A Dangerous gas mixture

[texey]

The Danger: Diving with a nitrox mixture higher than expected.

The Lesson: After getting doubles filled with nitrox where blending is done IN your tanks, be sure the Isolator Valve is OPEN before your analysis.

Here's what happened at the dive site:

While suiting up and getting my doubles ready, I turned on both tanks after getting my regulators hooked up and to my surprise the pressure read 480 PSI. My buddy said check the isolator valve and sure enough it was turned OFF! I never turn it off and should only be turned off in emergencies.

The History:
Apparently Employee A from my local dive shop had shut it off after putting 02 in the tanks. Then Employee B filled the tanks with air not knowing the isolator was shut off. So when the tanks were analized by Employee B and myself and the reading came up EAN33 we went with that.

Back at the dive site:
After opening the isolator valve the PSI went up to 1840. My buddy and I figured that would be sufficient for one dive. We entered the water.
Plan was I would run reel, but not ever being here before I was only partly sure as to which way to go. I made my initial tie off ok on a tree trunk and waited for my buddy. He was still in 15' of water trying to get his dive computer to work. I didn't know what was wrong but knew he was not in distress so I tried to run the reel to a secondary tie off which I did, but then could not figure out which way was into the cave. It was tight and silty every where I went. I decided to come up and wait for my buddy. On my way up from about 30 feet I felt a bit dizzy. Suddenly the realization came to me that if the tanks had been mixed with O2 while the isolator was off there was no telling WHAT my EAN was!!? That fact racing through my mind combined with my slight dizziness made me want to abort the dive and go to the nearest dive shop to get my tanks analized and then filled with air. When I met up with my buddy I gave him the thumbs up and began my 3 min safety stop.
Once head out of water I explained the situation to my buddy. Climbed out of water, put gear in trunk and drove to nearest dive shop. While there had the tanks analized and it came up 42% oxygen! Then filled them with air. Final O2 was 25%.
Back to dive site to begin dive again.

If my dive for the day was going to be deeper than 50' I could have had major trouble with the oxygen level in my tanks.

 

[ScubaSteve]

If I am not mistaken (I am at work and not near any tables) you would have been good to 75' or so with 42% Nitrox but I might have misunderstood the story.

Definitely a good point here though. I do not dive doubles so I can only store it for future reference but it is a good point and thing to watch for. That is also the problem with having more than one person trying to do the same job.

 

[cerich]

The same mistake has caused lives and worse(perm. vegetative states).

I'm glad you did OK, when I teach students to dive doubles I stress checking the isolator over and over. It MUST e checked before every fill, after every fill, before every dive, after s drills and during the dive. the isolator should only ever be shut during drills or real emergencies, otherwise it is ALWAYS in the open position. ALWAYS watch somebody fill your doubles, it happens time and time again that somebody will fill and shut the isolator by mistake or not check themselves before filling.

 

[ianr33]

If I had only been to 30 feet and realized that I had no idea what I was breathing I would have passed on the safety stop!

There was a nasty accident at Eagles Nest a year or 2 ago caused by an isolator that was closed during filling.
http://www.scubaboard.com/forums/accidents-incidents/113785-eagles-nest-accident-report.html

 

[Saudi-Diver]

Suprised you did not do an independent check of your Nitrox mix, just took what the shop gave you in blind faith.

 

[jridg]

This is why I have my own analyzer - I always re-check before the dive......

 

[JBRES1]

I too have my own analyzer, and re check my mix before a dive.
When diving doubles I also check both valves for the o2 %. I partial pressure fill my doubles, and have sometimes had readings that were 1 to 2% off from each other after the fill. So now when I fill my doubles, I fill the o2 with the isolator open, then top with air via each valve with the isolator closed. When I have both cylinders filled, I then re-open the isolator to balance the pressure.
You are lucky that you opened the isolator valve and balanced the pressure.. If you had not done so, you could have been on 100% o2 had it been the right post with the 100% o2.

Dive safe, Jim

 

[Katamuki]

Glad you found the problem early! When we fill our nitrox tanks here we are required to check the percentage and then the diver after witnessing the analysing logs all the tank info along with MOD into binders maintained at the fillstation then signs. In addition we usually tape the tank by the valve with the mix, mod and date too. Not that its the best way but as its a military fillstation paperwork is the norm.

 

[BabyDuck]

he *did* check in the store. the isolator being shut off then opened changed the final reading.

glad you're ok & thought it through.

 

[Jim Lapenta]

I knew there was a reason that even though I'm sure it's open I always take a sample from both valves.