Air Compressors and Air Tanks

[Oceaneer99]

I thought I'd start an air compressor and air tanks thread.

First, a caveat:  compressed air can be dangerous, and there are many examples of converted air tanks that have gone terribly wrong.  You can buy a real 5 gallon air tank at the hardware store for $30 or so, so it is usually not worth the risk to convert some other sort of tank.  I can easily shift my attachments over to one of those later and not lose too much investment.  One of the main issues is water, which can quickly rust out a tank that was not designed to hold non-dry gases (such as an old helium balloon tank or propane tank).  If you don't believe me, ask Ray.  He knows of a machine shop that used a converted propane tank, and one day it exploded and blew out all of the windows.

Also, keep a reasonable upper bound.  There is currently a sale on a DeWalt pancake compressor and nail gun for $150 at my local big-box store.  I'm told its a good unit, but I'm not ready for a pneumatic nailer, and find it hard to justify a $150 compressor to airbrush my little models instead of using $10 paintbrushes (they're nice ones).  When I was a boy, I ran a toy airbrush using an inner tube squeezed under a sheet of plywood with bricks on top (sometimes I just sat on the tube as I painted!).  It sounds ridiculous, but I painted some nice plastic models with it, including a full camouflage Me-262 jet.

This weekend, I finally got my fire extinguisher to air tank conversion to work without major leaks.  I started with a 2.5 gallon water fire extinguisher, which had a broken feed tube (through which the water shoots up when you use it as an extinguisher).  I paid $2 for the extinguisher, which was also missing its nozzle tube.  The tank is thick stainless steel and was last pressure tested to 300 psi.  Normal operating pressure is 100 psi.  My original intent was to make some sort of super soaker water cannon out of it, but because it was designed to hold compressed water, I though that it might make an acceptable compact air tank for light-duty air brushing.

I removed the trigger handle and the plunger from the cap on the top of the extinguisher.  I had to re-tap the nozzle tube fitting, which is close to a standard NPT thread, but not cut as deep.  First, I had to grind away the back of the hole, which closed down inside the cap.  After that, re-threading was easy.  The cap is made of brass.

I had a hole from the trigger plunger, so I tapped that 1/4-20 and put in a brass screw with a rubber washer for sealing.

I replaced the "Empty - Full - OVERCHARGED" gauge and replaced it with a 0-120 PSI one which I removed from a flow regulator.  There is a Schrader valve on the other side of the cap, which I kept in place to fill the tank using a bicycle pump.  I put a short threaded pipe into the nozzle tube fitting, then an adapter and finally a quick-release receptacle.

I had to replace the O-ring under the cap, then was able to pump up the tank manually using a bicycle pump to check for leaks.  There were many, so I disassembled everything and this time used more Teflon tape and also some thread seal putty.

I attached a quick-release plug to the inlet of a pressure regulator (as I learned, a FLOW regular and a pressure regulator are very different) and another quick-release receptacle on the outlet side.  The regulator has its own gauge to show the output pressure.

I put a jar of USN Gull Gray model paint on my airbrush and tried it out with the tank at 40 psi and the regulator set at 25 psi.  It's my first time using this airbrush (Paasche, bought used long ago), but eventually I figured it out and was able to do some fairly fine work, a simulated camouflage pattern on a tongue depressor. 

The pressure started to get a little low, so I turned on my two-stage compressor (my son and daughter), who took turns keeping the pressure up at 40 psi.  I think we could probably get some more reasonable use time out of 100 psi tank pressure, but I haven't tried airbrushing wit that pressure on the tank yet (it's currently at 96 psi doing a leak check). 

We have a small tankless airbrush compressor and tried to use that to pump up the tank, but found that it tops off at 25 psi.  There is a 5 psi drop through the regulator, so that really won't work.

Yes, I could use the airbrush compressor to do basic airbrushing, but I'd like to be able to control the pressure better and also have something quieter.  My hope is that this tank can hold enough air to do at least a few coats of paint on a small model, plus clean-up.  I have to admit that getting the pressure up to 100 psi takes a lot of work on the bicycle pump -- it's a bit of an exercise program for modelers, but no one is going to be disturbed by the sound of a compressor turning on.

Garet

[lastvautour]

My first airtank/compressor was a tire and a hand pump. It worked fine. I now have a proper 10 gal tank and I did need a brad nailer, hence the new compressor. My previous compressor was a $69.00 special at my local hardware store. It lasted me nearly 5 years before the seals let go. The pump still moves, but the output is nil. Either way, compressed air is a powerful substance and care is to be excercised. I regulate my tank to hold no more than 50 psi of pressure and spray at between 25 and 30 psi depending on enamel or acrylic paint. This last me for several projects before it needs topping up.

REGARDLESS CAUTION CAUTION CAUTION