Abstract
In the exposure test of plastics capable of absorbing water, data on the true relative humidity of specimen surface is important and necessary to consider the effect of water content on the photodegradation. Equations for obtaining the true relative humidity of specimen surface during light exposure test of plastics are derived using the measure ment data on absolute humidity of the air and on specimen surface temperature. The absolute humidity sensor element utilizes the temperature difference of two hot thermistor beads, one in dry air and one in wet air, caused by the difference of conductivity of air. Data of relative humidity of ambient air may also be useful to obtain the truerelative humidity of specimen surface, but data of ambient temperature will be equally required for calculations.
