At atmospheric pressure, most pressure gauges read __________.

• A. 14.7 psi
• B. Atmospheric pressure
• C. Zero psi
• D. 1 psi

Answer: (C)

Gauges on most systems measure gauge pressure, which is the pressure relative to the surrounding atmospheric air. The reading is P_g = P_abs − P_atm. When the actual pressure inside the system equals atmospheric pressure, there’s no extra pressure above ambient, so the gauge shows zero. If you used an absolute-pressure instrument, you would read about 14.7 psi at sea level, because that measures total pressure above a vacuum, not relative to atmosphere. So, under atmospheric conditions, the typical gauge reads zero psi.