Absolute pressure is defined as?

• A. Pressure measured at sea level only.
• B. Dynamic pressure in the cockpit.
• C. Pressure difference between inside and outside the cabin.
• D. Pressure measured from the reference of zero pressure, or a vacuum.

Answer: (D)

Absolute pressure is the total pressure exerted, measured relative to a perfect vacuum. In other words, you start from zero pressure and add all pressures around you—the atmospheric pressure plus any additional pressure from your system. This distinguishes it from gauge pressure, which only tells you how much pressure exists above the surrounding air; a gauge would read zero when the pressure equals the ambient atmosphere. It also differs from differential pressure, which is the difference between two pressures (for example, inside versus outside a cabin), and from dynamic pressure, which relates to the energy of moving air. So the definition that measures pressure from zero pressure, or a vacuum, is the correct one for absolute pressure. For context, at sea level the ambient atmospheric pressure is about 14.7 psi; absolute pressure would be that amount plus any extra pressure added by the system.