I've set up several Oxygen refill stations/carts over the years, starting with our hang gliding club in 1992. I think the easiest is a single cylinder with a simple transfill adapter from Mountain High. However, the single cylinder option, while
relatively inexpensive, drains quickly after multiple fills, resulting in much less pressure in the target cylinder. A better solution is a multiple cylinder cascade system that allows for lower pressure cylinders in the cascade to provide the bulk gas (
by volume) which is then topped off by the cylinders containing higher pressure. For example, three cylinders in the cascade might contain 800, 1,400 and 2,000 psi after several refills. You bring in a cylinder from your glider with 700 psi remaining.
First you connect it to the lowest pressure cylinder (800 psi) and bring it up to an equal pressure. Then, you close the valve on the 800 psi cylinder, open the 1,400 psi bottle and top off. Finally you close the second cylinder valve and open the 2,000
psi valve and bring your cylinder to the highest pressure possible. Then you close all the valves and bleed the O2 from the fill line and disconnect your cylinder. Typically, the three cascade cylinders might now show 750, 1,200 and 1,900 psi, with yours
also at 1,900 psi.
There will always be some wasted pressure, as it seems that everyone wants to refill as soon as their glider cylinder pressure drops below 1,000 psi' This means that the lowest cylinder in the cascade gets replaced to make room for a new high pressure
cylinder while still containing half its contents.
Mountain High sells all of the required transfill hose kits with end connections, bleed valves and pressure gauges. They can also outfit you with connections to set up a multiple cylinder cascade. (
https://www.mhoxygen.com/product-category/ground-support-
equipment/?msclkid=2577c5b9cd5e11ec8312274cbcf435f3) I consider three cylinders to be the minimum needed. Four, or even five cylinders in the cascade are much more efficient at keeping the top pressure as high as possible, but at a higher expense if you
are buying or leasing the cylinders. Also, the cylinders are very heavy (300 lbs.), and anything over three cylinders is a bear to move around.
For a trailer, I suggest an inexpensive flat trailer from a supplier like Harbor Freight (
https://www.harborfreight.com/automotive/trailers-towing/1090-lb-capacity-40-12-in-x-48-in-utility-trailer-62665.html) You will need to fabricate a cradle system to
securely hold the cylinders. You definitely don't want them coming loose and falling off while hauling it around the airport. "MythBusters" did a great experiment where they knocked the valve off a pressurized cylinder and watched the bottle blow through
a concrete barrier.
Make sure the cradle you build has some leeway to handle variations in cylinder size. Most are pretty close in diameter, but length can vary by several inches. "Aviators Breathing Oxygen" is preferred for the main cylinders, as their capacity is usually
higher (335 cu. ft. vs, 300 cu. ft.) and they are also generally filled to 2,200 psi. Welding Oxygen cylinders are normally 2,000 psi, And, in spite of what some people still believe, there is absolutely no difference between Aviators, Welding or Medical
oxygen. Except for the price, which is determined by the expected liability (read: lawyers) involved. Medical Oxygen falls under Food and Drug Administration (FDA) rules, and requires a prescription. Welding and Aviators O2 are under Department of
Transportation (DOT) protocols.
You will also probably need a set of adapters to go from the CGA-540 fitting on the end of the refill line to accommodate other fill valve systems typically found in gliders. The most common is a "Schrader" valve, which looks like the fitting on a car
tire (but isn't), a European style DIN-477 valve or a CGA-870 fitting. (Also referred to as a "Post type," "Medical," or "Puritan-Bennett.) You may also need a refill hose extension, as some gliders (notably Schempp-Hirth) have their onboard cylinders
buried deep in the fuselage and are difficult to refill and worse to try to remove.
You mentioned a pump as an option. From what I understand (having little experience with industrial pressure concentrators) they require an air compressor to power the piston pump and a source of oxygen (meaning a cylinder. The one I have used is from
Hydraulics International, costs about $5,000 and, with 175 psi. air pressure for the pump, can raise the available cylinder pressure about 50%. This means that you can increase the 2,000 psi in your fill cylinder to about 3,000 psi in your target
cylinder. This necessarily implies some risk, as 3,000 psi is above the rated capacity of most commercially available portable cylinders. Personally, I would discourage their use.
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