Best Nitrox Mix/ Profile at 100'

[Brewone0to]

I'm learning a ton on this thread.. the rule of thirds is: to leave 1/3 of your air for an ascent...

no you completely missunderstood.
If you are getting proper fills, 1000 psi isn't even close to 1/3.
Hides behind flameproof barrier.

 

[DaleC]

Let's see. Rec dive limit is considered around 130 ft. When I'm wreck diving at 80-130 depths, I make three safety stops during slow ascent. One at 60 (a couple of mins), one at 30 (3 mins), one at 15 (5 mins). That's for a bottom time of around 20 mins. It's not cave diving, but having 1000 PSI to perform these stops during ascent is what I do. Is it conservative? Hell yea. Prudent? Maybe.

Per DAN stats, OOA is a common cause of diving accidents, and OOA is often caused by not being conservative with air reserves. By definition, NDL does not require safety stops. But then again, smart folks say that all dives are decompression dives. Given the many unknowns of diving physiology, I tend to think the rule of thirds is useful, even for OW.

I just had a similar discussion with a friend a couple of weeks ago during a dive trip. He kept wanting to begin ascents with 800psi from 50' or less while at the same time slinging a 40 pony. He said he was just trying to be conservatively safe. I got that but questioned his logic and suggested he was being very vague about what he could do with the gas he had. Further, by doing so, he would not get a "real" feel how much gas he actually needed to perform certain activities. It was an interesting discussion. I convinced him to breath down half his pony to see how long that thing would last, and to add that time to how long 800psi would last, at those depths. About half the dive.

My point to him was that, without a realistic concept of what he actually needed, he might "freak out" on a dive if he dipped below his imagined safe volume limit, when in fact, he had a very large margin to work with. This actually occurred to a diver locally who bolted to the surface when she saw she had gone below 500psi in 40' of water. Her BF chased her and they both went for a ride in the chamber.

Every time I read about someone arbitrarily diving thirds for OW rec I sense over simplification instead of experiential understanding.

So, what exactly are you mitigating with 10 minutes worth of stops on a rec dive?

 

[bada3003]

Thanks for sharing your discussion with your buddy.

Regarding going to the chamber after bolting from 40 feet, I assume the dive was actually much deeper. Otherwise, not sure why going to a hyperbaric chamber was necessary. The problem with rapid (uncontrolled ascent) from shallower depth is mainly with pulmonary barotrauma.

As to what I'm mitigating with 10 mins of stops during rec dive, up to 130 feet (in some parts 100 feet) is considered rec diving, but that's an artificial threshold. There is no sudden hard delimiter from rec (i.e., NDL) to technical diving. Making safety stops after a 80-130 feet dive is most certainly recommended. How nitrogen builds up is physiology dependent and variable, and not worth messing with for a few minutes more of bottom time.

At 80-90' I skip the brief stop at 60 feet, but never the ones at 30 and 15. Even should my reg free flow in colder temps, I'll slowly ascend to 15' and perform as much of a stop as the remaining air allows. Hasn't happened to me in real life yet, but have practiced it. The air is very cold and it's different from practicing in shallow waters in a pool (e.g., PADI training). I may eventually turn to deeper dives with trimix, but till then the main danger I face at depths up to 130' is reg free flow. For others it may be different, say, nitrogen narcosis. As we all know, key element is not to panic. Stories are told of divers fumbling around with their reg to fix it, trying to switch to their octo (nonsensical since it's the first stage that freezes up), trying to reach a buddy 10+ feet away, etc. all the while expending air. And drowning or bolting to the surface uncontrolled become real possibilities.

My main reason for chiming in is the impression I got (maybe inaccurate) that the OP did not seem to know he was exceeding the recommended NDL with the 2-dive profile he described. It is one thing to know but disregard it (the tables are supposed to be conservative to begin with). To not know, or rely too heavily on dive computers, seems imprudent.

I just had a similar discussion with a friend a couple of weeks ago during a dive trip. He kept wanting to begin ascents with 800psi from 50' or less while at the same time slinging a 40 pony. He said he was just trying to be conservatively safe. I got that but questioned his logic and suggested he was being very vague about what he could do with the gas he had. Further, by doing so, he would not get a "real" feel how much gas he actually needed to perform certain activities. It was an interesting discussion. I convinced him to breath down half his pony to see how long that thing would last, and to add that time to how long 800psi would last, at those depths. About half the dive.

My point to him was that, without a realistic concept of what he actually needed, he might "freak out" on a dive if he dipped below his imagined safe volume limit, when in fact, he had a very large margin to work with. This actually occurred to a diver locally who bolted to the surface when she saw she had gone below 500psi in 40' of water. Her BF chased her and they both went for a ride in the chamber.

Every time I read about someone arbitrarily diving thirds for OW rec I sense over simplification instead of experiential understanding.

So, what exactly are you mitigating with 10 minutes worth of stops on a rec dive?

 

[HIGHwing]

What's really scary is this thread is searchable by future divers. I can see someone potentially reading this thread and becoming as confused as a few of the posters. The explanation of helium's impact on decompression was particularly precious.

 

[CurtSchu]

I was wondering if everyone was making an incorrect assumption as to the Caution Zone Statement. I was hoping the OP would clarify but has only posted one follow up. I was thinking he was reaching an O2 CNS Caution diving that rich of a mix at the limit of MOD for a square profile. Having dove Deco for so long I just don't get freaked out by the NDL on my computer. Planning for proper gas reserves is the key. Knowing what a 20 minute 15 foot hang takes is something all divers should know and practice. I agree that additional training is needed, one thing that make me suspect the OP is a troll is the RB statement and lack of follow up posts. Not being an RB diver but knowing several I get the impression you would have Adv. Nitrox and Deco prior to RB training. Sorry if I'm mistaken.

 

[Tigerman]

Theres the "recreational rebreather" setup and training you can get now.. No decompression rebreather diving with prepacked scrubber cartridges..

 

[mathauck0814]

Not being an RB diver but knowing several I get the impression you would have Adv. Nitrox and Deco prior to RB training. Sorry if I'm mistaken.

There's what the Agency says you need as a prerequisite and then there's what common sense dictates you ought to know before you get into it. The two are rarely similar.

 

[bada3003]

Amen.

There's what the Agency says you need as a prerequisite and then there's what common sense dictates you ought to know before you get into it. The two are rarely similar.