Bailout planning

[ScubaAl]

Myself and a couple of other rebreather divers were discussing bailout planning. I have seem some videos mostly in cave where the RB diver is only carry one bailout bottle(dil) no O2
I also did a search on RBW. I get the impression that many divers, doing relatively deep dives 200-250FFW are carrying 2 bottles, one with bottom mix, the second with maybe 50%. Some people had as rich as 80% for the second bottle. But not carry O2, if I understand correctly.
So anyway I was wondering what other RB divers are carrying or not carrying and why.

Al

[jleech]

I have no training on CCR for cave, but I have OC full cave.  The planning for bailout is often quite different from openwater.  Since most of the time, your return path has to be the same way you came in, bailout can be dropped along the way with relative safety.  You only need to plan bailout to your next bottle.  Your stages/bailout should be clipped to the guideline you need to follow to exit the cave, so even in total blackness you should be able to find them.  As far as O2, it is almost always dropped near the entrance (30 ft or so).  On the CCR, you are most likely planning for worst case team OC bailout but the preferred bailout would be semiclosed.

I have only recently been trained for trimix dives on the CCR, and in training we planned for self bailout, including bottom mix, intermediate deco mix and O2.

The only deep dives I have planned on the CCR post training we each carried bottom mix, with a reliance on the team for intermediate deco mix and O2.  The intermediate mix was 50%, mostly due to convenience.  I did some alternative planning using only 2 mixes, a bottom mix and something like 80% with no O2, to see what volumes would be required.  I was planning for 180 ft for 25 min, with a potential swim at 150-170 for about 10 min to get back to the ascent line given a problem.  Using 80% meant we needed more bottom gas than we each wanted to carry.  This was in a situation with potentially lots of current, so ascending on the mooring line was a key planning point, if at all possible.

I'm sure as I get more experience, I will try several different mix combinations.  Planning tools that can help with bailout gas volumes given different scenarios are great to try to work out what mixes will work well give your dive conditions and limitations.

Jeff

[ScubaAl]

[i][i]The only deep dives I have planned on the CCR post training we each carried bottom mix, with a reliance on the team for intermediate deco mix and O2.  The intermediate mix was 50%, mostly due to convenience.  I did some alternative planning using only 2 mixes, a bottom mix and something like 80% with no O2, to see what volumes would be required.  I was planning for 180 ft for 25 min, with a potential swim at 150-170 for about 10 min to get back to the ascent line given a problem.  Using 80% meant we needed more bottom gas than we each wanted to carry.  This was in a situation with potentially lots of current, so ascending on the mooring line was a key planning point, if at all possible.[/i][/i]

Thanks for the reply Jeff,
IDeco is my friend. Have ran several different scenarios just to see what is needed. I can see that if 80-100% is carried much more bottom gas would be needed. That's when carrying two BO bottles. IE bottom mix and 80%-100% O2.
When you say you were reliant on the team for intermediate/O2 are you saying one diver carried the O2 the other diver the intermediate gas?
I want to be able to carry all my own BO gas and not go with the team concept. Just to be on the safe side. In doing so figure out if I can carry one bottle of bottom mix, along with one of intermediate bottle. The O2 would then be clipped off to the line or hung from the boat.
Of course if not carrying the O2 I won't be able to plug it into my counterlung and inject O2 manually.
All this gas(hopefully) would never be used and is there just for bailout situations.

Later, Al

[john j]

Here's my opinion:

I agree with a lot of what Jeff said.  I will elaborate more.

The primary bailout mix must be something that is breathable at the MOD of the dive, so that if the CCR fails, you can go directly to this mix for bailout. On wreck dives, where one would start an ascent immediately when anyone is on bailout, some people are using a 1.6 PO2 for this mix, with the idea that the ascent starts immediately, so the PO2 drops fairly quickly.  In cave diving, a 1.4 PO2 is typically used because you might need to stay deep for quite a while, even when you are on bailout.  You have to carry enough of this mix (either as a team or individually) to get you to the next bailout decompression gas.  If the next gas is 50% oxygen and you are on a very deep dive, you have an awfully long way to go before you can start breathing that next gas, so the amount of primary bailout gas you need to get there, becomes quite large.  Also, as you are ascending, the PO2 in that primary bailout will drop to the point where it isn’t doing much for your decompression and you will have to stay even longer before you can get to your next mix.  My point is that the more oxygen you put in the second bailout gas, the more primary bailout gas you will need because you have to stay on it longer before you get shallow enough to start using the next gas.  There isn’t much choice in the primary bailout mix.  It’s based only on the MOD of the dive and whether you want to use a 1.6, 1.5, 1.4, etc. PO2 at the MOD of that gas.  With that gas chosen, run some dive plans using various secondary mixes and look at how much primary gas you will need if you use 50%, 80%, 100%, etc. for your second bailout gas.  Consider cylinder sizes when you do these calculations.  Do you want to carry two 40 ft3 cylinders, two 80's, a 40 and an 80, or whatever.  I did a lot of these kinds of calculations and found that for dives in the 250-300 foot range, around a 36% oxygen mix in the second bailout gas gives me the best balance between deco time and amount of gas that I need to carry.  The 36% mix allows you to start using it deep enough so that you don’t need such a large amount of primary bailout gas to get to it and it also allows you decompress faster than if you used the primary bailout mix all the way up to 70 feet or shallower like you would have to on richer oxygen mixes in the secondary gas.  On dives approaching 400 feet, we will carry or stage another bailout deco mix with a higher FO2 than the primary bailout.  This is for two reasons.  First, you need a huge amount of gas for bailout due to the depth and second, there is a very large difference in ambient pressure between the MOD and the place where the 36% oxygen mix is loacated, so an intermediate mix is needed to keep decompression times acceptable.

There is another issue too, which is counterdiffusion of nitrogen.  Years ago with open circuit, we all used to switch from our bottom mix trimix to EAN36 on the way up.  When we did that switch, the partial pressure of nitrogen (PPN2) spiked because there was a lot more nitrogen in the EAN36 than we had in our bottom mix.  In my early CCR days, I used to flush the loop with air on the way up to get the helium out of the breathing loop.  My dive computers and planning software all told me (and most still do) that this is good because it drastically reduced the deco time. It is mathematical hogwash though. Looks great on paper but it's very bad for your body. It is now recognized that increasing the PPN2 during the ascent is a bad idea and it may cause bends.  I’m told that the reason we all got away with doing this is because the bottom times were relatively short (less than 25 minutes or so).  Now with our CCR’s we sometimes do double that or even longer on very deep dives.  So now, many people are adding some helium to all of their bailout mixes and adjusting the nitrogen fraction to keep the PPN2 constant at the point of any gas switch on the ascent.  So on every gas switch you make, be it from the CCR to the primary bailout or from the primary bailout to the secondary bailout, etc. the PPN2 should not be allowed to increase.  One obvious solution is to use heliox for everything (no nitrogen at all in any of the gases you have).  I have been doing just that for a few years now.  However, deco on heliox needs to be very controlled and a slower than usual ascent rate, with deep stops that most computers won’t give you.  Buoyancy problems, uncontrolled ascents of just a few feet with heliox, can apparently cause subclinical tissue damage that adds up to real health problems over time, or worse, it can cause rapid onset of DCS, even right there in the water.  I have had 2 instances of pain immediatly after surfacing that may have been caused by diving heliox.  I don't know for sure.  In both cases, I felt fine after a few minutes on surface oxygen and I thus did not elect to have treatment.   This is still a learning process for me and discussions I recently had with an instructor who has connections to COMEX (European deep commerical diving research folks), have lead me to believe that putting a little nitrogen back in my mixes would be a good idea because it increases the tolerance for ascent mistakes.  The fraction of nitrogen (FN2) should be kept nearly constant in the bailout mixes all the way up so that the PPN2 does not increase during a gas switch with bailout deco.  The oxygen stop of course, does not need nitrogen.  Seems to me that as time goes by, all of this will start to get more mainstream and will begin to be incorporated more into the standard training courses.  I think there will be a push to use mixes that are more tailored to each dive rather than the generic EAN36, EAN50 mixes that have become the norm, because those gases aren't really the best for long deep CCR dives on helium.  Don't get too stuck on having to use the "standard" mixes.  Use the most efficient mixes for the dives you have planned.

On the question of carrying oxygen or not…if I am pretty certain that I can get to the oxygen when I need it, I will rely on surface supplied O2 from the boat (wreck diving example).  If I have some doubt about getting to it, I will drop it off somewhere on the way down or on the wreck somewhere.  If I am really uncertain about getting to it, then I carry it for the whole dive.

[ScubaAl]

Very well thought out and informative reply. Thanks for taking the time John. Really helps a lot.

Thanks, Al

[aknelsonone]

John,
Nice write up. I enjoy reding the explainations. I am wondering what you would recommend for the ranges up to 200 ft which I consider a very realistic range for a lot of dives around our lakes.
How do you configure your kit for such dives?

Andreas