Trim with 3 deco cylinders?

[rjack321]

On OC, where I only have 2 gases "up front", the chances of switching to the wrong gas is also pretty much zero.

There are quite a few wives, husbands and children who lost family members due to an erroneous gas switch that know you're wrong about this. Switching to 100% at 70ft will do it. Or back in the day when 32% was a more common deco gas than nowadays in 2021, switching to 100% or 80% at 130ft.

 

[inquis]

You will need a low ppO2 break for a few minutes at this point anyway

Cool, I'll put that in the Pro column for switching to back gas.

 

[rjack321]

I don't use cylinder position as indicative of anything. The actual switching procedure -- noting depth, confirming MOD on tank, pulling reg, tracing hose back to the regulator/valve, turning that specific valve, etc. -- wasn't my point of confusion. I do those things regardless of where tanks might be, though I appreciate you pointing out the importance of such. I completely agree with you.

Regardless, at the end of the day one of the tanks WILL be on top. I might as well put the first one I'll use there from the get go -- verify placement at that time and verify again before, during, and after deployment (with further checks during & after by the buddy).

I don't see that as any different than arranging for a "clean" set of stages as you described. Even when on back gas, it's easier to switch to and clean up a bottle when it's on top. Don't you strive to do that as well?

All I can say is you've been warned. You keep saying "verify placement" - I completely ignore placement. I verify the MOD right before I switch and don't give a damn whether the stage is top or bottom.

Stage to stage is a CF in the making. 99% of the time it's totally do-able. It's the rare time that will bite you hard

 

[inquis]

I verify the MOD right before I switch and don't give a damn whether the stage is top or bottom.

Couldn't agree more. I'm verifying contents/MOD before, during, and after deployment. I also verify contents/MOD as part of verifying placement, as it wouldn't be very useful to put the wrong contents in a desired position.

Stage to stage is a CF in the making. 99% of the time it's totally do-able. It's the rare time that will bite you hard

I'm certain there are good reasons that you feel as strongly about this as you seem to. I was really hoping you could describe a situation that points out the increased likelihood of error of stage to stage. Or perhaps an increase in convenience of switching to back gas. (The air-break was a good one.)

 

[stuartv]

I'm certain there are good reasons that you feel as strongly about this as you seem to. I was really hoping you could describe a situation that points out the increased likelihood of error of stage to stage. Or perhaps an increase in convenience of switching to back gas. (The air-break was a good one.)

My training has been that I only do air breaks after I am on O2. I have only done a couple of dives to 300', and none that were that long. I have yet to be on an intermediate mix where I thought I might need an air break before switching to O2. I would only do an air break after being on O2 for 15 minutes.

This may be my inexperience at diving that deep talking.

So, from a 300' dive, I would be on my intermediate mix when I arrive at my 30' stop. There, I would do a leisurely rotation to move my deep deco gas to the leash and my O2 to the front. While breathing my intermediate mix - so no chance of accidentally rotating the wrong gas to the back.

Then, when I ascend to 20', my O2 is there, ready to switch directly to from my intermediate mix. Further, the only regs that I can reach, that are not in my mouth, are my bungee necklace to back gas, my primary, clipped off, and my O2. Minimizing the chance that I would switch to anything but my O2.

As I said, you said, and others have said, none of any of this takes away the need to do proper verification of the gas each time you're going to change what reg is in your mouth. To me, it is just reducing gas switches and reducing the chance of preparing to switch and discovering you were about to switch to the wrong gas.

 

[tbone1004]

Intermediate switching to backgas - do you do that on the last switch, e.g. switching from 50% to 100%? With 10/70 on your back?
I don't.
Take the 50% back from around your neck, put the 100% around your neck and continue breathing. That stop is the longest and there is loads of time to stow your 50%. No rush, no entanglement, no clusterfcuk. I want to enjoy the reef, that's why I did a dive there.

why wouldn't you? You come off of the 50% bottle at 30ft with a ppO2 of ~0.2 which is perfectly fine. You load the O2 bottle on your way from 30 to 20 and when you hit 20ft you are at a ppO2 of .16 which is the same as what you experience inside of an aircraft cabin.

I'd argue that you shouldn't be doing those dives on OC anymore, but people are doing them on SCR which does require those gas switches, and even when I'm on CCR I have all of the bottles loaded and plugged into the rebreather so I go through those gas switches.

You do certainly want to drop your ppO2 during decompression to make sure that vasoconstriction doesn't impede off-gasing, but at the same time your 50% bottle is at a ppO2 of 1.0 when you are on your 30ft stop so it's not really all that high to where I'd say you need to be doing "air breaks" when you're on the 50% bottle-caveat being some crazy extended range stuff where you are actually at 70ft for 20mins, but you won't be doing those dives on OC and will likely have habitats, support divers, etc etc*.

For me? I switch to backgas at every switch. I like to put all "dead" bottles on a leash and get rid of them behind me as I find it more comfortable than trying to keep them loaded. Sure you can do it switching from one to the other, but I find it easier to take care of one before starting with the next. This may stem from using stage bottles in a cave where you are stowing a bottle while still using the trigger so you only have one hand for most of the actions so getting on your long hose is necessary and you don't have all the time in the world to dick around with both bottles like you do on deco, but either way I try to keep one procedure for all gas switches and that means going back to my long hose.

 

[inquis]

This may stem from using stage bottles in a cave where you are stowing a bottle while still using the trigger so you only have one hand for most of the actions

Excellent! I definitely favor consistency of process, so thanks for pointing that out.

 

[p_kos]

Oh, and, though it is off topic, since there was a bunch of discussion about using TX10/50 for a 300' dive, I will share this story. It reveals a screw-up or somewhat poor judgment on my part, but as I've said before, I'm not afraid to out myself here - especially if it somehow helps anyone else in the future.

On our final dive, yesterday, we hit 305'. Viz was great from 70' down to about 280 and then we dropped into a layer of bottom murk and viz was less than 10', I'd say. Our instructor was leading us. The plan was to hit the bottom by 13:00. That allowed time on the 60' ledge where we would be anchored to switch to back gas, clean up the travel gas cylinder, and swim to the edge of the ledge. Not necessarily the way to get the most bottom time, but we wanted to be conservative (versus, for example, switching to back gas and cleaning up while also descending). The day before, it took us until 16:30 to do the same thing and then drop to 250'. But, that was the first time we did it and the first time my buddy and I had really dived together (versus earlier class dives where we were diving with the instructor and doing skills at 60' - not really diving as a buddy team). We had talked through the first dive, sorted things out and felt good about getting to the bottom more quickly than the day before. And we did. We got to 300' at 11:00. Our plan was to start our ascent at 17:00. Not very long on the bottom, but we were constrained in our planning by the fact that I had only brought a 40 for O2 and we were only able to fill it to 2200psi after the first deep dive.

So, we got to the bottom, in crappy viz and started to swim. Almost immediately, I saw something and looked a little up and a little to the right and there was orange line floating there. Looking at my buddy, I saw that it was already starting to get wrapped around his feet. Fudge!! And I thought, "huh. That line looks new. And it's the same color as the line on my reel. Fudge!"

I had a newish (not too many dives on it yet) 400' reel, pre-rigged with an SMB clipped to my right waist D-ring. Except... not anymore. It was gone and I had no idea how.

I saw that our instructor was now in front of my buddy and already working the problem. I determined that I didn't care about trying to save the reel or the SMB (wherever that was - I never saw it), so I dug out my shears and swam up to my buddy's feet and started just cutting the line up. My buddy was stellar in his performance. He held totally still and kept his buoyancy. After the fact, I learned that my instructor had seen the line problem just before I did and it was coming from my reel, which was still clipped to me. The instructor unclipped it from my belt and I never even knew it. And, somehow, the line didn't get around me at all. My instructor got in front of my buddy and was trying to reel it up. He saw pretty much right away that things where too far gone for that and saw me cutting away at the back. On the boat afterwards, he said "I saw Edward Scissorhands back there, going to town." LOL! I was also watching the time and knew we still had several minutes left in our planned bottom time, so worked the problem at depth for the moment.

The instructor decided things were not where there was a simple and immediate fix, so at that point, he gave us the thumbs up and we started our ascent. At that point, the reel line was already cut, so he reeled in the last bit and clipped it off to himself. When we got back up onto the ledge and did our 2nd gas switch, the brake on the reel had loosened itself again and the line was coming unspooled again. My instructor pulled it back out and tried to get it reeled back in and secure again. It immediately started coming loose again. My buddy took it and tied a loop in the line and put that around the brake and tightened it all and that seemed to hold it. Meanwhile, my instructor was writing in his wet notes. When they both finished, the instructor took the reel back, clipped a noted into the gate of the double-ender on the reel and handed it all to me...

View attachment 610659

After it was all over, I was replaying how it all went and I can tell that I was definitely narced. I was functional. But, my situational awareness was definitely impaired. I am very glad that it all worked out without anyone getting hurt, or even causing us to have any variance from our schedule. Well, other than we began our ascent at 13:00, instead of 17:00. Planned runtime was 70 and we were out at 63, including about 4:00 of extra time on O2 at 10' at the very end (after my computer said I was cleared).

I will definitely not dive that deep again without a lot more helium. I think the way it went shows that I was "okay". But, I can tell that I was not "great", or even "really good" and, for diving deep, I want to be "great" in the future. That whole situation could have been a LOT worse and somebody dying because I simply didn't put enough helium in would be infinitely beyond stupid.

I said earlier the story reveals some personal shortcomings. What I mean by that is that, first, I splashed for a 300' dive with a reel/SMB setup that was not adequately secure. The SMB was only held to the reel by a bungee loop that is sewn to the SMB and which was around the SMB and the spool in the reel. It must have gotten knocked loose, so then the only thing holding it was the brake on the reel itself. The brake is clearly crap and loosened enough to let the line spool out and the SMB drop. I take responsibility for splashing with a reel/SMB rig like that. I should have seen the potential and rigged it in a way that was more secure. And, second, I didn't notice that it had come loose and was paying out line, nor did I even notice when my instructor came up behind me and took the reel off my belt. Another indicator of my level of narcosis.

So, I screwed up. Thankfully, in the end there was no harm, and I learned some valuable lessons for the future.

If anyone sees some other ways that I screwed up that I have not identified, feel free to point them out. I don't mind if you're brutal. I was there, and I'm here, to learn. A mistake that I don't learn from is an opportunity wasted....

Thanks for sharing! I’ve had a buddy unspool a real accidentally like than but also drag it on the bottom with a nice silt stream behind him. I’ve also clipped off an SMB/spool combo to my butt ring and never saw it again. The lesson there, and perhaps in your story, was to store spools and reals in pockets until you’re ready to use them. I also learned not to clip anything to the butt d ring with the gate facing my butt, lol.

 

[rjack321]

Couldn't agree more. I'm verifying contents/MOD before, during, and after deployment. I also verify contents/MOD as part of verifying placement, as it wouldn't be very useful to put the wrong contents in a desired position.

I'm certain there are good reasons that you feel as strongly about this as you seem to. I was really hoping you could describe a situation that points out the increased likelihood of error of stage to stage. Or perhaps an increase in convenience of switching to back gas. (The air-break was a good one.)

You have 2 bottles left clipped on your body
You're diving stages only with backgas in reserve cause you're scootering in a cave.
You go to switch stage to stage (this is the most common stage to stage type of switch, people with scoooters in caves are the sloppiest of all
You only have one right hand and the left needs to stay on the valve
So you deploy the 2nd stage's regulator off bottle #2, check it and then there's only one place for it to go - it ends up around your neck cause otherwise it'll dangle in the silt.
Remove the 2nd stage in your mouth (from bottle #1)
Now you have 2 loose 2nd stages around your neck and also your long hose which is clipped off
You put the 1st 2nd stage back in your mouth though. Maybe you even stow the wrong reg too and hit the trigger - and run OOA.
At least you were on a breathable gas, there are worse CFs you could do.

If you go through backgas you never have two second stages hanging around your neck in the first place. You only have 2 regs to manage. A stage reg and a long hose which is 1) always breathable and 2) has a bolt snap on it. You never get the stage hoses twisted, you never have a dead 2nd stage in your hand potentially getting distracted and putting it back in your mouth, you don't mix up the left hand being on one valve knob and your right hand holding a different 2nd stage. Every gas switch is exactly the same whether you have 1 bottle or 5.

Stage-to-stage encourages sloppy behavior, it has zero benefits and more than a few drawbacks. I'm done with this debate though so carry on and dive however you wish.

 

[inquis]

I wouldn't have called it a debate. Just a concrete discussion of pros and cons. I now see that going through back gas makes a ton of sense when both hands are not available. Thanks again for bearing with me.