The use of oxygen and oxygen-enriched gas blends is growing in popularity among sport and recreational divers around the world, especially in the United
States. Judging from the questions that Luxfer has received, a good deal of confusion exists within the recreational diving community about safely using Luxfer aluminum scuba cylinders in oxygen-enriched
and oxygen service. This document answers the most frequently asked questions and provides references for further research about this important subject. This page contains Luxfer’s recommendations
only for Luxfer scuba cylinders manufactured in the U.S. and may not be applicable for other countries.
1. Question: What are the differences among the terms oxygen service, oxygen mixture, nitrox , enriched-air nitrox (EAN) and oxygen-enriched air?
Answer: The term oxygen service generally means using pure oxygen (oxygen concentrations of 95% to 100%), but the term is sometimes used more broadly to refer to any gas mixture containing
more oxygen than air. The "standard air" we breathe is composed of 20.95% oxygen, 78.05% nitrogen and 1% of trace amounts of other gases, including argon, carbon dioxide, neon, helium, krypton
and xenon (the amounts vary depending upon your geographical location, altitude, etc.). For practical purposes, most technical literature rounds off these percentages and defines "standard air"
as 21% oxygen and 79% nitrogen.
Oxygen mixture generally refers to gas blends containing between 50% and 95% oxygen.
Oxygen - enriched air, nitrox , and enriched-air nitrox (EAN) are generally used synonymously in the diving industry since all refer to pressurized diving gas mixtures
containing various oxygen concentrations greater than standard air. For example, two common mixtures used by divers are EAN 32 (32% oxygen) and EAN 36 (36% oxygen). However, in the gas industry,
oxygen-enriched air is sometimes defined more specifically as gas mixtures containing more oxygen than found in standard air, but no more than 50% oxygen.
To avoid confusion, Luxfer prefers using the term oxygen service to refer only to pure oxygen usage and enriched-air nitrox (EAN) to refer to oxygen-enriched breathing mixtures used for diving.
Whatever term you choose, the main thing to remember concerning Luxfer scuba cylinders is that when you fill a cylinder with an oxygen concentration of 23.5% or more, that cylinder must be specially
cleaned for oxygen service as though it contained 100% oxygen.
2. Question: Can I fill my Luxfer aluminum scuba cylinder with pure oxygen so that I can use it for decompression?
Answer: Yes, if you follow the Luxfer requirements listed below, which must be met both for pure oxygen and gas mixtures containing more than 23.5% oxygen:
- The cylinder must have been properly cleaned, tested for cleanliness and maintained in a proper state of cleanliness for oxygen service in strict accordance with the Code of Federal Regulations (CFR),
specifically U.S. Department of Transportation (DOT) CFR49 173.302(b).
- The cylinder must be equipped with an oxygen-compatible valve and be filled only by oxygen-clean, properly maintained filling equipment dedicated to oxygen service.
- The cylinder must never be fast-filled or over-pressurized.
- Proper safety procedures for handling and dispensing oxygen prescribed by the DOT and U.S. Compressed Gas Association (CGA) must be followed at all times. In fact, Luxfer further recommends that
anyone in the scuba industry who handles pure oxygen in a Luxfer cylinder become familiar with and follow current guidelines concerning cleanliness, filling, storage, usage and maintenance of oxygen
cylinders provided by the U.S. Food and Drug Administration (FDA). As the world’s largest manufacturer of aluminum and composite medical oxygen cylinders, Luxfer has a great deal of experience
with cylinders in pure oxygen service and with the FDA guidelines that promote safe oxygen usage. Luxfer’s experience has shown that following FDA guidelines is a practical and effective way of
preventing oxygen-related accidents.
3. Question: Why is it necessary to clean scuba tanks for oxygen service?
Answer: To remove contaminants that can become the fuel for an oxygen-fed fire.
Oxygen, an oxidizing gas, does not burn, but it supports combustion and causes other materials to burn. In the presence of pressurized oxygen concentrations exceeding that in standard air, most materials
become progressively more flammable and ignitable as the oxygen concentration and pressure increase. Even materials that would not easily catch fire in standard air under normal atmospheric pressure will
ignite and burn furiously in a pressurized oxygen-enriched environment. It cannot be overemphasized that rising pressure increases the risk of ignition.
For an oxygen-fed fire to occur, three things must be present: oxygen, fuel (something to burn), and heat (an ignition source)—this is the classic “fire triangle.” When dealing
with pressurized oxygen or EAN in a scuba cylinder, remember this important variation on the classic three factors needed for a fire:
- Oxygen or gas containing oxygen, under pressure (higher concentrations of oxygen and significant pressure greatly increase the need for oxygen cleanliness and diligent safety practices).
- An ignition (flame, spark, heat source, etc., including impact from dropping or striking a pressurized cylinder).
- A contaminant (i.e., fuel—but the reason the term “contaminant” is preferred in this case is that in an properly cleaned oxygen system, no fuel is present. If
fuelis present, it is contaminating the system and making it no longer safe for use with oxygen). Common contaminants include:
- Machining oils (including residual oil film)
- Hydrocarbon-based grease and lubricants (including compressor oil)
- Some soaps, detergents, solvents and cleaning solutions, especially those that contain organic compounds
- Skin lotions and emollients and cosmetics
- Sun-tanning oils and lotions
- Human skin oil and bodily fluids
- Insects and insect body parts
- Paint, wax, and marking crayons
- Carbon dust from filtration systems
- Metal fines, filings, scale and burrs
- Chrome chips (usually from valves and other chrome-plated parts)
- Rust particles and dust
- Metallic oxides in general
- Airborne soot and dust
- Pipe thread sealants
- Residue from soapy water and leak-detection fluids used to check for leaks
- Lint from cloths used in cleaning
- Any other material containing organic compounds and hydrocarbons
Any of these contaminants—many of which are very difficult to detect—can be the initial fuel for an ignition event, the technical term for starting a fire. Once an oxygen-fed fire
gets going, even metal components—including a scuba cylinder itself—can burn vigorously. Aluminum melts at 1,220 degrees F (660 degrees C); after an oxygen-related cylinder fire, it is not
unusual to find large areas of melted aluminum—which shows just how hot and energetic such a fire can be.
That’s why it’s so essential to remove contaminants and to keep the oxygen system clean. Proper oxygen cleaning need not be excessively costly or time-consuming, but it does need
to be effective. Removing contaminants and keeping equipment oxygen-clean is the best way to make sure that an oxygen-fed fire never starts.
A number of agencies and organizations in the United States provide certification courses in oxygen cleaning. For example, American Nitrox Divers International (ANDI) offers a particularly comprehensive
training course that defines “oxygen cleaning” as “the process of contaminant removal from component parts or assemblies intended for contact with gases containing Oxygen concentrations
of 23.5% or more by volume.”1
4. Question: I’ve read in some scuba manuals, including those published by well-known scuba organizations, that it’s only necessary to clean cylinders and other scuba equipment for
oxygen service when the oxygen concentration exceeds 40%. Why does Luxfer require cleaning for oxygen concentrations above 23.5%?
Answer: Few concepts have caused more confusion and controversy in the recreational diving industry than the so-called “40% rule.” While there seems to be general agreement that
special cleaning is required when a pressurized oxygen concentration reaches a particular “threshold” percentage, there is disagreement about exactly what that threshold should be and at what
pressure it becomes important. Some say 40%; others say 23.5%; still others say anything more than 21% when a gas mixture is pressurized more than 100 psig. It would be helpful to explore the background of
this confusion briefly before discussing Luxfer’s position on this vital subject.
The 40% threshold is cited in a single Federal CFR published by the Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor: 29CFR910.430, which applies to “Commercial
Diving Operations” and states in the section titled “Oxygen safety” on page 854: “(1) Equipment used with oxygen or mixtures containing over forty percent (40%) by volume oxygen
shall be designed for oxygen service. (2) Components (except umbilicals) exposed to oxygen or mixtures containing over forty percent (40%) by volume oxygen shall be cleaned of flammable materials before
Please note that OSHA also provides a very specific definition about who should—and should not—be considered a “commercial diver” to whom the 40% threshold applies:
“Commercial diver means a diver engaged in underwater work for hire excluding sport and recreational diving and the instruction thereof” (46CFR197, page 409; italics added for
Even though OSHA clearly excludes sport and recreational divers from the CFR that specifies a 40% threshold, some professionals in the recreational diving industry have nonetheless been citing the
OSHA “rule” for many years and maintaining that special cleaning of recreational diving equipment is not necessary with oxygen concentrations of 40% or less. These advocates of the
“40% rule” have often stated that the U.S. Navy supports their position, which at one time was true—but no more. In the current applicable military specification (Mil-Std-1330D), the
Navy specifies a 25% threshold for oxygen cleaning. Compounding the confusion is the fact that two other OSHA documents, 29CFR1910.146 and 29CFR1910.134, specify an oxygen threshold of 23.5%.
For the sake of clarity, here’s a summary of the oxygen threshold percentages at which various key U.S. agencies and organizations require special cleaning of oxygen-handling equipment and
|U.S. Compressed Gas Association (CGA)
||CGA Pamphlet 4.4
|National Fire Prevention Association (NFPA)
||>21 – 25%
|American Society for Testing & Materials (ASTM)
||G126, G128, G63, G94
|National Aeronautical & Space Administration (NASA)
||Various KSC & JSC
|Occupational Safety & Health Administration (OSHA)
The reasons for Luxfer’s position: Luxfer supports an oxygen-cleaning threshold of 23.5% and does not support the alleged “40% rule.” This means that when a pressurized
oxygen concentration used in a Luxfer cylinder exceeds 23.5%, the cylinder must have been cleaned to the same cleanliness standard mandated for a cylinder containing 100% oxygen. Luxfer’s reasons
for this position are:
- All key U.S. regulatory and gas industry references except one advocate an oxygen threshold of 21% to 25%.
- Luxfer is a member of the Compressed Gas Association and therefore supports the CGA-specified threshold of 23.5%. Furthermore, Luxfer defers to CGA on all safety matters related to oxygen handling
and containment, as do OSHA and DOT.
- As an international manufacturer, Luxfer works with regulatory authorities and industry associations around the world—the overwhelming majority of which support a threshold value from 21%
to 25% (for example, this is true in the United Kingdom, Australia, France, Germany and Japan).
- United Nations compressed air packaging guidelines (see UN 1002) indicate that when compressed air contains oxygen as the only oxidizing gas and the oxygen concentration exceeds 23.5%, then the
entire gas mixture must be listed as an oxidizing gas.
Some within the recreational diving community contend that the supposed “40% rule” is justified by an excellent scuba safety record and should therefore be maintained—despite the
fact that so many agencies and organizations have specified much lower threshold values for oxygen cleaning. Luxfer finds this “history-of-use” argument unconvincing for the following
- Compared to other industries and organizations that use pressurized oxygen, oxygen usage in the recreational diving industry has not been sufficiently widespread, nor are available oxygen-use
statistics sufficiently comprehensive, to declare a successful history of use for the 40% threshold.
- Oxygen-related fires and explosions are inherently low-probability, high-consequence events—which means that they occur very infrequently, but are usually catastrophic when they do. Forensic
evidence has shown that undetected, non-propagating fires happen within scuba oxygen systems more frequently than is generally known. It is possible to operate “on the edge” of a fire for
years without knowing it—and to be lulled into complacency by seemingly “safe” performance.
- As use of higher oxygen concentrations at higher pressures increases in the recreational diving industry, the risk of serious accidents will also increase.
Under these circumstances, Luxfer believes that requiring thorough oxygen cleanliness at a threshold of 23.5% makes very good sense not only for the sake of individual safety, but also from the business
standpoint of prudent risk management.
5. Question: How clean does my cylinder actually need to be for oxygen service?
Answer: It needs to be “oxygen clean”—which means free from contaminants that can be the fuel for an oxygen-fed fire (see the list of contaminants in Question 3, above).
Cleanliness on surfaces is measured by milligrams (mg) of hydrocarbon per square foot of surface area (mg/ft2). In the U.S., DOT (in RRC 901c) requires that the cleanliness of cylinders used in
oxygen service be proven to be less than 2.5 mg/ft2, which is a level considerably below what can be seen by the naked eye. For this reason, a visual inspection after cleaning is not enough. A
test must be performed to verify that the required cleanliness level has been achieved. In CFR49 173.302(b), DOT specifies an approved cleaning method and a standard test to determine the cleanliness of
6. Question: What is the difference between “formal” and “informal” oxygen cleaning?
Answer: The terms “formal oxygen cleaning” and “informal oxygen cleaning,” which appear in some non-Luxfer scuba manuals, are intended to differentiate between formal,
government-mandated oxygen-cleanliness documentation required in the gas and medical industries, as opposed to less-formal documentation used in the recreational diving industry.
Unfortunately, some scuba technicians and divers have erroneously interpreted “informal cleaning” to mean that a less-rigorous, less-thorough level of oxygen cleaning is required for
scuba equipment—even though the scuba manuals that use these terms make it quite clear that the same careful cleaning procedures and acceptable cleanliness levels should be used with all
equipment in oxygen service. Moreover, the term “informal” is often associated with the so-called “40% rule,” from which some divers and technicians have wrongly inferred that
the recreational diving industry somehow has special dispensation to clean cylinders to a less-stringent standard and at a higher oxygen-content threshold than other industries (see Question 4, above).
Because of these erroneous and potentially dangerous misinterpretations, Luxfer chooses not to use the terms “formal” and “informal” when referring to oxygen cleanliness.
Oxygen clean is oxygen clean—there are not different levels of cleanliness.
As for documentation, it is always a good idea to thoroughly document all aspects of your oxygen-cleaning procedures and maintenance. Such careful record-keeping can be very valuable if an oxygen-related
7. Question: How do I keep my cylinder clean for use with oxygen or oxygen-enriched gas mixtures?
Answer: Here are some basic guidelines for keeping your cylinder clean and for using it with oxygen and oxygen-enriched gas mixtures (more detailed guidelines are available from CGA):
- Only use an oxygen-clean, oxygen-compatible valve installed by a properly trained, competent technician familiar with the procedures described in current CGA guidelines.
- Either use no lubricants at all—or use only lubricants that the lubricant manufacturer explicitly recommends for use with oxygen systems and cylinders containing oxygen under pressure. Once
you have found an oxygen-safe lubricant, always strictly follow the application and use instructions of the lubricant manufacturer. If a lubricant manufacturer does not specifically state that a
lubricant is suitable for compressed oxygen service, don’t use it.
- Have your cylinder filled only with contaminant-free gas pumped by oxygen-clean systems, including fill lines that are clean and dedicated only to oxygen service and compressors that will not
release oils or other contaminants into the gas.
- Make sure that the inside of your cylinder does not become contaminated by water, dirt, oil residues, body fluids or other contaminants during use and storage (see contaminants listed in Question 3,
above). If the cylinder does become contaminated, it must be re-cleaned and tested before being used with oxygen again.
- If a hydrocarbon or other contaminant is discovered on the outside of the valve-to-cylinder connection, the cylinder must be depressurized and tested to make sure that it is still oxygen-clean. If
the cylinder is not clean, then it must be re-cleaned for oxygen and tested again before being filled with oxygen or an oxygen-enriched gas mixture.
- Ensure that your cylinder is inspected and requalified only by properly trained, competent technicians who are familiar with government requirements regarding oxygen cleanliness and use. If you
suspect that your cylinder has been handled by someone who lacks this requisite knowledge, do not use the cylinder for oxygen or oxygen-enriched service until the cylinder has been tested by competent
- A valve should only be removed and reinstalled by a properly trained, competent technician who is familiar with procedures described in current CGA guidelines (see CGA P-38). Attempting to remove
a valve without following the proper safety procedures can cause death or serious injury.
- If a cylinder owner, user, inspector, requalifier, distributor or filler has any doubts concerning the proper cleanliness of a cylinder intended for oxygen or oxygen-enriched service, the cylinder
must be tested for oxygen cleanliness before being filled or used.
- When your cylinder has been exposed to possible contaminants, have it cleaned and tested for oxygen-cleanliness before filling or using it.
8. Question: Are Luxfer scuba cylinders ready for oxygen service when they leave the Luxfer factory?
Answer: All Luxfer scuba cylinders manufactured after January 1, 2005, have been cleaned for oxygen service at the factory and sealed with an oxygen-compatible plug.
9. Question: I’d like to start using nitrox (EAN) in a scuba tank in which I’ve only been using air. Can I do that?
Answer: Yes, if you have the cylinder properly cleaned and tested for oxygen and oxygen-enriched service (i.e., oxygen concentrations above 23.5%).
10. Question: It’s really important that I have enough pure oxygen in my decompression cylinder when I need to use it after a deep dive. Is it okay to put extra oxygen in my cylinder to make
sure that I don’t run out?
Answer: DOT regulations (see CFR 49) strictly prohibit over-pressurizing a scuba cylinder containing any kind of gas, but it is particularly dangerous to do so with high oxygen concentrations. The
reason is: The higher the pressure and the higher the oxygen concentration, the higher the risk of a fire and explosion if a contaminant is present.
Luxfer has received numerous and persistent reports that technicians in certain sectors of the recreational diving community routinely over-pressurize scuba cylinders, including cylinders containing
high concentrations of oxygen. This is sometimes described as “doing divers a favor,” offering “a little more down time” or “giving divers their money’s worth.”
Not only is this an unsafe practice, it is against the law! Under no circumstances should you allow the gas pressure in your scuba cylinder to exceed the service pressure for which the cylinder is designed
and stamped or marked. If a filler offers to over-pressurize your cylinder, you should not only refuse the offer, you should report the filler to the DOT. If you suspect that your cylinder has been
over-pressurized, you should have it depressurized and have it inspected by a competent technician to determine whether it is fit for further service.
For pure oxygen, DOT mandates strict pressure limits: Gas pressure in an aluminum cylinder containing pure oxygen must never exceed 3,000 psi (even if the cylinder is stamped for a pressure
above 3,000 psi).
If you are concerned about running out of oxygen, use a larger decompression cylinder filled to the proper service pressure—or carry more cylinders.
11. Question: I’d like to blend my own EAN mixtures. Is it all right for me to use Luxfer scuba cylinders for partial-pressure nitrox blending?
Answer: In all matters relating to gas-handling, blending and filling, Luxfer defers to CGA, of which Luxfer is a member. Partial-pressure blending with oxygen should be done only by those
properly trained to do so safely by a recognized, responsible agency that strictly follows CGA guidelines. Most accidents related to partial-pressure blending have involved people who have not been properly
12. Question: Can dropping or striking a scuba tank containing pressurized oxygen start a fire?
Answer: If a contaminant (fuel) is present and the oxygen inside a cylinder is under sufficient pressure, impact from a fall or blow can indeed be the ignition source for an oxygen-fed fire.
Although ignition of this type occurs very rarely, it’s still a good idea to handle a pressurized cylinder with care at all times. Always follow all applicable DOT guidelines when transporting
cylinders. Secure cylinders when transporting them so that they will not roll around, rattle about or strike other cargo or hard surfaces. Protect cylinders from falling and impact by using chain restraints,
padding, straps and dollies during transportation and use. Also, always exercise great caution when hand-carrying all pressurized scuba cylinders, especially those containing oxygen or oxygen-enriched
13. Question: What happens to an aluminum scuba tank in an oxygen-fed fire?
Answer: Obviously, no two fires are exactly alike, but based on forensic studies made on cylinders used with oxygen (including medical cylinders), it is possible to posit a likely scenario in
terms of combustion and metallurgical characteristics. Here’s what generally happens when a fire occurs inside or in close proximity to a cylinder containing a pressurized gas mixture comprised of
more than 23.5% oxygen:
- First, an ignition source of some sort—a flame or spark, excessive or prolonged heat, impact, etc.—ignites a contaminant, which serves as the initial fuel for the fire. (Please
remember: Many materials that might not ignite and burn in standard air under normal atmospheric pressure will ignite and burn, often furiously, in a high-pressure, oxygen-rich environment.)
- The initial fire begins melting the metal of the cylinder and/or the scuba valve, at which point the molten metal itself becomes fuel and starts to burn. The melt temperature for aluminum is 1,220
degrees F (660 degrees C), which gives you some idea of how hot an oxygen-fed fire can become—and it usually reaches these elevated temperatures in less than one second. All the time
that the metal is burning, oxygen is being consumed and the fire is intensifying.
- The action of the fire rapidly increases the pressure inside the cylinder, and all the while more and more aluminum is melting and starting to burn, fueling the fire and weakening the cylinder
to the point that it can no longer hold the growing pressure. All this occurs so quickly that the pressure-relief device on the valve does not have time to activate.
- By the time the pressure-relief device is ready to activate, either the valve is forcefully ejected at high velocity or the cylinder ruptures—or both.
- Afterwards, it is very difficult, if not impossible, to determine the nature of the original contaminant, because the contaminant has been completely consumed or driven off by the extremely hot
fire. At that point, a thorough examination of the total oxygen system, cleanliness records, cleaning methods, system use, care and maintenance records, filling procedures and other relevant data
may reveal possible sources of contamination.
14. Question: I’ve heard about fires and explosions occurring in tanks that were supposedly cleaned for oxygen. How could that happen?
Answer: There are two main possibilities. First, the cylinder might well have been properly cleaned and tested, but subsequently became contaminated in a way that was not detected before the
fire occurred. (Sometimes other elements of the oxygen filling or containment system become contaminated, and they, in turn, contaminate the cylinder; sometimes a valve is improperly installed into the
cylinder or an improper lubricant is used; sometimes contamination occurs during use—the possibilities are numerous.) Or, second, the cylinder was not cleaned and tested properly in the first
place, and a contaminant was still present to serve as fuel for the fire (see Question 5, above). One thing is certain: An oxygen-fed fire cannot start without oxygen, an ignition source and a
contaminant—so contamination must have been present (see Question 3, above, for a more detailed discussion of how fires start, why proper cleaning is crucial and a list of common contaminants).
15. Question: Do fires occur frequently with aluminum oxygen cylinders in general?
Answer: Absolutely not. Oxygen-related fires are extremely rare events, which is a testimony not only to the safety and suitability of aluminum cylinders for oxygen service, but also to proper
cleanliness, care and maintenance. Aluminum cylinders have been used in oxygen service for more than 35 years, and more than 20 million cylinders have been produced for such service in North America alone.
However, because oxygen-related fires happen infrequently, people can become lax about oxygen safety—which is the greatest danger of all.
1EAN and Oxygen Equipment Servicing Procedures, ANDI International: 1999, p. 19.