Temperature Measurement

What are thermostats? Different types of thermostats

What are thermostats?

The thermostat is a device for adjusting the temperature of the system so that the system temperature is maintained near the desired setpoint temperature.  The thermostat does this by switching the heating or cooling or deactivating the device or adjusting the fluid flow of heat transfer needed, to maintain the correct temperature.

A thermostat may be a control unit for heating or cooling and a system or component of a heater or air conditioner. Thermostats can be built in various ways and may use various sensors to measure temperature. The output from the sensor then controls the heating or cooling of the apparatus.

Common sensor technologies include:

  • Bimetal mechanical or electrical sensors
  • Expanding wax pellets
  • Electronic thermistors and semiconductor devices

Bimetallic type thermostat:

Image result for bimetallic thermostat

These thermostats are used In the steam or hot water of the radiator system, the thermostat may be entirely equipped with a bimetal strip. In general, this is an automatic valve that regulates flow based on temperature. Mechanical thermostats are used to regulate dampers in roof turbine vents, reducing heat loss in cold or cold building periods.

The thermostat is a type of valve that opens and closes automatically according to the coolant temperature. The thermostat is installed between the radiator and the engine cooling circuit.

Wax pellets:

Image result for wax pellet thermostat

This kind of thermostat is used in an internal combustion engine to maintain the core temperature of the engine at optimal operating temperatures by regulating the coolant flow to an external heat sink, usually an air-cooled radiator.

This type of thermostat operates mechanically. It makes use of wax pellets in closed spaces. The wax is solid at low temperatures but as the machine heats up the wax melts and expands. The enclosed space has an expansion provision that operates the stick that opens the valve when the operating temperature is exceeded.

The operating temperature is fixed but is determined by the particular composition of the wax, so this type of thermostat is available to maintain different temperatures, usually in the range of 70 to 90 ° C (160 200 ° F). Modern machines run the heat, that is, more than 80 ° C (180 ° F), to run more efficiently and reduce pollutant emissions.

Electronic Thermostat:

These are small, self-contained circular units with increased flange for attachments to plates or metal parts from heating or air conditioning, subject to ambient temperature. The internal sensor generally consists of a bimetal disk with electrical contacts in the centre.

Image result for electronic thermostat

At the switching temperature, the disk flips from concave to convex (or vice versa) causes the contacts to open or close depending on the mode needed to switch (normally closed or open normally). This device can also be used as an overheating prevention switch.

The interior of a thermostat of two ordinary household-only heat wires used to regulate gas-fired heaters through an electric gas valve. Similar mechanisms can also be used to control oil stoves, boilers, boiler zone valves, electric attic fans, electric furnaces, electric baseboard heaters, and household appliances such as refrigerators, coffee pots, and hair dryers.


Instrumentation Engineer

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