[Shop-talk] single vs. dual stage compressors

[Randall]

> if the die grinder says its needs 4 cfm @ 90 psi and the 
> compressor says 
> it'll provide 16 cfm @ 90 psi... What's the problem?

The problem is that the rating of the air compressor is in cubic feet
_after_ it has been expanded back to atmospheric pressure ... in spite of
what it says.  Another way to look at it is cubic feet into the intake
rather than into the tank.

You can easily see this by looking at how long it takes the compressor to
fill it's own tank.  For example, a typical 110v compressor might be rated 6
cfm and have a 20 gallon tank.  But 20 gallons is only about 2.6 cubic feet,
so if the compressor were really putting out that much air, it would shut
off in less than 30 seconds.  Instead it takes several minutes to even reach
90 psi.

Randall