I have noticed that lately my Sears 115VAC compressor is having more
trouble than I remember keeping up with my 1/4 inch die grinder. I
started taking some measurements and doing some calculations. I have a
few questions:

Why would a 1/4 inch die grinder use more air than a 6 inch dual action
sander? This makes no sense to me at all.

Are my calculations below roughly correct?

How big a compressor do I really need? I would like to be able to run
my die grinder at almost 100% duty cycle.

Richard



Air compressor calculations:

Tank Size: 25 gallons equals 3.3 cubic feet.

Experimental data:
The compressor took 1 minute 45 seconds to increase the tank pressure
from 90 to 120 psi.

The IR 1/4 inch die grinder, operating at 90 psi with no load, took the
tank pressure from 120 to 90 psi in 25 seconds, with the compressor off.
(For comparison, a couple of inexpensive die grinders from Taiwan drew
down the tank in around 19 seconds, so used even more air).

The IR 6 inch dual action sander, operating at 90 psi with no load, took
the tank pressure from 120 to 90 psi in 37 seconds, with the compressor off.

Calculations:

Ignoring the change in temperature as the pressure is changed, a 30 psi
pressure change on a 3.3 cubic foot tank is equal to 6.6 standard cubic
feet of air.

If the compressor takes 1.75 minutes to deliver 6.6 standard cubic feet
of air, that would be 3.77 SCFM.

If a die grinder takes 25 seconds to use 6.6 standard cubic feet of air,
that would be 15.8 SCFM.

If the dual action sander takes 37 seconds to use 6.6 standard cubic
feet of air, that would be 10 SCFM.

From the Ingersoll-Rand site, the SS3L3 compressor is rated at 11.3
ACFM at 40 psi.
11.3 ACFM at 40 psi would be 30.7 SCFM, as I understand it, again
discounting temperature effects. The SS3L3 is a single stage 220VAC
single phase compressor rated at 3 HP, (a more honest rating than most).

Altitude:

Since I live at about 5400 ft (1700m) elevation, any compressor in my
shop loses efficiency due to lower air density. I understand that the
air density here is 15-20% less than at sea level, so I would expect the
altitude to reduce compressor output by the same factor.