For a pic of the refilling hose, see my article.
The exact hose will depend on the fittings at the two ends.
I have a standard MH cylinder with a DIN 3/4" connector.
They are the same for welding in Germany. I use a transfill adapter available from welding shops.
It works for me. If everything goes tits up, I can get a 3l welding cylinder at a DIY shop and have enough time for a few roundtrips germany-UK with my O2D2.
Does anybody know how DA42 built-in oxygen solution can be filled? Can the tank be pulled-out and filled in or you have to use airport oxygen facility?
I think the clue is in your post….“built in”.
Of course they can be removed, as would be necessary if one was faulty, but the intention is that they are filled in situ. There is an oxygen service point and gauge in the front baggage compartment near the cylinder
Did once write a blogpost on flying on Oxygen. You can find it here: http://www.aeroplus.nl/flying-on-pure-oxygen
Does anybody know how DA42 built-in oxygen solution can be filled? Can the tank be pulled-out and filled in or you have to use airport oxygen facility?
Just have a 200l oxygen bottle stored at an airfield that allows it. They are not expensive, I pay around 40 € a year to lease it and it sits in my hangar. The oxygen production plant is in my hometown which has the advantage that I always breathe my home oxygen when flying abroad 
That helps a lot to keep your Swabian Identity. lol
Just have a 200l oxygen bottle stored at an airfield that allows it.
That’s the problem – my airfield is LDZA – international airport with no hangar.
It is certainly true that many owners of planes with built-in oxygen systems are flying with the cylinder empty and have been for many years. I know that from correspondence with TB21 pilots. When I was looking at planes in 2002, I found that a TB21 was going to cost an extra £1000/year for the cylinder inspection+test. I have no idea whether that is true currently (G-reg) but it would be a reason for some to just forget about the cylinder. There is a TB21 owner I know who replaced the heavy steel cylinder with a composite one and spent close to €10k on an EASA 21 company… but it only gets you a weight saving; the composite cylinder generally has a shorter life than a steel one.
They either don’t use oxygen (with all that implies) or fly with a portable kit which for many is far less hassle to refill.
Where I am hangared they do have o2 servicing for turboprops but I strongly suspect the price would be in 3 figures for a refill. And I would not be allowed to keep a cylinder in the hangar.
The advantage of a built-in system is that the pipework is neatly out of the way and you get a nice cannula connector for each person. One can achieve that “semi legally” by tucking the pipework for a portable system behind the trim. Anyway, some built-in system owners fly with an O2D1 demand regulator on each outlet, which saves a lot of gas.
That’s the problem – my airfield is LDZA – international airport with no hangar.
Croatia has a lot of nice small airstrips, that’s why I said “stored at an airfield that allows it”. You don’t need to refill very often if you have a Mountain High.
I found that a TB21 was going to cost an extra £1000/year for the cylinder inspection+test
My bottle needs an inspection every 5 years which is not expensive (ca 300 €, done by Hugen in the Netherlands). Some bottles have a lifetime limit which makes them more expensive. However, you can lapse the inspection because it will not render the aircraft unairworthy and there is no requirement to breathe oxygen from a builtin system.
