made to verify the humidity sensing characteristics of this sensor at temperatures over 100-degrees-C. We found that the ratio of the I(L2)/I(L1) (the ratio of the 2nd limiting current to the 1st limiting current) to the water vapor pressure was constant. Utilizing the constancy of the ratio, we can now measure humidity accurately regardless of the sensor deterioration or the performance variation among the sensor tips. It detects humidity linearly proportional to a water vapor pressure of 0 to 500 mmHg in a wide operation temperature range from 20 to 300-degrees-C. Additionally taking advantage of the planar structure of this oxygen sensor, a multi-functional sensor was easily obtained by arranging an oxygen sensor and a water vapor sensor on the same plane.