and for dehydration of refrigeration systems. The low-side gauge in a refrigeration gauge manifold indicates pressures below ambient in "inches of mercury vacuum" (inHg), down to a 30 inHg vacuum. Inches of mercury is also used in automotive cooling system vacuum test and fill tools. A technician will use this tool to remove air from modern automotive cooling systems, test the system's ability to hold vacuum, and subsequently refill using the vacuum as suction for the new coolant. Typical minimum vacuum values are between 22 and 27 inHg. Vacuum brakes Inches of mercury was the usual unit of pressure measurement in railway vacuum brakes. See also Torr (millimeters of mercury) Bar (unit) Mercury barometer References Units of pressure Mercury (element) Customary units of measurement in the United States