Dil switch off helium during deco

[tbone1004]

@beldridg why do you not want the high ppO2 hit at 70ft? It's the same switch you'd be making on OC, and you're only there for a few minutes while your body is dumping inert gas in the loop so while it may be 1.6 for a second or two, it's going to be falling pretty rapidly.

Personally I always change over and flush. The first flush that I do is typically at 70ft with the 50% bottle and it helps to get some of the He out *gets it out of the loop but your body is still dumping plenty of it into the loop*, but more importantly is the ability to validate cells at ppO2 of 1.6. since I run the bottom portion at 1.0-1.1 and don't want to run much higher than that until I can get a dil validation. One I get the dil validation, I know I can run up to 1.6 when I choose to and will typically bump up to 1.6 at each depth change and let it fall back down to 1.2 or so.

 

[macado]

I almost always carry Nx50 on those dives (and not Nx32). Any thoughts about flushing with Nx50 at 60' (I don't want the high PO2 hit at 70') instead of Nx32 at 100+'?

I'm guilty of doing this although I dil flush at ~65-70' to spike to ~1.5-1.55 and just breathed the ppO2 down in the loop.

Assuming a dil of 10/70 at 70' and 1.2 ppO2 the loop composition would be ~38.7% O2, 47.6% He, 13.6% N2 if my math is correct. There is still a decent amount of helium in the loop at 70' so depending on what theory you believe it should make a difference.

*Not an endorsement for doing it but just stating some numbers/thoughts.

EDIT: I missed @tbone1004 's response above. The flushing to 1.6 for cell validation at 70' (as opposed to waiting until 20' is an interesting idea).

 

[beldridg]

@beldridg why do you not want the high ppO2 hit at 70ft? It's the same switch you'd be making on OC, and you're only there for a few minutes while your body is dumping inert gas in the loop so while it may be 1.6 for a second or two, it's going to be falling pretty rapidly.

Personally I always change over and flush. The first flush that I do is typically at 70ft with the 50% bottle and it helps to get some of the He out *gets it out of the loop but your body is still dumping plenty of it into the loop*, but more importantly is the ability to validate cells at ppO2 of 1.6. since I run the bottom portion at 1.0-1.1 and don't want to run much higher than that until I can get a dil validation. One I get the dil validation, I know I can run up to 1.6 when I choose to and will typically bump up to 1.6 at each depth change and let it fall back down to 1.2 or so.

Really just a safety factor for a few reasons:

1) I have a rEvo so it is always leaking in O2 so it really wouldn't drop that much or at least not that quickly. But, as you note, I'd be dumping a bunch of helium into the loop as I off-gas so maybe it would be.

2) On my last deep dive, the 70 foot stop was 4 minutes which isn't terribly long but isn't also short. On a deeper, longer dive, it could be easily twice that.

3) I run 1.3 on the bottom and while I don't necessarily let CNS be the limiting factor on a dive, I do pay attention to it and it tends to increase pretty quickly on 1.6

But, I do like your idea of cell validation at 70 feet. Thanks for the ideas and feedback. I need to spend more time thinking about it.

Regards,

- brett

 

[tbone1004]

@beldridg

1-even with mCCR's it drops pretty quick once you start doing deco, especially on He.

2. not sure what your GF's are set to and I don't run that high of a ppO2, but you may want to consider going to your low setpoint for deco and let it decay naturally as you go through the stop and/or during depth changes. You need to be dumping a lot of gas out of the loop on ascent anyway so I don't like to let the solenoid run on deco since it makes the loop volume highly volatile which is properly annoying.

3. curious why you're running that high on that deep of a dive, but regardless a momentary blip at 1.6 isn't going to be that consequential. On that 4min stop I would be surprised if it doesn't drop to 1.3 before you are ready to ascent.

chew on it, it's very different than what most people teach because it's harder than just riding the solenoid, but I think it's much better.

 

[Wibble]

I also extend my deco on deeper dives. I usually add an extra 3-5 minutes of deco and then spend 3-5 minutes slowly coming up from the last stop (in my case, usually 6m/20'). However, I don't breathe O2, I just stay on the loop at 1.3.

Why not just decrease your GF-hi?

BTW what GFs do you dive with?

 

[beldridg]

@beldridg

1-even with mCCR's it drops pretty quick once you start doing deco, especially on He.

2. not sure what your GF's are set to and I don't run that high of a ppO2, but you may want to consider going to your low setpoint for deco and let it decay naturally as you go through the stop and/or during depth changes. You need to be dumping a lot of gas out of the loop on ascent anyway so I don't like to let the solenoid run on deco since it makes the loop volume highly volatile which is properly annoying.

3. curious why you're running that high on that deep of a dive, but regardless a momentary blip at 1.6 isn't going to be that consequential. On that 4min stop I would be surprised if it doesn't drop to 1.3 before you are ready to ascent.

chew on it, it's very different than what most people teach because it's harder than just riding the solenoid, but I think it's much better.

I run 50/80 nowadays but I'm thinking about going to 50/75 given my age and the actual profiles I end up running. I almost never exit the water with anything greater than 75.

I agree and I always go to my low set point when I start my ascent and run manually. I hate the solenoid always firing on ascent / deco and the loop volume spiking as you indicate so I started doing that many years ago.

I've thought a bit about which set point to use. It is a bit dated, but here is an article I wrote about it and my thoughts:

Musings on CCR set point

I had an interesting conversation last week with another diver about what set point to use for a specific dive profile on a rebreather. The other diver sometimes used 1.2 to cut down on the amount …

wreckedinmyrevo.com

I need to run more profiles in the 250-350' range to see what the overall runtime and CNS loading would be at different set points.

- brett

 

[beldridg]

Why not just decrease your GF-hi?

BTW what GFs do you dive with?

I run 50/80 but I'm considering changing to 50/75 because it more closely reflects what I actually exit the water with. I always check my SurfGF before exiting and it is 70-75 on most dives.

- brett

 

[Garth]

So does the data support the helium penalty or does it not? I've heard of people doing all kinds of things to avoid their computer from giving them a helium penalty.
Has anyone switched their computer without flushing all the helium out or did those people stop posting on here indefinitely?

 

[moose_grunt]

Adujsting one's practice with O'Dive - O'Dive

On the other side of the world, Australian diver Andrew Oakeley had the opportunity to use the O’Dive system put at his disposal… Read more

o-dive.com

Its referenced here and discussed by the author on ccrx fb page, with demonstrable results in terms of bubble score, although other changes to ascent habits were also incorporated.

I think it'll be really interesting to see how this discussion goes when more people have the O'Dive and share their data from different strategies.

I don't have an O'Dive, but I have seen one in use on dives I've been on. As I recall, it typically just said we needed more helium and less sawtooth (hardly a revelation for those dives).

From the post you linked, he did ultimately lower his score. Unfortunately he also incorporated a number of changes that seem likely to have affected the score as much, or greater than, a 32% flush. Kind of hard to pick out whether the flush itself had any great effect. Contrary to our conversation here, he says that he does the flush, but follows the deco profile given for his helium--he's not using a nitrox flush to shorten his deco time.

 

[rjack321]

I almost always carry Nx50 on those dives (and not Nx32). Any thoughts about flushing with Nx50 at 60' (I don't want the high PO2 hit at 70') instead of Nx32 at 100+'?

Same, I am never carrying 32% along on a deep trimix dive.

If you flush with 50% at the end of the 70ft stop (let's assume you actually get it up to 1.6) your ppO2 will drop to 1.4 after you move up to 60ft.

If you only manage to flush your 70ft stop up to 1.5 you'll be at 1.35 after coming up to 60ft.

So does the data support the helium penalty or does it not? I've heard of people doing all kinds of things to avoid their computer from giving them a helium penalty.
Has anyone switched their computer without flushing all the helium out or did those people stop posting on here indefinitely?

Paraphrasing Simon from past posts

The helium penalty doesn't seem to be supported by empirical data but there are other (ill defined physiological) factors which increase risk as you go deeper. So by following the helium penalty you at least partially addressing the rising risk of DCS for other reasons.