Which pressure unit is defined as absolute pressure?

The unit defined as absolute pressure is PSIA, which stands for pounds per square inch absolute. Absolute pressure is measured relative to a perfect vacuum, meaning it accounts for atmospheric pressure in its measurement. In practical terms, this means that if you have a pressure gauge that reads PSIA, it tells you the total pressure exerted by the gas or liquid within a system including the atmospheric pressure.

Understanding this is important in many applications, especially when precise measurements are necessary, such as in scientific research and engineering tasks. Since it includes atmospheric pressure, PSIA provides a true measure of the pressure in a system, unlike gauges that measure pressure relative to atmospheric pressure, which can lead to underestimating pressure in certain situations.

The other pressure measurements, while useful in specific contexts, do not reflect absolute pressure. For example, PSIG (pounds per square inch gauge) measures pressure relative to atmospheric pressure and will read zero at atmospheric pressure, making it unsuitable for representing absolute pressure. Understanding these distinctions is critical for effectively managing systems that involve pressure.