Thermostat is a device control to regulate indoor air temperature at certain temperature limits by opening and closing electrical contacts automatically. Set the temperature can be done from button, while the specific differential (fixed) and have been determined at the factory. Thermostat can be used for commercial refrigeration and to regulate cold temperature or temperature hot.
A lot of thermostat design with various models and shapes. Thermostat operation is very sensitive to temperature changes, it must use sensitive thermal sensor also to temperature changes. Elements that are sensitive to temperature changes and can cause electrical contacts or other mechanical devices that open or close, that is bimetal.
Bimetal is made of two metals that are not identical, but at room temperature have same length. When two metals are heated, one metal will be longer than others. This is due to both these metals have different coefficients of expansion. Metal for bimetal is made of brass and invar attached together by welding, so that at room temperature to form a straight rod and flat. Invar is a trade name, consisting of mixture of nickel, iron and other metals that have a very small expansion coefficient. Brass has coefficient of expansion is relatively greater, so the expansion change of brass is greater than invar.
When bimetal is heated, it will bend towards the invar. In contrast with decrease temperature will cause bimetal curved towards brass. Whenever there is change in temperature, also followed by change in direction of bimetal curvature. The amount of change in curvature can be used directly or indirectly to alter the position of mechanical and electrical contacts open. To get larger movement, bimetal is rotated spiral. The inner edge is bound and connected to its house, while the outer edge can move freely rotate in accordance with received temperature changes.