Thermostat, << THUR muh stat, >> is a device that helps control the temperature of an indoor area or of an appliance. Thermostats are used in many kinds of equipment, including air conditioners, heaters, electric blankets, ovens, and refrigerators.
A thermostat is set to keep an area or an appliance at a certain temperature. It reacts to temperature changes and automatically controls the heating or cooling unit of the equipment being used. For example, the thermostat in a home heating system turns on the furnace if the temperature drops below the desired level. It shuts off the furnace when the temperature reaches that level.
How thermostats work.
Most metals, liquids, and gases expand when their temperature increases. They contract when their temperature decreases. Some thermostats use such expansion and contraction to measure and control temperature.
Most thermostats used in home heaters and air conditioners have a bimetallic strip that “senses” changes in temperature. This strip consists of two dissimilar metals fastened together. When the temperature rises, each metal expands at a different rate, causing the strip to bend. The metals contract unequally when the temperature drops. These actions cause the strip to bend in the opposite direction. This action opens or closes the electric circuit that controls heating or cooling equipment.
Some thermostats use the expansion or contraction of a gas or a liquid to open or close a circuit. Other types use electronic detecting devices or infrared detectors that sense temperature changes.
Most thermostats turn heating or cooling equipment completely on or completely off. But some use a method called proportional control. These thermostats measure the difference between the actual temperature and the desired temperature. They change the amount of heating or cooling in proportion to this temperature difference. Proportional control thermostats can provide a nearly even temperature. They are used in industry and scientific research to control the temperature so that certain chemical processes can take place. The greatest precision can be achieved using proportional control along with other advanced controls that track the trend of temperature changes.
Uses.
Thermostats control air-conditioning and heating systems in buildings to help keep people comfortable. Thermostats keep refrigerators and freezers at the necessary temperature to prevent food from spoiling. They also control the temperature of household ovens, irons, hot water heaters, and heaters for fish tanks.
Industries use thermostats to control the temperature needed for manufacturing certain products, such as plastics and frozen foods. Thermostats control the industrial furnaces used in making such products as bricks and steel. Thermostats also provide the precise temperature needed for many scientific experiments.
Thermostats also control the flow of water in automobile cooling systems. The thermostat operates a valve that opens when the water reaches a certain temperature. The open valve allows water to circulate through the radiator and through the water jacket that surrounds the engine.
See also Automation (How automation works) ; Heating (Central heating systems) .
