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:

Ya know WHUT I'd really REALLY like to see more about your start/ stop setup. That is certainly one way to get around "not enough fusebox," etc, LOL

67 I was not interested in using electricity very much in my pole barn shop. It is out back and routing a high amp 220 run 200 feet from the main and the corresponding fees and certifications associated.

Basic concept:

Use the factory tank pressure switch which is already set to be on at 90 and off at 140, I adjusted for 85 - 150. When it kicks ON, I use that to trigger a timer which engages the starter for 12 seconds and maintains a relay to hold open a head air pressure relief solenoid on the output of the compressor B4 the check valve (3/4" plumbing). SO while the engine spools up it has no head pressure on the compressor.

The factory pressure switch being on the duration of 85 - 150 psi maintains a relay open while engine is running and closes the relay which grounds the kill switch on the engine when the pressure switch goes off subsequently killing the motor and turning off the system circuitry.

I have a switch in the shop that activates the system power, excluding the main starter leads it is all very low ampers.

There you have it. Once the engine is warmed up thru one cycle, the system operates completely automated. Simple as heck and works like a charm. That 11 hp harbor freight engine cost 302 dollars to my door. The Eaton compressor is USA made and cost less than 500 to my door. It is rated for a 5 HP electric motor.

About 89 times better than that crap where the engine runs all the time and revs up and down and uses a centrifugal clutch pulley triggered by pressure actuated throttle mumbo-jumbo.

That sounds great!!! I'm gonna have to file that away. In my case, all I'd be interested in is "not having" a 5hp or larger motor and the power to it. But I DO have power, and (such as) my snow blower use 120V AC powered starter motors, so I could do that with NO battery needed.

Thanks 67, glad you like it.

So, snow blower that starts on 120vac so as to be batteryless.

I rather like having a battery start. I wish my mig welder was battery start. Its a pull rope. But it is on a cart so I can take it all over the farm and over to the neighbor's.