thermostat

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thermostat,

automatic device that regulates temperature in an enclosed area by controlling heating or refrigerating systems. It is commonly connected to one of these systems, turning it on or off in order to maintain a predetermined temperature. Its operating principle is based on the fact that one of its components expands or contracts significantly during a temperature change. This expansion or contraction actuates a control on a furnace, cooling system, or piece of machinery. The thermostat sometimes uses mercury, which expands when heated and rises in a glass tube until, at a predetermined point, it touches an electrical contact to complete a circuit and thereby actuate a control; conversely, during a lowering of temperature the mercury descends in the tube and breaks the circuit. The thermostat often uses a bimetallic strip, which is made of two thin metallic pieces of different composition that are bonded together. As the temperature of the strip changes, the two pieces change length at different rates, forcing the strip to bend. This bending causes the strip to make or break a circuit.

Thermostat

An instrument, electrically operated, such as a bimetal strip, for automatically maintaining a constant temperature; commonly used in conjunction with heating and air-conditioning plants.

Thermostat

 

(constant-temperature chamber), a device for maintaining a constant temperature. A thermostat is a vessel made of metal, glass, or other material, and it is thoroughly protected by heat insulation against environmental influences.

The constancy of temperature in a thermostat is provided by thermoregulators or by a phase transition, such as the melting of ice, the boiling of water, or the solidification of a eutectic, that takes place at a specific temperature. When the difference between the ambient temperature and the temperature in the thermostat is small, the temperature of the working agent (a gas or liquid) that fills the thermostat remains constant. The object whose properties are being studied at a given temperature is in thermal contact with the working agent, whose temperature it acquires.

Thermostats filled with a working agent are usually equipped with a quick-response heater or cooler, an automatic thermoregulator of the required precision, and a mechanism that vigorously mixes the working agent to ensure rapid equalization of the temperature in the thermostat. Liquid-filled thermostats may contain alcohol (for temperatures from –60° to 10°C), water (10°–95°C), oil (100°–300°C), and salt or saltpeter (300°–500°C). Gas-filled thermostats with the same range of temperatures are used less frequently because of the difficulty in ensuring thermal contact with the object being studied.

Low heat exchange with the environment is provided in high-temperature and low-temperature thermostats. The object being studied is kept at a constant temperature under adiabatic conditions—that is, there is no working agent. In low-temperature thermostats, heat is supplied or removed by a special “thermal key”—a heat-conducting rod. An electric furnace with a thermoregulator and a heavy metal unit into which the object of study is placed is frequently used as a thermostat at high temperatures (300°–1200°C). A thermostat for the maintenance of low temperatures is called a cryostat.

In thermodynamics, a system whose heat capacity is so great that the quantities of heat supplied to it do not alter its temperature is often called a thermostat.

REFERENCES

See references under CALORIMETER.

thermostat

[′thər·mə‚stat]
(engineering)
An instrument which measures changes in temperature and directly or indirectly controls sources of heating and cooling to maintain a desired temperature. Also known as thermorelay.

thermostat

An instrument which responds to changes in temperature, and directly or indirectly controls temperature.

thermostat

A device that maintains the desired temperature in a system.

thermostat

a device that maintains a system at a constant temperature. It often consists of a bimetallic strip that bends as it expands and contracts with temperature, thus breaking and making contact with an electrical power supply