Zero psia is:

• A. Atmospheric pressure
• B. Zero gauge pressure
• C. A negative pressure
• D. A pressure which cannot be further reduced

Answer: (D)

Zero psia means there is no pressure relative to a perfect vacuum—the absolute pressure is at its minimum. psia stands for pounds per square inch absolute, so it measures pressure against a true vacuum rather than against the surrounding air. Because atmospheric pressure at sea level is about 14.7 psia, zero psia is far below that and represents the vacuum limit. Zero gauge pressure would correspond to ambient atmospheric pressure, not to zero psia, and negative absolute pressure isn’t possible on the absolute scale—you can have negative gauge pressure when you’re below ambient, but absolute pressure cannot go below zero. So zero psia is the lowest possible absolute pressure, the point at which it cannot be reduced any further.