The bimetallic thermometer consists of a bimetallic strip by bonding together two thin strips of two different metals. The differential change in expansion of the two metals results in bending of the bimetallic strips with change in temperature.
Working:
The bimetallic strip has one end fixed and the temperature changes causes the free end to deflect. The range of linear relationship between deflection and temperature depends on the combination of metal used. Invar alloy is a low expansion material. The other metals used can be steel, monel and brass.
The deflection of the free end is directly proportional to the temperature change and the length of the strip and is inversely proportional to the thickness of the strip.
If one end is fixed, the deflection of the free end is direct indication of the temperature measured. the sensitivity of the thermometer is increased by increasing the length of the bimetallic strip.
The size pf the thermometer can be made small by having the bimetallic strip arranged as the circular spiral or as an helix.
The temperature range that can be measured is -40 degree Celsius to +550 degree Celsius. The accuracy is within +or-0.5% to +or-2%.
Uses:
used in ambient temperature measurement devices.
Used to air-conditioning thermostats
Used in oil refineries.
used in Tyre Vulcanizers.
Used in hot work wire heaters.
Used in tempering tanks.
Advantages
Simple, robust and inexpensive.
It has good accuracy.
It can measure temperature in the range of (-40 to 550) in Celsius.
It can withstand 50% overage temperature measurement.
Disadvantages
Not recommended for measurement of temperature above 550 degree Celsius.