Question for the masters - TriMix / Manual CCR

[Merida Cave Diver]

Hey all, I’ll start with the disclaimer. I’m asking to learn, I will not be taking any replies as advice, nor will I be doing any of the things I’m talking about as it is outside of my current skill set and expertise. Ok…

So I’m a new CCR diver and I’m just building up my hours with uneventful practice / skill dives. My long term goal though is sustained dives of say up to 70m. Mod 2 CCR is in my future, tri mix is in my future, and I’m working my way through articles and exercises but I was wondering if there’s something you might recommend that I could read and or do, to get a better solid understanding of it all before I take the courses? I feel like I know a little but it’s also coming in piecemeal. For example, some of the things I’m wondering are:

So you plan an emergency bailout using a 1.6 (I get the why) and I see divers planning their dives at a 1.4, but for my CCR which is manual the recommendation is 1.2. Is that because it’s manual vs electronic?

When you plan your dive, and you’re working out your trimix, is there a reason why you would pick an END of say 30 vs 40 or 20?

This is why I’m feeling like the data I have is bits and pieces.

Thoughts, advice?

 

[Heat Miser]

You start by saying "I will not be taking any replies as advice" and you finish by saying "Thoughts, advice?"

The setpoint you run in CCR, in your case @ 1.2 manually, is a managed constant and independent of the depth in the water column. When you bail-out to O/C the Max pp02 (whether 1.4 or 1.6) is variable and directly proportional to your depth. Whether you choose 1.4 or 1.6 for bail-out mixed bottom gas is largely an individual risk assement about oxygen toxcity. The people that do use 1.6 will argue that if something goes wrong and they have to bail-out they will be moving up quickly, so bailing out to a 1.6 pp02 mix at 60 meters (197 feet) becomes 1.4 pp02 at 51 meters (167 feet). The decision about a 1.2 setpoint or 1.3 setpoint on your CCR is also a risk assessment about O2 Toxicity/CNS

With calculating the best Tx mix, not only do you have to work out the best Helium mix looking at END, for which 25 meters is a good starting point (due to Nitrogen narcosis), but also you have to consider Gas Density as well, so you may end up with a mix that has an END at 20 meters because you need more helium to reduce gas density.

Much of this constant tweeking of Best Mix can be overcome with the great invention of adopting GUE standard gases. However you might end up having to do slightly more deco on them than if you calculated your best mix for the dive.

The CCR Helitrox Course rated to 45 meters, is a great little stepping stone on the way to building up to CCR Mixed Gas and may be of interest to you.

 

[Merida Cave Diver]

You start by saying "I will not be taking any replies as advice" and you finish by saying "Thoughts, advice?"

This is a most excellent point... I am an idiot lol

I love your reply, thank you for the time you took to write it. I'm running a KISS sidewinder so "I think" my bailout would also be my diluent. At least it is right now for the dives I'm doing but it's just normal air.

With the reading I've been doing, I've seen example of both shallow and deep bailout mixes. Gas density though I think was the part I was missing.

Thanks for the tip.

 

[J-Vo]

SW diver here. As the SW uses "dilout" we can't have the the luxury of different bailout and diluent ppo2, so you may get mixed messages without specifying the configuration. For cave I generally run dilout max ppo2 @1.4, with the understanding it can make pulling down an O2 spike difficult. This way the the deco penalty for bailout is minimized. I generally run standard gasses, so 32% up to 110'(30 meters).

 

[nickbutcher]

I don’t plan my bailout at anything like a PO2 of 1.6
I would look at something closer to 1.0 and be more concerned about the gas density - preferably less than 5g/l

 

[211ratsbud]

From all that I can gather there might be some other priorities than just bailout deco times and profiles that go into choosing the proper DIL OUT.

It's been mentioned before but just connecting the dots;

Gas density and low WOB are important. This is a critical first step IMHO because the easiest thing to do is just start breathing hard(by default gas density or exertion or worse both) and it's widely known this is what facilitates CO2 problems, retention, or dwell issues. CO2 effectively lowers the threshold for CNS toxicity by some general margin. (reference roaring river report, and other examples) A case in point where you can see that the ppo2 might be over the 1.6 limit and not necessarily by a large amount that caused a seizure in a relative short amount of time.

I'm sure in several multi hour dives, somewhere between a recreational boat or shore dive and a dive with a habitat the stakes for choosing bailout ppo2 are great.

If you have a bailout that is separate from your 2/3l dil you would want to be able to dil flush with your off board worst case scenario and still be safe. Planning a with a 1.4 in the bottom phase offers zero tolerance for excursion safety margin. In my opinion.

I'm planning a 1.0 at most.

 

[berndo]

Thoughts, advice?

I have advice for you: If your instructor thinks it's a good idea to have deep BO in one of the sidemount tanks and shallow BO in the other, run for the hills.

 

[lostsheep]

I have advice for you: If your instructor thinks it's a good idea to have deep BO in one of the sidemount tanks and shallow BO in the other, run for the hills.

You mean like air on one side and Tx on the other ? What could possibly go wrong

 

[grantctobin]

You mean like air on one side and Tx on the other ? What could possibly go wrong

Let's at least invite @DiveTalkGus to hear about it.

 

[lostsheep]

Let's at least invite @DiveTalkGus to hear about it.

I'm pretty sure he's not coming back...see what I did there?