deco myths

[Tom_Ivan]

How do you keep from going crazy on a 60 minute deco stop? Sitting at 20 feet doing nothing for 3 minutes is hard enough for me!

Tech diving does not always mean long stops. Depending on the dive site, you can plan the profile so that the deco obligation is mostly gone before you start your final ascent. Then with 100% oxygen your stops can be very short indeed.

 

[Wibble]

Tech diving does not always mean long stops. Depending on the dive site, you can plan the profile so that the deco obligation is mostly gone before you start your final ascent. Then with 100% oxygen your stops can be very short indeed.

Or 80%. Or 50%. Or even nitrox 21

In any case, hanging around on a stop is very relaxing. Especially if you’ve got enough redundancy.

 

[ofg-1]

How do you keep from going crazy on a 60 minute deco stop? Sitting at 20 feet doing nothing for 3 minutes is hard enough for me!

You just make up little games to pass the time. As example, if you find some trash during the dive, see if you can tie it on your buddy without him noticing. Clip a dropweight on your buddy while hes not looking. See if you can hogtie him with his own reel. Steal his SMB. Zipties can be fun.

 

[admikar]

You just make up little games to pass the time. As example, if you find some trash during the dive, see if you can tie it on your buddy without him noticing. Clip a dropweight on your buddy while hes not looking. See if you can hogtie him with his own reel. Steal his SMB. Zipties can be fun.

I can see this quickly going down the "cut his hose" route

 

[Lorenzoid]

Tech diving does not always mean long stops. Depending on the dive site, you can plan the profile so that the deco obligation is mostly gone before you start your final ascent. Then with 100% oxygen your stops can be very short indeed.

I speculate that most "tech dives" done nowadays involve less than 30 minutes of deco. Seemingly everyone is at least dipping their toes into tech diving. People seek tech training when they realize they don't need to be confined to the somewhat arbitrary limits that have long circumscribed "recreational" diving, and want more flexibility. It would be interesting to know what percentage of "tech dives" are big dives with lots of deco.

 

[admikar]

OK, what is big deco? 20 minutes? 30? 60? 100?
One deco gas? 2? More?
If it's 100 and 2, I have never done a tech dive.
I consider tech dive if I need to hold more than 10 minutes at 3m. Deco that clears on my way up is not considered a tech dive.

 

[Angelo Farina]

OK, what is big deco? 20 minutes? 30? 60? 100?
One deco gas? 2? More?
If it's 100 and 2, I have never done a tech dive.
I consider tech dive if I need to hold more than 10 minutes at 3m. Deco that clears on my way up is not considered a tech dive.

Here in Europe we have training agencies, such as Cmas and Bsac, which always considered and still consider deco dives to be fully recreational.
Then a couple of US-based agencies started to push for a more restrictive definition of recreational diving, limiting the depth and the diving time and promoting instead the practice of doing several separate dives along the day. Which is more profitable for the commercial diving center...
But here in EU we continue to have a number of recreational divers, as me, who routinely plan and execute dives with some (short) mandatory deco stops.
Personally I consider recreational diving any dive down to 50 m max and with deco which you can plan and execute using a low-cost recreational computer, such as my Cressi Leonardo.
In my case this excludes accelerated deco with a different hyperoxygenated mix.
Which I use when available, but keeping the deco times the same as with the bottom mix.
Dealing with accelerated deco requires knowledge and equipment (computer) which I do not own.
And, from a scientific point of view, I am not fully convinced on those formulas employed for shortening the deco times...

 

[JMarc]

And, from a scientific point of view, I am not fully convinced on those formulas employed for shortening the deco times...

Could you expand on that aspect? Thanks.

 

[admikar]

Here in Europe we have training agencies, such as Cmas and Bsac, which always considered and still consider deco dives to be fully recreational.
Then a couple of US-based agencies started to push for a more restrictive definition of recreational diving, limiting the depth and the diving time and promoting instead the practice of doing several separate dives along the day. Which is more profitable for the commercial diving center...
But here in EU we continue to have a number of recreational divers, as me, who routinely plan and execute dives with some (short) mandatory deco stops.
Personally I consider recreational diving any dive down to 50 m max and with deco which you can plan and execute using a low-cost recreational computer, such as my Cressi Leonardo.
In my case this excludes accelerated deco with a different hyperoxygenated mix.
Which I use when available, but keeping the deco times the same as with the bottom mix.
Dealing with accelerated deco requires knowledge and equipment (computer) which I do not own.
And, from a scientific point of view, I am not fully convinced on those formulas employed for shortening the deco times...

Unfortunately, even CMAS is backing out. R3 training states that deco and PPO 1.4 depth involves additional training.

 

[Angelo Farina]

Could you expand on that aspect? Thanks.

The current models for computing deco stops are basically yet an evolution of the old approach of Haldane: a set of parallel and fully independent "first order circuits", usually called "compartments", each charactaresid by just two numbers: the emisaturation time and the max oversaturation ratio.
Even the more advanced Bulhmann 16-compartments model behaves substantially according to this basic approach.
After proper tuning of these two parameters for each of the 16 compartments, this model has shown to be able to plan diving profiles which are reasonally safe for the vast majority of recreational dives (which include deco stops, as already amply discussed, and usage of Nitrox).
But these recreational profiles do not include trimix or changing mixture along the dive.
Albeit this model has also been employed extensively also for diving with heliox and trimix and for accelerated deco with high-oxygen mixtures (after some re-tuning of coefficients), the phoenomena happening in our body in these conditions make the basic model quite far from reality.
In our body there is no "separation" between "compartments", we have instead a number of different tissues and organs which communicate each other. The components of the gas mixture migrate from here and there following complex diffusion and convection phoenomena.
These should be accounted for by means of a more complex equivalent circuit, with links between compartments.
These phoenomena are exacerbated when the gases are moving pushed by strong pressure gradients.
So, while for a normal dive in air, with deco also in air, the pressure gradients are small, resulting in negligible effects of this inter-compartment gas migration, when you speed up the deco forcing large pressure gradients thanks to breathing (almost) pure oxygen I think that the basic Haldane model becomes too distant from reality for being thrusted.
Said that, my knowledge is not deep enough for proposing a more realistic model suitable for accelerated deco. I see the problem, but I do not have a reliable solution.
So I did never shorten my deco stops even when an higher-percentage of oxygen was used during the last part of ascent and deco stops (a case which happened to me perhaps 4 or 5 times in over 1000 dives).
The benefit is usually just a few less minutes of deco, and I did never think that such "advantage" was worth the increased risk and the hassle of computing this deco reduction.