Chosing Deco gas (50, or 80+)

General topics about technical diving.
Argon
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Joined: Mon Nov 05, 2012 8:51 pm

Re: Chosing Deco gas (50, or 80+)

Post by Argon »

My concept for deco gasses is that they serve one primary purpose; to speed decompression. That being said, the more gasses one carries, the faster deco can be done, but the more cumbersome the equipment configuration. So the trade off has to be between faster deco, and simpler gear configuration.

Playing with different deco gas configurations in v-planner shows that dives with exposure of less than 7ATA (198fsw) incurring obligations of 45 minutes or less generally see very little reduction in decompression time by including a third deco gas (usually that would be EANX32). My experience carrying three deco bottles, as is usually done for dives below 200fsw, would be that they really begin to slow your swimming, and up your consumption rates, so there really has to be a significant advantage before I will do it. Two gasses are what most divers I know plan for dives less than 200fsw, and the gasses they would use are 50% and Oxygen. Most of my tech partners keep full bottles of these gasses on hand for impromtu diving, so gas matching is easy, and complications with getting gassed up don't prevent a dive.

Interestingly, for most dives in the 150ish range with bottom times of about 30min, v-planner shows that if only one gas were to be chosen, oxygen is not it. 50% is a better choice, so it isnt as simple as the richer the gas, the faster the deco. The idea of "rock bottom" already got mentioned, so I wont say anything else there.

I too am perplexed by your mention of 80% as a deco gas, but I will say that if only one gas were used for a 150fsw/30min bottom time dive, 80% gets you out of the water the fastest. Mixes leaner than 50% as the first of two gasses usually begin to add substantially to your deco time.

I already saw the mention that deco times are not that different regardless of the model used. That is a good point. I've got vplanner, my partners use vplanner. Everybody knows what everybody else is doing. I like that. If I had a different group of buddies, the choices might be different.
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