Measuring a compressor’s flow (pump-up test)
The pump-up test measures a compressor’s real flow without a flow meter: all you need is a pressure gauge, a stopwatch and a known volume. You isolate that volume, shut off demand, run the compressor, and time the pressure rise between two values.
SCFM = (V × ΔP_abs × 60) / (P_std × t)| Symbol | Meaning | Unit |
|---|---|---|
| V | Known volume being pressurised (receiver + isolated piping) | ft³ |
| ΔP_abs | Pressure change during the measurement (end − start) | psi |
| t | Timed duration | s |
| P_std | Reference pressure ≈ 14.5 psia (1 bar, SCFM reference) | psia |
| 60 | Seconds → minute conversion | — |
The formula assumes isothermal compression: over the test duration the air has time to settle to ambient temperature, so compression heating is neglected.
The method, step by step
- Identify a known volume: the receiver, ideally plus the piping you can isolate downstream. The larger the volume, the more accurate the measurement. Compute the volume if needed with the system-volume calculator.
- Shut off demand: isolate the points of use so all produced air goes into raising the pressure.
- Time the rise: record the start and end pressures of the measurement window and the elapsed time.
- Apply the formula: or enter the values in the calculator.
Worked example
A 240-gallon receiver (≈ 32.1 ft³), demand off. Pressure rises from 90 to 110 psig (ΔP = 20 psi) in 45 seconds.
- SCFM = (32.1 × 20 × 60) / (14.5 × 45) ≈ 59 SCFM
So the compressor delivers ≈ 59 SCFM under these conditions. You can compare this figure to the nameplate to detect wear (a flow well below the original value signals a tired compressor). To frame the need and convert units, see Flow units.
Delivered capacity or consumption: two uses
- Demand off → delivered capacity. That’s the measurement above: the compressor’s real flow.
- Compressor off, demand on → consumption / leaks. Letting the network bleed down, the same physics (in reverse) estimates what the plant draws, useful to quantify leaks. See also the leak calculator.
Why it’s useful in an audit. The pump-up test gives a real, field-measured capacity with no permanent instrumentation. It’s a classic starting point of a compressed-air audit: comparing real capacity to the demand and to the nameplate.
With the Onyx M3 tools
- Calculator: Flow test, manual or geometry-based volume, optional piping, built-in stopwatch or entered duration: it returns the flow in SCFM.
- Calculator: System volume, adds up the internal volume of the receiver and piping to feed the test.
References
- CAGI, Compressed Air & Gas Handbook: field capacity tests and verification
- ISO 1217, Displacement compressors – Acceptance tests: flow and reference conditions (SCFM, 1 bar absolute)
- Compressed Air Challenge, Best Practices for Compressed Air Systems: pump-up / pump-down test method