Bailout

[Inspirationdiver]

The problem I see with using the same trimixture for both your onboard and bailout gases is the optimum PO2 desired at depth. When planning gases for the rebreather, you want the PO2 of the mix to be around 1.0 or 1.1 at the maximum depth. Normally the rebreather is run at 1.3 PO2 at depth, but if you need to do diluent flushes, the diluent must be able to drive the PO2 down to verify the oxygen cells integrity. This is why 1.0 or 1.1 works well. In your offboard cylinder however, I think it would be inefficient to have such a low PO2. In the event that you had to bailout, you would only be at that depth for a very short time. You want to get shallower and start doing your stops. Even if your first stop is at 200 ft, 10/70 would not be a very efficient gas to be doing your deco on(only a .7 PO2 @ 200 ft). I usually try to plan my trimix bailout so that the PO2 at the max depth of the dive is around 1.5-1.6. This is still a safe gas to bailout to at the max depth and will be much more efficient for doing your decompression. I see your points about making everything the same for simplicity but sometimes the easiest way to do things is not always the safest.

Planning your gas for the second bailout cylinder is where things get a little more complicated. I think you should plan this gas to balance the consumption out of both cylinders equally. If you were going to use 100% O2, you would have to do all your stops on the trimix and then switch to 100% at 20 ft. You would use a whole lot of trimix and not very much O2. How about 80%, this would increase the consumption of this bottle and use a little less trimix. 70%, 60%, 50% and so on. The lower the FO2 in the second bottle, the more you are going to use out of that tank and the less trimix you will use. The shallower the dive you are going to do, the higher the FO2 can be in the second bottle. I don't have my dive planning software here right now but I seem to remember that for dives in the 300 ft range, 50% balanced the bailout requirements much better that 21% did.
Last week for practice and to compare how calculations match up to real life situations, I bailed out to OC at 200 ft and did my entire deco on bailout. I was carrying 80% and trimix. I was suprised to see how much of the trimix I went through and how little of the 80% was consumed.
I'm not trying to imply that this is the only way to plan bailout. I started this thread because I want to get some other opinions. Keep them up.