Bailout planning

[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.