Measure a pump's actual capacity in the field from a wet-well drawdown test — no flowmeter — correcting for the inflow that keeps arriving while you pump. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: This assumes the wet-well plan area is constant over the test band — run the test between two elevations with straight walls, clear of benches, the influent pipe, and pump-off/on floats. Inflow rarely stops during a test; measure it (pump-off fill) or isolate it, because ignoring it understates the pump. Run several cycles and average, and confirm only the intended pump(s) were running.
Results
Plan area
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Inflow (Qin)
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Net drawdown rate (pump − inflow)
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Pump capacity (Qpump = net + inflow)
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Per pump
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Percent of rated
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How the Numbers Are Derived
The drawdown test
With no flowmeter, a pump's capacity comes from how fast it empties a known volume. Plan area A times a level change d is a volume; divided by the time it took, it's a flow. In field units, Q (gpm) = 7.48 · A (ft²) · d (ft) / t (min) — the 7.48 converts cubic feet to gallons.
The catch is inflow. While the pump draws the well down, sewage keeps arriving, so the level falls slower than the pump is actually moving water. The net drawdown rate is Qpump − Qin. Measure the inflow first with the pumps off (time the level rise), then Qpump = (net drawdown rate) + Qin. Isolate the influent and Qin is zero and the net drawdown is the pump.
Field notes
Constant area is the whole assumption. Benched wet wells, the influent invert, and fillets all change the plan area with depth — pick a test band where the walls are vertical and uniform.
Keep the test band clear of the pump-off and pump-on float/transducer setpoints so the pump doesn't cycle mid-measurement.
Run three or more drawdowns and average. A single stopwatch read on a fast pump is noisy; the inflow read especially so.
For two pumps in parallel, the per-pump figure is only meaningful if the pumps are identical — and remember two pumps rarely make twice one pump's flow, because the system curve steepens. Cross-check against the pump system-curve plotter.
Compare the measured capacity to the pump curve at the actual operating head, not the rated point — a pump can test “low” simply because the head is higher than rated.
References
Recommended Standards for Wastewater Facilities (“Ten-State Standards”) — pump station testing.