deco myths

[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.

 

[Angelo Farina]

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).

As said, I perceive there is a problem of using many independent compartments when there are strong pressure gradients between them, neglecting any gas transfer directly from one compartment to another.
But I have not enough knolwedge for proposing a proper lattice model, where gas transfer from one compartment to another is possible.
As we treat these gases as perfect gases, the fact there is only one gas, two, or more is substantially irrelevant, as in perfect gas theory each gas behaves and moves only based on its own partial pressure, and the partial pressures of the other gases in a mixture is irrelevant.

 

[rob.mwpropane]

I always thought the same. One of the main reasons I never pursued technical diving.

Would rather make a couple of shorter dives separated by a warm, sunny surface interval while eating snacks and drinking ice cold water than doing one longer dive and then hanging on a line trying to stay awake for an hour or more.

You just don't know what you don't know. We do the same dives I would argue probably safer though. Going tech means you learned more about what's really going on when you dive, planning better, and being safer. You really don't have to do hour long deco, but you don't have to be scared of this voodoo thing called NDL either.

I am way more conservative then I ever was before because I know what's past that imaginary line and have a good (or maybe I should say better) understanding of how to handle it. There is TONS of freedom in that.