and Inches of Water: A standard method of pressure measurement, where pressures are compared to atmospheric or ambient pressure. Inches of displacement are recorded for a water or mercury column measured in a "U" shaped tube with one end open to the air and the other end connected to the test pressure. Commonly called a manometer, this pressure comparison device is quite sensitive and accurate. When mercury is used in the manometer tube, one psi differential from atmospheric pressure will displace 2.04-inches of mercury. However, when water is the liquid in the "U-tube," a substantial increase in pressure sensitivity is obtained: one psi will displace 27.72 inches of water. A water manometer is used to measure small vacuum and pressure signals.
Two very common scales used to measure vacuum pressures (“HgA and “HgV). The scale ranges from 29.92” Hg to 0” Hg and scale orientation depends on whether it is used as a gauge scale or an absolute scale. One inch of mercury equals 25.4 torr.
Vacuum level with a scale of 0" Hg to 29.92" Hg (gauge) with 0" being equal to atmospheric pressure at sea level. The absolute vacuum levels scale runs reverse with 29.92" Hg equaling atmospheric pressure. Often used to describe rough vacuum whereas torr is used for deeper vacuum level (i.e, use 20 torr rather than 29.13"Hg)