Dive Forum Blog

Nitrox Fills and O2 Cleaning Tanks

Recently I have had several inquiries regarding O2 cleaning of tanks, Nitrox stickers, banding etc. Hopefully this article will clear up some misconceptions with regards to Nitrox filling and tank labelling.

nitrox_scuba_tank_01

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

Well it depends... If the shop doing the filling will be partial pressure blending or not. When a shop partial pressure blends Nitrox they add pure Oxygen to the tank first and then top off with air to end up with Nitrox. In this case because pure Oxygen is being used to fill the tank it must be O2 cleaned and filled with Oxygen compatible air. Oxygen compatible air has fewer oils and hydrocarbons as compared to standard air and avoids contaminating the tank and valve.

If the shop has Nitrox in banks or uses a compressor with either a membrane system or Nitrox mixing stick to create the Nitrox then the tank does not need to be O2 clean to be filled with Nitrox up to 40%. In this case the shop will be decanting Nitrox which is already mixed into the tank. More and more shops are beginning to bank Nitrox and have it ready to decant into customers tanks. Some shops keep more Nitrox banked than they do air. Using this method the tank and valve never see 100% Oxygen and as a result they do not need to be Oxygen cleaned. Cleaning is not required as long as the tank and valve only ever "sees" Nitrox below 40%.

What if I want a Nitrox fill over 40%?

All tanks and valves that are to receive a fill with Nitrox 40% or higher need to be O2 cleaned 40% is the threshold where we consider a tank to require O2 cleaning. Any gas over 40% is treated as Oxygen in terms of handling procedures and cleaning requirements for tanks and valves.

My shop banks 32% and they require my tanks 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 tank for Nitrox. Either the tank has been cleaned or it has not. If the tank has been cleaned it should more correctly be stamped/labelled for O2 service. If a shop does not clean a tank and charged for a "Nitrox cleaning" well then the customer is simply being gouged.

Some stores may require a tank to be O2 cleaned in order to be filled with pre-mixed Nitrox. Although not technically required for fills under 40% it is not a bad thing to err on the side of caution.

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

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

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

Beyond these minimum requirements some people choose to place a Nitrox wrap on their tank to clearly indicate the tank is being used for Nitrox. Although nice to do some people do not want to dedicate a tank to only Nitrox and choose not to add the bands, Also these stickers may add about $10.00 or so to the cost of having a tank setup for Nitrox. If you plan to use a tank for only Nitrox then by all means put the wrap on it. In this way there is no confusion as to what the tank contains.

Some dive shops may require banding, and that is their choice, but in general banding is not absolutely required, and many people do not band their tanks. This is especially true for technical divers who may have either Nitrox, air or Trimix in their tanks on any given day.

What is hyperclean air and can I fill my non O2 clean tank with it?

"Hyperclean" 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 Grade E air, but are required to contain much lower oil/hydrocarbon levels. This is to ensure that when Oxygen clean tanks are filled we minimize the contamination to the tank and valve by using this "extra clean" air.

Many 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 "hyper clean" 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 acheive with standard breathing air compressor systems if one is dilligent about filter and oil changes and uses sythetic 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 tank can be filled with Oxygen compatible air. However if a 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.

Scuba Tank Markings in Canada since 1993

Canada made a number of changes to their tank marking regulations as of 1993.

As of January 1 1993 all scuba tanks manufactured for use in Canada must have the TC stamp on them. Any tanks manufactured prior to 1993 are grandfathered if:

The tank is marked CTC, DOT, or ICC

cylinder was in use in Canada prior to 1993

For tanks manufactured after 1993 a number of changes have taken effect which affect the VIP inspector or fill station operator.

The fill pressure is marked in BAR
3AAM (steel) tanks are stamped at their rated pressure (in BAR) and no longer have a 10% overfill (with the + mark) because the 10% equivalent has been included in the rated pressure stamped on the tank.
Steel cylinders may be stamped with a "K" after the current hydro stamp which means the cylinder may only be filled to 90% of it's rated capacity in BAR
All cylinders are hydro tested to 1.5 time their service pressure and not 5/3 their service pressure.

Filling Practices

A number of changes to filling practices have resulted from the new regulations. However the following general rules apply to all Scuba tanks:

Cylinder must have proper neck markings
Cylinder must have a proper pressure relief device
Cylinder must not be due for re-qualification
Cylinder may only be filled with the customers approval
All scuba tanks should be filled only by trained personnel

In addition all scuba tanks must be filled in such a way as to ensure they are at their service pressure at 15C. However both steel and aluminum tanks may be filled (hot) to 25% over their service pressure so long as the pressure returns to the service pressure at 15C.

Steel tanks with a "K" stamp next to their latest hydro test may only be filled hot to 15% over their service pressure and must return to 90% of their service pressure at 15C

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Which hose length is right?

hogarthia_hose_kit

With the number of different second stage and pressure gauge hose length available how do you pick the one that's right for you?

Well there are several standard hose lengths are are used for several distinct purposes.

Second Stage Hoses

7 foot - A seven foot first stage hose is used by the technical, wreck or cave diver on either their primary or backup regulator to allow a "long hose" on their doubles setup. The long hose is used to allow both the donating and "out of air" diver to be able to pass thorough a restriction in an emergency air sharing situation. in a single file mannor. Basically the 7 foot hose is long enough that one diver behind the other can pass through a restriction single file.

5 foot - This is the length of long hose used on a sport diving hogarthian setup for the octopus. It allows air sharing divers to be able to swim comfortably together. The excess hose is either looped under a canister light, or looped twice around the neck.

40 inch - This length allows for a comfortable length of hose when being used with a stage tank. It allows the user to have the hose go over their head and to breath the stage tanks with the hose over their head. This keeps the hose up and out of the way.

36 inch - This the length of a standard sport diving octopus hose.

24 inch - This is the length of the primary regulator hose on a sport or backup regullator on a technical diving setup. On a technical diving setup this backup regulator is usually attached to a necklace and is kept in place under the divers chin. Some technical divers even use a hose shorter than 24 inches.

Pressure Guage Hoses

24 inch - The standard pressure guage hose length of a sport or technical diving setup. In many cases this pressure guage or console is clipped off to ensure it is easy to locate, and does not dangle.

6 inch - This is the pressure guage hose length for a stage bottle. Usually the pressure guage hose forms a U shaped loop and it tied back to the regulator with bungee or inner tube in such a way as to make it easily visible to the diver carrying the stage.

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Scuba Tank Markings in Canada and Introduction

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Scuba tanks carry a number of legally required markings. These markings indicate important characteristics of the tank including:

It's manufacture date
It's serial number
It's pressure rating
It's last hydro test date
What it's made of
Who the tank was made by

Here is a photos of a typical aluminum scuba tank:

In the photo not all of the numbers are visible.

Let's examine these numbers:

TC 3ALM207

DOT-3AL3000 P12345678 LUXFER 08A06 S080

The TC3ALM207 indicates the following:

TC is the Transport Canada indication (the Canadian regulating body)
3ALM is the material the tank is made of (aluminum in this case)
207 is the pressure rating in BAR where one Bar is 14.5 psi

The second line contains the remaining information along with the DOT codes which means this tank is stamped for use in both Canada and the US.

DOT is the Department of Transport stamp (the US regulating body)
3AL indicates the tank is made from aluminum
3000 is the rated service pressure in PSI (divide by 14.5 to get BAR)
P12345678 the tank serial number
LUXFER is the tank Manufacturer
08A06 indicates the tank was originally hydro tested in Aug 2006. The symbolized A is the hydro testing facilities mark.

It should be noted that the original hydro testing date also indicates the date of manufacture since all tanks are required to be hydro tested as part of the manufacturing process.

Normally there be will other markings on the tank which are other hydro test dates and facilities.

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