Explain the difference between PSIG and psia.

Explain the difference between PSIG and psia.

• A. PSIG is gauge pressure above ambient; psia is absolute pressure above a vacuum.
• B. PSIG is absolute pressure; psia is gauge pressure.
• C. PSIG is vacuum pressure; psia is ambient pressure.
• D. PSIG and psia are the same.

Answer: (A)

Gauge pressure (PSIG) tells you how much pressure is present above the surrounding atmosphere, while absolute pressure (psia) tells you how much pressure exists above a perfect vacuum. The key is the reference point: ambient air for gauge, zero pressure for absolute. Because atmosphere itself adds pressure, you can relate them with psia = psig + atmospheric pressure. At sea level, atmospheric pressure is about 14.7 psi, so 0 psig equals roughly 14.7 psia, and 50 psig is about 64.7 psia. Absolute pressure cannot be negative, whereas gauge pressure can read negative when the pressure is below ambient. This is why the statement that PSIG is gauge pressure above ambient and psia is absolute pressure above a vacuum is the correct way to distinguish the two. The other descriptions mix up the references or claim they’re the same, which isn’t correct.

Gauge pressure (PSIG) tells you how much pressure is present above the surrounding atmosphere, while absolute pressure (psia) tells you how much pressure exists above a perfect vacuum. The key is the reference point: ambient air for gauge, zero pressure for absolute. Because atmosphere itself adds pressure, you can relate them with psia = psig + atmospheric pressure. At sea level, atmospheric pressure is about 14.7 psi, so 0 psig equals roughly 14.7 psia, and 50 psig is about 64.7 psia. Absolute pressure cannot be negative, whereas gauge pressure can read negative when the pressure is below ambient. This is why the statement that PSIG is gauge pressure above ambient and psia is absolute pressure above a vacuum is the correct way to distinguish the two. The other descriptions mix up the references or claim they’re the same, which isn’t correct.