Which statement correctly describes PSIG and psia?

Which statement correctly describes 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 tells you how much pressure is above the surrounding atmosphere, while absolute pressure tells you the total pressure relative to a perfect vacuum. In other words, PSIG is the pressure above ambient air, and psia is the pressure above a vacuum. A handy relationship is psia = PSIG + atmospheric pressure. For example, with typical sea-level atmospheric pressure of about 14.7 psi, a gauge reading of 5 psi corresponds to an absolute pressure of about 19.7 psi. This makes the described statement the best description: PSIG is gauge pressure above ambient, and psia is absolute pressure above a vacuum. The other ideas mix up which reference each type uses or claim they’re the same, which isn’t correct.

Gauge pressure tells you how much pressure is above the surrounding atmosphere, while absolute pressure tells you the total pressure relative to a perfect vacuum. In other words, PSIG is the pressure above ambient air, and psia is the pressure above a vacuum. A handy relationship is psia = PSIG + atmospheric pressure. For example, with typical sea-level atmospheric pressure of about 14.7 psi, a gauge reading of 5 psi corresponds to an absolute pressure of about 19.7 psi. This makes the described statement the best description: PSIG is gauge pressure above ambient, and psia is absolute pressure above a vacuum. The other ideas mix up which reference each type uses or claim they’re the same, which isn’t correct.