>-> The best by far is a unit off a central air conditioner. They're
 >-> quiet, fast and compact. Punch a hole into the top corner with your
 >-> air chisel
 > They really do work well Gerald, but I just use the normal low pressure
I use one. I estimate it's output at about 10 ft3/min.
 > connection and put a foam packed filter canister over it.   Pumping
Same here, but the problem though is you can't measure the oil level as the 
suction is on the side. So I add the port at the top and slide a welding rod 
down as a dip stick. Needs about an inch or so of oil. The pistons are 
horizonal at the bottom so don't get the oil level too high.
 > striaght UP to a surge tank where the oil can drain back down to the
 > compressor don't let as much of it get lost in the output air.
But how does the oil return to the tank? The reed valves in the outlet line 
will prevent back flow, so you'd have to add some sort of small bypass, no? 
What happens to all the water from compression? How much can you allow to 
accumulate with the oil?
 > We've learned to be very careful with the overpressure valve on AC
 > units.  Those things have no conscience and will pump up to SEVERAL
 > hundred pounds altogether too quick.  We have a local tale we tell about
I've tested these and very seldom do the open below 500 lbs. Way too high.
 > a guy named Jerry, who built what we now call a "Jerry Bomb" with a
 > refrigeration compressor and a water heater tank. He neglected to use a
This doesn't sound too good. 
 > RELIABLE pressure controller switch and launched his water heater
 > storage tank right through the roof of his shop!
 > We'd better not design a "Gerry Bomb" if we can avoid it. 
Compressed air is not to be fooled with. Good friend of mine tells a story of 
group of engineers testing a length of new plastic gas pipe. Why the they 
used air is beyond me but when it blew pieces flew all over the yard. They 
did clear the area beforehand and were standing inside the shop observing 
through the window of the overhead door. A chunk lodged in the door panelling 
right next to them. It doesn't take a lot of volume to be terribly dangerous.
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