CCR Heliox, deco AIR

narayaana

narayaana

New Member
hi guys,

I am a newbie en Tec, I was asked as a test, to plan a short dive at 60m for 20 minutes.
My base idea was to start CCR Heliox (please take it as the assumption that this is part of the exercise).

I chose deco with air. it gives almost the shortest deco time (deco air + 100% oxy at 6 m).
It made sense for me, because Helium is not nitrogen, hence no further on gazing of Helium.
Off gazing would be like blood helium vs air (virtually no helium) on ascent, which sounded faster.
A bit of nitrogen on-gazing during super short deco (less than 40 minutes).

it seems like nothing goes against switching from nitrox/air to oxy, but how about heliox/air ?
Is there any studies against switch this way ?
 
Dave1w said:
Have a look at isobaric counter diffusion?
Click to expand...
Well as a "problem" IBCD doesnt really exist if you keep your ENDs reasonable (<30m).

The OP could in theory switching from 18/45 dil to air at 60m though - which is a pretty bad idea for more than just the IBCD rationale. Gas density is quite high for a CCR at 60m
 
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I should make it explicit:
- Descent and bottom, CCR is main. OC is bailout.
- Deco, from 39m, is OC.

2x9L twins of air (bailout &deco 39m->6m), 2x2.5L oxygene (CCR & 6m deco & bailout), 1x4L Heliox (20/80 for CCR & bailout 60m->39).
but yeah, maybe add an extra tank for 60m-> 30m, with trimix :confused:
 
A
narayaana said:
I should make it explicit:
- Descent and bottom, CCR is main. OC is bailout.
- Deco, from 39m, is OC.

2x9L twins of air (bailout &deco 39m->6m), 2x2.5L oxygene (CCR & 6m deco & bailout), 1x4L Heliox (20/80 for CCR & bailout 60m->39).
but yeah, maybe add an extra tank for 60m-> 30m, with trimix :confused:
Click to expand...

sorry - looks like a horrible plan …

air is a rubbish deco gas - why carry 2x9L?
Oxygen is also rubbish ie you have to wait till 6 to get on it and most of your deco is done by then.

some models will give an advantage switching to nitrox from helium … bit appears to be purely theoretical. I gave up on switching off helium on the way up ~20 years ago as I always feel better with more helium and many other report the same as well as evidence seems to back this up …
 
Bobjr said:
A


sorry - looks like a horrible plan …

air is a rubbish deco gas - why carry 2x9L?
Oxygen is also rubbish ie you have to wait till 6 to get on it and most of your deco is done by then.

some models will give an advantage switching to nitrox from helium … bit appears to be purely theoretical. I gave up on switching off helium on the way up ~20 years ago as I always feel better with more helium and many other report the same as well as evidence seems to back this up …
Click to expand...

Thanks for your answer, I appreciate you question my choice with argument on oxygen 100%.
I looked at the plan with different mix of oxygen, 50% 80% etc. it did not give much difference, only 100% gave faster recovery and time to surface.

You are right to question 2x9L, the plan gave 1170L of air, so, 1/3 rule + 1.5 for buddy, should only be 1x9L. My previous calc gave double that volume at 200 Bar, which I would not carry as single, too long a tank for my size. I had found it nice to have the redundancy of twins.

How bad is air as a deco gaz was a bit of my initial question, @Dave1w gave me a very good hint about it, thanks again Dave, I learn something new for me.
Air theoretically made sense for deco vs Heliox bottom, 2 very different inert gas would have made deco optimum.
besides, If I carry only 100% oxy, air, and Heliox for CCR, the plan seemed to work, to be light in terms of gaz to carry, to be cheap and to be short.

I looked at adding a bit of helium in the mix, it noticeably increased deco time and slowed ascent rate. My instructor said too he would add helium. You are feeling better. My question is more about finding studies that substantiate this observation. Isobaric Counter Diffusion answers my question :D
 
Why have 1.5x for buddy and rule 1/3 for bailout ... bailout should be a rare event you don't have to have two additional levels of contingency - just plan for what you need?

Who is teaching deep ccr in Hong Kong these days?
 
Bobjr said:
Why have 1.5x for buddy and rule 1/3 for bailout ... bailout should be a rare event you don't have to have two additional levels of contingency - just plan for what you need?

Who is teaching deep ccr in Hong Kong these days?
Click to expand...
In my plan, Air is for deco, not only bailout.

I had my answer for my question already, thanks
 
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In reality when diving CCr - I tend to have a limited set of deco bottles which I keep full of deco gas and usually remain full.

for a 60 metre dive - I take a bottom mix bail out say -16/45 - either a 7 litre or 10 litre with a 10litre 50 %. I can always add an extra 3 litre O2 on the back of my rebreather as either open circuit BO to speed up deco or use it as back up O2 for my rebreather (would let me stay on loop)
 
Why are you planning on switching from CCR to OC air for deco? Stay on CCR.
When on CCR, OC is just for bailout. And if you are bailing at depth, better have a little helium mixed in so you can think about what you are doing as well. Rapid optimal deco isn't the highest priority on bailout, having enough to survive is.
Adding complexity to the deco to save a few minutes should be avoided as well. Keep the same DIL a few extra minutes of deco will be better than struggling to get the loop flush with a different DIL and switching the computer over to the new DIL.
 
broncobowsher said:
Why are you planning on switching from CCR to OC air for deco? Stay on CCR.
When on CCR, OC is just for bailout. And if you are bailing at depth, better have a little helium mixed in so you can think about what you are doing as well. Rapid optimal deco isn't the highest priority on bailout, having enough to survive is.
Adding complexity to the deco to save a few minutes should be avoided as well. Keep the same DIL a few extra minutes of deco will be better than struggling to get the loop flush with a different DIL and switching the computer over to the new DIL.
Click to expand...

Good question .
Switch from CCR to OC could be avoided when I don't bailout. I could switch DIL.
DIL heliox > DIL air.

Just need an extra hose and prepare the dive computer with 2 DILs.

I don't know yet how fast isobar counter diffusion comes into play. It could help to progressively flush (SCR mode).

Gaz narcosis is not immediate either. The idea is that because it is not immediate, I had time to got up to 30m. Ascent at 10m/minutes + deco at 39m takes 4 minutes. I actually don't know how fast Gaz narcosis comes in effects. By the way, has anyone an idea or information about how fast Gaz narcosis come in effects ?
 
You are still making it too complex. Just keep the same DIL for the whole dive. 60M 20 minutes isn't enough to mess with a gas swap. Get to 100+m with run times in 5+ hour range, might think about it.

You can alter the deco more with a change PPO2 than changing DIL.
 
Agree with above.
Bailing out is rare.
Narcosis does hit quickly - and if you are bailing due to an issue, with stress, high breathing rate, CO2, then you really will appreciate a clear head - (the worst narcosis appears under workload/stress/ high breathing rate from experience)

From a bailout perspective - have a bottom mix bail out similar to your dil (maybe slightly higher o2) plus generally have 50% to deco on for 60m dives and work out size of bottles needed dependant on bottom time etc

(& as I previously mentioned add a bottle of O2 if your bottom times at 60m get longer)eg 3 litre)
 
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