It is a curved metal tube with elastic properties, generally made of copper alloys or AISI 316L stainless steel, although they are also manufactured in other materials (Monel, Hastelloy ...).
This tube, generally C-shaped, is welded at one end to the inlet of the fluid to be measured, and at the other end it has been closed and hooked to a transmission shaft. When the fluid enters the inlet of the pressure gauge and creates pressure inside, the Bourdon tube is deformed at its free end and, by means of the transmission shaft, transmits the movement of the tube to the mechanism and in turn to the indicator needle of the manometer. When the pressure decreases, the manometer Bourdon tube returns to its original position.
The "C" shaped tube is generally used for pressures from 0.6 bar to 60 bar. For higher pressures (up to 7000 bar) the tube is used in helicoid form, wrapped on itself one or two turns.
The manometers for low pressures (also called capsule gauges), with ranges between 6 mbar and 600 mbar, mount a capsule as a sensitive element. It consists of two wavy sheets of circular shape, welded around its perimeter, creating a sealed chamber. The materials of these capsules are the same as those used for Bourdon tubes, and enjoy the same elastic properties.
Due to their large surface, they have a high sensitivity to low pressures.
When increasing the pressure inside the capsule of the ventometer, it swells, pushing in turn a toothed mechanism and thus transmitting the movement to the indicator needle of the manometer.
An overpressure greater than 1.3 times the range of the manometer represents a serious risk to the sensitive element of the manometer, since its internal organ can lose its elastic properties and not return to its original position once the pressure ceases. This is one of the main causes of failure of the pressure gauges.