Air Refills Mixing With NITROX Gas Remains...

[MXGratefulDiver]

Seems to me that it was you who changed the subject.

This ...

This weekend I dove with 32% NITROX in three steel tanks and my AL40 Pony. Average starting pressure was 3200 - 3300 and ending pressures were in the range of 750 - 1000. The Pony was only filled to 2500 and was not used. I was going to top that up to 3000 when I got home. As I was out of my usual area for diving I decided to get the tanks filled where I live after the dive so I brought them back with me. I had a long drive and something occurred to me.

The issue: If I follow my usual routine and just drop the tanks off for an air fill, the shop will not know I used those tanks for NITROX. In the case of the Pony, if I top it up another 500 PSI (with 21%) then the mix will be in the 30% O2 range roughly. The other tanks will be lower, but still a higher % O2 than 21%.

... is not the same as this ...

Yes, you are supposed to analyze your tanks. That is known as the "Well, Duh..." response. Now, lets put on our imagination hats for a second, c'mon you can do it. And pretend you don't have the NITROX course. You are an OW qualified vacation diver climbing onto a dive boat or liveaboard that has NITROX on board. Not only does that put you in the majority, but it also means you don't know you should test the mix. None of the tanks have NITROX stickers. The boat uses black and green caps on the tanks to tell the difference. However, the example diver does not know that. Heck, to them a tank is a tank and they forgot to mention it during the briefing. Besides, they have not dove in 18 months and are having a hard time remembering which end of the tank the BC goes on for their SCUBA refresher. They had a group of advanced divers on the trip before and all had NITROX in the tanks. Our example diver didn't pay for the enriched air on their trip because they didn't take the course and are on a budget. They only dive twice a day anyway so why bother. So the dive boat knows this and fills your tank with air...

Now, did the boat staff purge the remaining NITROX from the tanks before filling or was the crew busy with a diesel engine or mooring issue and just topped them up at the last minute? Was the deck hand going through a break-up and not thinking straight? Was he supposed to empty the tanks, but didn't cuz he just wanted the day to end so he could go home and crack open his 151 proof?

I am going to use three very carefully chosen words here: Duty of care... Off loading the responsibility onto the diver WITH the NITROX course is probably doable. He or she knows better. However, this probably does not apply with the diver described above. That is just one example. Others who get what I was saying have mentioned equally compelling alternates (Thanks).

So back to the two questions I asked originally: Has this situation ever occurred resulting in injury? Does your local shop have procedures in place to stop this from happening?

To answer your first question ... I doubt anyone really knows. Diving injuries are notoriously underreported, and usually when they are they are typically not analyzed to the point where a specific cause of the accident can be determined. In other words, it may be determined that someone was injured as a result of breathing "bad gas", but why they were breathing that particular gas will not be investigated. The incident at CocoView, where two people died, is a good example. We can still only guess at what really caused it.

To answer your second question ... I doubt it. Every place I've ever used nitrox ... especially in vacation destinations ... leave it up to the user to analyze their cylinders.

... Bob (Grateful Diver)

 

[Brewski]

The issue: If I follow my usual routine and just drop the tanks off for an air fill, the shop will not know I used those tanks for NITROX.

They will if you have them banded according to standards and left your analysys label affixed to the tank.

OK, so lets take this thought experiment further. As an example, lets assume I have a 75 foot dive planned and want to extend my NDL at depth to see something interesting. I get an HP 120 filled with 40% O2. I analyze the gas mix and off I go to the dive site. I start at 3500 psi and end up aborting the dive because of a problem and return to the boat with 3000 psi. I drop the tank off at the shop knowing I have a deep dive in a weeks time and they top it up 500 psi with regular air.

As mentioned above the tank would have been identified by the appropriate markings. The filler should treat it as such and top it off with safe air not regular air.
Also when you drop it off he should have asked you what percentage you wanted.

I had a shop tell me they normally empty any remaing gas and refill to maintain accuracy, I had to tell them I prefered a safe air top off and when I picked up the tank analyzed the contents myself.

By my crude math, that would create ~37% O2 mix in the tank. Due to it being a regular air fill, I would not think to analyze the tank when I got it back. So if I go past 90 feet (~1.4 @ 37%) on my deep dive thinking I am only on air then I am in potential trouble. If I go past 110 ft (~1.6 @ 37%) then I could be in serious doo-doo. Note: The math is rough and approximate for example purposes only.

I have never had a shop ask me what % mix was in the tank, nor have I seen them test it beforehand. They just fill it; creating a situation where I could end up diving past the recommended mix depth inadvertently.

Has this situation ever occurred resulting in injury? Does your local shop have procedures in place to stop this from happening?

Yes our shop only fills appropriately banded cylinders with nitrox. We ask what percentage do you want when you drop off the tank. We require you analyze using our analyzer before leaving and log your results.

 

[tedtim]

So back to the two questions I asked originally: Has this situation ever occurred resulting in injury?

I cannot answer that one in general. I do not know of any incidents in my area.

Does your local shop have procedures in place to stop this from happening?

Yes. They drain tanks to be filled with Nitrox and will not top up any tank (AFAIK) that has a Nitrox band/sticker until they ask the owner what is in the tank. They will top tanks with air, but consider them as Nitrox (which they are) and require analysis.

As for draining the tanks... the LDS uses partial pressure blending and chooses to fill from empty tanks - their shop, their choice, their cost. Nitrox tanks are placed in a separate location from tanks filled with air. The location is next to the analyzer.

Are there any tanks that leave the shop with other than air that are not tested? If someone chooses to tell the staff that their tank has air, instead of some Nitrox remaining then I am sure that happens. I don't see how this can be avoided.

 

[ianr33]

Yes our shop only fills appropriately banded cylinders with nitrox.

As in the huge bumper stickers?

Could you please tell me which shop this is so I can avoid it? Thanks

 

[TSandM]

Well, in the original post, you posited taking your OWN tanks, which you knew had had Nitrox in them, in to a shop for an air top off.

Now you're talking about a hypothetical diver on a boat, who picks up a Nitrox tank unknowingly and dives it because he doesn't realize it's got something in it other than air.

They are two very different scenarios.

I have never read a report of anybody picking up a Nitrox tank and diving it and toxing. It is certainly conceivable, and is the reason for the requirement of a large, conspicuously colored banner on the tanks. It's sort of funny that the shops that pump Nitrox (and other gases) most frequently, are usually the ones who DON'T require the ugly labels. But I guess they figure they have a larger percentage of educated consumers.

There was a recent report of a woman who toxed and died in a cave at 1.4. It was pretty carefully reported by her husband. She was known to be unusually sensitive to O2, apparently.

 

[tedtim]

As in the huge bumper stickers?

Could you please tell me which shop this is so I can avoid it? Thanks

Different thread. :no

And no, it is not the huge bumper stickers only. The tech folks that have the BIG WHITE MOD stickers on their tanks also get fills with O2. Banded was not the most appropriate word.

 

[Scubakevdm]

There was a recent report of a woman who toxed and died in a cave at 1.4. It was pretty carefully reported by her husband. She was known to be unusually sensitive to O2, apparently.

Wow how did they know she had an unusual sensitivity prior to the tragedy?

 

[Scubakevdm]

Also, regarding the new senario:
I would estimate that a situation as described above (diver not certified nitrox gets ahold of a recreational nitrox mix without knowing it on a dive charter) would generally not end in injury for a few reasons. One, it is more than likely that the dive op is not taking the passengers someplace deeper than the mod for what ever mix they packed for the nitrox guys to begin with. I concede that there certainly are many examples of instances where this is not true, but often the op bases he site on the mix. Another big reason is that most recreational charters are putting divers into depths where the only really deadly mix would be a high percentage deco mix, and deco bottles tend not to be easily mixed up with recreational back gas. Also, and perhaps most importantly, I think that reasonably prudent dive ops would shield themselves from the kind of liability that they would assume by not marking nitrox rental tanks very conspicuously.

So, I think that the dive charter situation is much less likely to result in injury than the original complacent experienced diver situation. This is not to say that it couldn't happen, but I think that the chances are slim enough that the industry as well as the public is comfortable not checking tanks designated for use with air.

It would b e nice if the nitrox training were included in the open water certification though, because it's really not that difficult, and I feel, is beneficial.

 

[ianr33]

Wow how did they know she had an unusual sensitivity prior to the tragedy?

A previous chamber ride.

The Deco Stop
This was very,very sad.

 

[Rainer]

Wow how did they know she had an unusual sensitivity prior to the tragedy?

Looking back, many took her chamber experience to be indicative of a sensitivity to O2.