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Temperature actuators control equipment based on temperature changes, using methods like metals, chemicals, or gases. Mercury switches were used in thermostats until the late 20th century, but are toxic. Electronic controls like thermistors have replaced mechanical devices.
A temperature actuator is any device that turns equipment on and off in response to changes in temperature. They can use a variety of methods to measure changes in temperature, including metals, chemicals, or gases. Temperature control devices can range from a simple home thermostat to control stoves or air conditioners, to complex systems that control chemical reactions in industrial plants.
The heating and air conditioning system 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 bimetallic 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.
The mercury was installed in a glass tube placed on one end of the bimetallic coil. Liquid mercury moved back and forth inside the tube as the temperature changed and activated electrical circuits to control heating or cooling. Only the bimetallic tape is affected by the temperature; mercury remained unchanged. The use of mercury, however, was disappearing in the late 20th century due to its toxicity.
Chemicals can be used to control a temperature actuator, by changing the size, phase or pressure of the vapor, and some chemicals expand and contract with changes in temperature. 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 substance changing from solid to liquid or liquid to gas. Vehicle thermostats used to control engine temperature use a wax seal that turns liquid as the engine warms up, opening a valve that allows engine coolant to circulate. This will solidify again when the engine has cooled down.
Steam pressure can be used for temperature control in two ways. One type of temperature actuator measures the vapor pressure 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 flip a switch.
A second type of steam actuator is a metal hydride controller. Metal hydrides contain hydrogen molecules which become hydrogen gas as the temperature increases. 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 to control water. Once the fire is extinguished, the hydrogen gas flows back into the metal, the gas pressure drops, and the sprinkler head closes.
Electronic temperature controls began to replace mechanical devices in the late 20th century. You can make devices called thermistors, which are temperature sensitive controllers with no moving parts, that provide control over a narrow temperature range. Thermistors can be connected to control loops that turn heating or cooling systems on and off and are used in many digital thermostats.
