Old 9kg gas cylinder = new air compressor tank

Submitted: Saturday, Jun 06, 2009 at 23:08
ThreadID: 69574 Views:30393 Replies:10 FollowUps:10
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Hi All,
Had an old 9 kg gas cylinder in good condition laying around. What I needed was a portable air tank to run the stapler and brad gun while doing odd jobs in the Goldstream Camper. I didn’t want to run long air hoses from my compressor, especially for a job that would take ten minutes.

I did a bit of research beforehand, and these puppies have a Test Pressure of 3 Mpa, or ~435 PSI to us folk. Mmmm, more than enough to make a compressed air tank out of I thought, considering I’ll be only using it at 100 – 130 PSI. Moreover, a 9 kg cylinder has the volume of about 22 litres – about half the capacity of the tank found on an “auto parts store” compressor.

I still wasn’t convinced, so I went to a website that listed pressures for LPG gas under pressure – just to be sure. Well, I found out that LPG in a full 9 kg bottle at 30° C is ~145 PSI. And that’s easily achievable in a bottle left in the sun or similar. So there should be no dramas using compressed air.

So, all is good, and here’s my solution;

The trickiest bit was converting the gas POL fitting on the bottle, to BSP fittings that air compressors use. It turned out quite simple. I had an old POL hose, so I cut the end off it, and that gave me the connection that fitted into the gas cylinder valve.

I then tapped the inner portion of the POL fitting (the bit with the O ring on the end) with a ¼” BSP die (to about 10mm long), to accept a “T” piece as seen in the photo. I then screwed the “T” piece onto the POL fitting (with thread tape) and screwed it to the cylinder.

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Go to major hardware store and purchase some air fittings. How you set it up is your choice, but I did it like this;

On the top of the “T”, I added an open/shut valve and a male air fitting. This is where I charge the cylinder from the air compressor, via a hose.

Next, I added a threaded barb to the end of the “T” piece, and attached a short piece air hose (blue in colour in the photos), that in turn went to the Regulator/Water Trap (Note: you don’t really need a Regulator, as the cylinder is only ever pressurised to what the compressor is set at – but I like gauges!).

On the output end of the Regulator, I fitted a standard Nitto female coupling ready to accept an air hose and the tool of my choice.

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Apart from a coat of paint and a note saying “Compressed Air Only”, jobs done! I charge the cylinder to 130 Psi (this is Max limit recommended by the makers of the Regulator/Water Trap), using the air compressor and then disconnect. The Regulator is set to 90 PSI (as recommended by the makers of the tools), and away I go!

Cheers, Matt
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Reply By: Members Paul and Melissa (VIC) - Saturday, Jun 06, 2009 at 23:15

Saturday, Jun 06, 2009 at 23:15
Have you tried it on the bradder and the stapler yet?? i am curious as to how long it runs them to the point that they will not drive the brads flush.
AnswerID: 368764

Follow Up By: Member - Matt H (SA) - Saturday, Jun 06, 2009 at 23:31

Saturday, Jun 06, 2009 at 23:31
Hi Paul,


I'm currently installing a 4 speaker sound system in the camper at the moment, and I drove ~ 2 dozen staples and a similiar number of brads without drama.

Why was I using staples and brads on a stereo system will become clearer later!

I'll test the system to it limits soon.

Cheers, Matt
FollowupID: 636283

Reply By: Skippype - Sunday, Jun 07, 2009 at 05:31

Sunday, Jun 07, 2009 at 05:31
That's a brilliant piece of work there. Simple safe and effective.
Well done.
AnswerID: 368773

Reply By: V8Diesel - Sunday, Jun 07, 2009 at 10:47

Sunday, Jun 07, 2009 at 10:47
Nice work!

Out of interest, have you tried blowing up tyres with it? I am a bit hazy today (hit the wooblah last night) and working out how many 305/70/16's I could reinflate from 15psi to 35psi with 22l @ 100psi is currently way beyond my shrivelled brain's ability.

AnswerID: 368810

Reply By: Member - Bushpig - Sunday, Jun 07, 2009 at 10:48

Sunday, Jun 07, 2009 at 10:48
Fantastic job Matt. Recon it would provide enough grunt for an air operated grease gun for ball joints, uni's etc? Would my Bushman air compressor be able to fill it?

I just handed in an empty gas bottle last week. Damm.

Thanks heaps for a great article.
AnswerID: 368811

Reply By: Ozboc - Sunday, Jun 07, 2009 at 11:12

Sunday, Jun 07, 2009 at 11:12
I am also curious, have you tried to pump up tires? would be great to have one of your gas bottle "reservoirs" in addition to the pump running to pump up tires after being on the sand - your take a 30 min pump up ( for me 6 tires ) and i am sure it would reduce the time to maybe 10 min

let us know !!

AnswerID: 368815

Follow Up By: Member - Roachie (SA) - Sunday, Jun 07, 2009 at 15:35

Sunday, Jun 07, 2009 at 15:35
I have a slightly smaller tank; 16 litres....a GME "Re-charge tank".... mounted in the back of my dual cab Patrol. It is fed by a decent-sized Grand Boss compressor.

In my experience, what happens is that you get a good blast of air initially but it wouldn't be enough to blow my 315/75s back up to 40psi from 15psi.

So, what happens is that I hook up the hose to the tyre and the pressure in the tank drops to less than 75psi (which is when the compressor cuts in) after about 10 to 15 seconds. From that time onwards, the compressor is trying to pump up both the tyre AND the tank.

Ideally, you'd have a bypass valve fitted so that once you've depleted the compressed air supply available in the tank, you shut it off so that the compressor is directing all it's efforts to the tyre instead of the tyre and the tank combined.

You'd need a huge massive tank (or a much larger pressure, like they have in those "Air To Go" tanks) to be able to pump up 4 or more tyres without a compressor to finish the job.

FollowupID: 636334

Follow Up By: Trevor R (QLD) - Sunday, Jun 07, 2009 at 16:00

Sunday, Jun 07, 2009 at 16:00
Roachie would you be better off having the air hose permanently connected to the tank? Make it is long enough to reach each tyre and just snap on the valve connection when you need to pump up the tyres? This way you would not loose all your volume filling up your hose when you connect it up. It may not make that much difference but I reckon it might????

Cheers, Trevor.
FollowupID: 636336

Follow Up By: Member - Roachie (SA) - Sunday, Jun 07, 2009 at 19:53

Sunday, Jun 07, 2009 at 19:53
G'day Trevor,

Yes mate, I already have the whole thing plumbed up permanently with Ryco females front and rear. The tank and whole system is always fully pressurised. As soon as I start the truck up (and let it warm up for a minimum of 3 minutes), I start flicking switches on. It's a bit like a pilot in a plane in my cabin!!! hahahaha

1.) Turn key on with right hand (and wait for glow plug light to extinguish) whilst using left hand to depress the reed switch (located down next to gear lever) that stops the oil pressure buzzer from going off it's teets until oil pressure has built up.

2). Switch on: Toggle for power to Nuvi 660 GPS : Toggle for air compressor : Toggle for voltmeter (switch between both banks of batteries to make sure both are getting charged-up) : check that air pressure is building up by looking in passenger's side mirror, as gauge is mounted on the front side of the steel canopy : turn of toggle for ipod base station etc etc

Air pressure is required for the air horns to operate, that's why I always have to have the system pressurised.

When I'm airing up, the hose is connected to either front or rear outlet. The other end of the hose has an automatic stop cock, so no air escapes until I hook it onto a tyre. That end of the hose also has a Ryco female fitting, so I can plug the blow-down dust gun etc into it as well.

Cheers mate

FollowupID: 636358

Follow Up By: Trevor R (QLD) - Sunday, Jun 07, 2009 at 20:30

Sunday, Jun 07, 2009 at 20:30
One good thing with a system like yours: no one can borrow your truck as you need a degree in ROACHIENOMICS to get the thing going LOL!!!

I should have guessed that you would have all bases covered but thought I would ask anyhow.

Cheers mate,
FollowupID: 636368

Reply By: Member - Ian F (WA) - Sunday, Jun 07, 2009 at 14:01

Sunday, Jun 07, 2009 at 14:01
G'Day Matt,
Have you thought of how to drain drain water from the tank which depending on the humidity could be considerable (ie corrosion in the tank)?
AnswerID: 368829

Follow Up By: Member - Ian F (WA) - Sunday, Jun 07, 2009 at 14:32

Sunday, Jun 07, 2009 at 14:32
I should imagine it would be excellent for fitting tubeless tyres in the scrub,
FollowupID: 636329

Follow Up By: Member - Roachie (SA) - Sunday, Jun 07, 2009 at 15:38

Sunday, Jun 07, 2009 at 15:38
Given that it's designed as a portable tank, it should be quite easy for him to open that top ball valve, invert the tank and the water would drain out.
FollowupID: 636335

Reply By: Member - Matt H (SA) - Sunday, Jun 07, 2009 at 21:45

Sunday, Jun 07, 2009 at 21:45
Thanks Guys for the feedback! I’m replying to you lot as a whole, otherwise I’ll be typing to midnight!

Anyhow, I only finished the conversion and brief testing hours before posting the pictures. I was chuffed with the results and wanted to show the ExplorOz fraternity as soon as I could. Self indulgent to be sure!

What an air compressor will do is only limited by two main things; one is the pressure (as delivered by the compressor pump), and the other is the volume of air available (ie; the size of the tank or reservoir). With that in mind, my tanks is *only 22 litres* and therefore is limited in its capability.

Hypothetically, I might be able to inflate a tractor tyre to 1 PSI but be able to inflate 10 bicycle tyres to 40 PSI. I figure a 4wd tyre would lay somewhere in between. That said, it might just give you that initial surge of air to reseat a tyre, and allow you to pump the tyre up normally. Bushpig ask whether his Bushman air compressor would work – sure, it will work, it will just take longer to recharge the cylinder before using. As long as the compressor can supply 100 PSI+ to the cylinder, all is good (even the elcheapo versions can do this, but not for that long!).

As for Rattle/Grease guns, yes, it would work – but would limited for the reasons I have mentioned above.

Draining the tank. In this case, Ian F asked the question, and Roachie has beaten me to the answer. Given that my compressor has a water trap fitted, I figure only a minimal amount of water will enter the tank when I pump it up. If water proves a problem, I will do one of two things; open the main valve and invert or, braze a drain plug into the bottom of the bottle. At this stage, I will see how it goes.

My local dump (or *transfer stations* as they now are called), always have a supply of old cylinders that are handed in for disposal. Or, ask others on the forum here, as I’m sure many others will have something available – well, they might not now after reading this!

For the record, a mate has converted an old household cylinder (the one’s about 4 ½ foot high), in the same manner as I have, and it works perfectly. To carry that a step further, I can see no reason why you couldn’t use an old car LPG gas tank (even the spare wheel well type) as a starting point.

You never know, it might be a “get out of jail free” card when you’re in the bush – providing you have a onboard compressor to charge it.

Happy Pumping!

Cheers, Matt
AnswerID: 368872

Reply By: austastar - Monday, Jun 08, 2009 at 18:44

Monday, Jun 08, 2009 at 18:44
I'm very nervous that 'clang' may turn into 'bang'.

Some what reassured by your research figures of 140 PSI for LPG and test pressure of 435 PSI in a NEW cylinder.

I'm not an engineer or any thing clever in mechanics, my only reason for doubt, is having seen the thickness of the walls in an LPG cylinder and the thickness of the walls in a commercial garage compressor.

Do treat it carefully when charged.

Brilliant article though, thanks for sharing, and sorry for being such a scaredy cat.

AnswerID: 368979

Follow Up By: Member - Matt H (SA) - Monday, Jun 08, 2009 at 22:21

Monday, Jun 08, 2009 at 22:21
No apology necessary, Austastar.

I did my homework as best I could. That said, I treat ANYTHING under pressure with caution – as you should.

By the way of comparison, I checked two other pressure vessels in my garage tonight.

1.A 1kg BCF fire extinguisher has a Test Pressure of 2 Mpa (290 PSI), and
2.My 40 litre air compressor has a Test Pressure of 1.35 Mpa (195.8 PSI).

It’s funny, we have to get LPG gas cylinders tested every 10 years, but not air compressors. Why is that? Well, it might have something to do with the gas involved, rather than the cylinder itself.

When a gas cylinder is tested, the testers are checking the valve more so than the cylinder itself. The brass valve assembly is the only component changed when a cylinder is tested. Hence, the cost of testing usually means most of us throw away a perfectly good cylinder as it’s cheaper to buy a new one!

To be honest, I’ll trust the integrity of a gas cylinder to 100 PSI long before I’d trust a air compressor tank at the same pressure!

If using a LPG cylinder doesn’t appeal to people, I know someone who used tanks from a truck braking system to good effect. Again, I’d trust the LPG cylinder first. If there is a weak spot in my “invention” it would probably be the flexible hose and clamps joining the cylinder to the regulator. I’d reckon that would fail first.

Caveat Emptor.

It works for me, but feel free to try something different.

Cheers, Matt

“Ingenuity is only limited by the imagination of the individual”
FollowupID: 636527

Reply By: austastar - Tuesday, Jun 09, 2009 at 15:34

Tuesday, Jun 09, 2009 at 15:34
I'm not sure how LPG cylinders are checked and tested, apart from a physical 'eyeball'.
I know diving cylinders (yep I know 1,400psi - scary) are hydro statically tested yearly. i.e. full of water, a measured quantity is forced in, and if less than that is returned, the cylinder is losing its elasticity and condemned.
The price I was quoted for having a 10year old LPG cylinder re-certificated was nearly the price of a new cylinder, which I ended up buying. At that price I had presumed a hydro static test would be involved, but I may be wrong there.

Did you unscrew the valve and check the interior surface?

It is usually done with a small torch bulb on a pair of wires inserted through the valve opening, and some of the inside can be seen reasonably well.

AnswerID: 369124

Follow Up By: Member - Matt H (SA) - Tuesday, Jun 09, 2009 at 22:36

Tuesday, Jun 09, 2009 at 22:36
Hi again austastar,

You'd be lucky these days if the "testers" did any more than repalce the valve assembly and stamp the bottle neck with the test date! Hydrostatic testing would a luxury that most testing stations could not, and would not wear. Time is Money, as you know!

I have easy accessability to a device called a Boroscope which is just 4mm in diameter and can supply illuminated colour video of whatever you wish to look at. For the uninitiated, it's a snake like cable with a camera lens on the end, controlled by a handpiece that can be stuck deep into small areas to view. Very similiar devices are used in hospitals for routine check ups, and include ones that end in the word "oscopy". You figure it out!

Given the cylinder I converted, has been empty - but sealed since it was emptied many years ago, I'm pretty confident that the insides are in good nick. In fact, I'd be more than a bit worried if I saw much (if any) corrosion on the inside of cylinder that had contained nothing but LPG in it's entire life!

Unscrew the valve? When the Governor of California next visits Adelaide, I'll see if I can borrow him for couple of minutes!

I'm aware they are a left hand thread, but I'm buggered if I could budge it! But for the reason mentioned above - I'm not that worried about looking insie of it.

Cheers, Matt

FollowupID: 636683

Reply By: austastar - Wednesday, Jun 10, 2009 at 09:22

Wednesday, Jun 10, 2009 at 09:22
Well Matt, it looks like you've covered all bases, and have made a very useful addition to both your workshop and the general knowledge base of EO members.
Thanks again for the article and sharing it with us.
AnswerID: 369267

Follow Up By: Member - Matt H (SA) - Wednesday, Jun 10, 2009 at 20:39

Wednesday, Jun 10, 2009 at 20:39

Pleasure mate. Sometimes coming from an engineering and tinkering background comes in handy!

Ask me to fix your computer and I stumped though!

I'd love to share more projects/ideas with you folk, but my work tends to get in the way a lot of time (Defence sort of does that at times!).

When I finished the stereo install in the Goldstream, I'll take photo's and post.

As I mentioned earlier, Ingenuity is only limited by the imagination of the individual!

Cheers, Matt
FollowupID: 636819

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