Effect of Pt Oxidation on Pt Resistance Thermometry

Abstract

The effect of Pt oxidation on the electrical resistance of four different Pt resistance thermometers is investigated. It is shown that this process can cause large, undesirable resistance changes in these thermometers in spite of the low partial pressures of oxygen contained within them. Two different types of Pt oxide are identified; both have their temperature ranges for formation and dissociation within the official operating range of these thermometers on the International Practical Temperature Scale. Changes in the resistance at 0 °C due to the formation of Pt02 can be as large as 9 milliohms, which could cause temperature errors of up to 0.3 °C under some conditions. The rates of resistance change due to oxidation are found to depend strongly on the particular Pt sensor in each thermometer, the operating temperature, and the amount of Pt oxide initially present on the sensor. For the 0 °C resistance these rates can vary from 0 up to as high as 109 microhms per hour. Specific procedures, which greatly reduce the possible temperature errors from this source, are recommended.