deco myths

[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.

 

[Angelo Farina]

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

True, currently Cmas (Fipsas here in Italy) has limited the 3-stars diver certification to 40m and within NDL.
However here Fipsas has some "advanced" recreational courses which allow for deco stops:
1) Advanced Nitrox allows for deco stops (using the same gas mixture as for the bottom, which can be either Nitrox or air) and for 40 m depth maximum - Cmas ** required

2) Extended nitrox allows for dives with deco stops, using air between 40 and 51 meters depth as bottom gas and using high-oxygen Nitrox for deco stops. Cmas *** and Deep required.

Both these courses are NOT technical yet. They are called "advanced recreational".

My current recreational certification (an old 3-stars Cmas instructor) corresponds substantially to the current Advanced Nitrox, albeit my certification also includes pure oxygen CC rebreathers (ARO), as this was the basic Scuba system employed here in the seventies for training new divers.

 

[Wibble]

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.

Must update this….

Hanging around at deco is relaxing.

Unless…. You’ve not fully connected your pee valve so the damn thing is closed. Then you’ve over an hour and a half to go knowing that you are busting for a pee and no matter how much you try, you cannot free the pipe…. A matter of when the explosion will happen, most definitely not if.

Thankfully you planned ahead with some large trash bin liner bags for the undersuit and spare underpants. Naked on the dive boat yay!

Once off the boat it’s sort out the rebreather then take another couple of hours to drive to the nearest launderette to wash and dry the undersuit and underclothes. Amazingly, all went to plan ready for tomorrow’s dive on a submarine that’s not been dived in twenty years.

Oh the things we do for diving…

Remember children, make sure the pee valve is properly connected. Only misery and embarrassment awaits. Inconvenience even.

 

[Blackcrusader]

I'm not ashamed to admit I have no clue what setting a computer to 90/90 actually means.

I will come back to this again. Before I bought my Perdix in 2018 I also was not aware. Once I got my Perdix I spent time reading and re-reading the manual, reading the threads here, updating my Perdix firmware and reading what was new. Then also Surf GF and what that is what it means and how to use it. I went back and checked hundreds of my dives to see what Surf GF I was ending dives with. Less than 80. Perhaps that is because on many dives I simply do 10 - 15 minutes on reefs at 5m for the last part of my dives.
Now I am an old school BSAC trained deco 21% Nitrox diver so being deco trained I am not worried if I decide to exceed NDL. Some of the people I dive with are TDI ANDP trained in addition to PADI and even BSAC CMAS. I don't want to do trimix dives as my dives are for vacations with divers who don't even do deco dives.

However on some dives it might just be me and a dive buddy or two who are all deco trained and yes we might do that planned 30m nitrox 32% 40 minute dive and do a deco stop. We also normally might switch to AL100 if available but some places don't have them so AL80's. Dive time stays under an hour so doesn't upset the routine of going out on a dive boat.

 

[L13]

I'm not ashamed to admit I have no clue what setting a computer to 90/90 actually means.

No shame, but at the same time: why are you commenting on a thread about deco?

 

[Angelo Farina]

No shame, but at the same time: why are you commenting on a thread about deco?

I did deco dives for decades before someone invented the GF concept.
Also today I plan deco dives using a computer running an algorithm which does not allow to enter GF values, and where conservatorism can be simply set to min-mid-high.
Here on SB it appears that there are some wrong deco myths, such as:
1) dives with deco stops are strictly technical, rec divers only stay within NDL
2) planning deco stops requires the usage of a Shearwater computer running Bulhmann algorithm carefully tuned with personal GF values
3) deco stops must always be accelerated using an high-oxygen mixture carried in an additional deco tank.

In reality, I and many other recreational divers who posted here routinely conducted rec dives with deco stops without a computer at all (using tables) or using computers where there is no GF setting and using a single tank, usually air (EAN-21), for the whole dive.
Albeit this practice is discouraged by some agencies, it is taught and supported by other agencies.

 

[L13]

I did deco dives for decades before someone invented the GF concept.
Also today I plan deco dives using a computer running an algorithm which does not allow to enter GF values, and where conservatorism can be simply set to min-mid-high.
Here on SB it appears that there are some wrong deco myths, such as:
1) dives with deco stops are strictly technical, rec divers only stay within NDL
2) planning deco stops requires the usage of a Shearwater computer running Bulhmann algorithm carefully tuned with personal GF values
3) deco stops must always be accelerated using an high-oxygen mixture carried in an additional deco tank.

In reality, I and many other recreational divers who posted here routinely conducted rec dives with deco stops without a computer at all (using tables) or using computers where there is no GF setting and using a single tank, usually air (EAN-21), for the whole dive.
Albeit this practice is discouraged by some agencies, it is taught and supported by other agencies.

You are right in all your bullets and still missing my point. The person I was quoting apparently (from earlier in the thread) doesn't do deco diving. From that quote, he apparently doesn't understand one of the more common deco algorithms. He has never expressed knowledge of any of the other algorithms. What is he bringing to the discussion?

Obviously you don't need to use GFs to do deco. But, to be knowledgeable about deco in the modern era, you should at least understand the idea, even if you don't buy into it. You can't say if GFs are good or bad if you don't even know what they are.

You can have experience, or knowledge, or both, and be an expert. You can't be an expert with neither.

 

[JMarc]

These should be accounted for by means of a more complex equivalent circuit, with links between compartments.

In the case of a single inert gaz, I'm not convinced by your argument. I've not make the computations be to be sure (I fear that's the kind of maths I'm no more used to do), but think of how Thévenin's theorem in electricity shows that any passive linear circuit is equivalent to one resistor, one inductance and one capacitor. Having a more detailed circuit could help to map compartments to physical organs, but I don't think it would give more meaningful dependency on the dive profile when you consider that the current one have already to take into account the variability of the sensitivity of any diver at different time, and between divers (for which GF is providing us a way to tune). My intuition is that you'd need more than cross-coupling factors, for instance some non-linearity in the DE, between compartments to get something new.

I'm less familiar with how multiple inert gaz is handled. My intuition is that even the little I've been exposed to would have dive-profile dependent effects in a more complex circuit (with several compartments having the same time constant but different cross-coupling).

 

[L13]

In the case of a single inert gaz, I'm not convinced by your argument. I've not make the computations be to be sure (I fear that's the kind of maths I'm no more used to do), but think of how Thévenin's theorem in electricity shows that any passive linear circuit is equivalent to one resistor, one inductance and one capacitor.

We are getting way beyond what should be in the "New Divers & Those Considering Diving" forum, but:

Thévenin's theorem would be correct for modeling the circuit as seen from outside at it's interface. If you are just modeling the flow of inert gas into and out of the body, and only cared about the instantaneous net flow in/out and instantaneous total dissolved gas load it would be appropriate.

But, for DCS, it isn't the apparent model at the interface that maters, but the actual state at each individual location in the body.

Using your electronic circuit analogy to DCS, it is like asking if a particular 1/8 watt resistor would blow in a complex RCL network and trying to answer the question by looking at the Thévenin equivalent seen by the power source, instead of the Thévenin equivalent seen by the particular resistor.