Pressure Gauge

A cut-away diagram of a Bourdon tube
pressure gauge is shown opposite. The
Bourdon tube is a hollow metal tube
curved in an arc. One end is open to
the pressure to be measured and the
other end is sealed. This free end is
coupled to a mechanical linkage that rotates the gauge pointer as the pressure in the tube changes its radius of curvature.

The gauge measures the pressure difference between the inside and the outside of the Bourdon tube and atmospheric pressure must be ADDED TO the gauge pressure to obtain the ABSOLUTE PRESSURE of the gas or fluid in the gauge.