Hydrotest Makeup Water (Testing Allowance) Calculator
Allowable makeup water for a pressure test on a gasketed-joint pipeline, and a pass/fail against the makeup you actually added. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: The testing allowance covers water absorbed as trapped air goes into solution and gaskets seat under pressure — it is not a leak budget. Any visible leak fails the test regardless of the makeup math. Test pressure, duration, and the allowable makeup formula come from the project specifications; where the spec sets zero allowable makeup (common for gasketed PVC), use the zero-allowance toggle.
—
Allowable makeup rate Q
—
Allowable makeup over the test
—
Convenience rate (per 1,000 ft)
—
Measured makeup
—
Margin (allowable − measured)
—
How the Numbers Are Derived
The testing-allowance formula
Q = S · D · √P / 148,000 gallons per hour — S the length tested in feet, D the nominal diameter in inches, P the average test pressure in psi. Multiply by the test duration for the total allowable makeup. Worked check: 1,000 ft of 12-inch at 150 psi average gives 1000 · 12 · √150 / 148,000 ≈ 0.993 gal/hr, about 1.99 gal over a 2-hour test.
148,000 vs. 133,200 — where the divisors come from
The current AWWA C600 testing allowance is written directly in terms of length: Q = S·D·√P / 148,000. Older editions and some specifications use the joint-count form Q = N·D·√P / 7,400, where N is the number of joints in the section. With standard 18-foot laying lengths, N = S / 18, so N·D·√P / 7,400 = S·D·√P / (18 · 7,400) = S·D·√P / 133,200. The 133,200 divisor gives a slightly larger allowance than 148,000 — they are the same formula expressed per-joint vs. per-foot, differing in the constant. Use whichever your spec cites.
Field notes
The allowance is for absorbed air and gasket seating, not leaks. A joint weeping visibly fails even if the makeup is under the number — write it up and fix it.
Temperature moves pressure. In a rigid, full line, roughly 1 psi per °F of change is a workable rule of thumb; a test that quietly bleeds pressure over a hot afternoon may be losing temperature, not water. Log air and water temperature at start and end.
Measure makeup with a calibrated container or a meter on the pump, not by eyeballing a drop on the tank stick. The allowable numbers are small — a couple of gallons on a long line — so the measurement has to be at least as precise.
Fill and stabilize before the clock starts. Fill the volume before test day and let trapped air work out; the pipe-volume and AWWA hydrostatic pressure-test procedures cover the fill and the pressure hold this allowance is checked against.
References
AWWA C600 — Installation of Ductile-Iron Mains and Their Appurtenances (hydrostatic testing and allowable leakage/makeup).
AWWA C605 — Underground Installation of PVC and Molecularly Oriented PVC Pressure Pipe (testing provisions).
Contract specifications — governing test pressure, duration, and allowable makeup (including zero-allowance provisions).