In loop gas change calculation

dpielsticker

dpielsticker

New Member
Hi I am a revo mod 3 diver and I'm looking for a way to do some dive planning with in loop gas changes.
The dive planning software I have( multideco) is great but unfortunately I can't do in loop gas changes with it.does anyone have an idea how to calculate these profiles or got any idea where I can find some information about the topic?

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Have a look back through your course notes, it should have been covered and then, especially at mod three level. I'm away from home at the moment but I'll check latter to see if I still have notes in my drop box.
 
Interesting, can you post any rules as you understand them...

The issue I see is if you change gasses, particularly the He/N2 ratio, because you can't do a 100% perfect flush, and most units dont have a helium sensor in the loop its basically impossible to know accurately what actual gas mix is in the loop. Given that, you can't accurately predict deco...
If you're going to keep the same inert gas ratio, then little point in changing gas mixes, just let rb inject more O2. Maybe easier to do with an ECCR...

I suspect most assume that good enough flushes / assume 100% loop gas replacement, or perhaps assume that small innacuracies in the mix will have little impact on deco
Personally I think if the loop gas composition is unknown, its an unknown risk, only way to try to (not really effective) hedge against this risk is to pad the deco, if you do this, might as well deco out on the original gas...

Has anyone got any good data on what % of the gasses get flushed on average per flush cycle? I suspect its meaningless, as theres such a variation on how good different people actually are for doing flushes...

The other issue - that may not be relevant for some/smaller dives, is sudden changes in gas mixes, e.g. IBCD, narcosis, gas density changes affecting WOB etc...

If you switch to another unit / go OC, then you CAN know the gas composition exactly i guess...
 
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That's some interesting stuff here guys.

During my mod 3 course I've done a in loop gas changes we also calculated our in loop gas like a miliion times.so I don't have a problem selecting the right gas,and I think if you do a proper flush efficiency should be pretty good.I would like to know if there is a desktop software which runs a buehlmann algorithm where I can do in loop gas changes.
For example:
Let's say I have a 5/75 in loop mix and to optimize my deco profile and want to do a in loop gas change to 10/50 on my way up.does anyone have experience with this sort of stuff?I can obviously do the change and see what my shearwater comes up with but I would like to do a proper dive plan before jump in the water.just curious what's possible and what you guys are doing?

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Multideco will do it. Just put in the gas you want to change, the depth you will change at, and enter 0 for the time.
 
dpielsticker said:
Let's say I have a 5/75 in loop mix and to optimize my deco profile and want to do a in loop gas change to 10/50 on my way up.does anyone have experience with this sort of stuff?
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This was fashionable about 10-15 years ago. In my experience it does not work. For your example 5/75 to 10/50 then I assume you are diving sub-100m and for this dive I would not come off he backgas CCR, I would focus on managing your CNS, your Set Point and your GF's.

I've made several dives of this type and have flushed at 42m to both air and 20/30. Neither gave me clean deco, although neither bent me like the guys using the old-Britannic table. After the 20/30 flush I was so tired I did not dive the next day (112m, 30 mins bottom). I have since made a series of 130 dives on back-gas; felt great, made the dives back to back with an 80 the day after and a 115 the day before.

Matt.
 
The work at NEDU highlights why you would want to keep a high helium content. In the end they decided to stay using Heliox rather than trimix as high He concentrations produced less or less severe decompression outcomes. Do a search and you will find the data. Switching does not provide benefits based on current research.


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Doesnt most software do it? It's just another level with a minimal/zero stop time and new gas.

I agree with Matt. Any time I played with dil switches it left me feeling wiped out and that wasnt even particularly extreme diving (75-100m). I stay on one dil through the whole range.

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Thanks for the feedback guys its some interesting stuff.

I worked out how to do gas gas switch with my planning software and a gas switch shaves off a lot of deco.I think I will try it once and see how I feel and go from there.

To garethingham I always thought that with heliox you have a much greater risk of hpns and I'd say the deco will be much longer as well.never the less I will have a look its probably an interesting read

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dpielsticker said:
I worked out how to do gas gas switch with my planning software and a gas switch shaves off a lot of deco.I think I will try it once and see how I feel and go from there.
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You may well be better off just changing the computer and reducing the deco and stay on the back-gas.
 
dpielsticker said:
Thanks for the feedback guys its some interesting stuff.

I worked out how to do gas gas switch with my planning software and a gas switch shaves off a lot of deco.I think I will try it once and see how I feel and go from there.
Click to expand...

And that is the issue. The math model says its "faster" through manipulating gradients and off gas rates. It's actually accelerated deco, taken to the most extreme position possible. Accelerated deco works everyday in smaller amounts, as in regular OC gas swaps. But deep CCR diving gives the opportunity for some really big changes in gradients. Now if your physiology can't keep up with the very fast gradient math, or your overall tissue state falls behind, or if the tissue inert gas reversal generates its own set of problems, then it's a bad situation - One that has to be avoided to achieve full and satisfactory deco.
 
Our mod3 instructor taught to actually switch to 32% at 100ft. In multideco, it shaves off a lot of deco.

I've never done it, been largely advised by close friends not to, so havent, nor plan to.
 
Jeff Pack said:
Our mod3 instructor taught to actually switch to 32% at 100ft. In multideco, it shaves off a lot of deco.

I've never done it, been largely advised by close friends not to, so havent, nor plan to.
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I have done it on a very big dive that went more than 4 hours over the planned 5 hours. It is a dangerous thing to switch off of high helium to abruptly and increase the nitrogen so quickly. I was fortunate to only end up with shoulder pain for several hours
 
DSix36 said:
I have done it on a very big dive that went more than 4 hours over the planned 5 hours. It is a dangerous thing to switch off of high helium to abruptly and increase the nitrogen so quickly. I was fortunate to only end up with shoulder pain for several hours
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It did shave off about 2 hours if deco
 
Jeff Pack said:
Our mod3 instructor taught to actually switch to 32% at 100ft. In multideco, it shaves off a lot of deco.

I've never done it, been largely advised by close friends not to, so haven't, nor plan to.
Click to expand...

It's like jay walking. Its perfectly OK, except when getting run over by a bus. :rotate: Likewise, as the experience shows above, sometimes its OK and sometimes not.

I added the IBCD ppInert change warning to V-Planner in 2004. And been getting sh1t for it in some circles ever since.

At the recent TekUSA, David Doolette put on a presentation on IBCD, although it was more along the lines of the shallow version, and the skin transfer version. Afterwards I had a private talk with him about the deep / tec version that we are concerned with here.

Turns out that we agree on the theory and causes of the deep IBCD induced injury. Hence I feel vindicated on this matter, and think the IBCD warnings in the program are important and relevant.
 
rossh said:
It's like jay walking. Its perfectly OK, except when getting run over by a bus. :rotate: Likewise, as the experience shows above, sometimes its OK and sometimes not.

I added the IBCD ppInert change warning to V-Planner in 2004. And been getting sh1t for it in some circles ever since.

At the recent TekUSA, David Doolette put on a presentation on IBCD, although it was more along the lines of the shallow version, and the skin transfer version. Afterwards I had a private talk with him about the deep / tec version that we are concerned with here.

Turns out that we agree on the theory and causes of the deep IBCD induced injury.
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No you don't agree. David does NOT agree with the position you have taken over the years which is fundamentally that IBCD can contribute to most types of DCS. You are misquoting him. I have published extensively with David on this issue, and the stance we have taken in those publications reflects our views (including his obviously).

I would draw your attention to his recent NEDU study:

Doolette DJ, Gerth WA. SAFE INNER EAR GAS TENSIONS FOR SWITCH FROM HELIUM TO AIR BREATHING DURING DECOMPRESSION. NEDU TR 12-04.

In which the conclusions read:

This study was designed to assess whether helium-nitrogen counterdiffusion after a heliox-to-air gas switch at 100 fsw during decompression causes unsafe potentiation of inner ear gas-supersaturation with undue risk of inner ear DCS. The results indicate that:
1. Dives to depths up to 220 fsw (704 kPa) with 60-minute bottom time and 651 kPa inspired helium partial pressure followed by no-stop decompression to 100 fsw (408 kPa) and switch to air breathing have low risk of inner ear DCS.
2. The LEM-h8n25 model may be used to compute decompression schedules with a heliox-to-air breathing gas switch at 100 fsw for heliox dives to depths up to 220 fsw and bottom times up to 60 minutes without special consideration of the risk of inner ear DCS. Additional man-testing is required to confirm that the model may also be used to compute schedules for such dives with longer than 60-minute bottom times and for such dives to depths up to 300 fsw with short bottom times

Why don't you plug that into you IBCD warning system and see what it says.

rossh said:
Hence I feel vindicated on this matter,
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Well, you shouldn't.

Simon M
 
I do it on all deep dives. It is not about reducing deco. For me, it is about reducing the risk of ICD if I have to bail out. Of course the curve will be smoother if you stay on your deep dil. But you will run the risk of slamming yourself with nitrogen if you bail out to a low He mix while still having a high He loop. Gradually bringing the loop contents closer to the bailout gas at each depth can reduce the risk of ICD. Use Multi Deco or V-Planner and run bailout scenarios and you will see the results. But this is all a very personal decision.
 
kwinter said:
For me, it is about reducing the risk of ICD if I have to bail out. Of course the curve will be smoother if you stay on your deep dil. But you will run the risk of slamming yourself with nitrogen if you bail out to a low He mix while still having a high He loop. Gradually bringing the loop contents closer to the bailout gas at each depth can reduce the risk of ICD
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Hello Ken,

"Slamming yourself with nitrogen" was the subject of the study by David that I quote above. The risk has almost certainly been overhyped, AND contrary to prevalent perceptions the only tissue known to be at risk in the process when the gas change is from high helium to high nitrogen is the inner ear.

This is the one of the most misunderstood and misquoted issues in technical diving. There has been a substantial amount of work on counter-diffusion by diving physiologists over the years because the phenomenon is so interesting. However, virtually all of the research and interest has focussed on scenarios that are physiologically fascinating but irrelevant to what we do as technical divers.

Most of the attention has been on the effect of breathing various gases when the body is surrounded by helium. This is not a scenario relevant to gas switching during decompression. There has also been some interest in the potential for problems if a diver saturated with nitrogen at high pressure changed to breathing helium (again, not really relevant to us). In contrast, there is general agreement that switches from helium to nitrogen (which is what we do) should (if anything) reduce tissue supersaturation with the one exception being the inner ear. David's development of a model for predicting inner ear inert gas kinetics in 2003 confirmed that a small increase in inner ear supersaturation does occur after gas switches in the helium-to-nitrogen direction, but his recent study (which I quote in my reply to Ross above) suggests that the risk of clinical consequences is low, even when the gas switches very much constitute a "slam" as you put it.

I have linked below to a previous post on the subject which attempts to explain it a little more fully.

https://www.thediveforum.com/showth...ounter-diffusion&p=16159&viewfull=1#post16159

Simon M
 
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