Bailout Planning

[ghosch]

I asked this question on a "national" board and got worthless responses, so I'll ask it local.

Bailout, what is the current thinking on gas choices.

Bottom mix obviously is breathable at maximum depth, but for the deco mix what are peoples thoughts?

I was taught (and I dive) EAN 80+, that can either be used for OC deco or off-bard injection into the loop.  Problem is when planning OC bailout you wind up needing a LOT of bottom mix, and very little of the deco gas.  Going to a weaker gas for the deco mix (air, EAN 32, 40, etc.) yields a better deco profile, but you can't plumb it in off-board and run in CC mode (though you could SCR).

Thoughts?

[drmike]

I normally carry a small tank of 100% O2  (say a 3L or more usually a 5L)  That's enough to cover  6m stop requirements on most dives.   If  no booster is available and the available pressure of O2 didn't give me enough volume I would top off with air usually 70% min.   I would check with Vplanner to ensure I have enough volume for my 6m deco given the resultant FO2. 

I carry a bottom bail tank and if a deep dive other intermediate bail tank(s).   The tank size and mixes depend on the deco obligation and I select to maximise PO2 at each gas switch.  My aim is to minimize the number of tanks and still have enough gas to do full bail.


As you describe you may find on some dives that you can get away with less or no high FO2 bail gas if you maximiuse FO2 in your bottom bail gasses.

If just doing a shallow bimble dive I might just carry one tank of nitrox or even air - but Id check with V planner first to ensure I had enough volume for worst case bail given the FO2 and tank size.



I found Vplanner to be very useful to optimise bail gases and match to available tank sizes. It gives you the volumes needed and you can play around with changing the bail gas mix to suit your tank sizes and the number of tanks you want to carry.

[Inspirationdiver]

For deeper dives(~300ft), I'll ususally carry a bottle of bottom mix and a bottle of 50%.  If you run through the calculations, I to try to match the gas consumption for each bottle.  That way you don't end up needing 100cu ft of bottom mix and 13cuft of deco gas.  For shallower dives you can raise the FO2 on the second gas, deeper dives you need to lower it. 

[Mark Y]

I know this is an old thread - but Oh Well!

Inspirationdiver - do you still use 50% for deep dives?

John J - do you still use 21% (air) for bailout?  Have you tried running the Inspiration with the offboard diluent?  If so, what are you're thoughts.

Anyone else - you're thoughts, comments, and experiences?

[john j]

I no longer use air for bailout from helium mixes.  For bailout from helium mixes, I use another helium mix that will give me a 1.4-1.6 PO2 at the MOD of the dive.  If the bailout route would require a significant time at the MOD, I would lean toward lower PO2.  The second bailout gas is then formulated to balance the gas usage between that and the first bailout gas.  Using a very high PO2 for the second bailout gas, requires the diver to get relatively shallow before switching to it and thus, you need a lot of the first bailout gas, and secondly, since you are on the first bailout gas for a long time, the PO2 is not optimized for most of that part of the bailout and the deco requirement is then actually longer.  .

Using the second bailout gas to manually fly the CCR is a good idea in theory, but I wouldn't give that too much weight when planning bailout.    Zach's post about balancing the usage of gas is exactly what I think is best.  You could carry a small additional cylinder of O2 if you want to have backup to manually fly the CCR. 

Switching from a low PN2, helium rich gas to a higher PN2 gas is not a good idea because it can cause inner ear decompression sickness.  While not life threatening, it can cause serious vertigo, perhaps vomiting, etc. possibly leading to drowning.  This is the reason why I no longer use air as a bailout gas from helium mixes.  To my knowledge, the inner ear deco problem is the only reason why air bailout is a bad idea in such cases.

Most deco software gives a much reduced deco requirement if you switch to air and that is why I did it in the past.  I had heard from a few people not to use air, but nobody could tell me why.  It's the inner ear deco thing.

I run on onboard diluent and I choose that gas such that at the MOD it will give around a 1.0PO2 for a diluent flush.  You need to be able to drive the PO2 lower than the setpoint during a diluent flush to properly test the system and to be able to remove O2 from the loop in the case of a high PO2 problem.

Since gas usage is so small on the CCR, I dive almost exclusively with heliox these days.  My understanding is that helium is a kinder gas to the body than nitrogen, so I just avoid the nitrogen all together.  The VR3 penalizes you heavily for helium mixes (and so does most deco software).  I am hearing about studies that show helium offgases way faster than the current standard models predict and that most deco software is generating far too much deco for helium mixes.  True or not, I don't know, but from what I know about the chemistry of He, it does make sense to me.  I am now using the HS Explorer because the RGBM algorithm produces longer stops at deeper depths but overall gives significantly less deco than the VR3.   The VR3 gets you shallow very fast, but then you hang out at 15 feet for ever.  I never liked that.

[Inspirationdiver]

Hey Mark,

I still stick by my original thoughts on planning bailout gases.  John J just added a few good points about the use of N2 based gases vs. Helium which I would agree with.  I've also been using heliox for all my gases on dives over 300ft.  I have to say that I get out of the water feeling great. 

What have you been using?

[Mark Y]

I've tried a couple of different bailout gases... for my next adventure, I'll be carrying a bailout gas similar to my diluent gas and 50 EAN for my second gas.

So far I've only used bailout on computer dives, never had the need in a real life situation (I've still done bailout drills, etc.).  Well, once I did bailout after getting real disorientated and thought I was having a breakthrough.  I quickly switched back to the loop though.

John, how do you like the Explorer?

[john j]

I like the RGBM algorithm and I like the display (large letters and all on one screen).  There is a whole bunch of stuff I don't like too.  I have had no problems with it so far.