Isobaric Counter Diffusion....Type III DCI.....Vestibular Hit.

[Scubadec / Dec Hanniffy]

What about this:

In my current project 450m from the cave entrance the depth is -72m. A hundred metres further the depth is already -80m and continues like that for another 250m (800m from the entrance total) . A planned exploration dive to 1km assumes that if my rebreather fails at the furthest point of exploration I'm 35 minutes of rather brisky finning to get out of -80m and another 10min to reach my first,out of three, deco tankat -50m. For that 45minutes I need a cover of 5 full Alu80. Would you take a chance here and put an air in it?

And someone said recently to me that I'm lucky coz cave diving is so much cheaper than deep sea diving...)

artur

Ah I was wondering where you had gotten too! Youve been very quiet lately! I was thinking you had some major mission, cant wait for the trip report.

[arturconrad]

Out of curiosity, Artur, what do you call cheap?

Hi Matthieu,

Well , the cost of helium for 20/30 in Alu 80 200B even with Irish BOC prices 480e/10000l is only:

(0.3x11x200)x(480/10000)=31.68e plus some o2 plus air and yes someone will probably charge you for preparing it so let's say 60e and that cost spreads on any number of dives you wish  - it's only an emergency bailout. And even if you have to use it one day, than hey, it's only 60 quids for avoiding the chmber and being out ou water for couple of weeks or months.

But back to the topic for a moment, I was actually thinking about that jump from 10/70 to Ean50. It's on 21m so you're jumping from 0.62 PPN2 to 1.55 PPN2 which, but I'm only guessing here, is probably an acceptable risk? I'm sure it's not recommended but well... Going back to Air would be a different story all together but again, I didn't have the pleasure of doing that yet so  can't tell. However I'm sure there are divers here who at least once were forced to go on a high nitrogen mix straight from a high helium one, it would be interesting to know what, if anything, happened...any volunteers? 

[patto_chan]

A planned exploration dive to 1km assumes that if my rebreather fails at the furthest point of exploration I'm 35 minutes of rather brisky finning to get out of -80m and another 10min to reach my first,out of three, deco tankat -50m. For that 45minutes I need a cover of 5 full Alu80. Would you take a chance here and put an air in it?
artur

Jaaaaaaysus!  Scary. I'm sleeping with the light on tonight.

[Stewart Andrews]

This is obviously something that each works out for himself (and be comfortable with) - and it obviously has to be relevant to what you are doing AND where you expect your bail-out gas to come from. ICD is a real issue when dealing with bailing-out off a rich He mix!

I do a bit of diving to (typically) 115m, so I have to plan for that. I stick with a pretty common system for most of the deep stuff. I use 10/70 as DIL in the rebreather and 13/55 for my left hand side.
It's a balance between ox-tox and ICD:
The ox-tox is not a likely issue if you 'jump' up 30m or so to get out of depth - vertically if away from the shot & preferable with someone else with more left hand bail-out gas. It is important to be hydrated and also to use buoyancy instead of finning to ascend (CO2 brings on a CNS hit).
The He % is designed to begin the step-down to a weeker He mix (drop sets), air or nitrox.

My RHS is air (not ideal) but can be life sustaining from about 70m, if only temporarily, till the drop set arrives. I also use it for dry suit and spidge gas.

[Fintan Lowney]

What about this:

In my current project 450m from the cave entrance the depth is -72m. A hundred metres further the depth is already -80m and continues like that for another 250m (800m from the entrance total) . A planned exploration dive to 1km assumes that if my rebreather fails at the furthest point of exploration I'm 35 minutes of rather brisky finning to get out of -80m and another 10min to reach my first,out of three, deco tankat -50m. For that 45minutes I need a cover of 5 full Alu80. Would you take a chance here and put an air in it?

And someone said recently to me that I'm lucky coz cave diving is so much cheaper than deep sea diving...)

artur

Arthur, your Fu*k"n Nuts  , by the way, the 33B goes to Portrane 
All the best with the Project.

[thumpr]

I think stewie's comments are good ones. It is one of things that you have to be happy with yourself.

I tend to dive with 15/50 and 50% for a two stage dive. Over 80m I am carrying 10/70, 15/50 and 50% to avoid the IBCD issue.

I am sure that 20/30 would get expensive over time if you are using it to fill lift bags... 

Maybe with enough helium those aqua elevation bags would lift themselves into the boat.

Mike

[Colin Parle]

Its time`s like this that i`m glad i only carry air for the lift bags because if i was to carry all the other bottles i may as well go back to OC.

[Stewart Andrews]

I understand Colin's view point entirely and have dived myself, on occassion, to over 100m with very little bail-out - usually due circumtances beyond my control - with risks that I am prepared to accept as I do the pre-check dive and only on rare occassions.

I have dived, sober-talked and pub-talked with a diver who had to bail out from 130m (his RMV went through the roof, understandably).
I have also been on a boat in the UK when one of the 'best of the best' divers had the most uncomfortable experiences of his life on CCR and took well over an hour to get over it sitting on the boat afterwards (sorry, I can not go into this more here).

My point is, and also my concern is, that many divers do not seem to be able to make logical, independent and thorough risk assessments of the factors involved and the what-if's that might happen, say in the event of bailing-out. Risk assessment requires training, in my view - it is not a natural instinct.
How many of us have completed diver training, at a reasonable high level, where the instructor's 'what-if' tips amount to a couple of anecdotes that happened to him or a mate of his???

What is also important is that you are aware of the residual risks once you make your decisions, along with the myriad of unknowns and permutations of unknowns that may befall you. These residual risks - ie. nagging doubts / the voices in the head! - often decide your approach to riskier sports. Mitigation in deep (sea) diving can come in the form of buddies, further training, agreed drop systems, bail-out cylinders, surface support, returning to shot line/tagging systems, staged cylinders, etc, etc.

Ref. deep SEA diving above: sorry Artur, I know nothing about cave diving - just enough to keep well clear of it!!!

[Stewart Andrews]

Getting back on-topic a bit more...

I have often toy'd with the idea that if you had to go from a rich trimix (and had enough left in the cylinder) to say air, you could do about 15 minutes of using both gases simultaneously (a few breaths from each reg). The lungs will see the 'neat' concentrations of each gas but the blood should average it out and that is surely where it counts.

A bit like when you explain how a rebreather works to a newbie - it is an 'ideal nitrox mixing machine'.
The same way above... only your lungs become the mixing machine. Has anyone ever tried this or even heard of it?

[soheil]

Its time`s like this that i`m glad i only carry air for the lift bags because if i was to carry all the other bottles i may as well go back to OC.

Just a matter of interest, What did you fill your lift bag with before you start carrying air?

[Paul Dowling]

Its time`s like this that i`m glad i only carry air for the lift bags because if i was to carry all the other bottles i may as well go back to OC.

Just a matter of interest, What did you fill your lift bag with before you start carrying air?

We brought buddies for that 

[Peter McCamley]

Well Stewie
Many thanks for your post. Its good information, logical, well thought out and without a doubt suits the type of diving that you do and fair play to you are up there with the best, at what you do.
Diving is never perfect. Technical stuff gets even less perfect. The deeper you go, you start to weight off the good with the bad. Having a full understanding of whats going on is paramount.
The reason for my post was just from an experience and to help enlighten anybody, who was not aware of the consequences of baling out onto air from helium at depth, without fully understanding the potential risks. If everyone reading my post was aware of what I put up then it was a waste of time, however if only one person was not, high lighting the risks was worth it.
Regards
Peter

[Stewart Andrews]

I'm very glad you started the post Peter - I has made me start to think agian about some of these things - which is always good.
Furthermore, I know that there are many people on this forum who are newish to this sort of stuff and the more information (hopefully practical and thought provoking) we can get out there the better.

The more I look back on my diving learning curve (and the learning ain't over yet by a long shot), the more it feels / I realise that I seemed to have to learn a lot of it from 'scratch' and suck-it-and-see. This is despite the great help I received from more experienced fellow divers.

Even the best books available are going out of date at a faster rate than ever as diving knowledge grows (theoretical and practical) - the practical bit interests me even more than 'diving' into the best texts.

On the subject of books - the best book for advanced diving that I have ever come across is Kevin Gurr's "From the bottom up". He has had a great diving career including being the maker of the VR series of dive computers (eg. VR3) and some rebreathers. I loaned it to someone a couple of years ago and never got it back! I went out and bought it again - testament to it's quality. Anyone in Dublin is more than welcome to flick through it - but not take it away - divers!!!!

[Stewart Andrews]

Its time`s like this that i`m glad i only carry air for the lift bags because if i was to carry all the other bottles i may as well go back to OC.

Just a matter of interest, What did you fill your lift bag with before you start carrying air?

We brought buddies for that 

I find that if you're short on spidge gas, you can position your buddy directly under the lift bag mouth, remove his loop & give him a good squeeze against the wreck - you'll get an extra 4.5 litres into the bag - at ambient pressure too!! Yeah for disposable buddies!!
On another matter, anyone want to dive with me tomorrow?

[soheil]

Its time`s like this that i`m glad i only carry air for the lift bags because if i was to carry all the other bottles i may as well go back to OC.

Just a matter of interest, What did you fill your lift bag with before you start carrying air?
We brought buddies for that 

That is a big jump from hot air to buddy.