What bail-out for normoxic trimix?

Sutty

Hi all,
I'm currently only MOD1 certified on my Kiss but am hoping to do my MOD2/3 this year (earlyish hopefully). I am not O/C trimix trained either.
So I know that the person to ask is my instructor in due course, and I will do, but a few questions in the interim (exercising my brain).
If on a normoxic trimix dil and not too deep e.g. <50m what bail-out gas would be acceptable?
1. Air 'cos its cheap & easy - but means a change of inert gas at depth if you bail out, but then you are on-gassing nitrogen and off-gassing helium, nitrogen could ?super-saturate tissues but shouldn't the helium off-gas quicker?
2. Same trimix as your dil?
3. trimix with same N2:He ratio as dil but with O2 adjusted to equal CCR set-point at deepest dive depth?
4. Anything else??

I'd be interested to hear what people use and the rational behind it.

Neil

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jhaaja

I would not use air. Trimix with at least the same amount of helium and O2 so that the ppO2 at amx depth would be around 1,2.

You want the bail-out to be lower O2 to get our ppO2 lower if you feel CNS problems. And when you are bailing you are on rouble allready so you want a lot of helium to keep your head straight.

I have not done MOD2 yet but have dive OC trimix.

JH
Finland

Digger

Personally I use 21/30 and 50% for most dives down to about 60m. Beyond that I start to think more carefully about bailout plans. AL80 of each. Works fine for me. I used to use richer bailout, but increasingly I find I'm in and out of things and might need a bit more time on the bottom, so a deeper gas switch helps.

I wouldn't use air. Bailing on to ai at 50m after you've been on mix can be a really shitty old time. You don't notice it as it builds on descent, but it can be sudden, and the last thing you want after you bail is a big wadge of narcosis to deal with!

Just my 2c.

Digs.

Mike

Air and 60% for me, 40cuft of each, down to 60m. Plus I have the onboard via an air2, although I don't assume I'm able to use that in doing the bailout calcs. However in a real bailout situation, I'd go to that first as it would have at least 300l of an appropriate trimix, which I could grab a couple of sanity breaths from before swapping to air.

I find that it's actually the rich mix that you need more volume of, hence the use of 60% rather than 50%

I also use the air for suit inflation (which is part of why air, rather than 20/30 etc.)

Using these gasses, my bailout deco profile is pretty close to my CCR deco profile (ie 1.3 setpoint, dil with somewhere around 50% He). Close enough to get me out of the water anyway if it all turned completely brown.

In the normal course of events, while I carry enough to get me out of the water, I'd also expect that buddies / boat are a source of additional gas. Therefore, when things have settled down and we are doing deco, I'd be looking for donations of O2 on the shallow stop, and would pad the deco to make me feel better about life. Ditto, at least one of the guys I dive with carries air and O2 for these type dives, so if he had to bailout and I was still on the loop, I'd be handing off my 60% at ~20m to give him a better deco mix.

Carry the minimum you need, and share it with your friends.

Mike

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Scuba_Vixen

Some good questions here, and no doubt there will be a range of answers from different philosophies.

One thing that will make a difference in strategies, is whether you're in an overhead at depth, or if you have no impediment to coming up 10 or 20m at the point where you feel the need to bailout.

The one thing I doubt anyone will argue with, is that as mentioned, bailing to air from a He mix at depth is somewhere between "less than optimal" and "plain old bad". I'd rule out idea number one.

A lot of guys using mix just use 10/50 for (almost) everything. At the depths you mention, bailing to that dil isn't much of an idea, although a lot deeper it would be, so they may have a different mindset about bailout strategy than might be reasonably practicle for 50m or less on normoxic dil.

I use 18/45 since the first time in the water, and I'm working my way into the 50 to 70m range after a good while in the 30 to 40 range. I set my kit up with a bailout second stage on a neck bungee connected to my dil. I carry 30/30 and 50/25 bailout, like I would carry deco bottles. Since this is in open water, if I have to bail at 50m to my 3l dil bottle, ..... I'm not worried that it won't be enough to get me up to the 30m to 40m range where I can switch to the 30/30. If I were diving in a wreck at 50m, I'd want a significant supply of bailout like 21/35 or so, since a 3l dill tin won't get you all that far staying at that depth.

In general, I think it's practicle when using normoxic mix at depths up to 50 or 60m, * in open water *, to bail to dil, and carry 2 OC bailout bottles .... with proprtionately less He and more O2 than your dil.

Maybe it's just me, but 45% He at 50m is about like 20m on air. If things went pear shaped enough to bailout, I'd want my brain as functional as possible. ..... Bailing to air just seems to be courting risk without reason to.

Darlene

lizardland

If I have air dil then I have air bail out, if I have mix dil then I have mix bail out. In the grand scheme of things you will go a long time before you need to use it so cost is not an issue.

One thing I'd add, after a flood at 60m on my third dive... stay on the loop as far as you can. Staying on OC I felt made the problem worse. Get out of the mindset of bailing to OC if there are other options. If anything, your bailout becomes a sanity check while you try to solve the problem so you do not want any increase in narcosis.

After you've been on CC, going to OC is a horrible experience. It's cold, it's mechanical and the work of breathing seems sooooo much higher. I personally wouldn't want to feel like I was sucking treacle through my reg at depth... go for some helium.

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