Nitrox Fills and O2 Cleaning Cylinders

Recently we've have had several inquiries regarding Nitrox fills, O2 cleaning of cylinders, Nitrox stickers, banding etc. Hopefully this article will clear up some misconceptions with regards to Nitrox filling and tank labeling.

Does a cylinder need to be O2 clean in order to receive a Nitrox fill?

Well it depends on if the dive shop doing the filling will be partial pressure blending or not. When a dive shop partial pressure blends Nitrox they add pure Oxygen to the tank first and then top off with air to end up with the desired Nitrox mix. Because pure Oxygen is being used to fill the cylinder initially, the tank and valve must be O2 cleaned and filled with Oxygen compatible air. Oxygen compatible air has been through extra filtration to ensure fewer oils and hydrocarbons as compared to standard air and avoids contaminating the tank and valve. (At The Scuba Doctor ALL of the air we use to fill all cylinders is Oxygen compatible.)

If the dive shop has Nitrox in banks, or uses a compressor with either a membrane system or Nitrox mixing stick to create the Nitrox, then the dive shop may generally believe the cylinder being filled does not need to be O2 clean to be filled with Nitrox up to 40%. (That is, the standard practice is to apply what is known as the 40 per cent rule.) In this case the shop will be decanting Nitrox which is already mixed at a fraction of Oxygen (FO2) less than 40% into the tank. More dive shops are beginning to bank Nitrox and have it ready to decant into customers cylinders. Some dive shops keep more Nitrox banked than they do air. Using this method the cylinder and valve never see more than 40% Oxygen. If the dive shop is following the 40% rule, they will not require the cylinder and valve to be Oxygen cleaned, as the tank and valve only ever "sees" Nitrox below 40%.

At The Scuba Doctor we use partial pressure blending for Nitrox and Trimix fills, so your cylinders and valves need to be O2 clean when getting these fills because they will see 100% Oxygen.

What if I want a Nitrox fill over 40%?

This question is asked because many people in the dive industry simply refer to and follow the 40 per cent rule.

The 40 Per Cent Rule
In essence, the 40 Per Cent Rule states that, "If a regulator, cylinder or valve will not be exposed to gas mixtures with an FO2 greater than 40 percent, at pressures greater than regulator intermediate pressure, they need not be O2 clean or service rated." The 40 Percent Rule is widely accepted by divers, dive shops, dive operators and government agencies. Any gas over EAN 40 is treated as Oxygen in terms of handling procedures and the cleaning requirements for cylinders and valves.

So if you require a Nitrox fill over 40%, most dive centres will require you to have an O2 clean cylinder and valve.

Nevertheless, some manufacturers specify that any regulators, cylinders or valves exposed to gas mixtures other than air be O2 clean and service rated. It's not a bad idea to ensure that any cylinder designated for Nitrox use be kept O2 clean and service rated as well, regardless of whether it will be filled using partial-pressure blending or not.

At The Scuba Doctor because we partial pressure fill all Nitrox mixes, we require all cylinders and valves that are to receive a fill with Nitrox 22% or higher, to be O2 clean.

Is the 40% Rule Adequate?

Within the recreational diving industry it is generally accepted that regulators, SPG's and alternate air sources can be used with enriched air mixtures up to 40% without modifications. On the other hand, equipment exposed to gas mixtures with more than 40% oxygen need to be cleaned to oxygen service specifications. This will include cylinders and cylinders valves that will be used for partial pressure blending (introducing 100% oxygen).

As you would have guessed, we wouldn't have brought up the subject if there wasn't room for any controversy.

Although the 40% rule 'seems' to have a proven safety track record, the 40% rule historically originates from a single entry in 29CFR910.430 by the Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labour, which applies to "Commercial Diving Operations". Within the same set of rules, Commercial diver means a diver engaged in underwater work for hire, excluding sport and recreational diving and the instruction thereof. Despite this clear exclusion, within the recreational diving industry this rule keeps being cited.

Looking into more US legislation, a threshold of 23.5 or 25% seems more acceptable. Worth mentioning here is NASA: their threshold is >21% />100 PSI (6.8 bar). Now those NASA chaps might have a little experience with oxygen handling and unfortunately learned plenty of it the hard way.

To add to the confusion, the International Marine Contractors Association (IMCA) recommends a threshold of 25%.

Here in Australia we tend to follow the international stardards, with our own modifications is some cases. Well, EN 12021:2014 mandates the use of oxygen compatible air in mixtures containing an oxygen concentration greater than 22%. Furthermore, in Europe both EN 13949:2003 and EN 144-3:2003 state that scuba equipment sold in CEE (European Community) countries over 21% and up to 100% will be manufactured with the special DIN M26 fitting.

Not only in geographical areas where legislation is not as strictly enforced or present, we believe it is foolhardy just to stick to the 40% rule. If large geographical areas or industries have significantly raised the bar, why should the recreational diving industry not follow?

Well actually, it is. We're aware that RAID, ANDI and BSAC have already raised this bar and thus deserve an honourable mention.

Regardless of the above discussion, let not lose out of sight the manufacturer's position and recommendations for the use of their equipment in oxygen rich environments. We suggest you read carefully some of their manuals and you'll get some fine surprises there. Do a little research and you’ll find that both major aluminium cylinder manufacturers' Catalina and Luxfer state that oxygen cleaning should be performed if their cylinders are used with oxygen concentration in excess of 23.5%.

In summary, we believe the 40% rule is inadequate. We recommend that if you're using diving gas mixes with oxygen concentrations greater than in air (21%), then get your dive gear oxygen cleaned.

My dive shop banks 32% and they still require my cylinders to be Nitrox cleaned. Why?

Each dive store may have its own policy with regards to Nitrox. Technically there is no such thing as cleaning a cylinder for Nitrox. Either the tank has been O2 cleaned, or it has not. If the cylinder has been O2 cleaned it should more correctly be stamped/labelled for O2 service. If a dive shop does not clean a cylinder and charge for a "Nitrox cleaning", well then the customer is simply being gouged.

Some dive stores may require a cylinder to be O2 cleaned in order to be filled with pre-mixed Nitrox. We believe this is a good thing as it's always better to err on the side of caution.

Does a cylinder being filled with Nitrox need to be banded and dedicated to Nitrox?

Oxygen Clean In Test Sticker
Oxygen Clean In Test Sticker
Any tank filled with Nitrox must be labelled as a minimum as to whether it is O2 clean or not, so that a dive shop knows whether the tank can be filled or not based on their filling methods (i.e. partial pressure or pre-mix).

Once filled with a Nitrox mix, a contents label must also be applied indicating what gas (percent O2) is in the cylinder. Normally the tank is also labelled with the date of analysis, initials of the person who analysed the cylinder and the maximum depth the gas may be used at safely (Maximum Operating Depth - MOD).

Nitrox cylinders
Nitrox cylinders
Using a large Nitrox wrap sticker for labeling an O2 clean cylinder that may be filled with a Nitrox mix is recommended by most dive training agencies, regulatory bodies and applicable standards. If you plan to use a cylinder for only Nitrox then you should put the Nitrox wrap on it. In this way there is less confusion as to what the tank contains.

Although a large Nitrox banding label is preferred, some people do not want to dedicate a cylinder to only Nitrox and choose not to add the Nitrox wrap bands. This is especially true for technical divers who may have either Air, Nitrox, or Trimix in their O2 clean tanks on any given day.

Some dive shops may require Nitrox banding label, and that is their choice, but in general banding is not absolutely required, and many people do not band their tanks.

What is hyper-clean air and can I fill my non O2 clean cylinder with it?

"Hyper-clean" air is actually not a standard and no one can have their air tested to this standard. The correct term is Oxygen Compatible Air, or Modified Grade E air.

Oxygen compatible air and modified grade E air are both basically the same as standard Grade E air (standard scuba air), but are required to contain much lower oil/hydrocarbon levels. Indeed Oxygen compatible air is virtually free of hydrocarbons and contaminants. This is to ensure that when Oxygen clean tanks are filled we minimise the contamination to the cylinder and valve by using this "extra/hyper clean" air.

Many dive shops use an extra filter to create Oxygen compatible air, and this is fine. The problem arises when the air is not actually tested to this standard. All dive shops are required to test their air several times a year, but many fail to test their Oxygen compatible air.

The easiest solution would be to have all the air coming from the compressor tested to the Oxygen compatible air standard. The standard is easy to achieve with standard breathing air compressor systems if one is diligent about filter and oil changes and uses synthetic compressor oil. Simply always testing to this standard means all the air coming from the compressor is Oxygen compatible and of the highest quality.

Any scuba cylinder can be filled with Oxygen compatible air. However, if a dive shop charges more for this service then you are wasting your money if your tank is not already O2 clean since standard Grade E air is just fine for non O2 clean tanks.

At The Scuba Doctor all of the air we supply is Oxygen compatible, modified grade E, "hyper-clean" air of the highest quality. We are meticulous about filter and oil changes and test the air several times a year. Plus we charge less for Oxygen compatible air than most other shops charge for their standard Grade E air.

Should I treat my O2 clean cylinders differently?

Yes. You should do everything possible to ensure they stay O2 clean, and only ever fill them using Oxygen compatible air.

Standard scuba air, although very clean and dry, may still contain trace amounts of hydrocarbons and other contaminants. If allowed to collect on the internal surfaces of cylinders, regulators and valves, these create a risk of fire and/or explosion. This is a risk that O2 cleaning and the use of Oxygen compatible air should eliminate. But if you get a standard scuba air fill your O2 clean setup may become contaminated and this is potentially dangerous if you subsequently get a Nitrox mix at a dive shop using partial pressure blending where your equipment will be exposed to 100% Oxygen.

Oxygen compatible air, on the other hand, is virtually free of hydrocarbons and contaminants. By making certain that the internal surfaces of cylinders, regulators and valves only come in contact with Oxygen-compatible air, or gas mixtures created using Oxygen-compatible air, you effectively eliminate the risk of fire or explosion, and maintain your equipment's O2 service rating.

It is impossible for a dive operator to determine, simply by looking at cylinder markings that indicate the cylinder was O2 cleaned within the past year, that a cylinder and valve have not been exposed to air or gas mixtures that would invalidate its O2 service rating. Thus, it is the responsibility of the divers who own or use this equipment to:

  • Only allow this equipment to come in contact with Oxygen compatible air or gas mixtures, or
  • If the equipment does come in contact with standard scuba air, to remove any markings that indicate it is O2 clean, and not represent to dive operators that any such cylinders are suitable to be filled with Nitrox through in-cylinder partial-pressure blending.

Nitrox divers often have their cylinders refilled with air for dives in which having Nitrox may not offer any appreciable benefit. If you do so, and if your cylinder is O2 service rated, you must use only Oxygen compatible air.

Does The Scuba Doctor treat O2 clean cylinders differently?

Yes. Our fill panel is has fill whip outlets for just doing air fills of non O2 clean cylinders, and fill whip outlets for filling O2 clean cylinders with air, Nitrox or Trimix. If your tank and valve are O2 clean, then we use the latter fill whips even if you're just getting an air fill, or topping a mix up with air. This helps to maintain the integrity of your O2 cleaning. We NEVER attach a non O2 clean cylinder to this latter set of fill whips.

What are the legal requirements with regards to Nitrox and O2 cleaning dive gear?

Whether or not to O2 clean other equipment items is dictated by a hierarchy of practices.

  • Local laws, regulations and standards of practice always take precedence over any less-strict requirements.
  • Manufacturer's specifications also take precedence over any less-strict requirements, as dive operators cannot defend a decision based on the fact they presume to "know better" than the engineers who designed a particular piece of equipment.
  • Absent the precedence of either of the above two factors, the standard practice is to apply the 40 Per Cent Rule.
  • Every dive operator is legally entitled to enforce a higher, more-strict standard than any of the above practices based on their own risk assessment and comfort level.

Get Your Dive Gear O2 Cleaned by Scuba Doctor Service and Repairs

Yes, we can O2 clean your dive equipment. See Cylinder Oxygen Cleaning.

The Scuba Doctor Service and Repairs