Every metal and alloy has its won coefficient of expansion. That means they are expanded in their size differently for same temperature increase. If we couple two strips made of two different metals or alloys, then due to dissimilarity in coefficient of expansion they will be expanded or contracted differently during temperature change and consequently the whole bimetallic strip assembly will bend or be deformed. As this deformation of shape of a specific bimetallic strip is due to temperature rise or fall, this deformation can also be measured in the scale of temperature. From this principle, the concept of Bimetallic Strip Thermometer came.
Bimetallic strip mainly used in industries in temperature control devices. It is assembled with temperature controller system. When temperature reaches to a preset value, the bimetallic strip is so bent it closes an NO contact which initiates the cooling system to decrease the temperature of the system. Bimetallic strip thermometer is also widely used in industries because of their simplicity and robustness.
There are mainly two types of bimetallic strip thermometer are available in the market. Both are same in working but differ in construction.
Spiral Strip Bimetallic Thermometer
Here the bimetallic strip used is in spiral shaped. When temperature rises, due to bimetallic property, the spring twists more. Due to this mechanical deformation of the spring, a pointer attached to the dial moves and indicates the temperature, as the dial of this bimetallic strip thermometer is calibrated in temperature scale.
Cantilever Strip Bimetallic Thermometer
Here a straight bimetallic strip is attached as a cantilever. When temperature rises or falls, the strip bends either sides and the movement of the front end of the strip is transferred to a pointer dial system via gear – lever system, to take reading of temperature.
Advantages of Bimetallic Strip Thermometer
There are mainly three major advantages of this instrument. One they are robust two they are simple and three they are fully mechanical devices no need of power source.
Disadvantages of Bimetallic strip Thermometer
Main disadvantages are they are not very accurate and they are not suitable for measuring lower temperature as the metals and metallic alloys show nearly same expansion or contraction in lower range of temperature.