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Absolute Pressure
Absolute pressure — also known as true pressure — is a pressure measurement relative to a perfect vacuum. The measurement includes atmospheric pressure and gauge pressure, or the system’s pressure.
Absolute pressure offers an accurate reading of the total pressure in a system. It’s essential in applications where minor pressure variations could impact process safety or product quality.
Several industries that use air compressor systems need to measure absolute pressure. For example, the pharmaceutical industry relies on sensitive processes, so absolute pressure may impact this work. The food and beverage sector also needs to measure this metric to maintain consistent product quality and safety, particularly in packaging and carbonation. The value may matter in aerospace manufacturing, which requires precise, repeatable pressure conditions.
FAQs
Gauge pressure is the pressure determined by most instruments and gauges — for example, those on an air compressor. This measurement is just one part of the absolute pressure. A “g” placed after the pressure unit (psig) typically represents gauge pressure.
Pressure gauges work by comparing the ambient air to the pressured air, so the pressure would measure 0 psig when the pressure inside a system is equal to atmospheric air pressure.
Atmospheric pressure, or barometric pressure, is the ambient air pressure at a particular location and altitude. Like gauge pressure, it is just one part of the measurement that makes up absolute pressure. Atmospheric pressure changes due to air temperature, altitude and humidity.
You can determine atmospheric pressure using a barometer or the ideal gas law: PV=nRT.
Here is what each component of the formula means:
- P: pressure of the gas (typically measured in pascals)
- V: volume of the gas
- n: number of moles of gas
- R: ideal gas constant
- T: absolute temperature of the gas
Here is the formula used to determine absolute pressure:
Pabs=Patm+Pgauge
Here’s what each part means:
- Pabs: absolute pressure
- Patm: atmospheric pressure
- Pgauge: gauge pressure
To perform the calculation, determine the gauge and atmospheric pressure in psi. You may need to convert these units. Once both units are in psi, add them to get absolute pressure, typically expressed as psia.
Here’s an example of how this might work:
Suppose the pressure gauge on your industrial air compressor measures 125 psig. If you operate the machine at sea level, the standard atmospheric pressure is 14.7 psi. Add the two numbers together to get an absolute pressure of 139.7 psia.
Operations can use an absolute pressure sensor or transducer with an air compressor to get a more accurate pressure measurement. These instruments use absolute zero as a reference point for the compressor rather than comparing the pressure change inside the compressor to the atmospheric air around it. One side of the transmitter has a permanently sealed vacuum chamber, and the other contacts the air in the compressor.
These instruments ensure greater accuracy by measuring the total pressure within a system, limiting the influence of atmospheric conditions on the consistency and quality of various industrial processes.
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