Which statement correctly defines absolute pressure?

• A. It equals the pressure relative to vacuum, equal to gauge pressure plus atmospheric pressure
• B. It equals the pressure relative to atmospheric pressure only
• C. It equals the sum of gauge pressure and atmospheric pressure
• D. It is always greater than atmospheric pressure by a fixed amount regardless of conditions

Answer: (A)

Absolute pressure is the pressure measured with reference to a perfect vacuum. It represents the total pressure inside the system, independent of the surrounding atmosphere. To connect this to what a gauge shows, remember that a gauge measures pressure above atmospheric pressure. So, to convert a gauge reading to absolute pressure, you add the atmospheric pressure: P_abs = P_gauge + P_atm. Saying absolute pressure is the pressure relative to vacuum and that it equals the sum of gauge pressure and atmospheric pressure captures this relationship exactly. It also helps explain why just saying “relative to atmospheric pressure” describes gauge pressure, not absolute, and why absolute pressure isn’t a fixed amount above atmospheric in all conditions—if the gauge reads below zero (negative), the absolute pressure can be less than atmospheric.