Tanks and O2: Maintain separate rigs?

[delta-v]

Hope this question's in the right section. My query relates to 02-cleaned tanks and the subsequent use of normal air potentially introducing contaminants.

I have reviewed two prior posts which neatly explain the myths of 02-cleaning regulators, and I understand that nitrox mixes up to 40% do not require O2-cleaned first stages, provided the regs are factory-certified to this level (which most of the latest gear is).

use of a Nitrox regulator vs. normal air [Archive] - ScubaBoard
Using Nitrox after Air? [Archive] - ScubaBoard

Let's assume that for the time being I won't be crossing the 40% or 50% mark, and thus can freely switch b/w air and nitrox as far as the first stage is concerned. The problem (as I understand it) is the tanks themselves which may be exposed to 100% during partial fills; which makes sense.

OK, so I get the tanks + valves are O2-cleaned and certified (o-rings etc), but what happens if I use them with normal air? Do I need to have the tanks/valves cleaned again prior to introducing 100% O2?

Or is it simpler to maintain two sets of tanks? I know all my dives won't be O2-enriched ones.

 

[knowone]

Are you going to make your decision on a reply percentage basis

on what you think

or on all the invaluable information a course rushes past your senses.

 

[willembad]

I'll nitpick first just to make something clear.

exposed to 100% during partial fills

The tank/valve will be exposed to 100% O2 during partial pressure fills, that term relating to a method of making nitrox - versus continuous blending which is another method.

To answer your question, the fraction of O2 in the fill has no bearing on whether the tank was contaminated or not. As long as the quality of the air used to fill the tank meets the criteria of air used to blend nitrox the tank stays O2 clean even if no additional O2 is introduced during the fill. EAN21 made with OCA (oxygen clean air) is perfectly acceptable fill for a O2 clean tank

Willem

 

[delta-v]

Are you going to make your decision on a reply percentage basis

on what you think

or on all the invaluable information a course rushes past your senses.

I don't know, hence the post here. Divers at the new club I joined pointed me straight towards the LDS for O2-cleaning of all my gear, but I know that's not required unless I'm going higher than 40% or 50%. For e.g. my Mares SPG is good for up to 50% out of the box.

I know I'll eventually use higher O2 ratios (and need to have appropriate gear) but for the mid-term I need to be clear on whether I should maintain two sets of tanks. I'd rather not find out the hard way.

 

[elan]

What willembad said. If you use the same air that is used for partial pressure blending there is no need to clean the tanks after. The rest does not matter if you are not using the final mix higher than 40%

 

[delta-v]

I'll nitpick first just to make something clear. The tank/valve will be exposed to 100% O2 during partial pressure fills, that term relating to a method of making nitrox - versus continuous blending which is another method.

Willem

Thanks for the input. From what I can tell, my dive club just has tanks of gas available to members. I don't think they are doing continuous blending coz from what I can tell folks just hook up to different tanks in turn. In this scenario, it would be nitrogen from one tank and O2 from the other.

To answer your question, the fraction of O2 in the fill has no bearing on whether the tank was contaminated or not. As long as the quality of the air used to fill the tank meets the criteria of air used to blend nitrox the tank stays O2 clean even if no additional O2 is introduced during the fill. EAN21 made with OCA (oxygen clean air) is perfectly acceptable fill for a O2 clean tank

Willem

I thought the whole point of O2-cleaning was to get rid of hydrocarbon etc. contaminants introduced via compressed air fills from a regular dive shop compressor? In which case when these tanks are hooked up to the club's compressor for a regular fill, they won't stay O2-clean?

*sigh* It's OK. I'll ask someone face-to-face at the club. It's just that some of the more experienced guys walk around with big chips on their shoulders and it's difficult to get a straight answer from them. I thought asking here would be simpler. Thanks anyway.

 

[willembad]

Sorry you didn't get that. I'll back up some and try to be more clear.

The "tanks of gas" available to members I'm going to guess are what's called banks. They are large storage tanks filled from a compressor and then used to fill smaller scuba tanks without the need to fill directly from the compressor. If the club is not doing any continuous blending then the banks will contain air. This air can be breathing grade air (Grade E) or with some additional filtering could be OCA that meets higher criteria for amount of hydrocarbons it contains. You should ask what filters the club compressor has and what grade air they are pumping. Whether it goes directly into your scuba tank or into the banks does not matter. If it is OCA then you can fill an O2 clean tank with it and keep the tank O2 clean. If it is grade E then it will contaminate the scuba tank and it will no longer be O2 clean.

If the club uses the same banks to make nitrox then you can be sure that it contains OCA and getting an air fill from the banks will be fine. By the way, nitrox is never made by blending O2 and nitrogen. Pretty sure that it could be done but would be cost prohibitive for a number of reasons.

Hope that was clearer. It's good that you are thinking about these things. Never hurts to ask.

Willem

 

[rjack321]

Thanks for the input. From what I can tell, my dive club just has tanks of gas available to members. I don't think they are doing continuous blending coz from what I can tell folks just hook up to different tanks in turn. In this scenario, it would be nitrogen from one tank and O2 from the other.

Sounds like they are partial pressure blending. But if they are using the same whips for air and nitrox fills then all their gas should already be suitable for O2 mixing. If you get "just air" from them it should not contaminant anything. If you go elsewhere for air you'd need to be cautious about where you are getting fills.

Where are you located? I understand the big shoulder chips, but none of us can really understand that club's system from afar so you need to ask them how they are making nitrox and if their normal air fills are partial pressure nitrox compatible.

 

[delta-v]

Sounds like they are partial pressure blending. But if they are using the same whips for air and nitrox fills then all their gas should already be suitable for O2 mixing. If you get "just air" from them it should not contaminant anything. If you go elsewhere for air you'd need to be cautious about where you are getting fills.

OK, it's becoming clearer now, thanks (Willem too). The 'banks' are not hooked up to the compressor. These huge cylinders are delivered from a gas factory and I'm pretty sure they only have the 100% gases. So if folks get the rest of the air mix from the club compressor then the club's comp at least can be considered 'clean'.

So: As long as I stay with the club for fills (air or nitrox) I don't need to get the tanks/valves cleaned again. But if I fill-up elsewhere, then I need to know if that comp's filteration is Grade E or OCA.

Does that sound right?

 

[willembad]

You got it. If they are delivered then you're right - it is likely 100% O2. If you see anything else delivered it would be Helium or maybe Argon. Forget about those for now. "Banks" are just storage and would be permanently plumbed to the compressor. If they are using the compressor to mix then you are fine to use the air.

Good luck and have fun.

Willem