This video explains What is a Pressure Gauge, different types of pressure gauge and working principle of pressure gauges.

Pressure gauges are divided into three types, based on pressure sensing elements.
1. Bourdon tube
2. Diaphragm
3. Bellow

Bourdon tubes are manufactured in C, helical, and spiral forms. Without pressure, the circular cross section of the bourdon tube is flattened. When these elements are pressurized, their cross section tends to become more circular, which tends to straighten their shape.

The diaphragm is an elastic membrane that elongates when pressure is applied to it. The diaphragm is a single sheet which elongate, Joining two diaphragm to form a capsule.

The bellows expands or contracts based on the pressure difference across the inside and outside of the bellows unit. When absolute pressure is to be sensed with bellows elements, it normally involves two bellows, one for measuring and the other for reference. Dual-bellows elements are also available as differential pressure sensors. If either the low or the high pressure side is left open to the atmosphere, the unit will detect either gauge pressure or vacuum.