Trimix and CCR

[Pierre Farrugia]

Hello,

Is it difficult for an inspiration diver after sufficient training with unit to pass to trimix diving (for an alreadt OC Adv. trimix qualified) ? Do all of you who dive it on trimix have done a different course apart first CCR course? Is there any trimix standard gas that a CCR diver normally uses?

Regards

Pierre

[beanie]

blimey you've not even got it yet

i'm kinda hoping it's just a case of changing gases thinking more about bailout and deco

i'm just starting to think about how i'm gonna go about the move back onto trimix apart from gentally so i'm alive to enjoy it

[Pierre Farrugia]

Hello it's true, but as my goal are deep dives, I like to think about what is involved. I year ago I was asking about a CCR, maybe I have a 1 year delay

Beanie, have you ever used it with trimix ?

Last time I was thinking that mixing a 3liter bottle must be difficult due to a so small cylinder! I think that helair is more easy to do.

Also about bail out, let's say that I am used to 60m 30min dives. This is normally done on twin 12's and 10l of 60%. Also I normally take a 10l 232bar with 60%, as 100bar are reserve. With a CCR you only need the 100bar if you had to deco on OC, so a 5liter should be enough. What you think ?

NB. Please note that I am not CCR trained so any comment may be not the best!

Regards

Pierre Farrugia

[Sandman]

Most CCR mix divers I know use 10/50 which covers most depths you would want to go to and it is fairly easy to mix.

As for OC bail-out I would assume the worst case scenario, having to get off the loop just as you are about to leave the bottom and going OC all the way to the surface including all stops. You would need a lot more gas than in a 5L cylinder.

Cheers

Simon

[Pierre Farrugia]

Hello,

You are right, but I was talking about deco only. I don't think one can take an effective deco mix which one can go to from max depth and do deco on. Then to do your deep stops you need an other cylinder but this can be small as you only need to reach 15m (for 60%). So I think 2 stages 5l each are enough. I think to be safe one needs 2 7l stages to bail out from 60m after a 30min bottom time!

[Rob Evans]

10/52 is about the fave gas used by YBOD mix divers...Half He, half air.
There's not much difference re: the courses - -on my CCR mix course we were consulting OC course notes for the usual physiological differences between mix & air. The only major differences were calculating bailout instead of deco gases (fairly similar if you think about it) and ENDs.
If you've done Adv Trimix on OC the crossover is very straightforward.

As regards bailout, on a max72m, 38min dive the other day, bud and I had respectively; 1*7l 10/52, 1*7l 50% ; 1*7l 17/30, 1*7l 100%. We work on the practise that only one of the YBODs may fail catastrophically during the dive - the tins we have between us will get that one to the surface with no lost stops or requirement for drop bottle. They're all LP whip-equipped, so any one could be plugged into the loop for SCR if so required.

[Darren Soothill]

I am curious about the reasons for taking the 10/52 with you?

Could you just use the 17/30 as a dil on the unit and not take the 10/52 with you?

Personally I would have taken 2x7l 17/30's between a buddy pair so we have more bottom gas with a sensible PP02 for breathing on the bottom and ascending upto 21m.

[Rob Evans]

I treat the 10/52 as a gas-for-all-seasons - if the planned dive gets weathered off, I know for a fact that the mix in the tin will be fine for whatever else I've got planned. At those sort of depths I like to keep the END to an absolute minimum - if it all goes to crap at depth, I don't want to be bailing to a mix that'll suddenly increase my END... Sooo.... Bail onto the 10/52, buddy sends bag up, start ascent, get onto the 17/30 at around 50m, onto the 50% at 20m, 02 at 6m. Totally self-contained, no drop bottles, keeps the PHe gradient (and any worries about counterdiffusion)sensible and an out-of-the-water time within 15min of the original.

I'm assuming you're factoring in deco mixes as part of your bailout, cos 2*7l 17/30 won't get you out of the water

[Darren Soothill]

It was the lack of O2 in the 10/52 mix I was wondering more about as that reduces the options for using it as a semi closed gas.

You are also using quite a low ppO2 on the 17/30 as you are only going onto it at 50m. I had assumed as the pp02 of the 17/30 was 1.36 at 70m that you would be using that gas alot deeper than you are.

Are there any particular reasons for using such low ppO2 values? Do you run with a lower than 1.3 setpoint for the bottom segment of the dive?

I hadnt really looked at the ppN side of things it was just the low PP02's that had caught my eye.

Yes I was expecting there to be 50% and 100% available between the pair of divers for the shallower water deco.

[Rob Evans]

I take your point re:the low PO2s, specially as regards SCR. There is an obvious penalty in terms of deco but our bailout tables are based around the 'Let's get the f*ck out of here & keep the deep stops to an absolute minimum' school of thought. Once we're at 21m and above, we can pad things out at our own discretion.
Most important to me is having a good clear head if there's a major problem at depth.

OK you got me, I admit it. I blew 10/52 into one of the 7s a couple of years ago - after a few whip-fills for the in-board dil & subsequent air-tops it's reduced to roughly 17/30. And yes, it's about time I blew it away and put a decent trimix in.

Tend to run at 1.3 for the whole of the dive - apart from the very last phase at 6m where I drop to the low set-point, flush through and run it manually. CNS% on the runtimes we're doing has thus far not been enough of an issue to consider dropping the PO2 for the deep phase.