Tank = Confusion AGAIN!!!!!!!

[rleslie]

You folks were kind enough to explain to me the differences in O2 clean, Nitrox ready, etc. tanks a few weeks ago. From the other thread my understanding was that "Any tank is nitrox ready as long as it is filled with 40% or lower and not filled using partial pressure method." So, as long as I use < 40%, the tank can be used for both air and premixed Nitrox without further O2 cleaning.

Upon your recommendations, I purchased a HP80 tank. When asked, I said that I needed normal air now but would want to use Nitrox also. And that I would only use premixed Nitrox.

SHOP: "No, only use partial pressure blending"

I questioned them about this and they continued to state that I could use air and Nitrox as long as the Nitrox was partial pressure blend.

So, now I'm confused again. That seems to be the opposite of what I understood from the other thread.

Comments?

 

[SteveAD]

You seem to have it right.

PP blending requires the tank and valve to be O2 clean and O2 compatible.

Premix nitrox <40% requires neither.

Assuming the tank has been appropriately set up and all fills are O2 clean/compatible then it doesn't matter if nitrox fills are PP or premix.

BUT!! If the tank/valve is not O2 clean/compatible and/or it gets filled with non O2 grade air then there is the POSSIBILITY that when adding 100% O2 as the first step of pp blending that there MIGHT be an ignition/explosion in the high O2 environment.

PP blending is the cheapest system to set up, but it is the system with the least tolerance for mistakes.

 

[MSilvia]

It's possible your shop is doing partial pressure blending using pre-blended EAN40 and air instead of pure O2 and air. That would be fine with a non-cleaned cylinder, and might be the source of confusion.

 

[skdvr]

If your shop only does Partial Pressure blending then you will need to get your tank and valve O2 cleaned. For some shops that is the only way that they do it. All of the shops around me only do PP blending. I am not sure why, I would think that setting up a NITROX stick would be pretty easy for them, and safer...

Phil

 

[SteveAD]

If your shop only does Partial Pressure blending then you will need to get your tank and valve O2 cleaned. For some shops that is the only way that they do it. All of the shops around me only do PP blending. I am not sure why, I would think that setting up a NITROX stick would be pretty easy for them, and safer...

Phil

Easier and safer yes, but more expensive and more work to set up.

The big storage tanks aren't cheap, plumbing them together to the compressor and fill station takes some work.

PP fills at their simplest only need a transfill whip from the O2 supply to the divers tank, an accurate pressure gauge and an O2 analyzer.

When I set up the system in my shop I thought it was a no brainer to go with pre-blend, and that's what I did, but, for most dive shops laying out several thousand dollars for new hardware really stings.

 

[ClayJar]

Upon your recommendations, I purchased a HP80 tank. When asked, I said that I needed normal air now but would want to use Nitrox also. And that I would only use premixed Nitrox.

SHOP: "No, only use partial pressure blending"

I questioned them about this and they continued to state that I could use air and Nitrox as long as the Nitrox was partial pressure blend.

Okay, obviously pure oxygen is oxygen clean (or "compatible"). O₂-clean air (often called "modified grade E") is also oxygen clean. Partial-pressure blending uses only those two ingredients, therefore partial-pressure blended nitrox is oxygen clean. Regular "grade E" air is not necessarily oxygen clean, so you don't want that in your nitrox cylinders.

The confusion comes with premixed nitrox. Most scuba cylinder fill station operators seem to follow the "40%" rule, where they assume that anything with less than 40% O₂ does not need to be oxygen clean (CGA uses the 23.5% rule). Unless their premixed nitrox is made by partial pressure blending into a bank, the premix does not fall within the 40% rule. As it does not necessarily "need" to be oxygen clean, you cannot assume that any given premix *is* oxygen clean any more than you can assume that any given compressed air is oxygen clean (which is obviously not a valid assumption).

If you have premix that is oxygen compatible, then it's no problem at all using it in oxygen-clean cylinders. Unlike partial-pressure blended nitrox, however, you cannot assume that premixed nitrox is oxygen compatible. (That was basically what he was saying.)

 

[oxyhacker]

Yes but remember you can buy a couple big tanks for the price of a hyperfilter system so if you do it right in the first place it's not that more expensive.

And you will sell a LOT more nitrox, when you can just shoot it into a tank like air, rather than have the customer leave it.

It's funny, because I've probably been responsible for as many people taking up PP mixing as anyone, but I really think any shop serious about selling nitrox (and if they aren't serious about it, why do it?) really ought to bite the bullet and do a premix system. I suppose there are some exceptions, like some little dive shop in some god forsaken spot like Vermont that is mixing up a dozen or so tanks a week for the use of the employees and a few special customers. But for anyone doing volume, besides being a lot more convenient for the customer, banked continuous mix is a lot safer. PP mixing is fine when you are a homebrewer mixing up a couple tanks, but a real drag when you've already put in a full day at the shop, and still have to PP mix a couple dozen tanks before you can go home. The temptation to keep edging up the fill rates becomes irresistable.

When I set up the system in my shop I thought it was a no brainer to go with pre-blend, and that's what I did, but, for most dive shops laying out several thousand dollars for new hardware really stings.

 

[spectrum]

So, now I'm confused again. That seems to be the opposite of what I understood from the other thread.

Comments?

Well I am hearing a very different issue here.

If you come into a shop that has say, 32% banked and have say 750 PSI of air (or even another nitrox mix) in your tanks then you will end up with some sort of dilution of the bank mix unless they spike your cylinder with O2 to level the mix. The O2 spike would require being 02 clean unless they count on a stick to premix the gas before it reaches the valve.

A shop doing PP blending can adjust the blend to compensate for whatever is in the tank.

The way to make alternating gas choices work on banked gas is to draw the cylinder down when going for the banked nitrox and analyze to see if it's off by a point. When diving air to significant depth analyze what you get since for some period it will be greater than 21%

Pete

 

[CompuDude]

You folks were kind enough to explain to me the differences in O2 clean, Nitrox ready, etc. tanks a few weeks ago. From the other thread my understanding was that "Any tank is nitrox ready as long as it is filled with 40% or lower and not filled using partial pressure method." So, as long as I use < 40%, the tank can be used for both air and premixed Nitrox without further O2 cleaning.

Upon your recommendations, I purchased a HP80 tank. When asked, I said that I needed normal air now but would want to use Nitrox also. And that I would only use premixed Nitrox.

SHOP: "No, only use partial pressure blending"

I questioned them about this and they continued to state that I could use air and Nitrox as long as the Nitrox was partial pressure blend.

So, now I'm confused again. That seems to be the opposite of what I understood from the other thread.

Comments?

Your description of the conversation isn't all that clear, but on thought that occurred to me is this: Could the shop have mean you can switch back and forth between oxygen-compatible air and partial-pressure blended Nitrox? Because that would be a perfectly viable situation. You can dive air all you want... as long as it's OCA (essentially EAN21) ... without messing up your tank's o2 clean status.

Putting in plain air from a non-hyperfiltered source contaminates your tank and prevents you from calling it o2 clean again. Putting "air" that is hyperfiltered and thus oxygen-compatible, however, is perfectly ok, and the tank can later be used for PP Nitrox without a problem.

Is that possibly what they meant?

Either that... or the shop has it exactly backwards. And is either misspeaking... or scary.

 

[ClayJar]

Is that possibly what they meant?

Either that... or the shop has it exactly backwards. And is either misspeaking... or scary.

What they meant is readily apparent (at least to a pendant such as I), but my earlier post must not have restated it well enough. The following statement is the basis of their rationale:

There exists nitrox that is not oxygen clean.​

If it is indeed true that there may exist continuous blended, banked, or otherwise derived nitrox that is not oxygen clean (as it never presents an oxygen percentage great enough to require oxygen clean equipment and processes), then such non-partial-pressure-blended nitrox could contaminate a cylinder and render it no longer oxygen clean.

On the other hand, since partial-pressure blended nitrox is made of oxygen and oxygen-compatible air, partial-pressure blended nitrox is necessarily oxygen clean and will not contaminate a cylinder.

(If you disagree with their base rationale, that there may exist some nitrox which may not be oxygen clean, we should discuss that topic. Whether you agree with their base assumption or not, it is plain that there is no logical flaw in their reasoning.)


(PS: As a pedant, let me also note that the word "nitrox" should be assumed in this post to mean "enriched air nitrox", but surely nobody would try pulling the air-is-nitrox-too misdirection. )