If the base test pressure is 3.5 psi and the water table is 2 feet above the sewer line, what is the total pressure in the pipe?

• A. 3.5 psi
• B. 4.5 psi
• C. 5.0 psi
• D. 6.0 psi

Answer: (B)

Total pressure in the pipe during a test is the base test pressure plus the hydrostatic pressure from the water column above the pipe. A water head of 2 feet adds pressure proportional to that height. Using the common exam approximation of 0.5 psi per foot, 2 feet adds about 1.0 psi, so 3.5 psi plus 1.0 psi equals 4.5 psi. If you use the exact value of about 0.433 psi per foot, the addition is roughly 0.866 psi, giving about 4.37 psi; rounded to tenths, 4.5 psi is the closest match. The core idea is that height of water above the pipe increases pressure inside the pipe by the hydrostatic head.