The Principle of Temperature Controller

The mechanic temperature use two layers of different metal thermal expansion coefficient together, when temperature changes, the degree of curvature change too, when  it bend to a certain degree, it  will connects (or off) the circuits, makes refrigeration (or heat) equipment work.

While Electronic Temperature through the thermocouple temperature sensing device, such as platinum resistance, puts the temperature signal transformation into electrical signals, uses the single-chip microcomputer. PLC controls the relay circuits and makes heating (or refrigeration) equipment work (or stop).

Another one is mercury thermometer model, when temperature reach to a degree, it will contacts the mercury .