How Bourdon Gage is used to measure Pressure??
Pressure or vacuums are commonly measured by bourdon gage. In this gage a curved tubeof elliptical cross section will change its curvature with changes in pressure within thetube. The moving end of the tube rotates a hand on a dial through a linkage system. Apressure and vacuum gage combined into one is known as compound gage.
The pressure indicated by the gage is assumed to be that at its center. If the connecting piping is filled completely with fluid of the same density as that in A. and if pressure gage is graduated to read in psi then PA (psi) = gage reading (psi) + 𝜸z/144
A vacuum gage or the negative pressure portion of a compound gage is traditionally graduated to read in inches or mm of mercury, for vacuums in of Hg vacuum at A = gage reading (in of Hg vacuum) - 𝜸z (29.9)/144 *14.7 Here, once again, it is assumed that this fluid completely fills the connecting tube. The elevation correction terms i.e. those containing z, may be positive or negative, depending on whether the gage is above or below the point at which the pressure determination is desired.
The expressions given are for the situation depicted. When measuring gas pressures, the elevation correction terms are generally negligible. The above equations, when written is SI units, require no correction factors; however care must be taken in dealing with decimal points when adding terms.