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It has been observed that several standard platinum resistance thermometers show resistance leakage due to humidity inside their sheath. This insulation leakage depends on the frequency of the Resistance Bridge that is used for the measurement of the platinum resistance thermometers, because of the polarization of the water molecules in the thermometer.

Therefore, a set of standard platinum resistance thermometers has been calibrated between the triple point of mercury and the melting point of gallium with D.C. and a.c. bridges at different frequencies, in order to investigate the polarisation effect due to humidity. At the triple point of water, a larger polarization effect was observed than at the melting point of gallium, while no such effect was observed at the triple point of mercury. This is due to the different values of the water vapor pressure at the temperatures of the calibration fixed points.

The calibration results so obtained are compared and analyzed, and information on the reproducibility and accuracy of the ITS-90 realization in this range is presented.