ABSTRACT
Leonardo da Vinci was the first to attempt the measurement of humidity by weighing a ball of wool. The wet and dry bulb hygrometer takes advantage of this fact to effect a relative humidity measurement. The temperature values indicated by the two thermometers are related to the relative humidity of the sample atmosphere. Cellulose strips or other shapes are used to measure humidity. The resulting resistance of the sensor is an indication of the relative humidity in the air. The largest number of manufacturers use some form of a capacitance measurement to detect humidity or dew point in air. The temperature of a surface is at dew point when the first molecular layer of condensation appears on it. Changes in humidity cause large changes in the impedance of the layer, and because the operating portion of the sensor is only its surface, its speed of response is fast-only a few seconds.
