Advanced Gas Switching

[stuartv]

the chance to mix gases based on the bottles "loaded" is different than having multiple regulators deployed though. If you don't fully stow the regulator before you switch, then chasing the hose back is more complicated, thus introducing higher risk of a bad gas switch. For me though, it's just less clutter overall and I'm all about less clutter.

@ChuckP switching to backgas to stow the previously used bottle isn't going to affect your deco time and efficiency. When we do "air breaks" we don't switch off of the deco mix on the computers. I don't know if there has been any actual research studies done on air breaks vs. not, but the general understanding says that the breaks help to open everything back up as O2 is a vasoconstrictor and that actually helps improve deco efficiency.

ICD concerns are valid if you're on that bottle for a long time, but you're not. If you watch the video, which is admittedly from one of the most competent divers in the world. That said, let's look at the time stamps
3:51-start
3:58-backgas in
4:58-move up to 6m
5:25-start switch
5:53-O2 in
Total time on backgas is 2 minutes, 30 seconds of which was moving up to the next stop. If you double the time to stow, a "new diver" would be on backgas for 3 minutes and that really is inconsequential in the grand scheme of things, certainly not enough IMO to cause worry about ICD since you aren't intaking that much He.

@stuartv you should still have a long hose when tech diving a CCR IMO, but I don't know how you have yours rigged. With a bmcl unit like the Revo it's not difficult to deal with a long hose and I much prefer having it when in any overhead

When I was practicing switching directly between deco gases, I still never had 4 regs around my neck at once. Once the 2nd deco gas reg was deployed and I was ready to actually switch what I was breathing, it was simple to dip my head and remove the one I was breathing, similarly to donating the long hose, then loop the next one around my neck and stick it in my mouth. I think the first time I did it, I ended up with the hose from one looped one half twist around the other deco hose. But, with only the 2 cylinders there on my left, it was simple enough to unwrap it and then stow it. And simple enough to realize what happened and not make that mistake again.

Your training was to leave your computer set to deco gas, even when you switch to back gas. My training was to switch the computer when I change what I'm breathing. Even on air breaks.

Regarding ICD, what I have read from the few sources where it has actually happened to someone, it sounded like ICD can hit very quickly and like a ton of bricks after you switch to the "wrong" gas. Is that not the case? It definitely sounded like something that I would really want to take all reasonable precautions to avoid.

I do not have a long hose on my BO regs for my rEvo. That was not part of my training. But, I am only at the MOD1 (with deco and Adv Rec Trimix) level. I would have completed my MOD2 by now, were it not for COVID hosing up the schedule. We did carry 2 BO/deco cylinders during my MOD1 training (for the last couple of dives). But, there was no discussion of using long hoses.

When would you use the long hose, rather than just give the bailed out diver your cylinder? That sounds like a REALLY bad day...

 

[stuartv]

I have the same question too - the video is a little different than I would expect as in it appears the stage tank is bottom mix and the doubles are travel gas - that's not what I would choose for a wreck dive. I'd think bottom mix in doubles and travel gas in a stage and I would not breathe my bottom gas during that gas change but I too defer to the folks doing that diving.

I have that same question as well and it's why I am (at this point - my hypoxic class hasn't even started yet) very NOT okay with a plan to switch to 10/50 at 20', in prep for switching to O2.

Analyzers are supposedly accurate to +/- 1%. That could mean even a properly working and calibrated analyzer could result in you thinking you have 10% O2 in your mix and you really only have 9%. At 20', that means you're trying to breathe with a ppO2 of 0.14. Even worse if you inadvertently ascend just a few feet as you're messing around "cleaning up" and prepping the next bottle, etc.. That just does not seem like a good plan, to me. But, maybe I'll learn differently in my future training...

As for the ISE video, yes, his ascent did start with him on his leanest deco cylinder and a reference to being on travel gas. I don't think that matters. I think the process would have demoed better if he started on back gas and included showing the switch to that first deco (or travel - whatever you choose to call it) gas. But, it started with him already switched to that and went from there. I think it still made very clear what the process is.

 

[stuartv]

I was just taught a much different approach to deco than what's shown with those gas changes - and I do not want to get into what's right or wrong. That video is obviously a fantastic smooth method of gas changes.

The whole concept of carrying three tanks and making the gas changes effectively is something - I never thought about can lights and suit heaters as I don't dive that way. Lots of variables....

Yes. Lots of variables.

And, I agree on not trying to say what's right or wrong. Different things are "right" for different people and for different diving situations. So, don't take this is a backhanded way of trying to say one way or right or wrong... What I am curious about, as far as carrying one (or more?) deco cylinders on the right, is this: If you get to the point of doing really deep or long dives where you are carrying 3 or 4 (or more) deco/stage cylinders, do you still carry one or more on the right? What is the max # of cylinders for on the right?

It seems like it could be a good, straightforward solution for when you're just carrying one deco cylinder on each side. But, my experience with 2 on the right made me feel like I would never feel good about carrying 2 or more on the right.

 

[stuartv]

I had not intended to do hypoxic training on OC. But, now I'm glad this opportunity came along and has prompted me to put some thought into it. I can see now that the way I have been carrying deco gas (OC or CC) has been somewhat sloppy and I think going through all this is going to result in "tightening up" what I'm doing, even when I'm just carrying 1 extra cylinder.

 

[tbone1004]

Your training was to leave your computer set to deco gas, even when you switch to back gas. My training was to switch the computer when I change what I'm breathing. Even on air breaks.

Regarding ICD, what I have read from the few sources where it has actually happened to someone, it sounded like ICD can hit very quickly and like a ton of bricks after you switch to the "wrong" gas. Is that not the case? It definitely sounded like something that I would really want to take all reasonable precautions to avoid.

I do not have a long hose on my BO regs for my rEvo. That was not part of my training. But, I am only at the MOD1 (with deco and Adv Rec Trimix) level. I would have completed my MOD2 by now, were it not for COVID hosing up the schedule. We did carry 2 BO/deco cylinders during my MOD1 training (for the last couple of dives). But, there was no discussion of using long hoses.

When would you use the long hose, rather than just give the bailed out diver your cylinder? That sounds like a REALLY bad day...

You dive with dual computers. Run one with switching and one without, see what the difference is on one of these dives. Measure with a micrometer, mark with chalk, cut with an axe...

ICD can happen quickly, but the suggestions from the agencies say that within the 100m diving realm and the gases used for that, they're pretty insignificant. It incurs risk and if you want to do the air breaks on your travel gas, then it's obviously less risk than the back gas, but I don't know of anyone who has had an ICD issue from "normal" hypoxic depths. Guys like @Bobby and @kensuf who have a helluva lot more experience that deep than I do may have different opinions on air breaks while on really lean mixes.

I have an aversion to handing bottles to other people, so I would use it any time I would have to share gas. My bailout in OW is on a rack so it's not something I can pass off, and if I'm diving sidemounted bailout then I would lose ballast by giving someone that bottle. It's one thing with AL80's, but a completely different thing with steel bottles.

 

[stuartv]

You dive with dual computers. Run one with switching and one without, see what the difference is on one of these dives. Measure with a micrometer, mark with chalk, cut with an axe...

If my instructor has me switch to back gas between deco gases, I will try that.

 

[tbone1004]

If my instructor has me switch to back gas between deco gases, I will try that.

I'm talking about for your air breaks, which he very much should be having you do...

 

[CptTightPants21]

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.

The GUE protocol has near zero risk of every being the wrong gas. The tank markings and contents are verified three times--visually, by pressure, and by buddy in a 7-8 step process that can be efficiently executed in 35 seconds on a slow day. It is difficult to state anything as 100% idiot proof, especially on the internet, but it is basically as good as can be in the real world.

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.

Your 'lists' and number of steps are arbitrarily longer. All the lists have switching bottles, stowing, rotate. You added an additional trading places of the O2 and 50% bottle into bottom or top position--which isn't done/necessary. All the other added 'steps' are unclipping and clipping primary regulator. We are talking about dives to 330ft (and beyond), clipping and unclipping a primary regulator should not be a serious point of consideration.

The process of going to back gas is about a minute--while you are stowing the previous bottle and switching to the next. In this example. the bottle rotation can be done while breathing the 2nd deco bottle. The benefits of going to back gas between switches are It keeps things cleaner, more clearly identifiable to yourself AND buddy, you don't have to worry about deco hoses getting twisted together, or other 'messy' hazards. You start at home base and it is the same procedure every time. It's easy to write list and 'say oh I know will I do this'. Every accident had a written down procedure, which is why it is important that there are certain, sometimes extremely subtle, redundancies baked in. A good example would be Chuck's above where he describes preparing for deco and draping the deco reg over his neck while he finished his ascent. What if he forgets to clip off his primary, he then has two regulators dangling around his neck, is it completely unheard of that he might pick up the wrong one? There are lots of little what-ifs you can play with--well I use my magic color coded regs, etc. I don't want to get into all that, but resetting to home base by going briefly to back gas is one of those subtle baked in redundancies.

I also want to talk about air breaks. Air breaks are 3 - 6 min gaps in your deco where you breathe a lower PO2. The idea behind them was to help alleviate some of the vasoconstriction caused by breathing high PO2 and improve deco efficiency; a little lower CNS also comes into play. Tbone says he does them during his deco. I was taught they get added in addition to deco and are part of the total run time. When I do a air break I switch my computers and I also keep track of how much time I am on the air break so I can add it to my total deco when the computers have "cleared". People use them to various degrees. Mostly done at the 20ft stop when PO2 is 1.6 for extended periods. Some people will do 12 and 3--12 on O2, 3 min air break; 15 and 5, 12 and 6, 16 and 4...whatever.

I called switching to backgas a 'mini air break' because that is what it is essentially is if you are worried about CNS or vasoconstriction, etc. I treated it more as a cherry on top vs one of the primary reasons to go to backgas for deco switching. I would not call the switching to back gas for the 45-60 seconds deco switch an air break in conversation. Air break implies a slightly longer time frame.

I have that same question as well and it's why I am (at this point - my hypoxic class hasn't even started yet) very NOT okay with a plan to switch to 10/50 at 20', in prep for switching to O2.

Analyzers are supposedly accurate to +/- 1%. That could mean even a properly working and calibrated analyzer could result in you thinking you have 10% O2 in your mix and you really only have 9%. At 20', that means you're trying to breathe with a ppO2 of 0.14. Even worse if you inadvertently ascend just a few feet as you're messing around "cleaning up" and prepping the next bottle, etc.. That just does not seem like a good plan, to me. But, maybe I'll learn differently in my future training...

People will do different things. Some are ok breathing the 10% briefly at 20ft and will do normal switch (.16 PO2 is still breathable), some will do the normal switch a little deeper (around 23-25' and finish the ascent to 20' on the O2. In the case of breathing <10%, some people will do the switch from 50% directly to O2, but that is the lone exception.

 

[PBcatfish]

A comment/question here from a guy who has zero experience with switching between 4 regulators & as many mixes, so please take this with a grain of salt.

As an outsider looking in, it appears to me that the primary question at hand is one of efficiency compared to reliability.

In the case of switching from one slung bottle to another slung bottle without going through back gas in the process, there are fewer steps & as long as the bottles & other gear are handled well, this should provide a fast & efficient change.

In the case of going to back gas, then getting the old bottle buttoned up & out of the way before switching to the new gas, you have fewer balls in the air at once & would seem to reduce your chances of tangling the gear. If you were in heavy cross currents, a kelp forest, or other difficult conditions, this may gain added importance. For those only casually experienced in the process, there may also be added importance here.

Of course, if the back gas in question is unsafe to inhale at certain depths, that would obviously change things.

Is that a reasonable assessment of what I have been reading here?

Now for the really stupid question: What is ICD?

 

[rjack321]

Question for anyone who actually does OC dives using mix with 10% or less O2 for bottom mix and 100% O2 as one of the deco gases-

Is bottom mix back gas or in a stage? If bottom mix is back gas do you breath this at 20ft while preforming the switch to 100%?

I’m nowhere near this level but genuinely interested and curious. Thanks.

10% is breathable at 20ft. Its not a big deal at this level to breathe 10% O2 with a ppO2 of 0.16 for 5-6 mins at 30ft and then again at 20ft during gas breaks from the O2.

and yes you would never have anything but your lowest O2 content gas in your backgas, in many cases you have a bottom stage as well but that cylinder will be empty long before you get to deco.