The pumping system shall be capable of developing a vacuum of 22 in. Hg at altitudes up to which height?

• A. Up to 5000 ft (1500 m)
• B. Up to 2000 ft (600 m)
• C. Up to 1000 ft (300 m)
• D. Up to 3000 ft (900 m)

Answer: (B)

Vacuum capability is the pressure difference you can create relative to the surrounding air. A pump that can develop 22 inches of mercury (inHg) vacuum is aiming to make the internal pressure 22 inHg lower than the ambient pressure. But as you ascend, the ambient atmospheric pressure drops. If the ambient pressure you start from is already low, you can’t guarantee a full 22 inHg difference because there isn’t enough surrounding pressure to pull against and the pump’s own limits and losses come into play.

At lower altitudes, where ambient pressure is still in the high 20s inHg, a 22 inHg differential is readily achievable, leaving a reasonable final absolute pressure inside the system. As altitude increases, ambient pressure falls enough that maintaining that full 22 inHg difference becomes unreliable for typical pumping gear, so the specification often caps the altitude at a level where the pump can still guarantee the target vacuum.

That’s why the standard practice sets the capability up to about 2,000 ft. Below that, ambient pressure remains high enough for the pump to maintain the 22 inHg vacuum reliably; beyond that, performance can drop off and the full vacuum cannot be guaranteed.