1. STOP the compressor unit

2. CLOSE the outlet valve on the tank/air receiver

3. DRAIN the condensate from air receiver until there is 0 PSIG -then close the drain valve

4. NOTE THE TIME– in minutes & seconds (Best to write it down.) Then START THE UNIT.
When the compressor unit stops and unloads – then NOTE THE TIME again – in minutes & seconds. Convert the minutes into seconds and then total the number of seconds it takes between START and STOP/UNLOAD.

5. NOTE the GUAGE PSIG reading

6. NOTE the Air Receiver/Tank GALLON SIZE


TANK GALLONS x .536* x PSIG divided by SECONDS

*.536 is a formula factor for the unknown that works to give you the CFM delivery.

EXAMPLE in the video:

You have an 30 gallon tank, your total start to stop/unload time was 3 minutes and 5 seconds.
Change the minutes to seconds timed (60 x 3= 180 seconds plus 5 seconds totals 185 - 22 seconds when the valve was closed = 163). You will use the total number of seconds 163 and the noted 175 PSIG within the formula as shown below:

30 multiplied by .536 = 16.08
16.08 multiplied by 175 (example PSIG) = 1608.00
1508.00 divided by 163 (total seconds)= 17.26 CFM delivered

You now know that your air compressor is delivering 17.26 CFM


Compressor Terms you should know:

Cubic Feet Per Minute (cfm) – Volumetric air flow rate.

“psig” means pounds per square inch, GAGE pressure. Gage pressure is the absolute pressure of something, with the atmospheric pressure subtracted. In practice, when someone gives a pressure in just “psi” they probably mean gage pressure. If they mean absolute, they should be using “psia.”

Gauge Pressure – The pressure determined by most instruments and gauges, usually expressed in psig. Barometric pressure must be considered to obtain true or absolute pressure

Load Time – Time period from when a compressor loads until it unloads.

Unload – (No load) Compressor operation in which no air is delivered due to the intake being closed or modified not to allow inlet air to be trapped.

Receiver – A vessel or tank used for storage of gas under pressure. In a large compressed air system there may be primary and secondary receivers.

Demand – Flow of air at specific conditions required at a point or by the overall facility.