psi vs. psig in Pipeline Pressure Testing

Question: Regarding pipeline pressure testing, what’s the difference between psi and psig?

Answer: Here’s my (relatively) simple answer: Although psi and psig are both units of measurement for describing the amount of pressure a gas or fluid is exerting, the difference is that psig specifies what the measurement is relative to, whereas psi does not. In both, the letters “psi” represent pounds per square inch (the pressure resulting from a one pound-force applied to an area of one square inch).

Pounds per square inch gauge (psig) is a unit of pressure relative to atmospheric pressure at sea level, which is 14.7 (the “gauge” designation indicates that the readings are already adjusted to ignore surrounding atmospheric pressure). When a pressure gauge gives a reading called psig, it’s the difference between absolute pressure of the connected pressure source and the local atmospheric pressure.

Example: When you check a car tire (valve stem) with a tire pressure gauge, the instrument is reading the pressure difference between the air inside the tire and the air outside the tire. That’s called gauge pressure. Suppose the air pressure in your car tire is 32 psig. Then the absolute pressure inside is 32 + 14.7 = 46.7 psi. Sometimes this is written as 46.7 psia (pounds per square inch, absolute) to remind us we’re talking about an absolute pressure. In either case, we could just use psi.

Tip: Temperature changes will affect pressure during a test. A temperature increase and the resulting increase in pressure can mask a small leak. A temperature decrease can indicate a leak when there isn’t one.

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