What term describes any pressure less than atmospheric pressure?

• A. Vacuum
• B. Ambient Pressure
• C. Absolute Pressure
• D. Gauge Pressure

Answer: (A)

Vacuum is the term used for any pressure below atmospheric pressure. Atmospheric pressure is the pressure of the surrounding air (roughly 14.7 psi at sea level). When the pressure drops beneath that level, the condition is described as a vacuum.

Absolute pressure is the total pressure relative to a perfect vacuum, so it’s not defined by being below atmospheric. Gauge pressure is the pressure relative to atmospheric; it can be positive or negative, but that describes how far the pressure is from atmosphere rather than labeling the state as vacuum. Ambient pressure refers to the pressure in the surrounding environment (usually atmospheric) and doesn’t specifically denote a condition below atmospheric.