ICD and gas swaps

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You have not specified if you use Buhlmann or VPM.
Gas density ( EADD ) is 43m / 142f. What's the problem?
pp = 1.6? This is normally used ... but in every deco planner you can adjust deco PpO2 as you want ..... ( however in deep stops you have a PpO2 = 1.6 for a very short time when switching, and it decreases while ascending, so this is not really a problem ).

If you calculate a RunTime with a max deco PpO2 of 1.4 you got 127 mins. ( + 6 mins on your Runtime, so not significantly longer ), but evitating
a brutal jump on PpN2 @ 190f and consequent ICD risks, with same number of gas swaps.

Sorry your plan is totally crazy; have you actually done this dive?
ppo2 1.6 on the bottom, huge END, high gas density, custom gases, (50/15 really?), VPM+2 is going to hurt on this dive and bad too.
All to avoid a non-existent IBCD "problem"

AJ laid out a real world plan that has actually been done, many times, with no IBCD. Simon laid out the known IBCD risks which are in the inner ear and not something that happens at a 190ft gas switch.
 
12/60 is in my eyes a good gas at 100m if you dive OC.
EADD is shown in the app 'CCR Mixer' too. But 43m as EADD is ok for me.
If you use 12/60 you can do a dive to 100m with 15 minutes bottomtime with 1 travelgas and 2 decogases. Then no 4th stage needed.
The PO2 in the bottomphase is important too for the deco. A PO2 of 1.1 (10% oxygen) will give you more deco than a PO2 of 1.3 (12%).
If you put in 12/60, 21/35, 40% and 80% with a 40/70 conservatism, the divetime will be around 105 minutes (I am not discussing icd here, just some examples of some gases for a 100m dive , no standard 'DIR' gases). If you use 10/60 instead of 12/60, the divetime will be around 110 minutes. Not a big difference, but in cold water it can be an issue, or with the gasreserves it can be a between 'possible' or 'better not'.

While not the way I would do it (END too high and density too high on 12/60 still), this plan is at least plausible.
 
Sorry your plan is totally crazy; have you actually done this dive?
ppo2 1.6 on the bottom, huge END, high gas density, custom gases, (50/15 really?), VPM+2 is going to hurt on this dive and bad too.
All to avoid a non-existent IBCD "problem"
AJ laid out a real world plan that has actually been done, many times, with no IBCD. Simon laid out the known IBCD risks which are in the inner ear and not something that happens at a 190ft gas switch.

The use of a 12/60 in a 100 meters dive is not in relation with ICD avoidance.
PpN2 = 3 and PpO2 = 1.3 it's perfectly acceptable.

The use of 1.6 as max deco PpO2 is a widely accepted procedure ( depending on training agencies )
PpO2 = 1.6 at bottom or at shallow depth still is 1.6.
Pp it is a relative unit of measurement !
( also, in deep stops when switching to deco stage and continuing ascent, exposure at 1.6 is very limited in time ( seconds ), instead of deco stops at 6 meters which take dozens of minutes ).

Note that there is a difference in PpO2 management between OC dives and CCR dives.

If I read previous posts you consider a END > 20 meters and EADD = 40 meters as crazy. It is your opinion that I do not agree at all.
In this case why do you make tech dive instead of recreational dive ? And for security reasons, it would be preferable to limit max depth at 15 meters.
If a diver is narked @ 20 meters ( 65f ) he just does not have to dive.

Seriously .......

If i apply your criteria, in a 120 meters dive ( 394f ), to avoid a non-existent EADD/Narcosis problem ( using your limits ), you have more than 20 minutes of additional deco time in your scheduled runtime, because of using a near-100% Helium mix, which dramatically increases decompression time ( what about gas quantity / number of deco stages to carry ???? ) ....

finally 50/15: yes. What's the problem?
 
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The use of a 12/60 in a 100 meters dive is not in relation with ICD avoidance.
PpN2 = 3 and PpO2 = 1.3 it's perfectly acceptable.

The use of 1.6 as max deco PpO2 is a widely accepted procedure ( depending on training agencies )
PpO2 = 1.6 at bottom or at shallow depth still is 1.6.
Pp it is a relative unit of measurement !
( also, in deep stops when switching to deco stage and continuing ascent, exposure at 1.6 is very limited in time ( seconds ), instead of deco stops at 6 meters which take dozens of minutes ).

Note that there is a difference in PpO2 management between OC dives and CCR dives.

If I read previous posts you consider a END > 20 meters and EADD = 40 meters as crazy. It is your opinion that I do not agree at all.
In this case why do you make tech dive instead of recreational dive ? And for security reasons, it would be preferable to limit max depth at 15 meters.
If a diver is narked @ 20 meters ( 65f ) he just does not have to dive.

Seriously .......

If i apply your criteria, in a 120 meters dive ( 394f ), to avoid a non-existent EADD/Narcosis problem ( using your limits ), you have more than 20 minutes of additional deco time in your scheduled runtime, because of using a near-100% Helium mix, which dramatically increases decompression time ( what about gas quantity / number of deco stages to carry ???? ) ....

finally 50/15: yes. What's the problem?
20mins of deco is absolutely nothing compared to the risk of high ENDs when deep and the risks of high gas density.

As to why do a tech dive instead of a rev dive,I don’t tech dive to get narced. I tech dive to see what’s there.

Backing off the END when deep is smart. Your situation is totally different. Delays can result in substantial additional decompression and of course the ever present risk of RUNNING OUT OF GAS in short order. You might have an hour to sort out some nonsense when shallow. You might have 15 when deep. It’s a different ballgame.

So you wanna expose yourself to all those problems (and then some) by diving in an impaired state to save maybe 20mins (and that’s up for debate).

Bro. Get it together.
 
20mins of deco is absolutely nothing compared to the risk of high ENDs when deep and the risks of high gas density.

As to why do a tech dive instead of a rev dive, don’t tech dive to get narced. I tech dive to see what’s there.

Backing off the END when deep is smart. Your situation is totally different. Delays can result in substantial additional decompression and of course the ever present risk of RUNNING OUT OF GAS in short order. You might have an hour to sort out some nonsense when shallow. You might have 15 when deep. It’s a different ballgame.

So you wanna expose yourself to all those problems (and then some) by diving in an impaired state to save maybe 20mins (and that’s up for debate).

Bro. Get it together.

Mmm....
I dont figure out what kind of courses you made and what manual you have read that says that a END = 20 meters is dangerous or a PpO2 = 1.3 is crazy ......
( and it appears, from your previous post, that you don't exactly know what a partial pressure is .... ).

25 minutes of additional deco time is a additional risk in your profile.
25 minutes of additional deco time means much more gas to carry / tanks to carry, which is an additional risk
25 minutes of additional deco time means more cumulated CNS
25 minutes of additional deco to avoid a non-existent NARCOSIS problem ( as you say maximum tolerated END is 20 meters ) is a non sense

your affirmations are corroborated by no manual, any scientific proof. It's just your interpretation.
 
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Let's just look at one tiny piece of this bad idea. Gas density. Do you know the density of your bottom mix? Have you read this scientific paper?
Gas density guidelines
 
Ok, we talk about OC diving. Max PO2 of 1.4 at bottomphase is the max in most agencies. 1.6 during deco. I always advice to lower the PO2 to 1.3 or maybe 1.2. Some divers want to use 1.0 or 1.1 at dives in full trimix range. You can do, but this means a longer deco, need more gas. Maybe it is not a lot, but consider it in your plan, look if all aspects fit.

END is a personal choice too. Some hold stricktly to 30m, others say 40 or 45m. Most agencies will say max END of 40m. For me personally I don't see any need to use a 30/30 at 30m, even not in currents. Somewhere between 40 and 50m I will add some helium to my mix. The 'deep air' courses go to 55m, and some divers will say I cannot deal with the narcoses, so it is too deep for them. I don't teach that courses as I am responsible as instructor, then will say take the normoxic trimix course. But I teach adv. nitrox to 42m.

More helium in your mix will give you longere decotimes too. There are some studies done that say there is no 'heliumpenalty', but use 'what works works'. So for me I accept that more helium will give me a longer decotime. More deco is not a problem as long as you have enough gas.
On CCR this means for me that I use on trimixdives 6/72 as diluent. This gas I have used up to 135m depth. A CCR gives you a constant PO2, so it works in a wide range. On OC it is not the best gas. If you believe in icd, more helium will make you maybe have to use helium in your first decogas too.

You can use on oc (even some on ccr) 'standardgases'. You don't have to do best mix calculations (which are not difficult), END always max 30m. The easy blend argument as it is helium+ean32 is most times ******** as trimix is made by partial blending mostly from what I see. But you can see it as advantage (poormansmix, heliair has the same advantage then). And you lower the ICD risks if they are there. But mostly you use a lot of helium. And helium is rare and expensive.
Here a nice example for people who believe in standardgases and DIR and say all other gases are wrong.
Go to 55m depth, use 18/45. PO2 ->OK, END=20.4-> OK. Plan with ratio deco. And you will have a nice dive.
Then you want to save some money, here it will be around 15 buck for a twin12. Take an 18/35, same depth:
PO2->OK, END=28->OK, still in the 30m range. Plan with ratio deco (you will be 1 or 2 minutes more conservative with this gas instead of 18/45 if you use a decoplanner, and ratio deco still can be used here), so still OK.
The only thing I will say with this example: be a thinking diver. 18/45 is not wrong, 18/35 is not wrong. And some other combinations of O2 and helium are not wrong either.

If you are afraid of high CNS load, take an ean80. This almost lowers the CNS half instead of pure oxygen. It seems that pure oxygen makes your more cold as the small capillairies will 'close', and EAN80 does not. If someone has proof for this, I am interested. This would mean that pure oxygen is maybe not as effective as some think (pure oxygen is theoretically the most efective decogas as the gradient are max). On dives to 100m, there is no difference in decotime if you use ean80 instead of 100% (1-2 minute differnce or so).

Think outside the box. If you only think in ean50 and 100% as decogases, you can be limited in bottomtime. Using for example 40% and 80% as decogas can mean you have 1 minute more bottomtime if you stay within reserves. And the bottomphase is the most worthly phase of the dive, that is the part of the fun. If you go for a 60m dive and you can do 21 minutes instead of 20 minutes, that can make a difference as such dives are not cheap. :D

Ok, my comments have mostly nothing to do with ICD, but I will say there are a lot of personal aspects to make a choice or make another choice. And all choices can be safe. Be a thinking diver, and are not afraid of thinking outside of the box. Dive safe.
 
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So, people, what are your criteria for preventing ICD? I have seen the rule: Do not increase the fraction of N2 by more than one fifth of the decrees in fraction of He. But that cannot be the full truth since technically any nitro dive where at any point you change to a lower percentage of O2 (yes, why would one do that, but theoretically) would trigger this rule. I guess there should also be a restriction to an absolute change in fraction of He (or anything like that). What would be a good criterium (for example to implement it in a dive planning software, hint hint)?
 
So, people, what are your criteria for preventing ICD?

Hello,

Constant PO2 rebreathers where gas switches are unnecessary for efficient deco: I don't bother doing diluent switches. If was going to do a diluent switch to air I would not worry about ICD unless I was doing a very deep dive (notionally > 90m / 300') in which case I would leave the diluent switch until a relatively shallow stop (notionally around 18m - 21m / 60 - 70') by which time it is unlikely that there will be significant pre-existing supersaturation in the inner ear.

Open circuit where gas switches are necessary for efficient deco: I simply would not worry about ICD, unless (maybe) performing a very deep dive (>90m / 300' as somewhat arbitrarily defined above - which I never do on open circuit these days). There is no validated rule of thumb for the latter situation, but I might consider carrying one intermediate trimix for such scenarios prior to getting onto a nitrox mix. Intermediate trimix or not, when I switch to nitrox, I would use a mix that provides the maximum safe inspired PO2 for the depth I'm at.

Simon M
 
https://www.shearwater.com/products/perdix-ai/
http://cavediveflorida.com/Rum_House.htm

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