Abstract
The oxidization characteristics of some standard platinum resistance thermometers are discussed on the basis of the Gibbs free energy diagram of the platinum-oxygen ensemble. The oxidization characteristics are estimated from the measurement of the resistance changes at the triple point of water. The resistance changes of thermometers are caused by pure chemical reactions of Pt to PtO2, PtO2 to PtO, and the deoxidization. The oxidized platinum of PtO on the platinum sensing wire does not grow directly from Pt but through PtO2. This hysteresis in the formation of PtO makes the resistance characteristics of the thermometers complicated, particularly in the range above 300 °C.