Gas switch using air

[Peter L]

I get a clenched testicular sensation around the absence of knowledge of what's in the loop to calculate deco without a HE or N2 cell and the repeat opportunities for ICD if this was done aggressively. HE is too fast a gas to go inviting it to run amok and with cold peripheral tissues thrown in to boot I think your way outside of any experimental guidelines that would be accepted for study in this century. You may be onto something or you might just have found an area of statistical miss-alignment on deco models, I would not want to know which in a hurry for the odds that it's the stats and the deco risk.

You want to play lab rat, do go ahead, take good notes, your outside of known science though..

 

[matt]

What do you mean with "significant gas shifts"? The blue line indicates such as from 70% to 60% at 100m and I think even smaller step is still very usefull. The curves are made with current V-Planner.

Jukka

I'm still unclear what the gasses in the charts are, but I'll add something else here on IDC. I agree that the evidence is more anecdotal than empirical - I guess the sample size is just too small.

Normally the maximum recommended jump in ppN2 is 0.5 (that's the guess bit, I reckon). It's generally accepted that the comparison is taken between the two stops and not at the same stop.

Therefore as well as knowing the gas we'd need also to know the depth of the stops.

Let's take my previous example of an Air flush at 60m:

10/52, 63m, SP=1.5, ppN2=2.449
Air, 60m, SP=1.5, ppN2=5.5

Difference is 3.051 which is definitely an ICD risk, and I'd say its why I felt bad afterwards.

The switch to 20/30 is better, but is still 0.9885, which is probably why it didn't work any better at that depth.

Most of what I learnt about IDC comes from John Bennett, BTW.

Matt.

 

[JukkaO]

Below there are five steps to reduce helium content in the mix. Not one as has been mentioned in the many examples with bad results.

The gasses in the charts (calculations) have PPO2 1.1 and from 60m 1.3 and He as in the table:

m He% "CCR" He% OC
120 70 70
100 69 60
80 67 50
60 65 35
40 60 20
20 50 0
10 39 0

I guess OC divers would be happy to have such mixes available and nobody would consider using “CCR” mixes.

Deco calculations were made using slightly different steps but the difference is not big.

Jukka

 

[Peter L]

Not sure I am getting you here......

 

[JukkaO]

Not sure I am getting you here......

Part of the question.

Would you see using the following mixes a big risk when ascending (CCR or OC) from 120m?

120m 9/70
100m 10/60
80m 12/50
60m 19/35
40m 26/20
20m 43/0
10m 65/0
etc.

Jukka

 

[matt]

I have not grasped the concept yet.

What is the purpose of He% "CCR" and then He% OC?

How does ppO2 slide from 1.1 to 1.3?

Matt.

Below there are five steps to reduce helium content in the mix. Not one as has been mentioned in the many examples with bad results.

The gasses in the charts (calculations) have PPO2 1.1 and from 60m 1.3 and He as in the table:

m He% "CCR" He% OC
120 70 70
100 69 60
80 67 50
60 65 35
40 60 20
20 50 0
10 39 0

I guess OC divers would be happy to have such mixes available and nobody would consider using “CCR” mixes.

Deco calculations were made using slightly different steps but the difference is not big.

Jukka

 

[JukkaO]

I have not grasped the concept yet.

What is the purpose of He% "CCR" and then He% OC?

How does ppO2 slide from 1.1 to 1.3?

Matt.

Majority likes not to flush and He% "CCR" + off gassing is the mixture used then. Hi He -> long deco.
He% OC is calculated using mixes selected using about the same END during the ascend -> 5h dive would be only 3,5h.

Jukka

 

[matt]

ppN2 differences below, assuming -3m for the previous stop and SP=1.3.

First 3-4 switches not great for IDC.

Matt.

120m 9/70
100m 10/60 -> OC=0.927 CCR=0.926
80m 12/50 -> OC=0.63 CCR=0.658
60m 19/35 -> OC=0.446 CCR=0.646
40m 26/20 -> OC=0.262 CCR=0.428
20m 43/0 -> OC=-0.072 CCR=0.240
10m 65/0 -> OC=-0.611 CCR=-0.3

 

[Peter L]

But unless you bail out there is no such thing as a pure loop without the rebreathed off-gassing components so the % inerts will never match your maths.

 

[matt]

But unless you bail out there is no such thing as a pure loop without the rebreathed off-gassing components so the % inerts will never match your maths.

Nothing ever matches my math I don't think the maths is relevant, I don't even understand the question yet!

I'd never do a flush CCR, never.

Matt.

 

[JukkaO]

ppN2 differences below, assuming -3m for the previous stop and SP=1.3.

First 3-4 switches not great for IDC.

Matt.

If you would need to bail out at 120m you would have a much bigger change in ppN2 as you would not have so many different cylinders for ascend. How do you compare these 5 steps to that situation?

Jukka

 

[matt]

If you would need to bail out at 120m you would have a much bigger change in ppN2 as you would not have so many different cylinders for ascend. How do you compare these 5 steps to that situation?

Jukka

I think we're moving into apple and orange territory. If you're bailing out then you're in big trouble, drowning is right on the cards way ahead of IDC.

I wouldn't dive to 120m OC, so the point is moot.

When I did dive to 80m OC 15/45 I would take 50% and O2 and the ppN2 for the first switch is 0.19, so also not great - but as I said before and as you agree with, choices are limited.

But the bottom time is tiddly in comparison; probably 75-90 mins in the water versus 180-240 CCR.

Myself I do not know the limits for IDC, for sure deep is problematic and I also think long maybe too. I've never had an inner-ear bend and I have no clue how relevant ppN2 calculation is for recreational diving; it's just something to be aware off and another input into deco-voodoo.

Peter L is right though, I also believe the reason flushing is flawed on CCR is that it is incomplete and ineffective due to off-gassing.

Matt.

 

[JukkaO]

I think we're moving into apple and orange territory. If you're bailing out then you're in big trouble, drowning is right on the cards way ahead of IDC.

Matt.

I am not too good with the prases in english. Do you mean that bail out does not help if you are deep? I guess most are planning to exit with OC if needed.

Many divers have survived from deep OC dives. I start to wonder how is that possible as so many are telling that the gases used on OC dives are so dangerous to use.

Jukka

 

[matt]

I am not too good with the prases in english. Do you mean that bail out does not help if you are deep? I guess most are planning to exit with OC if needed.

Many divers have survived from deep OC dives. I start to wonder how is that possible as so many are telling that the gases used on OC dives are so dangerous to use.

Jukka

All I'm saying is that in comparison to the number of deep long dives that are made today CCR there are relatively few OC ones to compare with. But you're asking the wrong guy as I think OC to 120m is foolhardy.

Matt.

 

[JukkaO]

One diver pointed some Advanced Diver Magazine articles for me. It seems N2 fraction in the mix should not be changed too much. Better to keep it quite constant.

I guess I will learn a bit more soon and try to find out where the optimal mix might be. Or someone who knows will tell.

Practical solution how to use the info is still another issue but it does not bother me now as I would just like to know more first. At the moment I believe that the safe way is not to do flushes. On a long dive the length becomes also an issue and there might be some better ways to manage mixes then.

Jukka

 

[matt]

One diver pointed some Advanced Diver Magazine articles for me. It seems N2 fraction in the mix should not be changed too much. Better to keep it quite constant.

I guess I will learn a bit more soon and try to find out where the optimal mix might be. Or someone who knows will tell.

Practical solution how to use the info is still another issue but it does not bother me now as I would just like to know more first. At the moment I believe that the safe way is not to do flushes. On a long dive the length becomes also an issue and there might be some better ways to manage mixes then.

Jukka

Yes, I think that's right. On CCR we don't have to worry about the mix, so for me that's not the way to optimise the profile.

Can you post the link the the article?

Here's some more info on IDC: Isobaric Counter Diffusion - What is it?

Matt.

 

[Simon Mitchell]

However, a change of gas at a compartment limit will result in transient increases in total inert gas pressure. How much increase? Dunno, I've seem little empirical data or studies to comment.

Hi John (I'm guessing),

See...

DOOLETTE DJ, MITCHELL SJ. A biophysical basis for inner ear decompression sickness. J Applied Physiol 94, 2145-2150, 2003

We made some estimates in relation to the sort of dive being discussed here.

You are absolutely on the money with your first sentence, and it is part of the issue that this discussion has ignored. It is He to N2 switches when a vulnerable tissue (specifically the vascular labyrinthe) is already supersaturated that maximises the risk, rather than the gas switches per se. That's why the term "isobaric" is not really correct in this setting. There is nothing isobaric about the circumstances underwhich these switches are made. We are in the process of ascending (which is not isobaric) and because of that there may be pre-existing supersaturation. A small transient increase in tissue supersaturation arising from a gas switch can be the "straw that breaks the camels back" under these circumstances.

Part of the question.

Would you see using the following mixes a big risk when ascending (CCR or OC) from 120m?

120m 9/70
100m 10/60
80m 12/50
60m 19/35
40m 26/20
20m 43/0
10m 65/0
etc.

Jukka

In theory no, not really. Except if I really was on O/C I would bump the oxygen fractions to produce a PO2 as high as you would be happy to accept as a set point on a CCR. A CCR would take care of that. This is all fairly theoretical though. The plan seems somewhat impractical as an OC dive, and difficult to "regulate" on a CCR dive.

If i don't respond to any replies it is because I am on my way to Bikini!!! and may not have internet access for the next 12 days.

Simon M

 

[divetheworld]

Hi John (I'm guessing),

It's Brent, John is the good looking one.

See...

DOOLETTE DJ, MITCHELL SJ. A biophysical basis for inner ear decompression sickness. J Applied Physiol 94, 2145-2150, 2003

We made some estimates in relation to the sort of dive being discussed here.

I will definitely take the to check that out. Thanks.

You are absolutely on the money with your first sentence, and it is part of the issue that this discussion has ignored.

(smug mode - on)

It is He to N2 switches when a vulnerable tissue (specifically the vascular labyrinthe) is already supersaturated that maximises the risk, rather than the gas switches per se. That's why the term "isobaric" is not really correct in this setting. There is nothing isobaric about the circumstances underwhich these switches are made. We are in the process of ascending (which is not isobaric) and because of that there may be pre-existing supersaturation. A small transient increase in tissue supersaturation arising from a gas switch can be the "straw that breaks the camels back" under these circumstances.

That's great, thanks for taking the time out to explain. As there is so much misinformation and even an incorrect term, the more the message gets out, the safer we will be.

If i don't respond to any replies it is because I am on my way to Bikini!!! and may not have internet access for the next 12 days.

Simon M

(smug mode - off)
That Simon Mitchell bloke, Bit of a tw@t really.

 

[alin]

I know I'll get a lot of bollok for this but here we go:
On long deep dives (long for me it's over 2 hours for ex), sometimes I flush a bit with air starting from 30 meters up. Not all in once but over a few minutes (don't want to get into a ICD debate here). I leave my computers on TX diluent until I make a final complete air flush. Even then, once every 10 min or so, I flush again to remove the traces of offgassed He. I feel a bit tired after sometimes but to be honest, most of the times I don't. Anyway I have insomnia and it doesn't bother me at all to feel tired, hopefuly it will help me to sleep better
I'm using 2 VR3's, plus a Vytec as bottom timer and slates but I run the dive on VR3 and I haven't been bent yet.
And why I am doing this? It's just to cut the deco shorter but I'm not giving advices here, this is just what I do and so far it worked for me.
All the best.
alin

 

[matt]

I know I'll get a lot of bollok for this but here we go:

Not at all - it's your dive!

On long deep dives (long for me it's over 2 hours for ex), sometimes I flush a bit with air starting from 30 meters up. Not all in once but over a few minutes (don't want to get into a ICD debate here). I leave my computers on TX diluent until I make a final complete air flush. Even then, once every 10 min or so, I flush again to remove the traces of offgassed He.

If you wish to accelerate your decompression then there are safer ways that this IMHO. Gas flushing on CCR is bad news, for me at least.

I feel a bit tired after sometimes but to be honest, most of the times I don't.

Do you normally dive warm water? I suspect water temperature make a difference. I'd made a few such flushes in a range of situations and just one time I was so tired it took 24 hours to recover.

And why I am doing this? It's just to cut the deco shorter ...

You get an almost identical saving by increasing the ppO2 to 1.5 before the ascent, and you do not need to flush.

If you crave the extra saving then I'd shift to air at 42m without flush, try it and see if you feel better (I doubt you'll feel worse).

If you want more then try watching the bag volume during the last 30 mins.

The more you shave the more likely you die, so try at your peril.

Matt.