A humidistat or hygrostat is an

electronic device Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other electrically charged particles. It is a subfield of physics and ...

analogous to a

thermostat A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint. Thermostats are used in any device or system tha ...

but which responds to

relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

, not

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

. A typical humidistat is usually included with portable humidifiers or dehumidifiers. It can also be included with combined air cleaner or humidifier units to control a home's humidity level or any other indoor space.

Usage

Humidistats are used in a number of devices including dehumidifiers, humidifiers, and

microwave ovens A microwave oven, or simply microwave, is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces Dipole#Molecular dipoles, polar molecules in the food to rotate and ...

. In humidifiers and dehumidifiers, the humidistat is used where constant relative humidity conditions need to be maintained such as a

refrigerator A refrigerator, commonly shortened to fridge, is a commercial and home appliance consisting of a thermal insulation, thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from its inside to ...

greenhouse A greenhouse is a structure that is designed to regulate the temperature and humidity of the environment inside. There are different types of greenhouses, but they all have large areas covered with transparent materials that let sunlight pass an ...

, or climate-controlled

warehouse A warehouse is a building for storing goods. Warehouses are used by manufacturers, importers, exporters, wholesalers, transport businesses, customs, etc. They are usually large plain buildings in industrial parks on the rural–urban fringe, out ...

. When adjusting the controls in these applications the humidistat would be what is being set. In microwaves they are used in conjunction with ''smart cooking'' one-button features such as those for

microwave popcorn Microwave popcorn is a convenience food consisting of unpopped popcorn in an enhanced, sealed paper bag intended to be heated in a microwave oven. In addition to the dried corn, the bags typically contain cooking oil with sufficient saturated f ...

. Humidistats employ

hygrometer A hair tension dial hygrometer with a nonlinear scale. A hygrometer is an instrument that measures humidity: that is, how much water vapor is present. Humidity measurement instruments usually rely on measurements of some other quantities, such a ...

s but are not the same. A humidistat has the functionality of a

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...

and is not just a

measuring instrument Instrumentation is a collective term for measuring instruments, used for indicating, measuring, and recording physical quantities. It is also a field of study about the art and science about making measurement instruments, involving the related ...

like a hygrometer is. For

heating, ventilation, and air conditioning Heating, ventilation, and air conditioning (HVAC ) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. H ...

(HVAC) of buildings, humidistats or humidity sensors are used to sense the air relative humidity in the controlled space and turn on and off the HVAC equipment.Engineering Manual of Automation Control, Honeywell, 1997, https://customer.honeywell.com/resources/techlit/techlitdocuments/77-0000s/77-e1100.pdf accessed 27 October 2014, Chapter "Electronic Control Fundamentals", Section "Relative Humidity Sensor", pg 125

References

{{HVAC Switches Temperature control Humidity and hygrometry