Absolute pressure is the sum of atmospheric pressure, which can be measured with a manometer, and the gauge pressure of the fluid.
In engineering, absolute pressure is the pressure of a system relative to the pressure of an absolute vacuum. In more practical terms, it is often expressed as the sum of the pressure of the atmosphere and the gauge pressure of a fluid. It is necessary in engineering calculations such as the Ideal Gas Law.
The most common expression for absolute pressure defines it as the sum of the gauge or gauge pressure of a system and the atmospheric pressure. The expression takes the form P absolute = P gauge + P atmospheric. Atmospheric pressure is defined as the pressure of the surrounding air at or near the surface of the Earth. This pressure is not a fixed or constant value and can vary with temperature or elevation.
Gauge pressure represents the system pressure measured by a pressure measurement device. These devices, or pressure gauges, can be classified based on how they measure pressure. The most common types are elastic element meters, liquid column meters and electrical meters. Unless otherwise stated by the manufacturer, most pressure gauges do not include atmospheric pressure in their readings.
In a typical chemical plant environment, absolute pressure and gauge pressure do not represent the same thing and different notations must be used to keep them separate. A common method of doing this is to add the letter a after the pressure unit to indicate absolute pressure and the letter g after the pressure unit to indicate gauge pressure. For example, an absolute pressure of 100 psi would become 100 psia. Likewise, a gauge pressure of 5 kPa would be 5 kPag. However, the US National Institute of Standards and Technology prefers that the clarifying letter not be applied to the unit, but rather to the letter P. For example, Pg = 25 kPa would be preferable to P = 25 kPag.
This measure of pressure is most commonly used in engineering calculations such as the Ideal Gas Law. When performing such equations, engineers must use the correct pressure to avoid costly mistakes or dangerous operations. The difference between absolute and gauge pressure is much more noticeable at pressures where atmospheric pressure is of the same order of magnitude as gauge pressure.
The error of neglecting the atmospheric component of absolute pressure can be demonstrated by examining a closed cylinder of an ideal gas with a temperature of 77° Fahrenheit (25° Celsius) and a volume of 1.0 m 3 . If the cylinder gauge reads 100 kPa and atmospheric pressure is ignored, the calculated number of moles of gas in the cylinder will be approximately 40.34. If the pressure of the atmosphere is also 100 kPa, then the absolute pressure is actually 200 kPa, and the correct number of moles is 80.68. The actual number of moles is double the original calculation, showing the importance of using the correct pressure.