Used Air Compressor: Questions before purchasing?

[G. Brad Schmidt]

I'm looking for any pertinent questions I should ask the seller and tell-tale signs to look for on the compressor?

From what I've researched checking the oil and air filter will both give a brief history. Should I take along an honest air-pressure gauge to check actual tank pressure?

I'm looking to buy:
220v 20-30 gal. oil lube in the 8 CFM range (for my metal working side of the shop). I have a small shop so mobility is a must.

[Brian Backner]

One of the most important things to consider with any used compressor is how religiously it was drained of moisture during its working life. I know one guy that has NEVER drained the water in over five years! It is a bomb waiting to go off in his garage as the inside of the tank will rust where the trapped water sits.

When I've looked at used compressors, I have it disconnected from any and all piping, fired up, and brought to full pressure. Can't be turned on for whatever reason? WALK AWAY.

Once up to pressure (I would take a short length of air hose and my own pressure gauge to fit on the end of it - I don't trust the gauges on the tank) I turn it off and let it sit for a minimum of ten minutes to see if there is any pressure drop. If there is ANY pressure drop, take some soapy water (I carry a Ball Mason jar half full with a chip brush for this) and wet down all of the fitting from the tank to compressor to your own pressure gauge. If you can find a leaky fitting, try tightening it with an adjustable wrench. Once you have all of the fitting tight and leak proof, wait another ten minutes and see if the pressure stays constant. Still leaking air? It might be a pin hole in the tank somewhere - or a leak in the compressor pump itself (think "dangereous" for the former and $$$ to repair for the latter). WALK AWAY

If it holds constant pressure for ten to fifteen minutes, open the drain valve and catch some of the water that pours out in another Ball Mason jar. If you see clear water with no rust at all, buy it if you can live with the asking price. If you see only rust tinged water, it is probably OK but you can use that to bargain the price down. If you see big flakes of rust - you got it - WALK AWAY.

If you buy a big commercial compressor from a business, I would ask to see their maintanence log first. I know it is required here in Massachusetts and maybe by OSHA as well.

If I were going to be buying a big compressor - say 25HP/200 gallon +, I would actually pay to have the tank pressure tested by an ASTME certified lab.

Why am I so anal about this - my brother's neighbor had a relatively small Sears Crapsman compressor ("5" hp and 30 gallon tank) in his garage. It was about 10 or 12 years old, had been drained maybe once or twice in its entire life and was always left plugged in and on (if you can believe it). One day it blew up and leveled most of the garage - the gas water heater that was located in the garage next to it got blown up in the process and the resulting fire took out the rest of his house and the back porch on my brother's house before the fire department could get it under control. Took something like two years to settle all the insurance claims.

This was ten or so years ago. For me to buy anything but a new compressor now, it would have to look like somebody had been keeping it on the mantle in their living room.

Brian
Taxachusetts

[G. Brad Schmidt]

I really appreciate your in-depth reply Brian. Especially about bringing some soapy water; I would have never thought of that ingenious little trick. A few other boards have stated rust is always a problem, but some added it is also almost always present (around the drain cock)?

With the gauge you brought to the seller, did you replace your gauge with the compressor's stock gauge or simply hook it up via an air line?


Thanks, Brad

[Brian Backner]

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The latter - I just take a short, 2' or so, air "whip" with standard quick disconnects as well as a 200 psi air gauge adapted to a male disconnect plug to go on the end of the whip..

Brian
Taxachusetts

[G. Brad Schmidt]

Thanks for the clarification.


Cheers, Brad