Inch of mercury

Inch of mercury (inHg or ″Hg) is a unit of pressure measurement. It is used primarily in the United States and is based on the pressure exerted by a one-inch column of mercury at the Earth's standard gravity. Its use spans various fields, including meteorology, aviation, and medicine, for different purposes such as measuring atmospheric pressure, aircraft altitude, and blood pressure, respectively.

Definition

The inch of mercury is defined as 33.8638866667 pascals, which is equivalent to the pressure exerted by a one-inch column of mercury. The exact value can vary slightly based on the temperature at which the measurement is made, as the density of mercury changes with temperature. In meteorology, measurements are often made at a temperature of 0°C (32°F), referred to as inHg(0°C).

Usage

Meteorology

In meteorology, the inch of mercury is a common unit for measuring atmospheric pressure. Weather reports in the United States often include atmospheric pressure readings in inches of mercury. High-pressure systems are associated with fair weather, while low-pressure systems can bring storms and rain.

Aviation

In aviation, the inch of mercury is used to measure altitude and atmospheric pressure. Pilots use altimeters calibrated in inches of mercury to determine their altitude above mean sea level. The standard atmospheric pressure at sea level is defined as 29.92 inHg in the aviation industry.

Medicine

In medicine, the inch of mercury is used to measure blood pressure, although the use of millimeters of mercury (mmHg) is more common. Blood pressure cuffs (sphygmomanometers) may have dials that display pressure in both millimeters and inches of mercury.

Conversion

To convert inches of mercury to pascals, multiply the value in inches of mercury by 3,386.389. Conversely, to convert pascals to inches of mercury, divide the value in pascals by 3,386.389.

Historical Context

The use of mercury in barometers and manometers for measuring pressure dates back to the 17th century. The inch of mercury unit originated from these early instruments, which used mercury because of its high density and low vapor pressure.

Safety and Environmental Concerns

Mercury is a toxic heavy metal, and its use in instruments has raised safety and environmental concerns. Alternatives to mercury-based instruments, such as aneroid barometers and electronic pressure sensors, are now widely used.

See Also

• Barometer
• Manometer
• Pascal (unit)
• Mercury (element)
• Atmospheric pressure