Normal Barometric Pressure

Atmospheric pressure can change with the weather and also depends on the altitude. For many reasons a standard reference condition for barometric pressure is needed. Unfortunately there is more than one standard and quite a number of different units of measurement.

Barometric Units

Barometers usually measure atmospheric pressure in inches or millimeters of mercury, which is the length of a column of the liquid element that the pressure of the air will support with a vacuum above it.

Other Units

Other common units for measuring air pressure are pounds per square inch, and newtons per square meter, which is equal to one Pascal. Meteorologists also use the millibar, which is equal to 100 Pascals.

The Atmosphere

Another common air pressure measure is simply called the atmosphere, which is 760 mm of Hg, 14.696 psi (absolute) and 101.325 kPa.

Standard Pressure

The International Union of Pure and Applied Chemistry defines standard atmospheric pressure as 100,000 Pascals. In barometric units this equals 29.53 inches (750.1 mm) of Hg and 1,000 millibars.

Normal Pressure

Normal pressure is used by engineers for fan performance and it equals one atmosphere, 29.92 inches (760 mm) of Hg, and 1013.25 millibars. This standard was also adopted by the International Civil Aviation Organization.

References

• EngineeringToolbox: The definition of STP - Standard Temperature and Pressure and NTP - Normal Temperature and Pressure
• University of Florida: Introduction to Gases