Roll Your Own Oxygen Cart

Longtime Kitplanes readers may recall an article about the benefits of using oxygen in an airplane and the relatively low cost of portable systems, especially if you roll your own (“Homebuilt O₂“, February 2014).

You may also be aware that all oxygen comes from the same source, providing a number of lower-cost alternatives for keeping oxygen supply bottles in the hangar, ready and willing to top off your portable bottle prior to the next flight.

However, while using large bottles to buy oxygen can be a less expensive way to keep portable bottles topped off, they do have some drawbacks, namely: (1) Multiple source bottles are needed to get the most out of your portable system and (2) They are big, heavy and aren’t easy to move around the hangar, especially if you prefer to use a built-in oxygen system in the airplane.

Multiple Bottles

While it is possible to use only a single large source bottle to refill the bottle for your airplane, this has the drawback that the peak pressure that can be obtained in the aircraft bottle is equal to whatever pressure happens to be in the source bottle, and after a few fillings that pressure drops, meaning less oxygen in the tank for flying.

To partially get around this issue, multiple source bottles can be used. When the aircraft tank is empty, it is filled first from the lowest pressure source bottle (source tank #1), then topped off with the next highest pressure in source tank #2 and then source tank #3. This leaves tank #3 at a higher pressure for longer, as the volume for the lower pressure fills are being handled by the other tanks.

When source tank #3 drops to the point where a fill is desired, source tank #1 (the most empty one) is taken to the oxygen factory (or welding supply shop or wherever it’s convenient) for refill and comes back as tank #3, with the prior tanks shifting down the line. In this way, the most recently sourced tank is always the highest pressure, and the tank taken in for refill is always the most empty. This minimizes trips to the oxygen factory and keeps the aircraft bottle at the highest possible pressure each time it is refilled.

The Oxygen Cart

At over 100 pounds each, large oxygen source bottles can be awkward to move around. This means either the aircraft bottles have to come to the source bottles, or some means to more easily move the source bottles to the aircraft bottles is required. This is where the oxygen cart comes in.

There are commercially available welding carts that will hold up to two large bottles and in fact Harbor Freight sells one for about $75. However, it is a bit unwieldy to use; it tips over like a furniture dolly and with over 200 pounds of bottles in it, the handle can get pretty heavy if tipped a little too far. It also is rated for only 187 pounds, which is a little light for two tanks. But the price is right and if you only have two bottles, it can be a good alternative. However, it’s pretty nice to have three bottles and for that, a different cart is required.

Fortunately, Harbor Freight sells polypropylene service carts that are rated for 500 pounds and their 16-inch-wide version is a perfect size to hold three Size H (also known as Size K or M250) 250 CF bottles. However, in typical homebuilder fashion, it requires a little customization.

Step by Step

Here are step-by-step instructions for how to turn an assembled polypropylene service cart into an oxygen cart suitable for holding three large (250 CF) oxygen source bottles.

1. Using a cardboard template, cut out three holes of slightly more than 9 inches in diameter, with the edge of each hole touching its neighboring hole. This will be a template for cutting the upper shelf of the service cart. Once the template is ready to go, line up the three oxygen bottles and make sure the template will lower down over the three bottles. It may be necessary to increase the size of the holes slightly to achieve a loose fit. The bottles do not need to be tight in the holes and the holes need to be large enough that the top shelf can be easily removed from the cart once the bottles are in it, so that the bottles can be removed when it comes time to refill them.
2. Place the template over the top shelf of the service cart. Make sure the three holes all fit within the shelf. Using a felt marker, draw the outline of the holes onto the shelf. Remove the template.
3. Using a drill bit with a large diameter (preferably at least ½ inch), drill some starter holes inside the drawn lines.
4. Using a saber saw and starting in the drilled holes, cut along the marked lines on the shelf. When complete, you should have three large, 9+ inch holes in a row. Test fit the shelf over the three lined-up bottles to make sure it slides easily over them.
5. Place the bottles on the lower shelf of the service cart and place the top shelf over the bottles, lining up the pins in the shelf with the sockets in the cart’s vertical arms. If desired, the mounting screws can be put back in to secure the top shelf to the arms, but these will need to be removed whenever it is necessary to fill the source bottles. I have found that as long as you don’t pull up on the handle of the service cart, the screws are not necessary.

At this point, the cart is ready to go and should provide a stable platform for wheeling the bottles around the hangar. If you have a single filler line available, you can connect it to each bottle one at a time. However, for just a little more money in fittings, much simpler and faster fills can be facilitated.

Plumbing the Bottles

There are a number of commercially available filling station hardware options with prices running up into a few thousand dollars, but there is a much simpler way to accomplish the same thing for about $125 in fittings.

Each bottle already has a valve on it, so all that is needed is a way to connect each bottle to a common line. This can be done with two T-fittings and two pigtails. Each pigtail consists of a short length of flexible hose with a CGA-540 fitting on each end.

Install a T-fitting on bottle #1 (the low- pressure bottle) and #2. Then use one pigtail to connect bottle #1 and #2 and the other pigtail to connect bottle #2 and #3. This leaves one side of the T-fitting open on bottle #1, to which the transfill adapter can be connected.

A transfill adapter is used to connect the CGA-540 fitting on the oxygen source tank to the fitting on the aircraft bottle. If you don’t already have one, they can be found on eBay for about $55, including a pressure gauge. If you are using aircraft bottles with a medical oxygen fitting, they will typically be a CGA-870, so the transfill adapter would adapt from CGA-540 on one end to CGA-870 on the other. Ensure you know what kind of fitting is on your aircraft bottle. If it is not a medical oxygen fitting, it will likely be something other than CGA-870.

Regardless of what kind of fittings and hoses are used, ensure they are rated for something in excess of 2000 psi, which is the typical full pressure for oxygen bottles.

The Filling Process

To use the system, connect the aircraft bottle to the open end of the transfill adapter and open the aircraft bottle valve, then slowly open the valve on source bottle #1, allowing oxygen to flow slowly into the aircraft bottle. Don’t open the valve very far; fast-flowing oxygen will heat up the aircraft bottle, and a hot bottle will not hold as much oxygen as a cold bottle will. Once the pressure has equalized, close the valve on bottle #1 and slowly open the valve on bottle #2. Repeat the process again for bottle #3. Close all valves, disconnect the aircraft bottle and the filling process is complete, with the aircraft bottle now at the same pressure as high-pressure bottle #3.

If you don’t already have source bottles, they can be purchased locally or from a variety of online sources. Prices vary significantly and sometimes attractive deals can be found, but be wary of expiry dates on the tanks. If you are buying your own bottles, this can be a good opportunity to pool resources with other local flying friends. Size H, also known as Size K or M250, are 250 CF size bottles. If you prefer to rent the bottles, most oxygen suppliers offer this service. I pay about $40 locally to fill up an owned 250 CF bottle and between myself and two other local pilots who use oxygen occasionally, we fill up a source bottle about once every year or two.