On Wednesday, July 17, 2019 at 7:45:58 PM UTC-7, wrote:
Wrong!

Medical cylinders, even though they are standard sizes, are invariably more expensive due to the influence of the legions of lawyers feeding off the health care system. (Rant off)

To expand on my earlier reply:

That may have been true back in the day, but for a variety of reasons medical cylinders have become very common, and consequently relatively inexpensive. Supply and demand and all that. I pretty regularly get such cylinders at yard sales for like $10 each, and they often come with electronic regulators similar in function to EDS. Sure, the medical posts and valves are an issue. But even with a valve change and hydro testing I come out way ahead of aircraft supply rates.

The usual problem is that the most common CGA540 valves are NPT pipe thread, while medical cylinders usually have a straight thread neck with a recess for an O-ring. But straight-thread CGA540 valves are available on the internet if you shop around. Make sure that the valve comes with an O-ring, or at least get the special O-ring from the supplier. The ones I've seen are some special white elastomer, and you probably shouldn't use a standard rubber or buna-n O-ring.

When you buy a medical cylinder without a valve, or with a valve change at the distributor, the cylinder will probably arrive empty--that is, filled with ambient air of unknown humidity. To make sure the cylinder is completely dry inside I will usually evacuate it with my vacuum pump and then do an initial fill from my welding oxygen bottle.

--Bob K.