Bailout

[Inspirationdiver]

Planning bailout for trimix rebreather dives is not a straightforward concept. There are some very different views on what is appropriate. Anybody want to share their opinions on what should be carried?
Assuming that you are going to carry 2 stage bottles, it is accepted that one should be a trimix bailout gas for at depth, and the other should be nitrox, deco gas, or oxygen.
The problem is that the ideal content of your second stage bottle is not universal to all diving depths. Adjusting the FO2 of your second stage bottle can greatly alter the amount of gas you consume out of each stage bottle.

[john j]

This is a good topic. My opinion follows and is in contrast I guess to what is currently being taught, but that's OK with me. I have to do what I feel is right and maybe someone can convince me that there is a better way.

If there is a problem at the MOD of the dive you have to have bottom mix to switch to. I think most would agree with that. Now, many of the very deep dives I do are at Wazee and if I have a problem, I would rather swim back along the line for my ascent rather than poping a bag and going straight up because we are way out there when we do these deep dives. This is kind of like a wreck penetration dive or a cave dive in that respect-you have to go back the same way you went out. Becasue of this, I need a generous supply of bottom mix, and so that is what I use in one stage bottle. For onboard diluent, I use the same mix too. I don't see any true advantage to having a different mix onboard than offboard. Maybe for really really deep dives you might want a travel mix onboard or something but I'm doing 320ft dives pretty often and don't need a travel mix for those. I have had thoughts of running the unit from the offboard cylinder becasue then if I have to donate the offboard cylinder, I still have a full bottle onboard, but for now, I still run the unit from the onboard cylinder. In a bailout situation, I would go to offboard OC becasue there is no way I'm going to a cheap BC hose regulator on a 25 cubic foot tank in a serious situation like that. That's another reason to keep the same mix in both cylinders. One more reason is that it makes fills easier, especially if you are at a remote site you only have to bring one kind of trimix with you and you can fill all your trimx cylinders.

As for the other cylinder, some use EAN 50, EAN 80, EAN 36 or 100%O2 and probably other gases too. I use air. I did calculations with Z-planner which showed that from a deco standpoint, you don't gain much by using richer gases than air becasue you can't switch over to them until you get fairly shallow. I still switch to air diluent at 150ft on the way up. I have been hearing that this is becoming a point of argument. Something about too high of pressure gradient for He offgassing forming microbubbles. I admit, I don't know much about that. OC folks do this kind of stuff all the time for example swithching to EAN 36 on the way up. Switching to air at 150 ft cuts 5-15 minutes off my deco time typically and in cold water, I'll take whatever I can get.

For some of our dives, we stash a cylinder of O2 somewhere too in case there was a total bailout situation.

So that's what myself and my main dive buddy do. If someone can convince me that we are doing something dangerous or stupid here I would like to hear about it. I can't find anything wrong with it in any text I have seen so far and haven't heard yet a convincing argument against this policy. The deco programs don't scoff at it and it does seem to work. What works works.

[Freedive WI]

John,

What about throwing some HE into that air bottle at 150'? Many divers I know are starting to using helium in their deeper stage bottles( 21/35).

I like what you said about a lot of the other stuff- having the same gasses in both bottles to eliminate confusion in a bailout situation.

Jon

[john j]

Jon;

An interesting suggestion. I guess in contrast to that idea i'd say that air is cheaper and easier to get fills and wouldn't the off gassing be faster on a mix like air becasue there is no He in it?

[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.