A humidity switch is a must for most humidifier installations and is mounted in the duct downstream of the dispersion assembly. It is usually set at 90 per cent relative humidity (RH). If duct RH rises above that setting, it will shut down the humidifier. Wetted media, also known as evaporative humidifiers, cannot oversaturate the airstream, so a duct high-limit humidistat is not a requirement. To ensure proper operation, the high-limit sensor must be located far enough downstream of the disperser to allow for full absorption and for humidity to equalize in the airstream. Otherwise, short-cycling of the humidifier can occur, resulting in an unsatisfied humidification demand.