The relative air humidity is the degree of saturation of air with water vapor. In meteorology, the measurement of rel. air humidity is standard for each weather station, in the meantime. Beyond the meteorology, the measurement and regulation of air humidity is an essential component of air conditioning technology, and in building control systems it is an important parameter for room climate. In green houses the control of rel. air humidity is indispensable for an optimal plant growth. The proper air humidity determines the perfect climate for sensitive goods also in store-rooms or drying plants.