Guideline for Measuring Actual Compressed Air Flow Using the Tank Filling Method

This method is used to determine the actual compressed air output (free air delivery) of a compressor. It is particularly useful for aged compressors or used/imported machines, where the actual air flow is often lower than the value stated on the nameplate.

Practical Guideline
Important notes:
  • Perform the test only when production is stopped and there is no air consumption.
  • It is recommended to measure 2–3 times and use the average value to improve accuracy.
  • All pressures used in the calculation are gauge pressures (bar g) as indicated on the pressure gauge.

Test procedure

Step 1: Stop the air compressor
Shut down the air compressor and make sure that no equipment is consuming compressed air.
Step 2: Isolate the air receiver
Close all valves downstream of the air receiver to ensure that, during the test, compressed air is supplied only to the receiver.
Step 3: Vent the receiver to atmosphere
Open the receiver drain valve to release all air and condensate, bringing the internal pressure down to atmospheric pressure (P1 ≈ 0 bar g). Then close the drain valve.
Step 4: Measure the filling time
Prepare a stopwatch and start the air compressor. Record the running time from the moment charging begins until the receiver reaches the target pressure (P2).

For variable speed (VSD) screw compressors, it is recommended to select P2 about 1–2 bar below the pressure setpoint to ensure the compressor remains fully loaded.
Step 5: Record the receiver volume
Record the nominal volume of the air receiver. If the pipe between the compressor and the receiver has a large diameter and significant length, its internal volume should be added to improve accuracy.
Step 6: Record the pressures
Record the following values:
  • P1: Initial pressure (typically = 0 bar g)
  • P2: Pressure reached when the stopwatch is stopped
All values are gauge pressures (bar g).
Step 7: Calculate the actual air flow
Enter the parameters P1, P2, V, T into the calculation tool below. The result is the actual compressed air flow (free air delivery), which can be compared with the compressor nameplate rating.
Technical tip:
If the measured flow rate is 15–30% lower than the nameplate value, the compressor may be affected by air-end wear, internal leakage, or performance degradation, and should be serviced or re-evaluated for its operating condition.

Actual Compressor Air Flow Measurement (Tank Filling Method)

Measure the time required for the compressor to raise pressure from P1 to P2 in a receiver with volume V. This tool calculates the actual free air delivery (FAD) in m³/min for comparison with the compressor nameplate.

Note: P1 and P2 are gauge pressures (bar g) indicated on the receiver pressure gauge. If the tank is vented to atmosphere, use P1 = 0 bar g.

IMX Tool
Vented to atmosphere: P1 = 0 bar g. Pressure unit converter
P2 must be greater than P1. Pressure unit converter
You may include piping volume if the piping is large or long.
Measure 2–3 times and use the average value.