The expansion of solids is employed mainly in bimetallic elements by utilizing differential expansion of bonded strips of two different materials. It consists of two strips of metals such as INVAR and BRASS welded together, each strip made from metal having a different coefficient of thermal expansion. INVAR is an iron-nickel alloy (35% Nickel) which has a very low thermal coefficient of expansion and BRASS is an alloy made of copper and zinc having
very high coefficient of thermal expansion. Whenever the welded strip is heated
,two metals will change length in accordance with their individual rate of thermal expansion. The two metals will expand to different lengths as the temperature rises.
This forces the bimetallic strip to bend towards the side having low coefficient of thermal expansion. One end of the bimetallic strip is fixed and it cannot move. The deflection of the other end is directly proportional to the square of
length of the
metallic strip as well as to the total change in the temperature.
The movement of the bimetallic strip is utilized to deflect the pointer over a scale which is calibrated in terms of temperature. The bimetallic strip is wounded in the form of a helix for industrial applications of temperature measurements. It consists of a tightly wound helical bimetallic strip located inside the thermometer with one end fastened permanently to the outer casing. The strip is attached to a shaft that will expand from the stem to the center of the indicating dial. A pointer is attached to the shaft. When the temperature increases,
bimetal expands and the helical coil unwinds which rotates the pointer. A thermo well is used with the bimetallic thermometer for the protection.
Bimetallic thermometers are inexpensive, rugged and simple to use. They usually provide a visual temperature reading. They are used for
cut-off switches, overloading and on-off control.
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