air compressor CFM comparison

[bdog]

It seems that there is a lot of variables in how CFM is stated on compressors. Different companies tend to overrate their numbers, etc. I am sure there is some sort of equipment out there that can test this but to me it seems that a quick and easy test for the end user to do is the pump up test.

I do not know the formula to translate it to CFM, but it seems that an easy way to compare units is to time them in how long it takes to go from 0 to 100 psi. Assuming you have the same tank size (or by accounting for differences) it should be easy to tell which compressor is better.

Maybe even some members could do this test so we could all look at it for comparisons sake and to help in purchasing decisions.

1) time in seconds from 0 to 100 psi
2) tank size in gallons
3) compressor details: brand / model / rated CFM

 

[Major Ramifications]

I would think that the 0-100 test would penalize compressors that pump up to 150 or 175 against the ones that shut off at 125. This is because the CFM@PSI curve starts to flatten out at higher pressure.

 

[bdog]

Ok lets change it then. two categories.

If you compressor goes to 125 pump up from 0 to 125
If your compressor goes to 175 pump up from 0 to 175

Just looking to get some real world numbers here.

 

[Iron-Iceberg]

Old Car Guy has a page on this site with his 2 compressors going up to 150 psi I believe. Cant find it now.

 

[PAToyota]

I would think that the 0-100 test would penalize compressors that pump up to 150 or 175 against the ones that shut off at 125. This is because the CFM@PSI curve starts to flatten out at higher pressure.

Actually, the pressure curve is going to be logarithmic. It takes exponentially more energy to get from 100 to 200 psi than it did to get from 0 to 100 psi.

So a compressor designed to run up to the higher pressures is going to get to 100 psi easier because it is designed with those exponential pressures in mind. The 0-100 psi compressor is going to be working hard at 100 psi because that is the upper limit it was designed for.

So looking at the 0-100 range of any two compressors would be a good comparison - unless one of them didn't go to 100 psi and you were extrapolating...

 

[Mugenlude]

I did something similar to this in the past on my compressor...
http://www.garagejournal.com/forum/showpost.php?p=261091&postcount=12

There is a link in that post to the page which I found the calculation I used... then I put them into Excel to save me time.