A humidity-measuring device for use with a room air conditioner has been developed. A special fin is installed on the evaporator in the indoor heat exchanger, and a system controller calculates the humidity from the temperature of the end portion of the fin. This is based on the fact that the temperature of the end portion of the fin varies according to the humidity change of the inlet air of the indoor heat exchanger. The unit structure is very simple, and the initial cost is low. Furthermore, reliability is high, because a temperature sensor, such as a thermister, is used to measure the humidity. The total error of the humidity detection is ±4.3% relative humidity within the temperature range from 24°C to 30°C and relative humidity from 50% to 90%.

Units: SI