The Bourdon Gauge
The Bourdon Gauge is a dial tool for measuring pressures both below and above the atmospheric pressure. It contains metal tube of oval cross section, bent into the form of a circular arc. One end of the tube is open, fixed firmly to the casing, and is linked by means of rigid or flexible tube to the pressure vessel in which the pressure is to be measured. The other end of the tube is blocked, and linked via a gear-and-lever mechanism to a pointer moving over a correctly graduated scale. An alter in pressure causes the oval section of the tube to tend to become round, and this causes a deflection of the free end, proportional to the change in pressure.
As the deformation of the tube cross section is caused by the pressure difference (p – patm), the Bourdon gauge measures gauge pressure, not absolute pressure. It might be used for measuring pressures either below or above atmospheric pressure.