50% O2 and and O2 for deco...........WHY? also labelling tanks........WHY?

[declan long]

So ok guys

i am wondering what the advantage of using 50%o2 and 100% O2 for deco is over other gasses like 40% which let you offgas at a deeper depth.

do any agencies use standard gasses because i know TDI don't?

also why do mainly dir divers permanantly label tanks for 50 and 100 percentwhat id you analyse it and it is 46 percent and the mod changed?

thanks

 

[SeaCobra]

One of the leading causes of accidents in tech diving is breathing the wrong gas at depth; proper labeling and gas switching procedures help greatly in that regard.

 

[kensuf]

On the "various" gases. Some people just like to keep the same routine. And when diving with like minded people it's helpful to have the same gases, that way if one has a problem (catastrophic gas loss during deco), you can still both deco out on the same schedule by buddy breathing if needed.

As for labeling. I knew two guys that died by breathing the wrong mix at depth. Proper bottle marking and analyzing the gases could have saved both of them.

One I had only met once or twice, and he tried to breathe 50% nitrox at 150'. His two bottles were labeled "1" and "2" and he didn't remember which was which, and he choose poorly. This was back in 1995 when people weren't thinking much about bottle labeling.

The other was a relatively good enough friend. We had done a dozen + cave dives together, and he had mislabeled a bottle which was filled with pure helium. He died from anoxia. This was back in 2001.

The fact that there are still people who die from breathing the wrong mix at depth (2013 Devil's Ear for example) just shows you that we still need to remain ever diligent about following protocols.

 

[PfcAJ]

100% is best because it doesn't have any inert gas. The less inerts the better.

50% makes sense as the next gas because 70ft is a reasonable spot to switch gases. Many a dive have longer stops starting around there. On dives that are real deep, introducing a 35/25 bottle makes sense 50ft deeper at 120ft.

GUE uses standard gases. 100%, 50%, 35/25, 21/35 for deco gases, 32% or 30/30, 21/35, 18/45, 15/55, 12/65 for bottom gas.

We permanently mark and dedicate bottles to certain gases to reduce the chance of error. The strength of having dedicated tanks to certain gases far outweighs the perceived "need" to relabel a 70 bottle if the analysis doesn't come out to exactly 50.00%. This stuff isn't that exact.

In the view of DIR, standard gases (and the flexibility they afford) outweigh the few minutes here and there gained by "best mix" gases.

 

[rjack321]

The issue with something like 40% is that: 1) you are still on-gassing at the 100ft switch (N2 for sure) 2) what's you next gas going to be? if it's 100% you have whole lot of stops to do where your gas gradient (as opposed to your pressure gradient is going to be nil). In other words you are wasting your time breathing 40% at say 50 and 40ft depth. There's plenty of N2 in you still, almost no gas gradient to enhance offgassing anymore, and you aren't yet shallow enough really get the pressure gradient going - although you might be at your ceiling for some tissues.

For multi-gas deco dives each gas is used over about 5 stops so that: 1) there's still a good gas gradient going, 2) its not too far between gases so you don't need to carry ridiculous volumes for you and a buddy (e.g. switching to 40% at 100ft means you now need to have enough gas to share it with a buddy all the way up to 20ft, that's potentially more than an AL80 which is putting a lot of eggs in that one tank), there's not so much N2 in the deco gas that it actually on-gasses you which you then need to eliminate later in the deco once you've switched to another gas.

Oh and if you aren't labelling your tanks you are definitely rolling the high risk dice. That's how OC divers die. In 2013 I think there were at least 3 deaths due to breathing the wrong gas worldwide. Completely preventable dumb deaths.

 

[Hickdive]

It's simple, you don't need to continually change cylinder MOD markings, you can leave them as 21 and 6 and if you're carrying two deco cylinders it halves the blending work

Remember, if you're going to use a deco ppO2 of 1.6 then it makes no difference if your using 80% at its MOD or 100% at its MOD, the ppO2 is still 1.6.

PS If I was aiming for a 50% mix and got 46% for one thing I'd be a bit shame faced and for another I'd tweak the blend to get 50%. Better to be on the same page as everyone else in the team than start messing about with differing MODs or calculating deco for 50% when you've actually only got 46%.

 

[PfcAJ]

It's simple, you don't need to continually change cylinder MOD markings, you can leave them as 21 and 6 and if you're carrying two deco cylinders it halves the blending work

Remember, if you're going to use a deco ppO2 of 1.6 then it makes no difference if your using 80% at its MOD or 100% at its MOD, the ppO2 is still 1.6.

PS If I was aiming for a 50% mix and got 46% for one thing I'd be a bit shame faced and for another I'd tweak the blend to get 50%. Better to be on the same page as everyone else in the team than start messing about with differing MODs or calculating deco for 50% when you've actually only got 46%.

Its not the 1.6 that makes oxygen great. It's the zero inert gas.

 

[declan long]

UPDATE

so i have just been running some profiles on v planner and see very little difference between 40 nd 80 as to 50 and 100, so fo example a dive to 60m for 30mins on 18/35 and a deco gasses of 40 and 80 is a 88 min runtime and 50 and 100 is a 92 min runtime, so if 50 and 100 is better then why is ti longer? surely 100 02 off gasses faster. wow

anyone care to explain.

 

[rjack321]

1) I wouldn't do a 60m dive on 18/35, not enough helium for me.
2) Is getting out of the water as fast as possible really your #1 priority?

 

[100days-a-year]

Like Ford Vs. Chevy there are proponents who absolutely do not recognize any thinking outside their box.This is one of those things.

50% and 02 are most logical for many reasons but if you die breathing the wrong gas then breathing the analogue of another philosophy would most likely be fatal as well,a piss poor argument.

Simplicity,mutual compatibility and familiarity of gasses,gear and plan help to avoid issues,gas choices are just a number once that is set in place.

I lost a friend who breathed the wrong gas from a properly marked 50% tank,just before they rolled the boat had a big wave rock them,he dropped his reg to hold on and picked up the 50% one when they rolled in.A few seconds of hurry up was fatal.