Its been a while since I looked at home air compressors. Recently,
Home Depot has been selling a Porter Cable pancake AC with these
specs: 6 gallon, 150 psi max, 2.6 scfm @90psi. There is not longer
mention of horsepower. It appears that in 2004 the government sued to
get more honest ratings. However other than specs, one way to get a
true measure of capablity is to measure the time it takes to pump a
tank of known volume from a known starting pressure to a known ending
pressure. Then the true CFM is the difference in starting and final
pressures, times the volume ot the tank, divided by the time it takes
to pump it up. The pump cycle can also use the cut-in to the cut-out
pressure for starting and ending points.
My old Craftsman has these ratings on its label: 4 gallon, 3 HP, 5.7
scfm @90psi, 150 psi max.
1. Tank Volume in cubic feet: 4 gallon divided by 7.48 gallon/cf => .
2. Cut-In: 130psig, Cut-out: 155 psig => +25 psig added or 25 divided
by 14.7 psi/atmosphere = 1.70 atm
3. Time to pump up => 9 seconds or 9/60 minute = .15 minutes
The calculation for my Craftsman is (.53 cf * 1.70 atm) / .15 minute 6.01 cfm (measured at 68 degrees F)