Compressor CFM actual under pressure.

I tested my compressor CFM today. Reasonably scientific process. Thought I would share.

Use a stopwatch and make 3 runs and get the average time (see below).



1 Get tank volume in gallons by reading the side of the tank hopefully.

2 Divide gallons of tank by 7.48 to get cubic feet of volume in tank (mine is 8.02 cubic feet or 60 gallons)

3 Take start and stop pressure (mine was 85 on and 150 off) so 65 psi increase.

4 Time the start and stop event. (Mine was 2 minutes 7 seconds or 127 seconds = 2.12 minutes.

5 Divide the psi increase by 14.7 to get the number of atmospheres increase or difference from start to stop. 65/14.7 = 4.422.

So mine yielded 4.422 atmospheres in 2.12 minutes.

6 Multiply atmospheres by cubic feet 4.422*8.02 and divide by minutes (2.12)to get cubic feet per minute.

My machine averaged 16.73 cfm from 85 to 150 psi. Which is pretty good for an under high pressure flow rate.

I built it from a 3 cylinder new eaton compressor running near 1000 rpms (high side rpm threshold), nice used tank and a basic circuit controlled 11 hp gasoline harbor freight engine @ max rpm which is supposed to be 3600 which starts and stops like an electric motor based on a pressure switch and simple circuit and relays.


Not a pretty looking rig but has been running flawlessly for a few years now: