Roll Your Own Oxygen

If you use enough of the stuff, buying your own bottles and filling an onboard or portable system saves a few bucks.

An actual conversation with an FBO counter person: Oh, you want some nitrogen for your struts, right?

Pilot: No, we want an oxygen fill for our onboard oxygen system.

Counter person: Is that, like, compressed air.

Pilot: No, its like oxygen. You know, the stuff thats in air. It supports life as we know it on earth. You may have read about it in biology class.

Counter person: Oh, we dont have that.

And so it goes for the owner of a non-pressurized, turbocharged airplane. Sometimes its just easier to cave in, forget the oxygen and slog around in the weather with everyone else. Explaining this stuff is just too painful.

And therein lies the problem with using oxygen. Sometimes you can find it on the field, sometimes you cant. Sometimes a fill costs $30, sometimes it costs five times as much. When oxygen availability begins to drive the flight planning equation, youve likely crossed into the area where its just not worth bothering with.

If theres a better way-and were not convinced that its always better-its to buy the transfill hardware, lease or buy bottles and fill your own system at homebase. Youre thus assured of a reliable supply so you can top the onboard bottle before departure with enough oxygen to last for inbound and outbound legs.

Low Pressure
Besides not being able to find the stuff or paying dearly for it, our chief beef with oxygen fills on the road is that you rarely get what you pay for, a situation thats somewhat dictated by the physics of compressed gas. Recalling those lectures on Boyles Law you slept through in physics class, to obtain a proper fill of an onboard bottle, you need whats commonly called a cascade filling system. Essentially, a cascade works by using one large bottle to fill the smaller bottle to a particular pressure and a second-or third or fourth-bottle to bring the onboard or portable bottle up to rated capacity or something approaching full.

The problem is that if the cascade system is approaching the low side of its pressure curve without a fresh, high-pressure bottle available and the best the system can provide is, say, 1400 pounds, thats all you get. (You actually get less than that if the system starts at that pressure.) In our experience, FBOs tend to flat-rate an oxygen fill; if its priced at $75, you pay that amount whether you get 900 pounds or 1900 pounds. (We once paid $80 for about a 400-pound top off that yielded a bottle a bit more than half full.)

If you set up your own hangar transfill system, youll face the same limitation. However, you become the bottle purchasing agent and can replace your bottles accordingly, depending on how full you want the onboard system to be for the trip at hand.

To make the effort of self-filling payoff, the hangar cylinders should have enough volume to charge the onboard system fully, thus avoiding having to buy oxygen away from home base.

What It Takes
Ignoring the supply cylinder for a moment, the basic hardware required is a transfill system consisting of valving and hoses to match the aircraft filler port-these are standard fittings known as either MS22066 made by Puritan Bennett or whats usually called the Scott, Airline or AS1219 fitting. The MS-type fittings are older and consist of a 3/8-inch 24 TPI nipple with a spring-loaded check valve that opens automatically during transfilling. The Scott-type fitting is larger, at 9/16-inch, and has a coarser 18 TPI thread.

The documentation with your oxygen system or aircraft will specify which fitting you have. Depending on the model, the transfill system also includes pressure gauges to measure the supply side pressure and a bleed valve to vent off pressure in the lines after transfilling is completed.

We bought our transfill system from Mountain High E&S in Redmond, Oregon, a well-known supplier of oxygen equipment for aircraft systems. We bought the TR95a transfiller and a T-fitting to connect to two supply cylinders. You can T in more cylinders, if necessary, but three are probably as many as most owners would ever need.

In addition, we bought a transfiller adapter to fit the AS1219 filler port on our Mooneys built-in system. Not counting the cylinders, the hardware came to $499, with shipping.

The TR95a is considered Mountain Highs top-of-the-line system because it includes a dedicated bleed valve and built-in pressure gauge, which we consider a must to monitor the filling process and to assess the content of the supply cylinders.

However, Mountain High offers less expensive options, including the TR55 for $75 and the TR75a for $125. The 55 has no gauge and no bleed valve, but it does have a large hand nut which can be slowly loosened to bleed off residual pressure when filling is complete.

The TR75a has no pressure gauge, either, but it does have a dedicated bleed valve. The line running from the supply bottle end of the transfiller to the aircraft end is of braided stainless steel and Mountain High can supply custom lengths.

Cylinder Choices
So much for the easy part. Filling your own system begins to get complicated when it comes to picking the supply cylinders. All compressed gas cylinders have what are called standard CGA-580 fittings that match those on the transfiller hardware but the bottles themselves come in a range of sizes, each corresponding to a particular volume.

Since its widely available locally and relatively cheap, we use welders oxygen. Some suppliers will have access or can obtain aviators breathing oxygen (ABO) but, as weve reported on several occasions, we dont see the sense of insisting on ABO since it costs roughly twice as much.

Weve been told that asking for supplemental breathing oxygen rather than ABO may yield a lower price. We havent tried that.

Welding oxygen is identical, with the same purity and moisture content as ABO. Indeed, medical, ABO and welding oxygen are all the same stuff. We recommend avoiding anything to do with medical oxygen, however, because its FDA regulated and may require a medical prescription. Local suppliers may vary on that point and your AME may happily give you an oxygen prescription. Nonetheless, we think its just easier and cheaper to use welders oxygen.

Although the most common standard size cylinders for welding oxygen are 281 cubic feet, theres no common terminology for these cylinders among manufacturers. For example, Airco/BOC calls its 281-cubic-foot cylinder an S while Praxair calls the same cylinder a K. Just to confuse things further, a 281-cubic-foot bottle may contain 240 to 260 cubic feet, depending on the pressure the supplier happens to use. Either way, these cylinders are 56 inches high and 9 inches in diameter. In contacting various suppliers, weve heard the cylinders called by the manufacturers code size, the height and by the volume. To avoid confusion, be clear on the volume you want. Our recommendation is at least two 281-cubic-foot cylinders if you fill several times a year. Larger cylinders are available and if the economics are attractive, the larger the better.

Full-charged pressures range from 2000 to 2400 PSI. Aircraft systems usually operate at a maximum of 1800 PSI. For the once-a-year fill, a single 281 CF bottle will be sufficient and so will the least expensive transfiller. Local suppliers offer a range of ways to obtain cylinders. You can buy them outright, lease them on yearly or multi-year basis or rent them on a monthly basis. There seems to be no standard or best deal here, in our view.

We think it makes sense to lease them, which is what we did. Our lease deal with a local welding shop for two 281 CF bottles delivered full was $300 for five years or $30 a year each. At the end of the five years, we can renew the lease or turn in the bottles. During the course of our lease, a refund is pro-rated at 20 percent per year. In other words, after a year, we would be refunded $240, after two years, $180 and so forth.

Purchasing the cylinders is also an option. However, our supplier sells only small bottles-83 cubic feet and smaller-and will only lease the larger bottles. With a 105-cubic-foot bottle in the airplane, smaller bottles make no sense. Further, if you own your own bottles, youre on the dime to have them periodically hydrostatically tested and you may have to drop them off for refill, rather than simply exchanging an empty cylinder for a full one under the lease arrangement.

Filling Up
Filling the onboard bottle is simple enough, requiring minimal tools but a careful understanding of basic compressed gas safety. (See the sidebar.) With both bottles at or a little above their 2000 PSI limit, you can start with either.

The transfiller is connected to the aircraft fill port and the valve on the supply cylinder is opened. You can hear a faint hiss as the oxygen streams from the higher pressure supply bottle to the lower pressure bottle in the airplane.

When the two pressures equalize, the aircraft cylinder will be higher than before, the supply cylinder lower than before. The second supply cylinder, whose pressure is presumably higher than the first, is then used to top the pressure in the airplane system to the desired value. (We top to 1600 PSI, enough to last one person for 12 to 16 hours of flying, using conserving cannulas.)

Once the fill is done, the supply cylinder valve is closed and the bleed valve on the transfiller is opened to vent the line of any residual pressure. As noted, the aircraft fill port has a spring-loaded valve that automatically closes when the pressure in the aircraft system is greater than that of the incoming oxygen from the supply cylinder.

How do the numbers work out here? Our two cylinders gave us three fills before one of them needed to be returned for a new bottle. Fill is a moving target since each time you use the system, you start with the lower pressure cylinder, then top with the higher pressure cylinder. This yields increasingly lower pressure fills. Below 1000 PSI or so, it may be time for a new cylinder. A full bottle costs $24.87 from our supplier so, conservatively, our fills cost about $9 each.

But-and this is a critical consideration-we had to spend $800 for the system in order to bask in the satisfaction of a $9 fill. So the important consideration becomes how often do you need an oxygen refill? If its once or twice a year, the economics of self-filling arent especially compelling, particularly if you can find $50 fills that charge the system to full pressure.

Another option is to buy an oxygen conserver. We know of three models. Mountain Highs EDS-D1A system costs $650, www.aeromedix.coms REACT sells for $450 and Precise Flight PreciseFlow costs $380. Only the EDS-D1A is suitable for onboard systems. The other two are intended for portable or onboard systems. Precise Flight is developing a conserver for built-in oxygen systems, which operate at higher regulator output pressure.

We think its critical to consider a conserver before buying transfillers and cylinders. These devices can reduce oxygen consumption by 80 percent or more, meaning a fill could last a year or more for moderate use by a single aircraft occupant.

For a family of four flying a lot of high-altitude trips, conserver economics arent as impressive. Its cheaper to buy oxygen for four than conservers for four.

For our purposes, well fill the onboard system about eight to 10 times a year, thus the convenience is as much an attraction as the lower cost. Theres value in knowing we can leave the hangar with a topped off oxygen system.

Were not big believers in boutique use of oxygen, say at night above 5000 feet, as some pilots do. But when it costs 50 cents an hour, you can spew oxygen around the cockpit like, well, air.

And thats not a bad thing.

Also With This Article
“Survival Notes: Cylinder Handling.”

• Aeromedix, 307-732-2642,
• Mountain High E&S, 541-923-4100,
• Precise Flight, 800-547-2558,