You need to establish your local refill options. They will vary anyway over time, as scuba shops etc change ownership / staffing.
I think that if there is no obvious way to choose, go for DIN.
I maintain a rented cylinder from British Oxygen and have a refill hose made up (details posted earlier) so could have whichever thread and it makes absolutely no difference. All the equipment comes from the US and 540 is much more common over there.
I will go for DIN then and in case it turns out I need CGA-540 I will get an adapter.
If someone still has a Scott system, I just listed an adapter on EuroGA’s marketplace
HBadger wrote:
I will go for DIN then and in case it turns out I need CGA-540 I will get an adapter.
I ran through this last year in order to put a refill cylinder in my hangar. I have done it in this order:
First, find out what kind of “input” you have (either mounted in your aircraft or for the portable system you want to buy). As I have a built-in Scott system, I went to the homepage of Aircraft Spruce to find some options to convert Scott to CGA-540 (both “female” or “male”, depending on how you will continue thereafter). Aerox also has some adapter and adapter sets. Primarily, from Scott you’ll have to go to CGA-540, I wasn’t able to find any other adapter.
Second, find out which “output” you can buy in the region where you live. For me it’s DIN here. And I intentionally went for welding oxygen. Funny thing is, that I am a hobby diver and did get to know some scuba teacher, and guess what they buy here as diver oxygen? You get it: welding oxygen. Interestingly I bought my 50 liter 200 Bar bottle from a diving shop. When my bottle will be half empty or so, I think I’m going to buy a second bottle, so that when I refill I start with the half-empty bottle and fill the rest on the full bottle.
To give another quote here: I paid 200 Euros for the full bottle of 50L/200 Bar (there is no rental fee of any kind), and it needs revision every 10 years. Normally, you exchange the bottle when it’s empty and you don’t pay any revision, just the price for the refill, which should be around 40 to 60 Euros (didn’t have to refill so far, but price range is like this). One bottle should last for at least 3 full refills (the built-in system of 10L has a maximum of 1750 PSI corresponding to 120 Bar), and quite a lot part refills, say I should make at least 10 refills in total (in fact even more, as I don’t fill it up to the max if not necessary).
Third, now you can buy the “intermediate parts” in between “input” and “output”. It may be possible that there do not exist all options to go through, and don’t get confused with male/female :-)
In terms of the built-in Scott system, there are two more points which are interesting to know. First is, that there is a limiting valve next to the “input” limiting the flow rate into the built-in cylinder. If the refill flow rate is too high, it will make some sound, then decrease the flow rate. It takes a while to refill. Second, as it takes so long, it’s easy to read the pressure level of the built-in cylinder on the gauge inside the cabin. So no need for another gauge outside.
Did you know, that a built-in oxygen system is labelled o.k. when it doesn’t leak more than 1% per day? It’s full to empty in 100 days. So even without using the system at all, it may be empty several times a year. And you should not let it drop to zero, as a revision of the bottle might be necessary (depending on the system). So a refill station in the hangar surely is an investment worth it.
I do the same – the only difference is that here in Croatia replacement tank (50 liter at 200 bar) is €20.
I pay not much more than that for the huge British Oxygen cylinder swap, but it is 80kg and I can’t transport it myself; I need help to do it.
Remember also that gas will flow only from a higher to a lower pressure 
so perhaps half of a cylinder is always wasted. A good partial solution is to fly with a large composite (lightweight) cylinder like the MH 48 cu ft one. Even half full they go to Greece and back a couple of times. Getting these cylinders has totally transformed the aviation oxygen situation, and if I had a local scuba shop which I could rely on I would not bother renting the BOC cylinder.
Peter wrote:
but it is 80kg and I can’t transport it myself; I need help to do it.
Mine comes empty at 66kg. It is just the weight I can handle to put one side in the car and push it in.
In order not to waste half of the cylinder, as I stated, I’m going to buy a second cylinder. So e.g. when the first cylinder is at 100 Bar, I can still fill the internal cylinder to some 85 Bar and add the rest from the second cylinder and so on. Thus you can go down e.g. to some 30 Bar or something before changing – or change the empty cylinder whenever the “full” cylinder is not sufficient to really fill the system (so when the “full” cylinder has around 120 Bar).
My TB21 oxygen bottle is closing in on the 36 month hydrostatic testing. Can someone recommend a place to get this done?
Thanks
Peter wrote:
Presumably you are looking for a test in Denmark?
Well the bottle will be taken out anyway, so I guess it can be shipped somewhere else for testing. I did have a local company at EKCH do it last time but they could not find the tourqe values for the bottle and gave me the bottle back without regulator in and charged almost a 1000 EUR for this :-( So I had to dig hard to find a CMM for the bottle and a friend working at the airlines had the equipment to get the regulator back. Must be something better out there?
Peter wrote:
I currently use a scuba cylinder test outfit. Great value, but in the UK.
Is this a scuba shop you use? We have scuba shops here but I would be worried if they know how to handle an aviation O2 bottle? Regular scuba bottle are treated quite differently and made from much thicker material.
