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