Anyone here know of a table or spreadsheet anywhere online that will
allow you to estimate the actual approximate CFM rating of a
compressor? Seems to me you ought to be able to measure the length of
time it takes for the tank to reach several different pressures and
then plug that along with the tank size into an equation to get a
reasonable estimate of the true (approximate) CFM. Anyone have
Avogadro's number memorized? If so maybe I could give him a call...

Dave wrote:

Anyone here know of a table or spreadsheet anywhere online that will
allow you to estimate the actual approximate CFM rating of a
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
--
Motor Modelle // Engine Models
http://www.motor-manufaktur.de
DIY-DRO - YADRO - Eigenbau-Digitalanzeige

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:
Dave wrote:


Anyone here know of a table or spreadsheet anywhere online that will
allow you to estimate the actual approximate CFM rating of a
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


--
http://www.fergusonsculpture.com
Sculptures in copper and other metals

Richard Ferguson wrote:
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

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 ?

You bet! I may not remember much from college, but 6.023 times 10 to the
twenty third has worked every time when I needed Avogadro's number.
Thanks for waking the old memory cells
Bert
"Dave" wrote in message
oups.com...
Anyone here know of a table or spreadsheet anywhere online that will
allow you to estimate the actual approximate CFM rating of a
compressor? Seems to me you ought to be able to measure the length of
time it takes for the tank to reach several different pressures and
then plug that along with the tank size into an equation to get a
reasonable estimate of the true (approximate) CFM. Anyone have
Avogadro's number memorized? If so maybe I could give him a call...

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:




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 ?



--
http://www.fergusonsculpture.com
Sculptures in copper and other metals

The Tagge's wrote:
You bet! I may not remember much from college, but 6.023 times 10 to the
twenty third has worked every time when I needed Avogadro's number.
Thanks for waking the old memory cells
Bert
"Dave" wrote in message
oups.com...

Anyone here know of a table or spreadsheet anywhere online that will
allow you to estimate the actual approximate CFM rating of a
compressor? Seems to me you ought to be able to measure the length of
time it takes for the tank to reach several different pressures and
then plug that along with the tank size into an equation to get a
reasonable estimate of the true (approximate) CFM. Anyone have
Avogadro's number memorized? If so maybe I could give him a call...





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

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."

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


Richard Ferguson wrote:
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:

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 ?

Dave writes:

Anyone here know of a table or spreadsheet anywhere online that will
allow you to estimate the actual approximate CFM rating of a
compressor?

http://www.truetex.com/aircompressors.htm

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

Richard



Richard J Kinch wrote:
Dave writes:


Anyone here know of a table or spreadsheet anywhere online that will
allow you to estimate the actual approximate CFM rating of a
compressor?


http://www.truetex.com/aircompressors.htm


--
http://www.fergusonsculpture.com
Sculptures in copper and other metals

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

Richard



Richard J Kinch wrote:

http://www.truetex.com/aircompressors.htm