Gauge pressure is measured relative to what reference?

• A. Vacuum
• B. Ambient atmospheric pressure
• C. Absolute zero
• D. A sealed reference pressure

Answer: (B)

Gauge pressure measures how much pressure a system has above the surrounding air. It’s the difference between the system’s pressure and the local atmospheric pressure, so P_gauge = P_absolute − P_atmosphere. A tire example helps: if the gauge reads about 32 psi, and ambient atmospheric pressure is around 14.7 psi at sea level, the tire’s absolute pressure is roughly 46.7 psi. If the system’s pressure is below ambient, the gauge can show a negative value (a slight vacuum relative to the surroundings).

This is why the reference is ambient atmospheric pressure—not a vacuum, not absolute zero (which is a temperature), and not a sealed internal reference (which would yield a differential or absolute-style reading, not a typical gauge reading).