Absolute pressure is the pressure of a system relative to an absolute vacuum and is needed in engineering calculations. It is the sum of the relative pressure of a system and atmospheric pressure. Gauge pressure is the system pressure measured by a pressure measuring device. The difference between absolute and gauge pressure is important in engineering calculations. Neglecting atmospheric pressure can lead to costly errors or dangerous operations.
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 needed in engineering calculations such as the ideal gas law.
The most common expression of absolute pressure defines it as the sum of the relative, or measured, pressure of a system and atmospheric pressure. The expression takes the form Paabsolute = Pgauge + Patmospheric. 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 measuring device. These devices, or pressure gauges, can be classified by how they measure pressure. The most common types are spring-loaded meters, liquid column meters, and electrical meters. Unless otherwise indicated by the manufacturer, most gauges do not include the pressure of the atmosphere 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. Similarly, a gauge pressure of 5 kPa would be 5 kPag. The US National Institute of Standards and Technology, however, prefers that the clarification letter should not be applied to the unit but to the letter P. For example, Pg = 25 kPa would be preferable to P = 25 kPag.
This pressure measurement is most commonly used in engineering calculations such as the ideal gas law. When running such equations, engineers must use the correct pressure to avoid costly errors or dangerous operations. The difference in absolute and gauge pressure is much more apparent at pressures where atmospheric pressure is of the same order of magnitude as gauge pressure.
The error in 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 m3. If the pressure gauge on the cylinder indicates 100 kPa and the pressure of the atmosphere is ignored, the calculated number of moles of gas in the cylinder is approximately 40.34. If the pressure of the atmosphere is also 100 kPa, then the absolute pressure is indeed 200 kPa and the correct number of moles is 80.68. The actual number of moles is double the original calculation, demonstrating the importance of using the correct pressure.
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