The pressure to be measured is connected to the fixed open end of the bourdon tube. The appilied pressure acts on the inner walls of the bourdon tube. Due to the appilied pressure, the bourdon tube tends to change in cross – section from ellipitcal to circular. This tends to straighten the bourdon tube causing a displacement of the free end of the bourdon tube.
This displacement of the free closed end of the bourdon tube is proportional to the applied pressure. As the free end of the bourdon tube is connected to a link – section – pinion arrangement, the displacement is amplified and converted to a rotary motion of the pinion.
As the pinion rotates, it makes the pointer to assume a new posistion on a pressure calibrated scale to indicate the appilied pressure directly. As the pressure in the case containing the bourdon tube is usually atmospheric, the pointer indicates gauge pressure.