OC Gas switch protocols?

[stuartv]

I did a search on this subject. I found one thread that I think asked the same question I'm going to ask. That thread rapidly spun out of control into OT discussion of rebreather topics.... Can we please stick to discussing of OC only??

The subject is doing a gas switch from the first deco gas to the second deco gas.

Method 1: Some teach to switch from deco 1 to back gas, then switch from back gas to deco 2. Generally (I think) using deco 1, then switching to back gas at the end of the last stop immediately prior to ascending to the depth where the switch to deco 2 will occur.

Method 2: Others teach to switch directly from deco 1 to deco 2.

In the previous thread, the most persuasive reason (to me) given for using Method 1 was that it provided a necessary "air break" between high ppO2 mixes. One poster specifically stated that these air breaks were something like "the only proven way to reduce the risk of OxTox."

But, having read this the other day:

Air Breaks… what are they, and do people take them for the wrong reason?

and holding @Doppler's opinion in high regard, I am skeptical that switching to back gas in between has any benefit with regard to risk of OxTox.

I also wonder whether using Method 1 in some scenarios might present significant risk of vestibulary DCS as the result of ICD. It's anecdotal, but I seem to remember reading in Mark Ellyat's book, Ocean Gladiator, that he had a DCS hit from that on a deco stop and the onset was very quick after doing a gas switch. And he spent the rest of his (long) deco time completely dizzy and vomiting through his regs.

I also think that the planning software I use does issue ICD warnings, but it would not be checking for ICD risks from switching to back gas. I'm pretty sure it assumes a direct switch from deco 1 to deco 2. So, IF (and I'm not saying there is) there is a risk of ICD from switching to back gas briefly, the planning software could not warn of it.

So, which way to you do it and why do you do it that way versus the other way?

 

[PfcAJ]

You're probably referencing me as the one who says gas breaks are a proven way to stave off oxygen toxicity.

The guys doing absolutely massive dives with insane oxygen exposures (like 1000%) have been using this method for decades without incident. It's not a coincidence.

 

[CptTightPants21]

Method 1.

Although, I was taught to take air breaks more for the vasoconstrictor component. While I like Steve, he certainly does a nice job setting up the strawman argument of "resetting" the O2 clock--which is something I have never heard.

I was also taught that if you have something small, like 15 or 20 min of deco,you can finish whole deco on O2 and taking the airbreak is divers choice..... Also notice how Steve keeps his example at 21 minutes of deco.

 

[boulderjohn]

I have learned gas switching protocols from three different instructors in three different agencies, and in doing so, I learned three different protocols. None of them involved going to back gas.

To me, the most important thing is making darn sure you are making the correct switch at the correct depth. In that regard, I will say, completely ironically, that the most complicated the procedure (with the goal of being extra safe), is the least safe. The reason is that when you are no longer under the bright light of instruction, the tendency is to abandon those elaborate procedures, leaving you with no procedure at all. I saw that myself. When just plain diving with the instructor who insisted on the most elaborate (and safe) protocol, we didn't do anything like it. I just takes too long. You can take an entire deco stop just getting everyone on the team switched in absolute safety.

I prefer a more streamlined approach that ends with everyone on the team checking to make sure that everyone is now on the right gas.

 

[RyanT]

I prefer a more streamlined approach that ends with everyone on the team checking to make sure that everyone is now on the right gas.

I just finished my trimix course and that's what we were taught.

Here's another couple of things I learned. First, both my deco regs have green hoses. I'm going to swap out my 50% deco reg to a different color (still retaining the green 2nd stage cover). This makes it much easier for both yourself and your buddy to ensure that you're not switching to O2 at 70'. Second, I learned to tuck the O2 bottle up behind the 50% on a separate D ring, making it harder to get to. Your 50% bottle should be in an easier, more natural location to reach. While deco'ing on 50%, you now have plenty of time to reach back and prepare your O2 bottle for your 20' stop. Doing both of these things adds safety and facilitates gas switches in a timely manner.

 

[RyanT]

@stuartv, I have not heard of the intermediate back gas step. From a logistics/toxcity standpoint, its not really helpful. Let's say you're using a deco set up of 50% and 100% O2. From a gas switch-toxicity standpoint, your 70' stop is the only one that matters. If you switch onto O2 at 70' that could be bad. If you did this and didn't tox by the time you reached your 20' stop, then switching to 50% at that point isn't going to matter. Maybe doing the convoluted backgas middle step would help you identify your prior gas switch mistake, but again from a toxicity standpoint, it doesn't matter. As for an air break, for any kind of moderate tech dive, taking an air break between your 50% and 100% O2 stages is also not that helpful. On O2 at 20 and 15' is where your CNS begins to climb again most rapidly, so doing air breaks then is when it really matters.

I looked at the blog you posted and I've never heard anyone say that the air break "resets" your CNS clock. If they do claim that, it's a rather silly notion. Any air break that reduces your CNS load during a dive would have to be a long break at a shallow depth indeed. Instead, the standard air breaks help to slow down or manage the overall toxic effects of O2.

 

[stuartv]

Method 1.

Although, I was taught to take air breaks more for the vasoconstrictor component. While I like Steve, he certainly does a nice job setting up the strawman argument of "resetting" the O2 clock--which is something I have never heard.

@stuartv, I have not heard of the intermediate back gas step.

So here:

You're probably referencing me as the one who says gas breaks are a proven way to stave off oxygen toxicity.

The guys doing absolutely massive dives with insane oxygen exposures (like 1000%) have been using this method for decades without incident. It's not a coincidence.

Yes, you were the one whose posts I was talking about. I didn't want to call you out on it, if you didn't feel like talking about it again.

I take "gas breaks are a proven way to stave off oxygen toxicity" to mean exactly what others are referring as "resetting the CNS clock". I don't take it to mean resetting it to 0. But, it must mean resetting it to "lower" or it wouldn't mean or do anything.

If I postulate that it has some value for people doing huge dives with insane O2 exposures, I could still believe that it has virtually no signficant value for the itty bitty dives that are, say, 240' or less, and only require levels of deco consistent with carrying 2 AL80s of deco gas.

I'd still like to know what you or anyone else thinks about the possibility of ICD DCS brought on by switching from a deco gas to back gas.

Also, if we stipulate that switching from a deco gas to back gas for a break before going on to the next deco gas has some value on some dives, does it make sense to just lump that "gas break" in with the deco gas switch?

I mean, my training thus far is that doing an "air break" is something I would calculate and perform independently of what I do for deco gas switches. Generally, at my current level anyway, I would only do an air break while at a deco stop, after being on my deco gas for at least 15 minutes. And when I do the air break, I would start from a deco gas, switch to my back gas for 5 minutes, then switch back to the deco gas I was using before. No depth change during the gas break and no intermediate gas involved. Why would I not treat the gas breaks you're talking about the same way? I.e. do my deco gas switches in a more streamlined manner (ala Method 2) and do "gas break" switches to and from back gas on a separate schedule?

 

[PfcAJ]

So here:



Yes, you were the one whose posts I was talking about. I didn't want to call you out on it, if you didn't feel like talking about it again.

I take "gas breaks are a proven way to stave off oxygen toxicity" to mean exactly what others are referring as "resetting the CNS clock". I don't take it to mean resetting it to 0. But, it must mean resetting it to "lower" or it wouldn't mean or do anything.

If I postulate that it has some value for people doing huge dives with insane O2 exposures, I could still believe that it has virtually no signficant value for the itty bitty dives that are, say, 240' or less, and only require levels of deco consistent with carrying 2 AL80s of deco gas.

I'd still like to know what you or anyone else thinks about the possibility of ICD DCS brought on by switching from a deco gas to back gas.

Also, if we stipulate that switching from a deco gas to back gas for a break before going on to the next deco gas has some value on some dives, does it make sense to just lump that "gas break" in with the deco gas switch?

I mean, my training thus far is that doing an "air break" is something I would calculate and perform independently of what I do for deco gas switches. Generally, at my current level anyway, I would only do an air break while at a deco stop, after being on my deco gas for at least 15 minutes. And when I do the air break, I would start from a deco gas, switch to my back gas for 5 minutes, then switch back to the deco gas I was using before. No depth change during the gas break and no intermediate gas involved. Why would I not treat the gas breaks you're talking about the same way? I.e. do my deco gas switches in a more streamlined manner (ala Method 2) and do "gas break" switches to and from back gas on a separate schedule?

The assumption here that needs questioning is if the concept of the o2 clock is valid. I contend that it isn't. It's widely used, but that doesn't mean it's an effective measure of tracking oxygen exposure. Now we're talking about manipulating a poor measure of exposure. Very difficult.

I think that breaks are of value for shorter dives, and especially if you're doing multiple dives over say the course of a week. Assuming there's some validity to the clock, even 30mins on oxygen puts you super high on the exposure scale. In my mind I want to use the same methods used on long dives. It doesn't hurt, and it damn well just might help.

 

[tbone1004]

so in my opinion, like everything there are pros and cons. Pros to switching back to backgas before you switch over is that you can fully stow your deeper deco mixes before going to another one. In OW I think this probably has a bit more benefit than in a cave, but saying "ok, I'm ascending from my 50ft stop, I'm going to switch over to backgas, fully stow this regulator then bottle on my butt *either during, before, or after ascent, then deploy my next deco gas".

In a cave, you can breathe it up, then hang out at the next bottle and swap over while you stow by half picking up a bottle. @PfcAJ may or may not agree with that, unsure of his switch protocols, but my O2 bottles live on my right hand side while the rest live on the left.

In terms of ICD, I think the risk goes up the higher the He percentage and the deeper the depth. A switch from 12/70 straight over to 32% is probably not your most brilliant plan, nor is going back to 12/70 after you've been on a 50% bottle for a while.
If I were to be silly enough to do something like this on OC I'd probably do the following.
If I had a 12/70 bottom mix, and 32% or 35/25 or 30/30 pick your 100 ish foot deco gas mix, I would keep that on the left, then pick up my 50% bottle at 70ft and keep that on my right. When done with that, I'd go back to the 30 ish mix to keep the ICD risk low, then fully stow the 50% bottle on my butt before picking up the O2 bottle which would again be on the right. My "air breaks" would be on whatever that 30 ish percent bottle was as opposed to my 12/70 backgas because of my fear of ICD *whether rational or not*
obviously that dive should be done on a CCR and that changes things.

I'm with AJ on this one

 

[PfcAJ]

I think Icd is one of those textbook risks that doesn't really happen in real life.

I've actually done a fair share of 220-300ft dives on 15-10/60-80 with 35/25, 50%, and oxygen as the deco gas with no problems at all with using bottom gas as an intermediate between deco gases.

All my compadres do the same thing all without issue as well.