Question about filling a tank with enriched air

[WaterFox]

Ok, I just signed up for the PADI Enriched Air Diver class and have read about half the book. I'm sure we will go over this in class, but I have a burning curiosity and want answers now . Plus, I'd rather sound stupid anonymously on the internet than face to face with an instructor and class.

Hypothetical situation...
Suppose I have a non 02 clean tank that I get filled with 32% using the non-partial pressure method. I dive with 32% in the tank. At end of dive I have ~1000 psi remaining in tank and want to get the tank filled for dive #2. But there is no dive shop close by that does nitrox fills. So I would be stuck with filling it with plain old air from a non 02 clean source. Is this ok to fill a tank that previously held enriched air with regular air since my tank is not 02 clean anyway?

Now for part 2...
Will the dive shop even fill my tank with regular air even though there is ~1000 psi of 32% left in it? If dive shop does fill my tank with regular air (I assume they just top off the tank), how on earth would I know what the new % mixture is in my tank (1000 psi of 32% plus 2000 psi of 21% = math, hmmm...)? Would I have to have it analyzed again, I assume so, but if the shop does not fill nitrox they probably don't have an analyzer... Would I need to carry my own analyzer with me in a situation like this?

The more I learn about diving, the more question I have...

 

[charlesml3]

Q1: Sure, it's perfectly fine to fill the tank with Air after the dive. You'll wind up with a mixture of something less than 32% and something more than 21%. Just analyze it so you know what it is. Then just don't exceed the Maximum Operating Depth for that mixture.

Q2: I see how you could get into that situation but almost all shops fill Nitrox these days. They'll have an analyzer there for you to use.

-Charles

 

[sheck33]

Part 1: I dont see any reason why you cant top off a tank that contains something other than air with air. My doubles alternately contain air, enriched air or trimix, no problems there. As long as the fill source is clean all should be ok.

Part 2: Anytime you fill your tanks with something other than air you need to analyze the tank, period. I dont do my own filling but I do have my own O2 analyzer.

 

[vicdiver656]

Pt 1 Yeah, you can fill it with air. I believe, however, that you would have to get the tank O2 cleaned before you fill it with O2 again.

Pt 2 The dive shop should fill it with air. They will have an analyzer, so there would be no problem there. Then you would have to analyze the air, and, if you dive on a computer, set the mix percentage of that computer (presuming it's nitrox-capable) to the mixture in your tank.

 

[616fun]

Ok, I just signed up for the PADI Enriched Air Diver class and have read about half the book. I'm sure we will go over this in class, but I have a burning curiosity and want answers now . Plus, I'd rather sound stupid anonymously on the internet than face to face with an instructor and class.

Hypothetical situation...
Suppose I have a non 02 clean tank that I get filled with 32% using the non-partial pressure method. I dive with 32% in the tank. At end of dive I have ~1000 psi remaining in tank and want to get the tank filled for dive #2. But there is no dive shop close by that does nitrox fills. So I would be stuck with filling it with plain old air from a non 02 clean source. Is this ok to fill a tank that previously held enriched air with regular air since my tank is not 02 clean anyway?

Now for part 2...
Will the dive shop even fill my tank with regular air even though there is ~1000 psi of 32% left in it? If dive shop does fill my tank with regular air (I assume they just top off the tank), how on earth would I know what the new % mixture is in my tank (1000 psi of 32% plus 2000 psi of 21% = math, hmmm...)? Would I have to have it analyzed again, I assume so, but if the shop does not fill nitrox they probably don't have an analyzer... Would I need to carry my own analyzer with me in a situation like this?

The more I learn about diving, the more question I have...

You'll get lots of variation in answers on this. For me it boils down to this:

The only reason to O2 clean a tank is:
1. You partial pressure blend your mixes
2. You plan to use the tank with higher concentrates of O2 in them (+40%)

For a more text book version of the above see Nitrox FAQ

If the tanks are not O2 clean, you have nothing to worry about. I top my nitrox tanks with air all the time. The only issue you will have is IF the tanks are O2 clean and you fill air from a source that is not O2 clean compliant. If you do, you have lost your O2 clean on your tanks.

The math isn't that hard to figure the mix you will recieve by topping a 1000 psi 32% tank with 21% air. It's the same calculations someone would use to partial pressure blend a mix. That said, people pay to take classes to learn those math formulas, so likely you'll not see them posted here. Invest in a O2 analyzer and you'll have your answer after your cylinder is filled. Likely it'll end up somewhere around 27% (***DISCLAIMER*** math in my head is subject to being totally wrong).

I strongly suggest you get an analyzer if you plan to dive nitrox a lot. If you're handy, save money and build Oxycheq's El Cheapo model. It will likely save you between $200-$300 and you'll be the envy of your friends:
Analyzer-DIY-ElCheapoII

 

[WaterFox]

Now just for fun,

I have dive shop top off my tank containing 1000 psi at 32% topped off with regular air. New mixture is unknown but somewhere between 21-32%. Assume dive shop doesn't have an analyzer or won't let me use it for whatever reason. Is it considered acceptable to use my own analyzer to check the mixture myself or does the diving community frown upon this?

 

[Walter]

Of course it's acceptable, besides who would know?

 

[JeffG]

Is it considered acceptable to use my own analyzer to check the mixture myself or does the diving community frown upon this?

Yes...its bad form to check your own breathing gas.

 

[616fun]

Now just for fun,

I have dive shop top off my tank containing 1000 psi at 32% topped off with regular air. New mixture is unknown but somewhere between 21-32%. Assume dive shop doesn't have an analyzer or won't let me use it for whatever reason. Is it considered acceptable to use my own analyzer to check the mixture myself or does the diving community frown upon this?

Not only is it recommended, I believe training organizations deem it a requirement for the user to check his/her mix prior to diving it.

To put it lightly, it's not the guy running the fill station that's gonna be using that mix. It's in your best interest to explicitly know what's in YOUR tank before YOU dive it.

 

[616fun]

Yes...its bad form to check your own breathing gas.

Let's be nice. He's a new diver and just signed up for a Nitrox course. Hasn't even started the class work yet. Just asking what experienced divers deem noob questions. We were all (or still are) noobs at some point.