A temperature actuator is any device that turns equipment on and off in response to temperature changes. They may use a variety of methods to measure temperature changes, including metals, chemicals or gases. Devices to control temperature can vary from a simple home thermostat for controlling heaters or air conditioners, to complex systems that control chemical reactions in industrial plants. Heating and air conditioning equipment is connected to a thermostat inside the building. Until the late 20th century, thermostats used a mercury switch to control the system. This temperature actuator used a bi-metallic strip, which was a coiled strip of two metals fused along their length. As the temperature changes the two metals expand or contract at slightly different rates, and the coil changes shape. Mercury was installed in a glass tube placed on one end of the bi-metallic coil. The liquid mercury moved back and forth inside the tube as the temperature changed and activated electric circuits to control heating or cooling. Only the bi-metallic strip was affected by temperature; the mercury remained unchanged. Use of mercury, however, was disappearing by the late 20th century due to its toxicity. Chemicals can be used to control a temperature actuator, either by changing size, phase or vapor pressure, and some chemicals expand and contract with temperature changes. When sealed in tubes with a piston at one end, changes in temperature can cause the piston to move and activate a switch. A phase change refers to a chemical changing from solid to liquid, or liquid to gas. Vehicle thermostats used for engine temperature control use a wax seal that becomes a liquid as the engine warms, opening a valve that permits engine coolant to circulate. This will turn back into a solid when the engine cools. Vapor pressure can be used for temperature control in two ways. One type of temperature actuator measures the pressure of vapor from a solvent sealed in a tube and connected to a chemical process tank or pipe. As the temperature increases, the solvent vapor pressure also increases and can activate a switch. A second type of vapor actuator is a metal hydride controller. Metal hydrides contain hydrogen molecules that become hydrogen gas as temperatures increase. The increasing hydrogen gas pressure can push against a piston and be used as a temperature actuator. Fire sprinkler systems can use these devices to open and close sprinkler heads for water control. Once the fire is put out, the hydrogen gas returns to the metal, the gas pressure drops and the sprinkler head closes. Electronic temperature controls began replacing mechanical devices in the late 20th century. Devices called thermistors, which are temperature-sensitive controllers with no moving parts, can be made that provide narrow temperature range control. Thermistors can be connected to control circuits that turn heating or cooling systems on and off, and are used in many digital thermostats.
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