Trim with 3 deco cylinders?

[kensuf]

My normal process is 2 stage bottles on my left side, anything extra goes on a leash. Swimming with two bottles on a leash isn't the greatest when they're full, but when they get floaty it's not a problem.

Here's a bad screen grab of me with 5 bottles, two are on a leash clipped on the butt, two are on my left (I'm in the process of pulling the 220' bottle off to throw it on the leash), and I have an O2 bottle that is clipped onto my hip as well. This particular situation was during an incident where a friend took a CO2 hit on our way back up from a 100m dive, the O2 was hastily clipped onto my hip and one of the bottles on the leash was stripped from my friend when he cashed it.

 

[kensuf]

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.

This is the correct way to do gas switches. Go to your backgas reg, clean up the first bottle, then go to your second deco gas. This should be taught in a trimix class.

 

[grantctobin]

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.

Is that 8.05g/L gas density for contingency depth? Would someone care to check my math?

 

[stuartv]

Is that 8.05g/L gas density for contingency depth? Would someone care to check my math?

That's what I get. Class hasn't started yet, so I had not gone that far in the planning.

 

[rjack321]

Is that 8.05g/L gas density for contingency depth? Would someone care to check my math?

Yes 170ft of density and 150ft of narced too.

I would not do this dive with less than 65-70% helium in the backgas and I don't see the need for 10% O2 either for a 300ft plan. Slipping to 330 on 12/65 is a ppO2 of 1.3 and I would be fine with that for the short bottom time here.

The fact that we are talking about how to be in better trim plus not going to backgas between bottles is evidence to me that perhaps the OP isn't ready for this many bottles or this magnitude of a dive.

 

[stuartv]

The fact that we are talking about how to be in better trim plus not going to backgas between bottles is evidence to me that perhaps the OP isn't ready for this many bottles or this magnitude of a dive.

Pretty good. Only took 3 pages to get there.

I suppose I should have been diving with 3 deco cylinders long before I sought training on it.... Or learned by now not to turn to SB to try and find help in preparing for an upcoming class...

I'll have to spend some time thinking about this. I'm not sure I have ever posted on SB with questions related to an upcoming class and NOT eventually gotten some posts that boiled down to "your instructor doesn't know what he's doing" and "you're obviously not ready for this class." You'd think I would have learned by now. Maybe this will turn out to be the time when I finally do die because I didn't listen to strangers on the Internet. If so, I can assure you I won't make that mistake again...

 

[CptTightPants21]

Yes 170ft of density and 150ft of narced too.

I would not do this dive with less than 65-70% helium in the backgas and I don't see the need for 10% O2 either for a 300ft plan. Slipping to 330 on 12/65 is a ppO2 of 1.3 and I would be fine with that for the short bottom time here.

The fact that we are talking about how to be in better trim plus not going to backgas between bottles is evidence to me that perhaps the OP isn't ready for this many bottles or this magnitude of a dive.

That is a little unfair. I do normally see a more even progression of added complexity--from 1 bottle to 2 bottles to 3+ bottles. If not through cert cards then through continued diving experience and adding the bottles themselves gradually. I don't know what Stuart has been up to, but I know he was 1 bottle OC then went to CCR and I don't believe he has cave. The jump back to 3 bottles is a larger step than usual and more than I would be willing to do given the increased depth as well, but it isn't exactly unheard of. The gas mixes on the other hand...

Pretty good. Only took 3 pages to get there.

I suppose I should have been diving with 3 deco cylinders long before I sought training on it.... Or learned by now not to turn to SB to try and find help in preparing for an upcoming class...

I'll have to spend some time thinking about this. I'm not sure I have ever posted on SB with questions related to an upcoming class and NOT eventually gotten some posts that boiled down to "your instructor doesn't know what he's doing" and "you're obviously not ready for this class." You'd think I would have learned by now. Maybe this will turn out to be the time when I finally do die because I didn't listen to strangers on the Internet. If so, I can assure you I won't make that mistake again...

You have a point, but some of the criticism is justified. If someone told me they calculated those gas mixes and that dive plan I would be looking for a spreadsheet calculation error.

You have excess helium in deco/travel gases and not enough in your bottom gas. I jested before that you like your exotic mixes, but I would be interested in hearing the calculation and dive parameters the instructor went through when selecting those gas mixes.

 

[stuartv]

That is a little unfair. I do normally see a more even progression of added complexity--from 1 bottle to 2 bottles to 3+ bottles. If not through cert cards then through continued diving experience and adding the bottles themselves gradually. I don't know what Stuart has been up to, but I know he was 1 bottle OC then went to CCR and I don't believe he has cave. The jump back to 3 bottles is a larger step than usual and more than I would be willing to do given the increased depth as well, but it isn't exactly unheard of. The gas mixes on the other hand...

I did TDI Trimix (i.e. Normoxic Trimix) training and certification a little over 2 years ago and have dived with 2 deco bottles a number of times, on OC and CC.

I do not have any cave training.

I practiced last weekend with double 120s and 3 deco cylinders. After the 2nd dive, I was talking to my buddy about what we'd just done and saying that I felt like I was total crap in the water. We had spent a bunch of time hovering on a 20' platform and I was practicing dropping cylinders and picking them back up and doing gas switches between them in the order I would during the dive. My buddy's comment was that he thought my buoyancy was actually fine. He noted that I never touched the platform itself and never went up more than a foot or so during all that. My problem was not staying exactly in one spot while I was doing it. I did a bit of swimming forwards and backwards that ended up with some sideways displacement from my starting point - because my tail weighting was still off and I was still fighting my trim somewhat. On the third dive, with a bit more tail weight, I think that problem was pretty well solved.

Which is all simply to support my feeling and statement that I think I am ready to go on to the actual training for full trimix.

 

[stuartv]

You have a point, but some of the criticism is justified. If someone told me they calculated those gas mixes and that dive plan I would be looking for a spreadsheet calculation error.

You have excess helium in deco/travel gases and not enough in your bottom gas. I jested before that you like your exotic mixes, but I would be interested in hearing the calculation and dive parameters the instructor went through when selecting those gas mixes.

Criticism to me is justified?

I said much earlier in this thread that I didn't choose the gases. And class hasn't started yet - I just needed to know the gases so I could show up with properly prepared cylinders. So, I cannot yet answer questions about why those specific gases were chosen. I didn't mention the specific gases for any other reason than as context for the question about whether changing from no helium in skills practice to the specified mixes in the real dives is going to throw off my trim enough to merit any kind of weight change (amount or distribution).

Things took a typical SB Downturn from there...

I can add that I did full gas blender training a few months ago and have been doing all my own fills, including trimix and O2 since then. The shop where I do the fills is where I did the gas blender training so they let me drive their fill station to do what I need. The only have banked air and O2, and a single cylinder of He. Everything is partial pressure blending, with a lot of boosting. So, all the input about some gases being better because of the ease of blending when having banked 32% is really self-centered twaddle that does not apply to my situation. I wish my shop had a bank of 32% and I'm happy for those of y'all that do.

 

[grantctobin]

I don’t know that the ease of blending of standard mixes has been mentioned in this thread at all. The gas input is directed at a selection of glasses that are grossly inappropriate for the dive outline as planned.