Advanced Gas Switching

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stuartv

Seeking the Light
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This is a spin-off from my other recent thread, based on some OT (for that thread) thoughts that were shared.

I want to specifically talk about gas switches in the context of an open circuit dive to a depth requiring a hypoxic gas. I do not want this to be about the specific gases used. Just the process of doing the gas switches.

To make the discussion a little easier (hopefully), let's just say you're using these gases:

10/70 (bottom mix)
21/35 (lean deco)
EAN50 (intermediate deco)
O2 (rich deco)

First stop depth on ascent is 140. Again, just for the sake of discussion.

The question is, what is your protocol for your gas switches and why? Based on the other thread, I will start by throwing out 3 options:

One depth w/Air Breaks:

- Arrive at your depth for your switch.
- If not already breathing back gas, you switch to back gas and clean up the cylinder you were breathing from.
- Switch from back gas to the new deco gas.

Two depths w/Air Breaks:

- Arrive at the last stop before you switch to the next deco gas. E.g. arrive at 30', before switching to O2 at 20'.
- If not already breathing back gas, you switch to back gas and clean up the cylinder you were breathing from.
- Ascend to next stop.
- Switch from back gas to the new deco gas.

One depth, no Air Breaks:

- Arrive at your depth for your switch.
- Switch from the gas you're breathing to whatever new gas you want to breathe.
- Clean up the cylinder you were breathing from. If it was back gas, that just means clipping off the primary 2nd stage.

Note: "no Air Breaks" does not mean that you would not actually do "air breaks" while on O2. It just means you're not switching to your bottom mix in between deco mixes. Once on O2, you could/would still do whatever "air break" procedure you use. My current training would have me breathe O2 for 15 minutes, then switch to back gas for 5 minutes. But, in this particular example, rather than switch to 10/70, I think I would switch to 21/35 for the "air break". Breathing 10/70 at 20' seems inherently risky, since it would only have a ppO2 of 0.16.


In the other thread, it seemed that the SB Communal Wisdom is that one of the first two protocols is the preferred approach. A couple of people said or implied that switching directly from one deco gas to the next is problematic because of the number of regulators that would be in your "golden triangle" (to pull in a sport diving term) - i.e. 4 versus 3. I have tried doing the direct switch, while just on a couple of shallow practice dives, and it didn't seem any harder than switching to back gas, cleaning up, then switching from back gas to the next deco gas. But, that was just a couple of practice dives.

So, which way do you do it? And, what specific things about your process do you think make it superior to the other processes I outlined?

At this point, I am not trying to advocate a specific process. I have tried them all and have a leaning. But, I think I am insufficiently experienced to have a strong feeling on which way is better. I'm curious about the subtle points that make some of you prefer a certain way, since most of you are vastly more experienced with this stuff than I am.

Thanks!
 
I think the two options above that include what I called air breaks are basically the same except for whether you switch to back gas, THEN ascend 10'/3m to your next gas switch, or ascend all the way to the depth for your next gas switch, then do everything there. So, for practical purposes, I'll consider them the same.

I did watch an ISE video that gives a slightly different process than what I envisioned for the process with "air breaks".

So, the options seem to break down to these steps. Starting with 2 bottles slung on the left and the O2 bottle on a tail leash:

First two options from above (depth changes and light management omitted):

- switch to 21%
- clip off the primary 2nd stage (bottom mix)
- unclip the primary
- switch to primary
- stow the 21% reg
- switch to 50%
- clip off the primary
- rotate the 21% and O2, so they have traded spots
- unclip the primary
- switch to primary
- stow the 50% reg
- switch to O2
- clip off the primary

ISE method (almost the same as above, just with moving the 21% to being nose-clipped at the left waist D-ring after it's finished):

- switch to 21%
- clip off the primary (bottom mix)
- unclip the primary
- switch to primary
- stow the 21% reg
- move the 21% bottle to nose clip position
- switch to 50%
- clip off the primary
- unclip the primary
- switch to primary
- stow the 50% reg
- rotate the 50% and O2, so they have traded spots
- switch to O2
- clip off the primary

Method from above with no "air breaks":

- switch to 21%
- clip off the primary (bottom mix)
- switch to 50% (the only not-in-use 2nd stage on the left)
- stow the 21% reg
- rotate the 21% and O2, so they have traded spots
- switch to O2 (the only not-in-use 2nd stage on the left)
- stow the 50% reg

The processes that are according to SB Conventional Wisdom appear to have twice as many steps and almost twice as many gas switches as the "no air break" process. And that doesn't even include the steps that could be listed for light management (i.e. clipping and unclipping a primary light during these other processes).

How is either of the processes that includes switching to back gas safer than the process that doesn't?
 
Ok, this not about the number of steps you need to do a task is about to make sure the task is done correctly.
Yes, you could go directly from deco to deco without going back to backgas but would you be sure you will never have a chance to mix up the order of the gases?
By shifting the tanks one by one, by having a single deco tank in the proper position and going back gas, you reduce the potential of mixing something because it might happen when you are diving.
It is just about training and practising and like everything else, after a while it becomes second nature.
 
Do you use the necklace backup reg instead of unclipping/clipping the primary reg during switches?
 
Do you use the necklace backup reg instead of unclipping/clipping the primary reg during switches?
I don't use the necklace reg as it is your backup. The reg in your mouth is the one you might need to give to your buddy in case of problem. So if you are on back gas, it is the long hose and if you are on deco gas is the deco reg.
Unclipping/clipping takes longer to type than to do :)
 
Ok, this not about the number of steps you need to do a task is about to make sure the task is done correctly.
Yes, you could go directly from deco to deco without going back to backgas but would you be sure you will never have a chance to mix up the order of the gases?
By shifting the tanks one by one, by having a single deco tank in the proper position and going back gas, you reduce the potential of mixing something because it might happen when you are diving.
It is just about training and practising and like everything else, after a while it becomes second nature.

Even the ISE protocol (and GUE, I think) starts with a switch to a deco cylinder that is one of two that are slung on your left. I haven't seen anybody teaching that you always only have 1 deco cylinder on your left. So, I don't think your comment about never having a chance to mix up the order of the gases is really valid.

On the other hand, everyone teaches some process that is designed so that you select the cylinder you're switching to and confirm it is correct as well as get your buddy to confirm it is correct, and then switch to it in a way that ensures you do actually switch to the chosen cylinder. With that process in place, having 2 cylinders slung on the left for all gas switches, versus just the first gas switch, does not seem like a safety hazard that needs additional mitigation. Does it?

I understand your point about not being the number of steps, but making sure the task is done correctly. But, I also think that the number of steps can have a direct correlation to the ability to accomplish the task correctly. Every step is an opportunity to make a mistake. Twice as many steps is twice as many chances for error.

I have always heard/read that the biggest risk for an OC tech diver is switching to the wrong gas. So far, in all the protocols I have identified, the first gas switch is to one of 2 cylinders slung on the left. So, they all have equal risk for that first gas switch.

After that first gas switch, the shorter protocol I outlined seems like it does at least as much to mitigate the risk of switching to the wrong gas as any other option. When you switch, there is only one reg on your left side to switch to. As long as you have managed your cylinders correctly (which is true for all the processes), you can't really switch to the wrong gas.

If you are using a protocol where you always have 2 cylinders slung on your left, then switching to back gas in between deco gases results in MORE risk, because when you go to switch from back gas to your deco gas, there are 2 regs slung on your left to choose from. All the protocols have this risk at the first gas switch. But, only some (one?) of the protocols maintains this risk through each and every gas switch.
 
I don't use the necklace reg as it is your backup. The reg in your mouth is the one you might need to give to your buddy in case of problem. So if you are on back gas, it is the long hose and if you are on deco gas is the deco reg.
Unclipping/clipping takes longer to type than to do :)

In this context, we're talking about being on back gas in between being on 2 different deco gases. If your buddy suddenly needs gas right then, why wouldn't you give them one of the 2 deco gases you are currently working with (instead of the bottom mix reg in your mouth)? In which case, being on your necklaced alternate right then seems like it would be okay?
 
Not yet. That's why I'm taking the class this weekend. :)
2 day class for hypoxic dives?
No offense and it’s just my opinion but maybe you should just do the class before asking about it on an internet dive forum as you might get different inconsistent opinions from internet divers who aren’t qualified in that area.
Good luck and enjoy :)
 

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