Sensor types Mechanical thermostats

Early technologies included mercury thermometers with electrodes inserted directly into the glass so that when a certain (fixed) temperature was reached, the contacts were closed by the mercury.These are accurate over a range of temperatures.

Common sensor technologies in use today include:

• Bimetal mechanical or electrical sensors.

• Swelling Wax Beads

• Electronic thermistors and semiconductor devices

• Thermocouples

These can then control heating or cooling devices using:

• Direct mechanical control

• Electrical signal

• Pneumatic signal

Mechanical thermostats 

This covers only devices which both sense and control using purely mechanical means.

Bimetal

Domestic water and steam based central heating systems are traditionally controlled by bimetal thermostats, which are dealt with later in this article.Purely mechanical controls are localized steam or hot water radiator bimetallic thermostats that regulate individual flow rates. However, the Thermostatic Radiator Valve (TRV) is now widely used.Purely mechanical thermostats are used to adjust the dampers of some rooftop turbine vents, reducing heat loss from buildings during cool or cold periods.Some automotive passenger heating systems have a thermostatically controlled valve that regulates water flow and temperature to adjustable levels. In older vehicles, a thermostat controls the application of engine vacuum to actuators that control water valves and dampers to direct air flow. In modern vehicles, vacuum actuators can be operated by small solenoids under the control of a central computer.

Wax pellet

Automotive

Perhaps the most common example of purely mechanical thermostat technology in use today is the internal combustion engine cooling system thermostat, used to keep the engine near its optimum operating temperature by regulating the flow of coolant to the air-cooled radiator.This type of thermostat operates using a sealed chamber that contains a pellet of wax that melts and expands at the set temperature.Expansion of the chamber operates a rod which opens a valve when the operating temperature is exceeded.The operating temperature is determined by the composition of the wax. Once operating temperature is reached, the thermostat gradually increases or decreases its opening in response to temperature changes, dynamically balancing coolant recirculation flow and coolant flow to the radiator to maintain engine temperatures within the optimum range.On many automobile engines, including all Chrysler Group and General Motors products, the thermostat does not restrict flow to the heater core.The radiator's passenger side tank acts as a bypass for the thermostat, flowing through the heater core.This prevents steam pockets from forming before the thermostat opens and allows the heater to run before the thermostat opens. Another benefit is that if the thermostat fails, there is still some flow through the radiator.

Shower and other hot water controls

A thermostatic mixing valve uses a wax pellet to control the mixing of hot and cold water. A A common application is to allow electric water heaters to operate at temperatures high enough to kill Legionella (above 60°C, 140°F), while the output of the valve produces water cool enough not to burn immediately (49°C, 120F Spend).

Analysis

Wax bead actuated valves can be analyzed by plotting a bead hysteresis curve consisting of two thermal expansion curves; extension (movement) versus temperature increase, and contraction (movement) versus temperature decrease.The distribution between the upper and lower curves visually illustrates the hysteresis of the valve; there is always hysteresis in wax-actuated valves due to phase transitions or phase transitions between solid and liquid.Hysteresis can be controlled by specifically mixing hydrocarbons; tight hysteresis is most desired, but some applications require a wider range. Wax pellet actuated valves for scald protection, freeze protection, over temperature purge, solar thermal or solar thermal, automotive and aerospace applications, etc..

Gas expansion

Thermostats are sometimes used to regulate gas ovens.It consists of a gas-filled bulb connected to the control unit by a slender copper tube.The bulb is normally located at the top of the oven.The tube ends in a chamber sealed by a diaphragm. As the thermostat heats up, the gas expands applying pressure to the diaphragm which reduces the flow of gas to the burner.

Pneumatic thermostats

A pneumatic thermostat is a thermostat that controls a heating or cooling system through a series of air-filled control tubes.This "control air" system responds to pressure changes (due to temperature) in the control tubes to initiate heating or cooling when needed. Control air is usually maintained at 15-18 psi on the "main line" (although it can often be as high as 20 psi).Pneumatic thermostats typically provide 3-15 psi of output/branch/post-restrictor (for single-pipe operation) pressures that are piped to end equipment (valve/damper actuator/pneumatic electric switch, etc.).The pneumatic thermostat was invented by Warren Johnson in 1895, followed by the electric thermostat. In 2009, Harry Sim was granted a patent for a pneumatic digital interface that allows pneumatically controlled building integration with building automation systems to provide similar advantages to direct digital control (DDC).