O2 clean: Is it really?

[g1138]

I had an epiphany that's giving me conflicting thoughts on O2 clean cylinders.

If you have a recreational nitrox tank (<40% fills). You're required to O2 clean it to fill.

So you have 2 options when filling it. Either:
A) Nitrox is banked from storage tanks, premixed
B) 100% O2 is mixed with air to create your %.

How does the tank stay O2 clean if regular air is pumped into the tank? (ie. method B for filling)

 

[adelman]

The cylinder and valve have two states, O2-clean or not-O2-clean. You can "move" a cylinder from not-O2-clean to O2-clean by disassembling, cleaning, and inspection to be sure that there are no residual hydrocarbons present. You "move" a cylinder from O2-clean to not-O2-clean by exposing it to a gas which contains excessive hydrocarbons.

Regular Grade E air (scuba air) is presumed to have such excessive hydrocarbons, and introducing it into your cylinder will make it not-O2-clean. Oxygen-Compatible air (OCA) is Grade E air that has undergone one additional level of filtration (and a more sensitive test for hydrocarbons) to further reduce the residual hydrocarbon level. The introduction of OCA into your cylinder does NOT hurt the O2-clean status.

An O2-clean cylinder and valve are required whenever they are to be exposed to gasses which have concentrations of O2 greater than 40%.

So to answer your question, when Nitrox is mixed *in* the cylinder, O2 is added and then topped off with OCA, and your tank stays O2-clean.

But it gets more complex...

When Nitrox is premixed and put into your cylinder, your cylinder doesn't need to be O2-clean. HOWEVER, depending on how the Nitrox is mixed and how it is filtered, it is possible to get Nitrox that itself is not Oxygen-compatible and will make your cylinder not-O2-clean.

For example, I mix Nitrox in my bank by partial-pressure mixing. I add O2 to my bank bottles and top them off with OCA. The resulting mix is itself Oxygen-compatible and I can pump it into a scuba cylinder which is O2-clean (and it will stay that way) or not-O2-clean (because it is less than 40% O2 at that point).

But, Nitrox can also be mixed by blending the O2 and Air at ambient pressure and then fed into a scuba compressor (continuous blending). In this case, if the output of the compressor doesn't receive that extra filtration (and there is no reason it would need to), then the Nitrox coming out of the compressor is no cleaner than Grade E air and is not Oxygen-clean. Feeding this into your O2-clean cylinder gets you a nice Nitrox fill and a non-O2-clean cylinder. I believe that this is common in some Southern California dive shops and San Diego dive boats. This can be very annoying since to get a Nitrox fill "at home" you may need to keep your cylinder O2-clean, but if you get a Nitrox fill elsewhere you may lose the O2-clean status.

In the Monterey Area there are some shops that always sell OCA for all of their air fills, and some shops that sell Grade E air unless they are filling a cylinder that is obviously labeled for Nitrox and is O2-clean. Just remember that the visual inspection sticker only indicates the O2-clean status as of the date of inspection. If you have an O2-clean cylinder and want to maintain it that way, it is the responsibility of the *DIVER* to know if the gas being filled is O2-compatible, and if not, mark the cylinder as not-O2-clean.

Personally, I think the industry should be a visual inspection (EOI ) sticker that has a tear-off corner that indicates the O2-clean status. All fill station operators that serve non-O2-compatible gasses should use a knife to remove the corner of the sticker indicating that the cylinder is no longer O2-clean.

 

[adelman]

I had an epiphany that's giving me conflicting thoughts on O2 clean cylinders.

I just noticed you're from Santa Cruz.

It is my understanding, but I would confirm it with the fill station operators, that:

Adventure Sports -- air fills are NOT O2 COMPATIBLE. Does not sell Nitrox.
Aqua Safaris -- all gas sold, air as well as Nitrox, IS O2 COMPATIBLE.
Aquarius Dive Shop on Del Monte Blvd in Monterey -- all gas sold, air as well as Nitrox, IS O2 COMPATIBLE.
Nelson Dive Center in Morgan Hill -- Nitrox IS O2 COMPATIBLE. Air fills are not unless requested.

 

[ajduplessis]

Hyper filtrated air is used to top up partial pressure nitrox fills.

 

[Bryan Cunningham]

Adelman has more than covered the question. Good reply. But note to add to original question. If you are diving with a mixture of less than 40%, the main danger is to the person filling if using partial pressure blending. As a diver using the mix I would not worry too much, you are safe.

 

[adelman]

Adelman has more than covered the question. Good reply. But note to add to original question. If you are diving with a mixture of less than 40%, the main danger is to the person filling if using partial pressure blending. As a diver using the mix I would not worry too much, you are safe.

I tend to be a little pedantic about this issue because I also have a (private) fill station. The fill station operator has to trust the diver to know if his or her tank is O2-clean, and since a good fill station operator is something a diver needs, it is better not to get a reputation for blowing us up .

It can also be argued that regular old Grade E air is clean enough to mix with O2 and the industry specification on Oxygen Clean Air is much tighter than it really needs to be. This point is debated regularly.

 

[Scared Silly]

Historically, it was believed that any equipment (regs, cylinders) had to have special treatment if used with any gas that contained more than ~23% O2 (I think it was and still is a NOAA rule). When it came to cylinders that made life easy as any and all NITROX fill had to done with O2 compatible gas. So folks had dedicated cylinders for NITROX and air. But then they used the same regulator. Which BITA had to be converted to viton o-rings instead of buna-N for NITROX. Using the same reg for up to 40% seemed to be okay regardless of the gas source (Grade E or Mod Grade E). So then folks started banking NITROX or using a membrane system - neither of which need to produce Mod-Grade E gas. At the same time regs became NITROX compatible out of the box as did cylinders for premixed NITROX.

Mean while the VIP stickers have remained either air or nitrox. As Adelman, noted that is not the best system given that once cylinder is filled with gas that is not O2 compatible the cylinder is no long O2 compatible (note I use the term O2 compatible versus O2 clean - nothing in the general scuba world is truly O2 clean, just compatible).

Here is problem: if an inspection sticker says Air service the fill monkey will probably not put premixed NITROX in it. If the inspection sticker says NITROX the person doing a partial pressure mix has zero guarantee that premixed NITROX that was not mod Grade E gas was put into it.

As such, the system has problem. My solution for my cylinders which I inspect is to put a NITROX sticker on the cylinder but add a note that says "Premix NITROX ONLY." For cylinders that will get higher percentages (i.e. deco bottles). I put "O2 Compatible gas ONLY."

The above is an issue that the dive industry should address given the popularity of premix NITROX. But given the fubaring of cylinder inspections currently I doubt that folks could agree. The only saving grace is that partial pressure blending is not that common except at the more technical shops.

 

[g1138]

Great and well informed answers guys, and all within the first page.
Thanks for your help.

 

[Wookie]

It can aloso be noted that a hyper filter is not required to meet OCA. I don't have hyperfilters and I have never failed the OCA test when performed, which is every time I test air.

 

[adelman]

It can aloso be noted that a hyper filter is not required to meet OCA. I don't have hyperfilters and I have never failed the OCA test when performed, which is every time I test air.

Indeed. I have a Bauer MVT-20 with the K15 block and a Securus filtration system. Although I do have a hyper filter installed outside of the compressor chassis, I sample my air prior to the hyper filter and perform an OCA test on it. Always passes. As the compressor's design criteria is Grade E air, I still want that filter, but it is nice knowing that I'd pass without it.

Other types of compressors that aren't oil-lubricated produce OCA by design.