Bimetal Temp Gauge

Bimetal temp gauge Basic Principle: These thermometers use the following two principles: 1. All metals change in dimension, that is expand or contr act when there is a change in temperature. 2. The rate at which this expansion or contraction takes place depend on the temperature co-efficient of expansion of the metal and this temperature coefficient of expansion is different for different metals.Hence the difference in thermal expansion rates is used to produce deflections which is proportional to temperature changes.

The bimetallic thermometer consists of a bimetallic strip. A bimetallic strip is made of two thin s trips of metals which hae different coeffcients of expansion. The two metal strips are !oined together b " bra#ing, welding or reeting so that the relatie motion between them is arrested. The bimetallic strip is in the for m of a cantileer beam. An increase in temperature will result in the deflection of the free end of the strip as shown i diagram. This deflection is linear and can be related to temperature changes. The radius of the cur ature of the bimetallic strip which was initiall" flat is determined using the following relationship. $% t&'(1)m*+ ) (1)mn*m+)1mn/0(h-l*(T2-T1*(1)m* +

where, $% radius of the curature at the temperature T2. T % total thickness of the bimetallic strip % (t1)t2* m%t1t2 % Thickness of lower  expansion metalthickness of higher  expansion metal. l% coefficient of expansion of lower expansion metal. h% coefficient of expansion of higher expansion metal. T1 % 3nitial temperature. T2 % temperature. The following are the important proporties a material should hae to be selected for bimetallic thermometers.

1.

4oefficient of expansion.

2.

5odulus of elasticit".

'.

6lastic limit after cold rolling.

7.

6lectrical conductiit".

8.

9uctilit".

0.

5etallurgical abilit".

Different common forms of bimetallic sensors are listed. 1.

Helix t"pe.

2.

piral t"pe.

'.

4antileer t"pe.

7.

;lat t"pe.

5etals used in bimetallic strips are:

High expansion 1.

iron*.

Application of bimetallic strips and thermometers 1.

The bimetallic strip is used in control deices.

2.

The spiral strip is used in air conditioning thermostats.

'.

The helix strip is used for process application such as refineries, oil burners, t"re ulcanisers etc.,

Advantages of bimetallic thermometers 1.

The" are simple, robust and inexpensie.

2.

Their accurac" is between )or- 2? to 8? of the scale.

'.

The" can with stand 8@? oer range in temperaures.

7.

The" can be used where er a mecur" in-glass thermometer is used.

Limitations of bimetallic thermometer: 1.

The" are not recommended for temperature aboe 7@@4.

2.

Bhen regularl" used, the bimetallic ma" permanentl" deform, which inturn will introduce errors.

Bimetallic Thermometer  3n an industr", there is alwa"s a need to measure and monitor temperature of a particular spot, field or localit". The industrial names gien to such temperature sensors are Temperature 3ndicators (T3* or Temperature Cauges (TC*. All these temperature gauges belong to the class of instruments that are known as bimetallic sensors.

Two basic principles of operation is to be followed in the case of a bimetallic sensor. The" are

1.

A metal tends to undergo a volumetric dimensional change (expansion/contraction), according to the change in temperature.

2.

Diferent metals have diferent co-ecient o temperatures. !he rate o volumetric change depends on this co-ecient o temperature.

Working

The deice consists of a bimetallic strip of two different metals and the" are bonded together to form a spiral or a twisted helix.