Air Refills Mixing With NITROX Gas Remains...
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For my cutomers that regularly get nitrox fills my first 2 questions when they come in for a fill are what have you got left? and what do do you want? unless they respond "fill it with air (or 36) and I'll take what I get" I still throw an analyzer and a gauge on it before filling.
Oxycheq has a build it yourself analyzer kit for $100. At that price, everyone who dives nitrox should have an analyzer! If you get a mix at the local cave diving shop and don't analyze it yourself or use the shops' analyzer, but just ask the guy what mix he's putting in to write down on the gas log....pretty much you've just lost all of your credibility as a competent diver 
I often dive 36% and use the tank down to around 800-1000 psi then drain it to where I need it to be to have 25% or 26% for a future deep dive. It works well but there are some qualifiers:
1. You need to let the shop know you are doing this so that a) the air they fill it with is really nitrox compatible to preserve the O2 clean status of your tank and b) so that the person doing the fill, takes some care to ensure the pressure in the fill whip is greater than the tank pressure so that you are putting nitorx upstream into the air system (although realistically, the mall amount of gas exhcanged in the whip will not be measurable in an air bank or even in a single scuba tank.)
2. You need to analyze your own gas to ensure you got what you expected and then mark the tank appropriately as nitrox with the mix and the MOD.
3. If what your really want is air (21%) for your next dive, then you need to drain the tank down to about 100 psi to ensure that the resulting mix is close enough to 21% so as not to pose any significant issues at recreational depths - but since it is a nitrox tank that probably had the previosu mix on it, you still need to analyze it to ensure the shop did not fill it up again with 36% or some other mix that could get you in trouble at deeper depths.
4. This is a good idea at any shop that does both air and nitrox fills, as mistakes happen and the wrong mix can get put in the wrong tank. If air gets put in a nitrox tank, it gets caught when both the shop and the diver confirm the mix and discover a 21% reading, but if the assumption is made that air went into the tank, no one checks and if your tank got the other guy's 40% mix, you could have a problem and not even see it coming.
5. Owning and using your own analyzer is cheap insurance.
1. You need to let the shop know you are doing this so that a) the air they fill it with is really nitrox compatible to preserve the O2 clean status of your tank and b) so that the person doing the fill, takes some care to ensure the pressure in the fill whip is greater than the tank pressure so that you are putting nitorx upstream into the air system (although realistically, the mall amount of gas exhcanged in the whip will not be measurable in an air bank or even in a single scuba tank.)
2. You need to analyze your own gas to ensure you got what you expected and then mark the tank appropriately as nitrox with the mix and the MOD.
3. If what your really want is air (21%) for your next dive, then you need to drain the tank down to about 100 psi to ensure that the resulting mix is close enough to 21% so as not to pose any significant issues at recreational depths - but since it is a nitrox tank that probably had the previosu mix on it, you still need to analyze it to ensure the shop did not fill it up again with 36% or some other mix that could get you in trouble at deeper depths.
4. This is a good idea at any shop that does both air and nitrox fills, as mistakes happen and the wrong mix can get put in the wrong tank. If air gets put in a nitrox tank, it gets caught when both the shop and the diver confirm the mix and discover a 21% reading, but if the assumption is made that air went into the tank, no one checks and if your tank got the other guy's 40% mix, you could have a problem and not even see it coming.
5. Owning and using your own analyzer is cheap insurance.
Two things jump out at me --
1) Why would you not analyze your mix after getting your tanks back? There is only one person who is responsible for the mix that the diver breathes -- THE DIVER. If you don't analyze it, it's your problem, not the shop's.
2) If you had a 40% partial fill topped off with air, why would you plan your dive using less than a 40% mix?
The issue here is sloppy planning, not what did the shop do wrong. If you ask for an air fill, the shop has no obligation whatsoever to test the mix.
Nitrox tanks should be clearly marked as such and the diver using the tanks must analyze the mix himself before use.
1) Why would you not analyze your mix after getting your tanks back? There is only one person who is responsible for the mix that the diver breathes -- THE DIVER. If you don't analyze it, it's your problem, not the shop's.
2) If you had a 40% partial fill topped off with air, why would you plan your dive using less than a 40% mix?
The issue here is sloppy planning, not what did the shop do wrong. If you ask for an air fill, the shop has no obligation whatsoever to test the mix.
Nitrox tanks should be clearly marked as such and the diver using the tanks must analyze the mix himself before use.
Hah, Cave Adventures has a sign which says all tanks will be treated as HP 130s and filled to 3600 with 32% unless otherwise specified.Oxycheq has a build it yourself analyzer kit for $100. At that price, everyone who dives nitrox should have an analyzer! If you get a mix at the local cave diving shop and don't analyze it yourself or use the shops' analyzer, but just ask the guy what mix he's putting in to write down on the gas log....pretty much you've just lost all of your credibility as a competent diver
pickens_46929
Contributor
So I'll add my $0.02
1) Sounds like the tanks aren't marked or labeled as NITROX tanks, along with the appropriate sticker for MOD and PERCENTAGE; how did they get filled with Nitrox in the first place? Taking them to a different shop?
2) ANALYZE, ANALYZE, ANALYZE... always, always, always; some people are more sensitive to OxyTox and the difference between a PO2 of 1.3 to 1.4 can send them into seizures. So please make sure you analyze the tanks after they are filled.
3) Depending upon how they were cleaned and prepped, simply topping them off might make them unusable for partial pressure fills (eg. add pure O2 first, then top with hyperair to get your nitrox). So if they are clean for partial pressure fills, topping off might not be prudent unless you know the quality of the air your topping off with.
4) If it were me, and someone came into the shop and said I've got EANx40 in here and I want them topped off... unless I filled the original tank with EANx40, knew the person, etc. I would be more than willing to tell them, "I've got to dump your gas first, then I'll fill them with air."
1) Sounds like the tanks aren't marked or labeled as NITROX tanks, along with the appropriate sticker for MOD and PERCENTAGE; how did they get filled with Nitrox in the first place? Taking them to a different shop?
2) ANALYZE, ANALYZE, ANALYZE... always, always, always; some people are more sensitive to OxyTox and the difference between a PO2 of 1.3 to 1.4 can send them into seizures. So please make sure you analyze the tanks after they are filled.
3) Depending upon how they were cleaned and prepped, simply topping them off might make them unusable for partial pressure fills (eg. add pure O2 first, then top with hyperair to get your nitrox). So if they are clean for partial pressure fills, topping off might not be prudent unless you know the quality of the air your topping off with.
4) If it were me, and someone came into the shop and said I've got EANx40 in here and I want them topped off... unless I filled the original tank with EANx40, knew the person, etc. I would be more than willing to tell them, "I've got to dump your gas first, then I'll fill them with air."
Randy43068
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This is the most accurate answer, including the check with an analyzer. Break it all down into PSI's of O2, then sum.
I used to do this all the time, and the shop owner knew that I'd visit the other shops who banked 36%. I knew what mix I'd have depending on the pressure of the fill, but would still analyze to verify. The mix was never off more than 1% IIRC.
Now I have my own compressor and nitrox stick, but as long as your tank isn't 02 cleaned, and the nitrox you're getting isn't partial pressure blended, and you know how to do the math correctly, I don't see a problem with it.
If you're going to do this frequently, take a gas blending course or pick up Oxyhacker's book for more clarification.
If tanks come into the shop I instruct for and I am filling them, non marked tanks are treated as air only. Any other label, ie. NITROX; SAFEAIR; CUSTOM MIX; DECO; etc etc etc get analyzed, unless and this is a big UNLESS the customer requested 'air', then the cylinder gets emptied and filled with air. Labelled cylinders get analyzed and logged, even if it is a 21% fill.
CYA
CYA
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