# ⓘ Inch of mercury is a unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. I ..

## ⓘ Inch of mercury

Inch of mercury is a unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.

It is the pressure exerted by a column of mercury 1 inch 25.4 mm in height at the standard acceleration of gravity. Conversion to metric units depends on the temperature of mercury, and hence its density; typical conversion factors are:

In older literature, an "inch of mercury" is based on the height of a column of mercury at 60 °F 15.6 °C.

1 inHg 60 °F = 3376.85 Pa

In English units: 1 inHg 60 °F = 0.489 771 psi, or 2.041 771 inHg 60 °F = 1 psi.

### 1.1. Applications Cooling systems

In air conditioning and refrigeration, inHg is often used to describe "inches of mercury vacuum", or pressures below ambient atmospheric pressure, for recovery of refrigerants from air conditioning and refrigeration systems, as well as for leak testing of systems while under a vacuum, 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 systems 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.

### 1.2. Applications Vacuum brakes

Inches of mercury was the usual unit of pressure measurement in railway vacuum brakes.

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