Chinese CFM ? -- Compressor

Volume (of tank) * pressure (in tank; value in bar) = aspirated volume. If the tank is pressure-less, start your stopwatch, stop it when it is full. Divide aspirated volume by that time (in minutes). That's only a guesstimate, because efficency will decrease with rising pressure and the formula also doesn't consider rise of temperature. But you have a good ballpark.

Nick

A variation on that theme is to measure the time required to increase the pressure of the tank by 15 psi (14.7psi is atmospheric pressure at sea level.)

Let me try a sample calculation.

25 gallon tank x 0.1337 cuft/gal = 3.34 cubic feet

If it takes 30 seconds to increase the tank pressure 15 psi, it would take one minute to increase the tank pressure 30 psi (disregarding efficiencies). 30 seconds is half a minute.

3.34 cubic feet / 0.5 minute = 6.68 CFM

Richard

Nick Müller wrote:

Thanks, that is very straightforward. So if I measure the time from

60psi to 75psi and plug that in I should get a conservative value of true CFM for 60psi ?

I don't know about a conservative value, but you would have a ballpark value. You can use the reverse process to figure CFM for tools, turn off the compressor and measure the time it takes the tool to drop the tank presssure from 105psi to 90 psi. As others have noted, the measured CFM for a compressor will probably be less than the published CFM, while the measured CFM for a tool will probably be more than the published CFM. :-( In other words, get a bigger compressor than the CFM numbers suggest would be adequate.

You would need to convert gallons to cubic feet, but the sample calculation showed the conversion factor.

Richard

Dave wrote:

It's also 781-721-1130, our second home phone line. I listed that line under "J. Avogadro" when we got it years ago, further proof that I'll never grow up.

Before they outlawed residential telemarketing it was fun to respond to callers asking for "Mr. Avogadro" with, "Sorry, he passed away." They usually offered condolences and I'd say, "We'll we're pretty much over it, he died in 1856."

I looked recently and found there's three other phone numbers for Avogadros in the US, all in California. One of them is in the name of "No Avogadros", obviously another jokester, and the other two are listed in the name of "Amadeo Avogadro", but my references to the number guy all show his first name as "Amedio."

Jeff

Oh, I meant it would be a conservative value because it is being measured above rather than at the specified pressure.

Richard Fergus> I don't know about a conservative value, but you would have a ballpark

Thanks for posting the link to that detailed explanation. It confirmed my understanding and calculations. ;-)

Richard

Richard J K>

I'm getting the feeling that an actual Chinese CFM is about 50% of what the rating claims. The Truetex link suggests a big fudge factor. Is this all due to the unknown temperature change? What if you start with a cool compressor, run it, and then let it cool down again?

Richard Fergus> Thanks for posting the link to that detailed explanation. It confirmed