Tech Diving Mag - issue 27

[Red Sea Shadow]

Available for download @ Tech Diving Mag | Free online technical diving magazine

Contents:
* Temperature & DCS
* Kobanya
* Namibian Diving
* Diving Fads
* VPM-B: shaping the curve

 

[Red Sea Shadow]

Currently looking for new, well-illustrated articles for the next issue. If you have something to contribute drop me a line.

 

[Michael Guerrero]

For the shaping the curve article, I was disappointed that the author did not discuss the research by Doolette indicating that nitrogen and helium have similar on-/off-gassing rates in aqueous tissues. That should be relevant to any decompression algorithm that seeks to differentiate between those two inert gasses and their impact on ascent curves.

 

[JureZ]

Let's not jump the conclusions here. I agree that nitrogen/helium rate ratio is an assumption made on first mixed gas models, and should be, like any assumption made in the past, subject to revision. But the article you mentioned does not say that nitrogen and helium on-/off-gassing rates are the same. It shows that the deco obligation on helium and trimix is the same.

But is it true? The need for mixed gas algorithm was a result of air (nitrox) algorithms not give adequate results for trimix. If the conclusion you made would be true. We would all be fine diving trimix dives with nitrox algorithms. Agree?

The trick is that the shape (and not necessarily the total duration of deco stops) for helium rich gases is different.

Regarding the article you mentioned. It is interesting reading, but I do not see how we can use it in solving our problem. It again shows only that heliox gives better results than trimix on a dive profile generated by an algorithm we know nothing about.

 

[Michael Guerrero]

I'm not sure I'm understanding your response JZ, assuming it was directed toward my post. And I'm not sure we're talking about the same research. The research I'm referring to looked specifically at washout rates for Nitrogen and Helium.

My point is that that research should influence the shape of the curve, particularly in the early stages of an ascent, where the "fast" tissues are most affected.

 

[JureZ]

You are right. Slower on/of gassing of helium would have reduce the deep stops and increase the shallow stops duration and that is exactly what I wanted to achieve. It is quite possible that we are talking about different research. Could you please give me some reference on research you have in mind.

 

[Patoux01]

But is it true? The need for mixed gas algorithm was a result of air (nitrox) algorithms not give adequate results for trimix. If the conclusion you made would be true. We would all be fine diving trimix dives with nitrox algorithms. Agree?

Assuming:
- the modeling of the physics is correct
- the maximum stress allowed (M-values) is correct
- a whole bunch of other stuff
Then yes, using a nitrox algorithm could be the right way to go. That is what is done by some that want to skip the helium penalty (eg when diving helium in the shallows).

The current state was summarized as "We're doing the right amount of deco, but not for the right reasons" ( Eliminating The Helium Penalty - Shearwater Research ).


I suspect Michael refers to Altering blood flow does not reveal differences between nitrogen and helium kinetics in brain or in skeletal miracle in sheep. - PubMed - NCBI

I didn't read the tech diving mag article though, so pardon me if I'm wrong, simply seems this is what you guys are talking about.

 

[JureZ]

Looks like Michael and me were talking about different Doolette's article

I did some simulations with VPMOpen (OC) planner and results are:
All profiles are 70m/25min
Nx17-Nx50 rt= 146min
Tx17/40-Nx50 rt= 120min
Tx17/80-Nx50 rt= 155 min
All three profiles are almost the same - no helium penalty

But the good stuff comes when running single gas profiles (that would be somewhat similar to CC profiles):
Nx17 rt= 274 min
Tx17/40 rt= 271 min
Tx17/80 rt= 613 min
The profiles are almost the same on the deep stops. The huge difference is on the shallow stops, because of higher on gassing of slow tissues during the 21m and shallower stops when using heliox and assuming classical on/off gassing rates.
Of course the differences would be smaller if CC planners were used.

I will try to do some shaping and see what hapens.

 

[rossh]

Coming soon..... VPM-LS (Low Stress). with all the latest adjustments inbuilt for recent studies.

 

[JureZ]

Ross, I know you do not like to do that, but could you share the phisics behind it?