Rating Air Compressors

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 => . 53 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)
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150 psi max.

I only saw one on the HD site and it was a with two nailers. That does not appear to be a very beefy motor.
I think the size of the motor will go a long way towards the duty rating and the life of the unit.
My old CH extreme duty pancake is rated at 1.5 and the motor is big and heavy. So heavy that I often take a smaller unit for small one gun jobs. It will drive an air chisel which greatly exceeds its specs for short bursts of use.
I concur in your opinion that the time it takes to bring an empty tank to full pressure is a good indicator of performance.
Colbyt
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