Thermometers & Heaters

A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by controlling the flow of heat energy into or out of the system. more...

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That is, the thermostat switches heating or cooling devices on or off as needed to maintain the correct temperature.

Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus.

Common sensors include:

Bi-metal mechanical sensors; Expanding wax pellets; Electronic thermistors; Electrical thermocouples;

These may then control the heating or cooling apparatus using:

Direct mechanical control; Electrical signals; Pneumatic signals;

Mechanical

Bi-metal

On a steam or hot-water radiator system, the thermostat may be an entirely mechanical device incorporating a bi-metal strip. Generally, this is an automatic valve which regulates the flow based on the temperature. For the most part, their use is now rare, as modern under-floor radiator systems use electric valves, as do some older retrofitted systems.

Mechanical thermostats are used to regulate dampers in rooftop turbine vents, reducing building heat loss in cool or cold periods.

An automobile passenger compartment's heating system has a thermostatically controlled valve to regulate the water flow and temperature to an adjustable level. In older vehicles the thermostat controls the application of engine vacuum to actuators that control water valves and flappers to direct the flow of air. In modern vehicles, the vacuum actuators may be operated by small solenoids under the control of a central computer.

All thermostats have thermometers. In a classical mechanical thermostat, the thermometer is a coiled bimetallic strip. A bimetallic strip is made by uniting several layers, made up of two different types of metal, together. The metals that make up the strip get bigger or smaller when they are heated or cooled. Each type of metal expands at its own specific rate, and the two metals (usually iron and copper) that make up the strip are selected so that the rate that the strip gets bigger and smaller aren’t the same. When the strip cools off, one side of the metal on the inside of the coil gets smaller and the strip will wind more tightly. When the coil tightens, the circuit is completed by a switch attached to the coil, and the furnace turns on. The switch in the circuit is typically a mercury switch (a bead of liquid mercury metal inside a glass bulb with two leads at one end). As the temperature of the room rises, the opposite occurs, and the coil unwinds, opening the circuit and turning the furnace off. Most modern thermostats are digital, with a solid-state temperature sensor rather than a bimetallic strip, and a transistor switch instead of a mercury switch.

Read more at Wikipedia.org