Sam, a private pilot in Montana, writes: How do I know how long a supplemental oxygen bottle will last in flight? Or, more correctly, how do I know it has enough for my flight before I takeoff?
That’s a great question. One, oddly, that doesn’t seem to have a great answer. Or at least not a quick and simple one, but let’s all take a deep breath (of oxygen) and dig into it.
Looking at training materials, one commonly taught acronym for the preflight check of oxygen systems (installed or portable) is the PRICE check where:
P stands for Pressure, an indicator of system volume
R stands for Regulator function test
I stands for flow rate Indicator test
C stands for properly secured tubing Connections
E stands for Emergency briefing of passengers
The FAA’s Civil Aerospace Medical Institute’s pamphlet OK-21-0375 helpfully tells us that P comes first, and that pilots should “ensure that there is enough oxygen pressure and quantity to complete the flight.” But then it’s mute on how to actually go about doing that. As are all other FAA training manuals and documents.
Of course, this is because there isn’t really any one size fits all answer. Oh, sure, there is an oxygen industry formula for tank duration. You just take the current tank pressure, subtract something called the residual pressure (the amount a given tank needs to deliver the remaining contents), then multiply that by the tank conversion factor.
What’s a tank conversion factor?
Well, the volume of compressed oxygen is measured in liters, but tank pressure is measured in pounds per square inch, so some conversion is needed, as math only works when playing with the same units. This conversion factor is standardized for various tank sizes, but isn’t standard across the board for all tanks.
Then you divide all of that earlier math by the flow rate required for the pilot to get the expected duration of the oxygen in the tank.
Naturally, if more than one person is breathing off the tank, you have to monkey around with the flow rate side of the equation. All of this challenging math is compounded, ironically, by the fact that pilots, as a group, aren’t very good at math.
This is why I just fly low. Just sayin’…
Naturally, there are apps, lots of them, to help you crunch the math. But it’s all ground-based math.
In flight, things get trickier, as both temperature and altitude have an effect on tank pressure. Oh, and you need more oxygen — generally — the higher you go, so the flow rate isn’t necessarily constant over the duration of a flight. I think at this point you can see why it’s basically impossible to have an accurate tank gauge that simply reads in hours.
Oh. Right. I almost forgot one other complication: Pilots and passengers, being people, vary in their biological needs when it comes to oxygen, plus, as mentioned, how much oxygen any individual pilot or passenger needs varies at various altitudes.
Realistically, if you are using oxygen, you should calibrate your need in-flight with a pulse oximeter. It’s a little finger tip device that can be had for $20 or $30. Outside of the USA, Apple watches have pulse oximeters built in, but for legal reasons they were shut down for us. That said, I believe the Garmin watches still double as oximeters.
On top of all of this, there are yet more variables introduced by system architecture.
Briefly, there are two types of oxygen systems used in GA airplanes: Installed and portable. Both actually use oxygen bottles.
The installed type — generally found in aircraft designed to fly in the altitudes where oxygen is needed — just have a slicker, cleaner interface with all the plumbing neatly hidden away, while we bottom feeders need portable systems when we venture higher into the sky.
As a side note, many older installed systems — due to the regulations in place at the time — are designed to deliver the volume of oxygen the pilot would need at the service ceiling of the aircraft in question (based on 1-liter per minute for each 10,000 feet above 10K MSL), so they can be rather wasteful at lower altitudes, where a half liter per minute might be fine.
Although, on the positive side, oxygen is relatively cheap, and I guess that makes tank duration easier to calculate.
But you won’t need to. If your airplane has an installed system, the POH should have an oxygen duration chart. Just like with any other performance chart, you can look up your numbers to see what’s needed, and then compare it to how much is in the tank to see if you have enough — same as you do with fuel.
I keep talking about tanks, so I guess I should segue into the various modalities of oxygen storage.
So what’s in the tank?
In the big universe, there are three ways to store oxygen: As a compressed gas, as a liquid, or as a solid. It’s also possible to distill oxygen from the atmosphere.
Most GA systems use compressed gas, although, ironically, liquid oxygen is the source of that compressed gas. The tanks are generally made of aluminum, but some are composite materials. Actual liquid systems are used on some jets, but not commonly in our ecosystem.
Oxygen stored as a solid is usually in the form of sodium chlorate, a substance that, when heated, releases oxygen at a whopping 600-to-1 expansion ratio, making it a lightweight way to store oxygen, but it’s one-and-done.
By the way, that’s the tech used on airliners to supply the emergency masks that drop from the ceiling.
In military aviation, oxygen concentrators are used to distill oxygen from the surrounding air. It’s basically the same tech that Grandma’s little living room unit uses to separate the oxygen from the more voluminous nitrogen in the surrounding air, “concentrate” the oxygen, then deliver it to boost the percentage of oxygen that the person is breathing.
OK, so let’s say you are plying the skies in a typical GA airplane that wasn’t designed to spend the bulk of its life high up in the atmosphere, but you are taking a hop over the Rockies. How much air do you need in the tank?
Well, we’ve only scratched the surface on the variables and, actually, the biggest one is downstream from the tank, where the system connects to the pilot. Nasal cannulas and oxygen masks vary tremendously in their oxygen use requirements from model to model, even with what are called continuous-flow systems. On top of that, there are systems that are designed to only deliver oxygen when you inhale.
On the cannula front, my favorite, just based on name alone — I’ve never actually used it — is the Oxymizer, a riff on the word mizer. Apparently this miserly little device, which is sort of a rebreather pillow, can triple tank duration over run-of-the-mill cannulas, although it’s not a good look for Top Gun selfies.
Still, it beats running out of O2 and going hypoxic.
Now, like airframe-installed systems, if you are using a single-source solution — rather than mixing and matching components — most manufacturers provide some sort of guidance to help you predict how long a given tank system will last in flight.
Some are as simple as, “hey, this tank and cannula should last four hours.” Well, if the pressure valve is at 50%, then you’ve got two hours of oxygen. If the flight is three hours long, you have a problem and should top off the tank. Other manufacturers provide more detailed, altitude-based guidance.
Speaking of altitude, there is one clever device from Aerox that seems to get good reviews from pilots that use it, and that’s a simple altitude-based flow meter. Assuming that your portable system allows you to control the delivery rate (some cheaper ones only deliver a fixed volume), you can dial in a flow rate to match your altimeter. Think of it as a Kollsman window for your lungs.
Other things to know: When you buy a portable oxygen tank, it will be shipped to you empty. That’s because dropping tanks is… uh… bad, and you know how those delivery folks are.
What is “bad?”
Remember how Roy Scheider killed that shark in Jaws? Yeah. Never a good idea to rupture anything filled with something under pressure.
The good news is that most decent-sized FBOs fill oxygen tanks as part of their service lineup. The corollary bad news is that, like all services from decent-sized FBOs, the price might hurt.
Now, and I’m not trying to lead anyone down a path of evil here, but while the FAA makes a big deal in training about the need to use “aviator’s breathing oxygen,” which is supposed to be different from medical oxygen and welding oxygen, that’s not entirely accurate. At least not nowadays.
Back in the day, there was a difference between the three oxygens. I recommend the old 1941 black and white movie Dive Bomber, with Errol Flynn, for understanding the issue.
But according to AME and high-altitude pilot Dr. Brent Blue, for the last three decades, all pressured oxygen — aviation, medical, industrial — is the same stuff. In his words, “oxygen is oxygen,” so in a pinch…
Lastly, while on the subject, what about those little disposable cans of oxygen sold at drug stores, big box stores, and online? Yes, I’m talking about the “as seen on Shark Tank” Boost bottles.
Boosts are pre-filled disposable oxygen “tanks” that come in three sizes, each with a mouth-covering cup that allows the user to take a huff of pure oxygen. They are very light weight, and it’s impossible to tell how empty they are, but depending on the tank’s size, the Boost people say you get between 60 and 200 one-second huffs per tank.
Somewhat bizarrely, at least to me, the tanks also come in a wide range of “flavors,” such as peppermint, pink grapefruit, and eucalyptus. Uhhh… OK. I don’t even know what to say about that. So moving on….
Obviously, Boost won’t meet the regulatory requirements for continuous oxygen use at altitude, but I know a couple of pilots who carry a can in their flight bags and take a few huffs when coming into the pattern for landing to reportedly maximize their mental sharpness. I’m all for sharpness in the pattern, and if eucalyptus-flavored air is what it takes, who am I to argue?
One final word on oxygen: Contrary to popular belief, oxygen isn’t flammable.
That said, pure oxygen is the ultimate oxidizer, which means that it can serve as an accelerant for fires, making them burn hotter and faster, and even causing materials that don’t normally burn at all to go up in flames — all bad things when thinking about in-flight fires.
So save the cigars for the post-flight, once you are sure that you shut that oxygen tank off.
