Which term describes pressure measured relative to atmospheric pressure?

• A. Gauge pressure
• B. Vacuum pressure
• C. Dynamic pressure
• D. Absolute pressure

Answer: (A)

Pressure measured relative to atmospheric pressure is gauge pressure. A gauge zeroes at atmospheric pressure, so the reading shows how much pressure is above or below air pressure. Mathematically, gauge pressure equals absolute pressure minus atmospheric pressure, so it directly reflects the pressure relative to the surrounding atmosphere. If the absolute pressure is higher than atmosphere, the gauge reads a positive value; if lower, it reads negative (a vacuum condition). Absolute pressure, by contrast, uses a perfect vacuum as the reference, and dynamic pressure relates to fluid motion, not reference pressure. Vacuum pressure terms are used for pressures below atmospheric, but the concept asked for is specifically gauge pressure.