Sharda University

Submitted By :-

Name – Shashank NarainRoll No. – 090103199Branch – Electronic & Communication ( 3rd Year)

BIMETALLIC STRIP

A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases brass instead of copper. The strips are joined together throughout their length by riveting, brazing or welding.

The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled

VOLUME EXPANSION OF SOLIDS

The volume of a solid is slightly dependent on temperature. Let the length Lo be the length of the solid when the temperature is To. When the temperature rises the length will change Providing the temperature increase is not too great the length of the solid will change by an amount where the constant a is the thermal expansion coefficient, which accounts for the different solids that could possibly be used.

change in L = a * Lo * (T - To)

BIMETALLIC STRIP - DESIGN

For any metal, the thermal expansion coefficient is very very small, and for any reasonable length of material, and for any reasonable temperature change, the increase in length is very small, too small to be of practical value in making a thermometer. However, by fusing two strips of different metals together we can make use of the difference in their thermal expansion rates to make a useful device. It is termed a Bimetallic strip. When heated the difference in the expansion of the two metals makes the strip bend to one side by an appreciable angle. (It will bend the other way if the strip is cooled.) The angle of deflection is a measure of the temperature change, that is the Bimetallic strip acts as a thermometer.

Material Expansion co-efficient

Aluminium 2.4 x 10-5 K-1

Iron(steel) 1.2 x 10-5 K-1

Copper 1.7 x 10-5 K-1

Brass 1.9 x 10-5 K-1

Glass 3.3 x 10-6 K-1

APPLICATION OF BIMETALLIC STRIP

• Bonding two metals with dissimilar thermal expansion coefficients can produce useful devices for detecting and measuring temperature changes.

•Bimetallic strips are made in coils to increase their sensitivity for use in thermostats.

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