Relative humidity effects on rectangular waveguide thermistor sensors

A correlation between the relative humidity and the effective efficiency of several rectangular waveguide thermistor sensors has been observed. The difference between effective efficiency measurements made at 10 % and 30 % relative humidity can be 10 times the k = 1 reproducibility uncertainty of these devices. The time scale for the effective efficiency change is several days. The effect appears to be reversible. It has not been observed in any of NIST's coaxial calorimeter standards.