Gas switch using air

[JukkaO]

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

It is good to hear about practices divers do. And also others comments especially if there is some real content in them. I did some reading and found same discussions over and over again.
The issue is still unclear. If no flush long CCR profil is working it does not mean that there can not be anything more to learn on the subject.
It is not nice to suggest alin full air flush deeper than alin has done partial ones. I am interested in the idea doing it how alin does. Small flushes first. It means no fast switch. The word “switch” has been used hundreds of times in these discussions. Perhaps one day it would be possible to discuss about other methods.

Jukka

 

[Peter L]

From a pub, deal with it: Change your setpoint not your inerts, f'k with the unknown variables & your ars's dragon fodder, t's going to get bit some day, why only comes after 1K bent goats, t's you or the goat, choose...

Ps. Subject to sobriety, don't believe t'net..

 

[Iain3]

From a pub, deal with it: Change your setpoint not your inerts, f'k with the unknown variables & your ars's dragon fodder, t's going to get bit some day, why only comes after 1K bent goats, t's you or the goat, choose...

Ps. Subject to sobriety, don't believe t'net..

Translates as

I am in the pub writing this, I hope you can accept that: Change your setpoint not the mix of inert gasses in your loop, if you muck around with the unknown variables you're liable to get hurt, 1000s of goats have been bent testing these theories, you can do tests on yourself or on goats, choose...

Ps. Subject to sobriety, don't believe what you read on the internet.

 

[JukkaO]

Excellent choice to think about goats and diving in a PUB. I do it almost every night in bed. It is also very good that this issue gets all the possible attention as divers are still doing flushes and vomiting when ascending from 200m. All the experiences and opinions are welcome. I need to find out where is the line or dark 3D space between different risks. My current setup do not support complete mix management but the equipment will be much more developed after some time. Will there never be no need for measuring other gases than O2 and CO2 in the loop?

Jukka

 

[alin]

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 know but don't forget that sometimes people do bad things. Remember that there are many alpinist divers over here, even if they don't say it. They all know it's not good but sometimes they still dive like that. Even instructors.


[/QUOTE]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.[/QUOTE]


Yes, I dive just warm water in the last 12 yeras or so. I used to dive a lot in cold water but nowadays I don't go much at home and when I go I don't stay long enough to dive.


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


You're right

 

[Iain3]

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

Just a thought, if you are running deco at a 1.5 setpoint you are far more likely to start needing air breaks. I presume you would use diluent for these, not air, for the same reason of not increasing you partial pressure of Nitrogen too fast.

 

[Peter L]

Just a thought, if you are running deco at a 1.5 setpoint you are far more likely to start needing air breaks. I presume you would use diluent for these, not air, for the same reason of not increasing you partial pressure of Nitrogen too fast.

In this scenario an OC air break is to lower the PPO2 of the inhaled gas, on a CCR again you just need to push your set-point down and viola, air break....

 

[matt]

Just a thought, if you are running deco at a 1.5 setpoint you are far more likely to start needing air breaks. I presume you would use diluent for these, not air, for the same reason of not increasing you partial pressure of Nitrogen too fast.

My air breaks look more like silencing the alarm. I'm happy with 150%, which in practice gives 4-5 hours which is about as much as I'm fussed with.

If I'm heading over that then I'd put the SP down and either wait or Dil flush.

The only time I'd go OC is to drink - but I gave that up sometime ago as it is more hazardous than necessary. Good hydration is not lost in 4 hours, IMHO.

Matt.

 

[JukkaO]

We do OC air brakes starting from 10m using one air cylinder for two.
We are afraid of having too much O2. Diluent is 10/60...70 or already flushed lower.

Jukka

 

[JukkaO]

Matthew mentioned twice the risk of gas shifts on the compartments, ok it was subtle but no one seemed to pick up on it.
Making significant gas shifts close to your compartment limit runs a high risk of ICD. The anecdotal posts above give credence.
As a side note;
What can be surprising is the effect on computed deco when working with gases in some older computers. If you were running a 10/52 for example, at the last stop at 3m (sp 1.3) - some older computers would show different TTS if you changed to OC 100%.
Interesting how they handle inerts in the calcs. I bring this up in light of Matts post above with the loop mixes. Ponder away.

How to define what is close to compartment limit?
I have seen the suggestion not to exceed 0,5bar in PPN2 at a switch. If we add the number of switches the rule is not valid anymore?

Jukka

 

[matt]

How to define what is close to compartment limit?

At or near to a ceiling.

 

[JukkaO]

Ok. If we are near the limit but do not do any big shift but just small ones so how to see the effect of a change in He/N2 portion?

It is written also that the shift is not that serious if done deep or shallow but more of a consern if done in between. Why so? 0,5 rule is more limiting at deep than shallow.

By the way... our V-planner in laptop do not give ICD warning. It is activated among other warnings. The graph is drawn right.

Jukka

 

[matt]

The mechanism that causes the problem at the switch I cannot properly describe - you can look at the referenced paper if you want to try and understand it for yourself: Biophysical basis for inner ear decompression sickness

This is a relevant quote:

The flow of nitrogen into the vascular compartment via the arterial blood exceeds washout of helium in venous effluent. The transfer of helium into the vascular compartment by diffusion from the perilymph and endolymph exceeds the counterdiffusion of nitrogen in the opposite direction and temporarily exceeds the washout of helium in the blood.

The calculation is normally done from stop to stop, so a switch from 10/52 to Air at 42m would be calculated as the difference is PN2 between PN2 10/52 at 45m and PN2 Air at 42m (the switch depth) and if the difference is >0.5 then that may indicate ICD if 42m is at or near a ceiling (i.e. a stop depth).

The OC calculation is therefore 2.09-4.108=2.018 and the CCR calculation at SP=1.3 is 1.77-3.9=2.12. The CCR calculation assumes you never breathe the air directly and that the SP come immediately up to 1.3 (PO2 is 1.092 and is therefore not exactly true).

Matt.

 

[JukkaO]

Calculations are done from stop to stop... as all the examples so far.
If we add the number of stops there is no sudden change present anymore.

How to estimate kind of a "pressure" to have problems as I have not seen how the calculations are done.

I just made fills of 13/55 and 50/20 for one diver and I guess there are many many others doing such ultra dangerous choices all the time. I would get stomach problems if I would have more time to think this issue.

Jukka

 

[matt]

I'm not entirely sure of the question - are you routinely doing deep (over 80m) dives with these flushes?

 

[JukkaO]

I can see the example where the pressure difference is not that critical in deep and shallow... just a bit bigger work to do the calculations... and understanding it. I am just not ready to accept that CCR would automatically give the optimal mix.

I am sorry for confusion.... If I do a mix for some diver it has nothing to do with my diving. It was just an example of the very big difference between this discussion and the real life especially among OC divers.

Jukka

 

[matt]

I can see the example where the pressure difference is not that critical in deep and shallow... just a bit bigger work to do the calculations... and understanding it.

I think that's right, that's why you really need planning software and not just a calculator!

I am just not ready to accept that CCR would automatically give the optimal mix.

Optimal for PO2 perhaps, but agreed, not necessarily for inerts.

I am sorry for confusion.... If I do a mix for some diver it has nothing to do with my diving. It was just an example of the very big difference between this discussion and the real life especially among OC divers.

Agreed, internet forum is not reality. But some people have had such decompression problem so it is not all theoretical at least!

Matt.

 

[JukkaO]

I think that's right, that's why you really need planning software and not just a calculator!
Matt.

Is there a planner doing it? Shift size alarm is a different approach.

 

[matt]

Is there a planner doing it? Shift size alarm is a different approach.

You get ICD warnings in Projection, but I didn't figure out the parameters as I don't flush. I think VPlanner does something too. I have a (very basic) XLS too.

 

[JukkaO]

In V-Planner it is possible to take He% down fast without warnings if you do it in many steps and stay below the set alarm level.

Jukka