DC and Low-Frequency Humidity Dependence of a 20 pF Air-Gap Capacitor

We have measured both the 0.01 Hz (quasi-dc) and 1.0 kHz values of a 20 pF room-temperature air-gap capacitor as a function of the environment. Its dependence on pressure and temperature is found to be small, in contrast to a significant effect of relative humidity. Moreover, this humidity dependence greatly differs for the two measurement frequencies, both in sign and magnitude. This indicates that the surface layers of adsorbed water have a significant effect on the frequency dependence of the capacitance value of air-gap capacitors.