Checking a used compressor

A good quite accurate way

There's a good rough-and ready check of pumping capacity. As atmospheric pressure is 15 p.s.i., every time the air tank increases in pressure by another 15 p.s.i. the compressor has pumped that tank's volume of free air into the tank. To a good approximation. The air is a bit heated up in the tank after the pumping, but not by much. It's cooled down a lot after coming out of the pump and is nearly back to room temperature. And other such minor effects. But we are only looking for a rough measure that all is well.

So you get a watch with a seconds hand and time how long it takes to pump from 90 p.s.i. to 105 p.s.i.. We want FAD at 90 p.s.i. and by timing from 90 p.s.i. to 105 p.s.i., your figure is for "at least" 90 p.s.i.. If the compressor's air tank has a capacity of 50litres, that is how much air from the atmosphere has been pumped in the tank. Covert from litres to cubic feet and divide by the time taken in minutes and you get the FAD in c.f.m.! Rough but shows whether all is well.

A calculation I did for my compressor is here. It's expressed in the computing language Lisp, which might mean you are best only seeing the general idea rather than looking for an example to copy.

Less exactly, if the compressor took a long time to come to cut-out pressure, that would indicate all was not well. A 3HP portable with a 50litre tank should take around two to three minutes to come to cut-out pressure.

By the way - a 3HP portable air compressor will not continuously keep up with the air demand of most air tools - so there is no basis for a test here.

Someone told me at a bus-stop once... checking the valves

Someone told me at a bus-stop once that there is a way to check the valves of a compressor. It isn't something one could ever recommend doing. If you take the air filter off a compressor and the run the compressor, if you put your hand over the air inlet (!!!) you should feel a constant suction on the palm of your hand. If any of the valves are defective, you will feel an alternate sucking and slight blowing. Now, this practice would definitely be forbidden. For a start, on a good working compressor, it is going to more feel like "something grabbed your hand". For two, you are going to have to pull your hand off the compressor. The maximum force that can be exerted is 15 pounds force for every square inch of passage way you are covering, by reason of what atmospheric pressure is. So if the passageway is a 20mm by 15mm hole, you are going to be able to pull your hand away. But don't blame anyone else if you die of a heart attack. And don't do this to any compressor above one you might find in a garage (auto-repair shop).

(Richard Smith, June 2005)