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Re: [Shop-talk] single vs. dual stage compressors

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Subject: Re: [Shop-talk] single vs. dual stage compressors
From: "Karl Vacek" <>
Date: Fri, 21 Nov 2008 07:38:00 -0600
I can't address the meaning of the Chinese website's specs.  My electric die 
grinder has at least the power of most air ones, and it uses a few amps at 
110 volts.  So are you telling us that this wonderful piece of Chinese 
technology takes the equivalent of over 25 amps at 220 volts ?

I don't want to belabor the point further, and I bet most of the list 
doesn't either.  For anyone interested, there are lots of good explanatinois 
on the Internet.  Here's oone, and it includes not only an explanation of 
CFM and the various misconceptions, but it also goes into horsepower.  It 
includes an example of evealuating a Chinese brand's claimed performance vs 

To the CFM point, I suggest the fifth paragraph in the "example" section:

"When a compressor pumps one "CFM" (cubic foot per minute), that means the 
intake port inhaled one cubic foot of "free air" (air at atmospheric 
pressure). (Note: A CFM does not mean in any sense the compressed volume.) 
So the unit really measures the mass of air flowing per minute, not volume. 
Some people labor under a stubborn misundertanding that these units refer to 
the flow of compressed volume (as opposed to free air volume), but this is 
flatly wrong."

Back when I worked in the paint spray industry, we assembled and sold the 
range of Quincy compressors (in addition to a few little diaphragm ones for 
airbrushes, etc.).  That was the real Quincy, not whatever the name has 
become today after being sold.

We always preferred the term "free air", as it's easiest to understand. 
SCFM is sort of a standard, but everyone who uses it tends to mean something 
different.  In the end, it means free air under atmospheric pressure and 
normal temperature and humidity.


>> That's not quite right.  The die grinder needs its air supply
>> at 90 PSI, but in a minute it will exhaust 4 CFM of free air,
>> not 4 CFM of 90 PSI air or 28 CFM of free air.
> Note where it says "4.5 cfm (32 scfm)".
> Randall 
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