The Flow of Oxygen
Based on the number of questions I get on the subject, there is a lot of confusion among pilots concerning the flow rates for oxygen.
The FAA rules are simple—1 lpm (liter per minute) of oxygen for every 10,000 feet MSL. Most certified aircraft with built in systems have their flow rate set for the service ceiling of the aircraft. For instance, if your aircraft has a service ceiling of 25,000 feet, when you plug in your oxygen connector, you will receive 2.5 lpm even if you are at 15,000 feet. This empties your oxygen tank quickly!
Some aircraft with built in systems (mainly Beechcraft) have an altitude compensation valve which uses barometric pressure to mechanically adjust the oxygen flow rate. These systems work with variable success and are found inconsistently even within the same model line.
Why save oxygen? The main reasons are cost and hassles. Officially, built in oxygen systems are “supposed” to be filled or signed off by A&Ps. This makes dirt cheap oxygen absurdly expensive. Portable systems are easier to fill at most compressed gas stores (albeit some erroneously require prescriptions—if they do, write me an Email—I will send you a RX for those bozos) and usually the charge is minimal but it still takes your time.
Many aircraft owners have started filling their own aircraft renting a couple of H cylinders and keeping them in the hangar. The fill procedure is relatively simple and requires a onetime purchase of hoses so the system can be filled sequentially. The first H cylinder is used to fill the system and the second to “top it off.” The sequential filling is similar to the technique used by scuba shops. It is not a difficult process but “officially” is supposed to be signed off.
Can you use too much oxygen? No--except if you have a running out problem. Can you use too little? Absolutely! That is where people can get into trouble especially when no oxygen is used at all.
Although pilots consider themselves “above average,” they still tend to underestimate the need for oxygen especially since the pilot population tends to be of a more “mature” age. In informal studies, we have seen that some pilots will need oxygen much lower than the 12,500/14,000 feet MSL FAA requirements while others do not drop their oxygen saturations till higher levels. Factors which contribute to needing oxygen at lower altitudes include underlying lung disease, upper respiratory infections (e.g. colds), allergies, obesity (a big gut tends to keep the pilot from taking deep breaths particularly in a sitting position), heart problems, anemia, and of course, smoking.
About 25 years or so ago, Nelson started marketing an adjustable oxygen valve which could be placed in line with the oxygen tubing for both build in and portable systems. The Nelson valves were marked by altitude (e.g. 1,000s of feet MSL) which eliminated the simple 1 lpm/10,000 feet math and saved a considerable amount of oxygen. Now there are several flow meters on the market which are “in line” or part of the regulator at the tank.
One of the neatest devices to come out for saving oxygen is the Oxymizer cannula. This is a patented product made by Chad and sold by several aviation oxygen shops as well as medical stores. The basic principal is the Oxymizer saves oxygen that is flowing during the exhalation and pause portions of the respiratory cycle. This savings allows for a 50 to 70% in the oxygen flow depending on the users oxygen needs and their respiratory rate. Oxymizers are cheap—about $27—and although disposable, they can be reused for years if protected from high heat (like the cockpit in open sun).
There are also mechanical conserving devices which essentially open up the flow of oxygen sensing the negative pressure generated by the inspiration of the user. The amount of oxygen delivered when the valve opens is adjustable. There are also electronic versions. Personally, I think these devices are ridiculously expensive ($400 to $700 per user) since the amount you may save over the Oxymizer is minimal.
The most important point to remember from this column is the only way to know how much oxygen to use is by using a device call a pulse oximeter. The best is made by Nonin which is a finger tip device, the size of a cigarette pack cut in half, which shows how much oxygen is in the blood in real time. Using anything less than an oximeter means you are purely guessing whether you need oxygen and how much oxygen to use if you do. It is 21st century technology and anyone who flies above 12,500 feet needs one.
0 Comments
Click here to sign up now.