For an air test of drain and vent systems, the test pressure is 5 psi, or is sufficient to balance a mercury column of how many inches in height?

• A. Five inches of mercury
• B. Ten inches of mercury
• C. Twenty inches of mercury
• D. Thirty inches of mercury

Answer: (B)

When air-testing drain and vent systems, you apply a known pressure and compare it to the hydrostatic pressure of a mercury column. The height of mercury in inches corresponds to the pressure in psi, with about 2.04 inches of mercury per 1 psi (since 29.92 inches Hg equal 14.7 psi).

So 5 psi balances roughly 5 × 2.04 ≈ 10 inches of mercury. That’s why the correct choice is about 10 inches of Hg.

For context, 5 inches of Hg would only be about 2.5 psi, 20 inches Hg is about 9.8 psi, and 30 inches Hg is about 14.7 psi, none matching the 5 psi test pressure.