Hygrostat

A hygrostat is a device that senses ambient humidity and controls equipment to maintain a chosen relative humidity. It functions as a humidity regulator, turning on or off humidification or dehumidification systems to keep the environment near a set point. The term is sometimes used interchangeably with humidistat, though some contexts distinguish between a humidistat (which controls humidification) and a hygrostat (a broader humidity regulator).

Principle of operation: In mechanical hygrostats, a sensing element such as treated hair or a polymer strip changes shape with humidity and actuates a switch through a linkage mechanism. Electronic hygrostats use capacitive or resistive humidity sensors that feed a controller, which drives a relay or solid-state switch to modulate a connected device. Most offer on-off control with hysteresis to reduce frequent cycling; more advanced models provide digital setpoints and sometimes proportional or modulated output.

Types: Common varieties include mechanical hygrostats with simple relays and a fixed hysteresis, and electronic hygrostat controllers with digital displays, adjustable setpoints, and multiple output options. Some units are paired with humidifiers or dehumidifiers in HVAC systems, greenhouses, or storage rooms, while others are designed for standalone use in terrariums, incubators, or museums.

Applications: Hygrostats are used in horticulture and greenhouses to maintain plant-friendly humidity, in museums and archives to protect sensitive materials, in electronics manufacturing and cleanrooms to control moisture, in homes and offices as part of humidification/dehumidification systems, and in terrariums, incubators, and storage facilities.

See also: Hygrometry, hygrometer. While a hygrometer measures humidity, a hygrostat or humidistat provides active control to regulate it.