How do you convert psiA to psiG according to the material?

• A. psiG = psiA - 14.7
• B. psiG = psiA * 14.7
• C. psiG = psiA + 14.7
• D. psiG = 14.7 - psiA

Answer: (C)

Gauge pressure is the pressure above ambient air, while absolute pressure includes both the pressure of the system and the ambient atmospheric pressure. At sea level, atmospheric pressure is about 14.7 psi. Because absolute pressure equals gauge pressure plus atmospheric pressure, converting from absolute to gauge means subtracting the atmospheric part: psiG = psiA − 14.7.

For example, an absolute pressure of 29.4 psi corresponds to a gauge pressure of 14.7 psi. If the absolute pressure is 7 psi, the gauge pressure is −7.7 psi, indicating a vacuum relative to the atmosphere. The key idea is gauge = absolute minus atmospheric (with atmospheric often taken as 14.7 psi for standard calculations).