# Do CFM ratings on HF compressors 'lie'?

• posted on October 27, 2011, 5:04 pm
We all know about the 'peak horsepower' lie... but what about CFM?
My \$100, 8-gallon HF compressor is rated at 4.5 SCFM @115 psi. There are 7.48 gallons per cubic foot meaning that my 8 gallon compressor is just a little over 1 cubic foot.
Assuming I am basically at standard pressure & temperature conditions, this would seem to imply that my compressor should fill in under 15 seconds, i.e., (8/7.48)*(1/4.5)*60
But it takes more like 2 minutes to fill...
What am I missing here?
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• posted on October 27, 2011, 5:35 pm

The charging flow's resistance isn't linear as the pressure increases in the tank. The answer to the original question is no.
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• posted on October 27, 2011, 8:36 pm
On 10/27/2011 1:04 PM, blueman wrote:

You appear to be expecting approximately 1 CF of air to fill the tank. And it will, but not to 115PSI.
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• posted on October 27, 2011, 8:50 pm
On 10/27/2011 3:36 PM, Mike wrote:

And, when account for that, (115 + 15)/15 = 8-something so the SCF input required is 8X times the final volume. Thus 8X the 15 seconds is for 1 SCF yields about 2 minutes for 8. Amazing!!! :)
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• posted on October 28, 2011, 6:31 am

DOH me!!!! of course I forgot the old pv=nrt and therefore it's not 1 SCF in there but as you say (115+15)/15...
That makes a lot of sense of course...