Here’s an interesting page about some aspects of oxygen:
http://www.ozonesolutions.com/info/is-medical-oxygen-different
They are not doing too well on the first sentence
21%
(they also cannot spell “gases” but hey it could be much worse; they might have written gas’s 
)
They are right in that somebody could put some dirt in a cylinder and take it back to the welding gas depot, as a joke, and hope that when that cylinder goes back to the oxygen manufacturer for a refill, they don’t spot it. How likely is that, however? It would eventually be spotted because welding oxygen needs to be very pure, and even welding oxygen cylinders go back for a periodic inspection of the internals.
In most cases I am aware of, there is no way to keep your own full size cylinder and get it refilled. I have seen it done (in Greece) but every commercial user who uses cylinders just swaps them out for fresh ones. It’s much more practical.
Yes, it’s 21%. we all know that :-) (They have it right in the picture next to the sentence though)
Get friendly with your local gliding club….they will have a refill setup…
They are right in that somebody could put some dirt in a cylinder and take it back to the welding gas depot, as a joke, and hope that when that cylinder goes back to the oxygen manufacturer for a refill, they don’t spot it. How likely is that, however?
I think you might be missing what they are trying to say. I think they suggest that an oxygen cylinder shouldn’t be left open, allowed to go to ambiant pressure. Instead of removing the valve and inserting dirt as you seem to suggest. If their is no pressure difference and the valve is open regular air might enter (with a higher mosture content). The only way to remove this is to create a vacuum first.
This is also described in manuals of aircraft that have a fixed oxygen bottle onboard. A maintenance organisation is not allowed to fill up when the bottle is completely empty. Quite some can fill up, but can’t fill completely as they haven’t got the equipement to create a vacuum first.
Yes, that is true.
But almost nobody returns a cylinder that is completely empty because the connected equipment will stop working long before then.
In oxy-acetylene welding (the main use of oxygen from welding depots) when the 3000psi drops to something like 50-100psi, the cylinder is finished. A lot of e.g. jewellers use oxygen cylinders too (with propane, because it is much cheaper and they don’t need the temperature to melt steel) and they too won’t run the cylinder right down.
Finally, any residue in the cylinder is going to get massively diluted by a 3000psi fill. That is 200 bar. Even if the cylinder was full of air at ambient pressure (say 80% nitrogen, plus all the usual impurities) the 200 bar refill will squeeze that nitrogen from 80% to 0.4% so you will still have 99.6% oxygen. (Compare this with the zeolite-based oxygen concentrators – lots of discussion here on EuroGA about them – which deliver something like 90% oxygen, by stripping out most of the nitrogen but leaving all the other impurities in place, and these are widely used in aviation including military). Similarly any other atmospheric contaminants will come out of that cylinder diluted 200x (i.e. to 0.5% of their earth-surface concentrations). If there is water, at 200 bar that will most definitely not be vapour and it will be compressed into a few drops of liquid at the bottom of the cylinder, and none of it will escape as vapour through the cylinder valve which is normally at the top (the big cylinders are hardly going to be hung upside down).
The “empty cylinder risk” does exist when you take your aircraft cylinder abroad with you on an airline, or when it is delivered to you. Then it has to be empty and sometimes even has to be shipped minus the valve (though none of the ones I bought from the USA came minus the valve, and all were ready to use). These cylinders, full of air, are then always refilled with oxygen (by the scuba shop or whatever method you use) without being evacuated first, and this is completely standard. The air in them becomes insignificant by a factor of 150 (see below).
And finally, when you open an empty aircraft cylinder to the atmosphere (which happens when it is being shipped to you from say MH) the partial gas pressure law will ensure that any contaminants will equalise in pressure against the ambient. So if e.g. the manufacturer left 20% carbon monoxide in there, by the time it arrives there will be just the usual “x ppm” CO in there. AND that CO will be diluted by 200x (actually 150x for aviation-labelled cylinders; they are redlined at 150 bar) when you fill it up.
So I think the risk is miniscule – unless you are the President in which case somebody might just fill the whole cylinder with cyanide at 3000 psi 
in which case somebody might just fill the whole cylinder with cyanide at 3000 psi
…where it will quickly polymerise into a non-toxic solid. Probably plugging the valve, too.
Very refreshing to see someone [in Europe] that’s done their homework on O2 and shooting down all the OWTs on the subject – thanks Peter !
Not all equipment stops working, for example, free flow systems will continue untill their is a pressure balance. This does occur on aircraft with fixed bottles installed. I have seen this a number of times.
These systems are far less efficient than the modern on demand systems. I do agree with Flyer59 advice to go for a modern system. It’s investment will be returned quickly.
The water will typical add up as it in water droplets form not as vapour.
The main cause of contamination in an oxygen system is moisture. In very cold weather, the small amount of moisture contained in the breathing oxygen can condense. With repeated charging, a significant amount of moisture may collect. Additionally, systems that are opened contain the moisture from the air that has entered. Damp charging equipment, or poor refill procedures, can also introduce water into the system. Always follow manufacturer’s instructions when performing maintenance, refilling, or purging an oxygen system.
Extracted from FAA AMT Handbook Chapter 16
This is what manufactures will also write in the maintenance manual for fixed systems.
I think it is odd that often mechanics are said to take all kinds of short cuts as with Peter’s avionics experiances, but when something would be done according the book it would be idiotic.
