This is about the dumbest thing I have heard for a very long time. A typical air compressor won't reach much above 120-150 psi, or roughly 10 atmospheres at best. So, just eyeballing it, I'd say you might get something like 5 or 6 cubic feet of air into a 20 lb CO2 tank and a whole lot less than that in the 10's and 5's. The pressure will drop steadily as the air is depleted, unlike a cylinder filled with liquid CO2. So, lets say you have it hooked up to a nail gun or an air impact wrench. You might be able to run the air impact wrench for about 30 seconds before the pressure dropped to a worthless level. The nail gun might go a little further as they use less air, but it will still be a short run and hardly worth the effort. You also risk introducing moisture into your CO2 tank which can cause interior corrosion problems. Not a good thing. Now then, regarding using it for beer, IIRC, air contains something like 19% oxygen, 79% nitrogen and 2% other gasses including CO2. So, you would be introducing a lot of oxygen and very little CO2. IOW, it won't work at all for force carbonating or serving. The exception would be for serving from a keg that will be consumed in short order. That would be no different than using an air type party pump on a keg. Then there is the risk that the air compressor may actually introduce some lubricating oil into your CO2 tank along with the air. Again, not a good thing and it would be expensive to correct properly. I don't know where the a-hole got the notion to time the air compressor. This, like the rest of that shiit, makes no sense at all. Air compressors vary widely in their output and pressures, making the timing suggestion nothing short of stupid. CO2 is not very expensive and a tank typically lasts quite awhile. JMO...nothing else.
