Bellows pressure gauge is used to determine vapour pressure. This is calibrated on the basis of a U-shaped oil pressure gauge. These are made by forming thin-wall tubing. To obtain a reasonable fatigue life and motion that is more linear with pressure, a coil spring supplements the inherent spring rate of the bellows. These spring-loaded bellow gauges are generally used in pressure ranges having spans to 100 psi and to 1 in. Hg. This consists of an ash croft pressure gauge that is used for measuring low-pressure form 10 H2O to 10 psi.