McLeod Gauge is designed for general laboratory and industrial use. It is normally positioned with the measuring capillary and scale in horizontal position. For taking measurement the head is rotated through 900 to bring tube and scale in vertical position .The rotation cause the mercury to move in capillary tube. When mercury in open capillary is leveled at zero mark then system vacuum is measured by height of mercury column in the closed capacity which is calibrated directly in mbar.