The Bourdon tube
Bourdon tube pressure gauges are intended for measurement of pressure and vacuum and are generally suitable for all clean and non-obstructive liquid and gases media.
C form tube is commonly used for ranges from 0.6 to 70 kg/cm2 (bar). One end is fixed and connected to the pressure circuit and the other is sealed and free to move. When pressure is applied to the tube, the free end is forced outwards.