CCR Trimix Diving in a NutShell (for OC Trimix qualified divers)

[JPTaylor]

OK. CCR Trimix isn't that different to OC Trimix, I think it's easier (as I commented on "Trimix Simplified" thread).

When I did my first CCR Trimix cert that theory paper was from the OC course & I had to plan an OC Trimix dive!!

If you have an OC Trimix background this should cover it!

My thoughts only:

Apply basic skills/techniques (i.e. O2 rebreather) that was taught on your MOD 1 (MOD1 & cross-over are worth doing)
Pack a fresh scrubber for each dive
To calculate your diluent, it's like calculating an OC bottom gas but use a partial pressure O2 of 1.1
Consider using HeliAir (16/24, 15/29, 10/50, 8/62, 6/72) calculate their max. depth as above & then just pick one for dive.
CNS/OTU tracking is easier than OC as your partial pressure of O2 is constant
If you bailout your dive has just become an OC Trimix dive, plan as such with residual N2/He loading from CCR phase of dive
A SP of 1.3 is good enough, a lower SP on bottom may reduce narcotic depth (especially if using HeliAir) & EADD which may help with CO2 issues
Don't deco on a SP greater than 1.4 (and let eCCR do the work)
70m is probably the maximum depth where you can carry all your own bailout
For dives deeper than 70m its ("back to the shot/lazy shot dives") then team bailout, drop tanks, support diver, tanks on deco station stuff...
SCR is useful to get back to the shot, then it's OC for ascent
Use you buddies (if you have one) deep bailout to get back to shot then your own to ascend, rather than other way around (easier)
Lots of free & paid for deco software available to compute CCR/OC bailout deco plans, take your pick
Other stuff like using slates/bottom timers, wreck navigation, use of deco stations etc as per OC diving....


You also need to think about what you'll use each cylinder (well not O2) for?

On-board diluent: Diluent, SCR, OC via BOV? I've run unit from my off-board stage & used in-board for just suit/wing inflation.
Off-board stage: OC bailout, connected to BOV, SCR via MAV, use a diluent via MAV

Depending on what you intend to use each cylinder for will influence your gas choice for the cylinder.


Please amend, criticise as you wish!! I've most probably forgotten/over looked something.

FYI - I don't use a BOV, but if I did I'd have it connected to inboard & use as a transition reg to get to OC bailout as taught on my Sentinel crossover.

PS. I also think Bozniac's book on rebreathers is a waste of paper as well ;)

NOTE: THIS IS A PERSONAL VIEW AND DOES NOT REPRESENT ANY TRAINING STANDARDS FROM ANY TRAINING AGENCY.

[nickb]

This is a good post and I like it but I'm conscious that people could read it, and posts elsewhere, that make out that CCR trimix is trivial, especially for those already trained to normoxic OC level. I've alluded to the opposite and will expand on it when I get the opportunity.

After doing open-circuit dives in the 60-70m range, strapping-on a CCR can make you feel invincible and if things pan-out right, as they will 999 times out of 1000, you are. But unless you're prepared to go Alpinist, you are still limited to the available OC gas that's on-hand and if shit happens that may well not be anywhere near enough to what you'd expect to use for a vanilla OC ascent. Just carrying that amount of gas around as luggage is a ballache and has consequences.

It's not a free ride. I didn't move to CCR as a gas-extender as a lot of people do; I did it primarily for the logistic advantages. I was quite happy doing 30+ minutes at >60m and always felt a lot more comfortable about getting back to the boat with the available gas than I do now with the bailout I'm carrying, even having done one for real and had plenty left over.

You mentioned CO2 as a throwaway comment - it needs to be expanded-on until people get tired hearing about it. It's my number 1 priority - numbers 2, 3 & 4 too!

[nickb]

Oh, and heliair is the diving equivalent of walking around with bog paper trailing from your shoe. Don't do this people, you'll look like a fucking idiot ;)

[Major Clanger]

John if you were to standardize on gas choices for dil and bailout for ranges down to 100m, what would they be as this is where I see some divergence from OC choice?

[JPTaylor]

John if you were to standardize on gas choices for dil and bailout for ranges down to 100m, what would they be as this is where I see some divergence from OC choice?

I'm not really a "standardize" type person; I'd improvise, adjust & top-off whatever I had to hand!!

:think:

0 - 40m : Air diluent & 32% (dived this recently on a 36m wreck & in the pit/34m at Stoney )

40m+ - 55m : 15/29 diluent with 15/50 & 50% [1]

40m+ - 75m : 10/50 diluent with 15/50 & 50%

75m+ - 100m : 6/72 (possible 8/62 if at shallower end of range) diluent with 10/65 or 12/60 deep bailout., then.....


12/60 better OC bailout & can use SCR, but not that good to run unit from off-board (not good for flushing)
10/65 can cover bailout, SCR & able to use as a diluent via MAV [2]

Now it depends on how many cylinders your prepared to carry & dive set-up. A trimix travel gas to avoid HPNS then a deco nitrox mix (32% - 50%) depending on how brave you are!!

I've carried just 10/65 & 20/35 & then relied on drop tanks for deco gas, but understand not everyone's cup of tea....

I've run a Sentinel off 10/65 for a couple of dives @100m, then Air topped it to use as bailout on shallower stuff (75m - 80m). But that was mainly for gas logistics reasons & dive schedule planned (i.e. have to be flexible)

[1] possibly air top a used 10/50 cylinder, use bailouts I've got as don't want to dump gas!

[2] Understand a lower ppo2 for bottom bailout is better is bailing due to a CO2 hit??


This is what I do/done, please figure your own gases out from basic Trimix OC/CCR planning principles & your own level of risk!

.

[DiverMike]

I'm not really a "standardize" type person; I'd improvise, adjust & top-off whatever I had to hand!!


My standard gasses for this season are air to 30ish m

18/35 to 50 ish m

14/55 to 60 ish m and I don't plan on going deeper than that this year.

Gases chosen for a low pO2 for the bottom phase, reasonable END and also equiv air density of less than (IIRC) about 35m to the gas is an easy breath.

Bail out I have 27%, 18/45 and 60% which I can mix and match as the run time needs.

I've got a spreadsheet with max run times for self bail out and buddy bail out so I can stick within those and run the dive off my two computers.

[Peaslake]

Oh, and heliair is the diving equivalent of walking around with bog paper trailing from your shoe. Don't do this people, you'll look like a fucking idiot ;)


care to elaborate on that? or is it just that it's hypoxic?

[notdeadyet]

I don't have a five grand dog so I can do the barking hence I dive heliair.

Anyhow, I don't want to divert a very good thread with a tangent on personal preference (but couldn't resist :D) so: another good resource ... Rich Pyle's Learner's Guide to Rebreather Diving is (I think) still one of the best things ever written on rebreather diving and should be (but isn't) essential reading.

[dwhitlow]

care to elaborate on that? or is it just that it's hypoxic?
Heliair Poor Mans Mix (http://www.advanceddivermagazine.com/ezinefreearticles/HeliairPoorMansMix.pdf)

[Major Clanger]

Heliair Poor Mans Mix (http://www.advanceddivermagazine.com/ezinefreearticles/HeliairPoorMansMix.pdf)

Does it mention you don't need an o2 clean cylinder? Very important to us tight wads :)

[dwhitlow]

Does it mention you don't need an o2 clean cylinder? Very important to us tight wads :)

Keeping a single twinset in O2 clean isn't a big cost..

I have a twinset of 15/55 as diluent for all depths to around 60m and select bailouts depending on the dive (AL40 of airish, AL7 of 21/35 with AL7 50%, AL80 of 18/45 with AL80 of 60% and O2 drop bottle). If this lot don't suffice then I'll not be doing the dive.

[Major Clanger]

I have a twinset of 15/55 as diluent for all depths to around 60m and select bailouts depending on the dive (AL40 of airish, AL7 of 21/35 with AL7 50%, AL80 of 18/45 with AL80 of 60% and O2 drop bottle). If this lot don't suffice then I'll not be doing the dive.

I have a J of 15/55 for dil, which I'll use as is or air top the 3 litre to 16/40 to give me a bit more. Bails are 10ltr with 10/50, ali 80 with 21/30, ali 40 for dives to 35m, ali 80 with 50% and drop 12 ltr with 80% in it.

[Peaslake]

Heliair Poor Mans Mix (http://www.advanceddivermagazine.com/ezinefreearticles/HeliairPoorMansMix.pdf)

ah ta. guess its bog roll stuffed in the trainers for me then........