A device used to indicate or to control the relative humidity in the air in a closed area: Jack decided to buy a humidistat or hygrostat, which he learned is an electronic instrument similar to a thermostat, but which reacts to moisture in the air, but not to the temperature.

1. An instrument that measures, or controls, the relative humidity of the air.
2. A device used for regulating the relative humidity of an enclosed space.

A hygrostat contains a sensor that detects a preset level of humidity and is connected to a heater that is turned off, and on, as needed in order to keep the humidity at the preset level.

Hygrostats are used in humidifiers, and dehumidifiers, and are placed in the ductwork of laboratories and other facilities where climate control is necessary.