Trim with 3 deco cylinders?

[tbone1004]

I'm also curious about why switching directly to the other mixes. That is a righteous PITA especially with multiple bottles. Far easier to switch to backgas, stow the first bottle completely, then get the next one out.

 

[Imla]

Have to agree with others here. Gas choices are kinda odd.
Personally I'd sling two S80s on my left, and the O2 on a leash.
If the bottles are full, and you're using them as BO, not OC, you will notice a slight difference. The O2 will be heavy and ride below your knees (Depending on leash length)
Two bottles on a leash if there is room on the left hand side is just bothersome.
If you are diving OC, the bottle rotation will cause lighter (empty) bottles (especially S80s) to end up on your rear. If significantly top heavy this might cause stress. However, at this level you should be able to handle shifts in trim by positioning arms/legs to counter act the shift.
Switching from one stage to another without going to backgas and stowing in between is asking for a flustercluck. Keep things tidy. When done with a bottom stage/deco bottle, go to back gas, clean up. Then do a proper gas switch.

 

[tbone1004]

oh on the point that @Imla brought up. A half full AL80 is a lot easier to deal with an a full AL40...

 

[CptTightPants21]

A little bit of a pile on going on. Anyone who has been on this board for a reasonable amount of time knows Stuart likes his gas mixes exotic

--Steel plate matters. With that many tanks, go aluminum or carbon fiber
--Yes helium content will affect things
--Yes, the left side and leash combo all help a great deal.
--From what I recall you don't have any cave experience, diving that many tanks takes some time in the water to get used to.

--What is your bottom mix? The travel gas is generally one of the non-100% deco gases (in this case 50%). It is only needed to get you deep enough not to be hypoxic.
-Without a specified dive plan--the three stages would be al40 pure o2, al80-50%, al80-bottom stage. Is 21/30 strictly travel gas?

--What class is this? Any reason you are not doing it on CCR?

 

[Compressor]

@stuartv: Trimix Class I assume.
Reading the thread with interest. Very nice contributions so far.
The stuff is over my skill ability; I admire those of you who are so advanced.

 

[NothingClever]

Strongly disagree with how PADI and TDI teach it in this manner. There shouldn't ever be multiple non-stowed hoses on one person at one time.

Edit: how the TDI and Padi standards are written for this procedure.

My question is sincere and not intended to be antagonistic.

Where do you see the “no backgas between deco gases” stipulation in TDI’s Decompression Procedures and/or the Trimix course? I reviewed the course standards and didn’t see any wording to that effect.

I just finished my Trimix course through TDI and my instructor didn’t train me to avoid my backgas after getting on my deco schedule.

I can see this being a much hotter topic in the Adv Trimix course where a low PPO2 in one’s backgas is inadequate for base metabolic needs at deco depths.

 

[NothingClever]

- double steel 120s

Good gravy....what kind of course profile do you intend to dive with 6,796 liters of backgas (which is slightly more than 3 x AL80s with 6,600 liters).

Sounds like you’re intending to rack up enough deco obligation that you’ll get to practice air breaks.

Is your course in an OW environment?

 

[grantctobin]

Have seen it taught a few different ways by TDI instructors, but I stand corrected at least as far as this is direct from TDI: How to Switch your Diving Gas - SDI | TDI | ERDI | PFI

My question is sincere and not intended to be antagonistic.

Where do you see the “no backgas between deco gases” stipulation in TDI’s Decompression Procedures and/or the Trimix course? I reviewed the course standards and didn’t see any wording to that effect.

 

[CptTightPants21]

In my research of how different people/agencies teach it, I have not found anyone that recommends that. And I do seem to recall a video from the ISE guy that specifically mentioned swimming with 2 on the leash is annoying (my interpretation - not his wording).

I have swam (swimmed? swum?) 2 on the left before and I'm okay with that. Also, I talked to my instructor and we both agree that we don't want to switch to bottom gas in between deco mixes. So, once I get on my first deco gas, I want to switch to the next one directly from the first one, and so forth. Which means having 2 "up front" anyway, at least during gas switches.

Switching to back gas in between the deco switches is absolutely the way to go. Switch from Deco-->backgas-->clean up used deco bottle--->switch to new deco bottle.

Keeps everything clean and controlled. No dangling regs to get get in the way, no picking up the wrong regulator, etc. The benefits even extend past the physical. You are on deco and you have loads of gas and nothing but time. There is no reason to rush a gas switch relax, collect your thoughts, take your time, and chill.When you are doing a dive that requires multiple deco gases the time on back gas is essentially a mini air break anyway.

 

[stuartv]

Bottom gas is 10/50. Deco gases are 21/30, 50/10, and O2. Target depth is 300', with contingency allowance for 330. I don't mean contingency as in, we might change the dive plan mid-dive or get in with a plan for 300, but possibly go to 330. I mean contingency as in filling tanks today for this weekend and, for Monday's dive, we might end up deciding to plan it for 330, using the gas I already filled my cylinders with.

I did not choose any of the gases. They were communicated to me by my instructor. That said, the idea of switching to 10/50 at 30', and staying on it to 20', then switching to O2 makes me very uncomfortable. Or whatever your process is. Being on 10/50 at 20' is not something I really want to do.

The class is TDI Advanced Trimix. Yes, open circuit. I had not planned to pursue hypoxic OC training, as I am really only interested in this kind of deep diving on CCR. But, the opportunity came along to do it at the right time, for the right cost, so I decided to do it. I'm working towards becoming a tech instructor and this training and experience seems like a good thing to have under my belt, even though I don't expect to ever get to the point of teaching hypoxic trimix on OC myself.

I have practiced switching directly to one deco gas from another and it didn't seem any harder than switching from back gas to a deco gas. Maybe easier. There's a lot less gas switches that way too, which seems like a good idea. And no frequent clipping and unclipping of the primary back gas 2nd stage.

Once I switch from back gas to the first deco gas, the primary back gas 2nd gets clipped off and stays clipped off. With it clipped off, the alternate on its bungee, and a deco reg around my neck and into my mouth, it was pretty straightforward to deploy the next deco 2nd stage, confirm the gas, etc., etc., and then take the 1st deco reg out (and from around my neck) and put the 2nd one in - all without tangling the hoses, of course. Maybe I just don't have enough experience with that yet, to see the true shortcomings.

Anyway... that was to answer some questions raised, but is all OT.

From the responses that addressed my questions, it sounds like I should probably be fine with my Kydex BP and the same tail weight I was using this past weekend (i.e. 5#). We'll be doing some shallow skills dives before we actually really go deep, so I will have plenty of opportunity to make adjustments and practice all this. I just came here to consult the Scubanati to get some validation of my plan for my starting point. Thank you.