Here is a hair raising story that could have had a very bad ending were it not for the sharp thinking of the owner!

So, I'm in Memphis for a couple of days visiting my daughter and her family and ask the FBO to put in the O2.

I get to the airplane the next morning to check things out and the first thing I look at is the O2 pressure. It's below 400psi. I gently back away from the aircraft and tell the others to get back as well. Clearly there was a leak (the FBO definitely had topped  it off). Apparently they didn't snug the cap back on the fill fitting and the check valve had a slight leak. They said they'd top it off again and snug it up.

They are incredulous when I tell them to leave it open for an hour before messing with it.  But I have my reasons: There is an entire aircraft interior oxygen soaked overnight. Anybody that thinks this country boy is gonna climb in and crank up is out of their ever lovin' mind. Oxygen blowing out of a line is one thing.  Combustibles long term soaked in 100% of the stuff (it's down in the gaps of the fibers and *in* the fibers) is "sort of flammable".

On reflection I wasn't sure an hour would do it, so I called them back and told them to leave it for three (with the doors open). I wasn't leaving until the next day anyway. I'm sure they still think of that crazy Texas guy. Call me chicken, but not fried!

Contrary to a common and published myth, there are no different grades of oxygen being produced or contained in cylinders maintained under DOT regulations. By the very nature of the state-of-the-art process in which oxygen is produced, it will be better than 99.99% pure.

In other words, it all originally comes from the same spout no matter what the purpose. In addition, oxygen for medical use does not have any more moisture than oxygen for any other purposes, nor is it added. Furthermore, because of the chemical nature of oxygen it must be as pure and dry as possible if stored under pressure or else the cylinder and equipment may be damaged, or worse, personal injury or death may occur.

Therefore, a hygiene protocol is necessary in order to prevent oxygen from being contaminated or to help keep contaminated oxygen from being used. This, perhaps, is why many believe there are different grades of oxygen.

REQUALIFICATION OF OXYGEN CYLINDERS

Most pilots who use oxygen cylinders in their planes have a general sense that their cylinders need to be re-tested and re-certified every so often. But few of them actually know what is required. This article is intended to give a general overview of these requirements.

Re-testing and re-certification is technically called "requalification." Requalification of oxygen cylinders is governed by regulations of the Department of Transportation (DOT) and not by those of the Federal Aviation Administration (FAA). The DOT regulations govern cylinders used for transportation of hazardous materials.

        Compressed oxygen is a hazardous material. (49 CFR Section 172.101, Table).  Each cylinder used for the transportation of compressed oxygen must meet DOT specifications and bear DOT specification markings and be "qualified."  No one may use an unqualified cylinder in transportation.

        Such cylinders must be "requalified" at certain times and under certain conditions. Requalification must be done by a person holding DOT approval. (Section 180.205). Cylinders which are not qualified or requalified may not be refilled. And federal OSHA regulations require that anyone who works with oxygen or oxygen systems receive special training. (29 CFR Section 1910.253(a) (4).

        Regardless of time, a cylinder must be tested and inspected if:

1. The cylinder shows evidence of dents, corrosion, cracked

     or abraded areas, leakage, thermal damage, or other conditions that might render it unsafe for use it transportation;

2.  The cylinder has been in an accident and has been damaged

    to an extent that may adversely affect its lading retention

      capability, or;

3.  The cylinder shows evidence of or is known to have been

   over-heated.  (Section 180.205(d)).

This is called "conditional requalification."

        Regardless of condition, a cylinder must be tested and inspected after

the passage of a certain period of time.  This is called "periodic requalification."  How often a cylinder needs requalification depends on the kind of cylinder-and there are many kinds of cylinders. However, the most commonly used cylinders in aviation are Types 3A, 3AA, 3HT, 3AL and E(xxxx).

        Type 3A and 3AA cylinders are heavy steel, must be requalified every 5 years, and have an indefinite lifetime. Type 3HT cylinders are light steel, must be requalified every 3 years, and must be condemned after 24 years or 4,380 fills. Type 3AL cylinders are aluminum, must be requalified every 5 years, and have an indefinite lifetime. (Section 180.209).

        Aluminum cylinders made from type 6351-T6 alloy have special requalification requirements because they have had failure problems. (Section 180.205(f) (4)). There is a proposed regulation to prohibit the use of this alloy in cylinders.

        Type E(xxxx) cylinders are Kevlar or a similar material, must be requalified every 3 years, and must be condemned after 15 years.

These time periods may be extended under certain circumstances. For example Type 3A or 3AA heavy steel cylinders can be requalified every 10 years, instead of every 5  years, if  "removed from any cluster, bank, group, rack or vehicle each time it is filled," meets certain other specifications, and is subjected to the dreaded "hammer test" at every filling.

(Section 180.209 (b)).

        Each cylinder that is tested and inspected must be "requalified", "rejected" or "condemned." If a cylinder is rejected, the owner must be notified in writing and have an opportunity to have it requalified under more rigorous terms. (Section 180.211). If a cylinder is condemned, the tester must stamp a series of Xs over the DOT specification number and mark it "CONDEMNED." Alternatively, with the consent of the owner, the bottle may be rendered incapable of holding pressure-usually by drilling. No person may remove or obliterate a "CONDEMNED" marking. (Section 180.205(i) (2)).

A cylinder must be condemned if:

1. It fails the visual inspection;

2. It leaks through its wall, or:

3. There is evidence of cracking. (Section 180.205 (i) (1)).

        In addition to both internal and external visual inspection, each cylinder must be pressure tested. This is commonly called "Hydro-Testing" or "Hydro-Static Testing." During a hydro-static test the cylinder is placed inside a test jacket filled with water, and a specified internal pressure is applied. Most cylinders are tested to 5/3 their certified pressure. Thus, a cylinder having a service pressure of 3000 pounds per square inch would be tested to 5000 pounds per square inch. How much the cylinder expands during pressure testing is measured. And how much the cylinder has permanently expanded from the pressure testing is also measured. (Section 180.205(g)).

If a cylinder passes its requalification testing, it must be marked with the month and year of requalification. It must also be marked with the requalifier's identification number (RIN). (Section 180.213).  If a cylinder was requalified in November, 2003 by a requalifier holding a RIN of A221, the requalification mark would look like this:

                                                  B3 2   05

                                                  52

Cylinders are also marked with Crown Markings which are normally on the shoulder of the cylinder. Suppose for example a cylinder was marked: DOT-3AL2216 HH34087 03/01 LUXFER TC-3ALM153. These markings mean: The cylinder is a DOT approved Type 3AL aluminum cylinder with a service pressure of 2216 pounds per square inch.  It has a serial number of HH34087. It is made by LUXFER. It was made in March, 2001. It was certified by Transport Canada as a Type 3ALM aluminum cylinder with a service pressure (in bars) of 139.

Another example might be: DOT-3AL2015 NN0366194 M4002 08 02 CATALINA M15 TC-3ALM139. These markings mean: The cylinder is a DOT approved Type 3AL aluminum cylinder with a service pressure of 2015 pounds per square inch. It has serial number NN0366194. The M4002 is the DOT manufacturer's number issued to CATALINA. It was manufactured in August, 2002. It is an "M" cylinder having a capacity of 22 cubic feet. It was certified by Transport Canada as a Type 3AL aluminum cylinder with a service pressure (in bars) of 139.

So go out to the hangar and check your oxygen cylinder. You will find out a lot more than you already know. If your cylinder is out of date, you must have it requalified before it can be re-filled.  The manufacturer of your oxygen cylinder can requalify it for you for around $60, but you will have to ship your cylinder to them.  However, many fire equipment businesses and dive shops can also do the requalification for you. Usually these businesses are local, easily accessible, and will charge around $25.

Maintaining your oxygen cylinders in top-notch shape is important. A leaking oxygen cylinder can cause instant and spontaneous combustion of nearby flammable material. A catastrophic failure of an oxygen cylinder will almost certainly be fatal. There is a reason for requalification of oxygen cylinders, and given the implications of a failed oxygen cylinder, the price of admission is low.

1. Individuals should consult the regulations themselves respecting the requirements for their own cylinders.

2. All references are to specific sections of Title 49, Chapter 180 of the Code of Federal Regulations unless otherwise indicated.