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?
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?