A metallic bellows is a series of circular parts which are formed or joined in such a manner they are expanded or contracted axially by changes in pressure. Materials commonly used are brass, bronze, beryllium copper, alloy of nickel and copper etc.
Bellows pressure gauge: - Bellows elements are used for the measurement of absolute, gauge and differential pressure. A bellows pressure gauge is shown in fig.
There are two bellows. A and B. The pressure applied to bellows A is Pa and to bellows B is Pb. In case it is desired to measured the absolute pressure.
When gauge pressure is to be measured, than bellows B is opened up to atmosphere with pressure Pb is equal to the atmospheric pressure and therefore the reading of scale is the gauge pressure.
The differential pressure P=Pa-Pb is measured when pressure Pa is applied to bellow A and pressure Pb is applied to bellows B.
The gauge records pressures by means of expansive and construction of flexible metal bellows, these being transmitted to an optical lever of high sensitivity and then to the pointer.