Air Refills Mixing With NITROX Gas Remains...

[shadragon]

First, if this topic is not in the right area, please feel free to move it. I could find no NITROX specific thread this covers a few fields including dive shops and general safety.

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%.

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. 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?

 

[Scubakevdm]

First, if this topic is not in the right area, please feel free to move it. I could find no NITROX specific thread this covers a few fields including dive shops and general safety.

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%.

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. 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?

Maybe you should always check your mix if you are diving with tanks used for nitrox?

 

[Jimmer]

Well for starters, you should have your cylinders labelled with your mix and MOD, which they should notice when checking your hydro date etc. Secondly you should let them know what is in them, and a third option is just drain the tanks and get them filled or remixed with the appropriate EAN%.

 

[TSandM]

Due to it being a regular air fill, I would not think to analyze the tank when I got it back.

But it wasn't a regular air fill. It was an air top off on a tank full of Nitrox, and you know it. Why would you not analyze the gas when you picked up the cylinders? This is not a shop problem. This is a diver problem. It is always our own responsibility to know what is in the cylinders we are breathing. If the shop pumps Nitrox, it should have an analyzer, even if you don't own one, and you should analyze the tanks before taking them home, and label them with their contents and MOD.

 

[jviehe]

Best practice is to tell the shop what mix and pressure you want and check what you got before leaving the store. When I do fills at the shop, I always check the mix before filling with 02. If you drop off a tank and dont specify what you want it filled with, you get topped with air to rated pressure. Youre responsible for analyzing your tank.

 

[ScouterJT]

As I understand the math, if you had 3000 psi of 32% O2 (so, 960 psi O2) and added 500 psi of air (so, 105 psi of O2), you would have a total of 1065 psi O2 out of 3500, for roughly EAN30.

But I would check it with an O2 sensor.

 

[String]

I have air top-ups all the time.
Quite often the first dive is on nitrox, then the 100 bar or so of it left is topped up for the second dive.

Its a cheap way of getting a weak nitrox mix so better than air on the dive at no extra cost for mix.

If you have tanks with nitrox in then analyse them - its 30 seconds work.

Id resent a shop draining off gas just to put normal air in!

 

[Scubakevdm]

I would also definitely tell the shop that there's still "X"psi in a nitrox tank and that you just want to top it off with air. If you don't, they may end up blasting some of it into their air banks,and messing up all of their air.

 

[Steve R]

........... 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. ........

Bold is mine.

As long as you know who's fault it is, it doesn't matter to me what you do.

How you mange to rationalize an air fill on top of a nitrox mix as a situation that you would not think to analyze it is beyond me.


Kev:

I'd like to think that all shops have check valves in their fill whips for these reasons (and more).

 

[shadragon]

Guys, RTFQ...

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

What I wrote was hypothetical and for example purposes. If everyone followed reasonable, common sense things then dive injuries would be non-existent. However, that was not the question.