Join Date: Dec 2001
Location: Belleville, Illinois
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Re: OBA calculations..
Compressors are rated in SCFM at some output pressure. SCFM stands for standard cubic feet per minute. "Standard" means normal atmospheric pressure and temperature - it's really defining how much air the pump will take in.
With your 50 gallon tank, every time you put another 50 gallons of air into it, the pressure will go up by about 15 PSI. To get your tank from atmospheric pressure to 150 PSI you will have to put in 10 times 50 gallons. There are about 6.4 gallons per cubic foot, so your compressor will need to pump 10 X 50 / 6.4, or about 78 cubic feet. With a 1.2 SCFM compressor you're looking at 65 minutes.
Now, there are two more factors to consider. First is that single stage compressors usually get pretty inefficient above about 125 PSI, so you will typically see them rated so many SCFM at 90 PSI or 100 PSI. In that case, they might only pump half their rating at 125, and might never make 150.
Second, if you're using an AC compressor, it's not designed to pump air to continuously higher pressures, and doesn't have the cooling fins that air compressors do. As it gets hot it will loose efficiency and pump less.