Will http://www.ncbi.nlm.nih.gov/pubmed/25525213 change deco procedures?

[100days-a-year]

Interesting,I may start diving Mix on my EDGE to try this out.What could happen,HE is our friend right?

 

[dreamdive]

If I tell shearwater that I dive a 7/20 in my CCR Dil instead of the 7/70 that I am actually diving, my profile is SHORTER. Same dive ... just adjusting the Helium concentration and I get a shortened deco profile. That is what I understood 'helium penalty' to mean. I know what Doolette said, but I am not sure that we are talking about the same thing.

 

[boulderjohn]

Some agencies have taught (and still teach) that when you dive with helium, your deco times will be longer because helium is absorbed into tissues so much faster than nitrogen. Yes, it leaves faster as well, but off-gassing is supposedly not as fast as on-gassing.

 

[PfcAJ]

Some agencies have taught (and still teach) that when you dive with helium, your deco times will be longer because helium is absorbed into tissues so much faster than nitrogen. Yes, it leaves faster as well, but off-gassing is supposedly not as fast as on-gassing.

Most popular deco algorithms show this, as well.

 

[Agility]

What I learnt from the thread on CCRX is that using the common deco models we have a larger risk of DCS on deeper dives than shallower ones using the same model or table.
I had assumed they had been adjusted to give long enough deco on deep dives to result in constant DCS probability across all depths.

 

[rossh]

In a nutshell

1 - You might need the longer deco that the 'helium penalty' gives you (maybe even more), but not because of the helium, but because longer and deeper dives (where you're apt to use helium) carry a higher % of DCS risk.
2 - For a given gradient factor setting, decompression profiles are not iso-risk across depth and time combinations.
3 - Common deco algorithms aren't great on very long and/or very deep dives.

Hmmm. Here is some thoughts on those ideas.


1/ If helium is no longer the culprit causing DCS, then what is causing us the DCS that is to be avoided ??? Are they now suggesting that DCS is no longer caused by excessive gas pressure stresses, but instead now its being caused by other (external) influences like cold, or physical activity stresses, or what?? Are they suggestion He now has non-linear kinetic rates with pressure changes?

When we pressed Simon and David on this issue, they respectfully declined to answer, or clarify this conundrum.


2/ Problem with GF in this situation, is that it's over riding the ZHL model. So you cannot be comparing random x/y GF values here. You need to go back to raw ZHL as a comparison point. Also see next answer re; iso-risk.



3/ ZHL has issues with long dives, because the ZHL formula (paired A/B coefficients matched to a fixed half time) is rather inflexible. When the formula is extrapolated to longer dives, then is tends to go off scale or out of context. Plus if we add GF to the result, it compounds the exaggeration as GF has its own set of issues with expanding times.

VPM-B, on the other hand, is far better at keeping context with longer dives, as the physics behind the models controlling ascent limitations (micro-bubbles), are not bound in the same way as ZHL is. VPM-B calculations are more dynamic and better able to match the dive conditions.

---------- Post added December 15th, 2015 at 05:02 AM ----------

I wonder whether anyone will adapt the Bühlmann algorithm for Trimix diving?

Can it change ZHL Bühlmann into something new? Not really.


The ZHL calculation comprises of 16 (or 32) sets of 3 matched values (a halftime, with a matched A/B value). In order to modify ZHL successfully, one would need to redefine all three. That would result in a whole new algorithm and model. And when its finished, its still a range bound and inflexible design.


But, VPM is more than capable of handling any gas kinetic changes that might come our way, without any real modifications needed. Maybe if the science can come up with enough tested date points, we can have a VPM-C one day

 

[Agility]

Adjusting the slope via the a/b coefficients should do the trick, should it not?

 

[rossh]

Adjusting the slope via the a/b coefficients should do the trick, should it not?

Hi, that will give you a different result. Maybe if the 'new kenetic' proves to be linear then it could be merged in. But it needs a whole test program, because the A/B is a fitted value to achieve the desired angle.

But no one much wants to do real world ZHL Buhlmann anymore. It's all patched up and fudged out longer on GF adjustments now. But whatever is derived, it can't overcome the inflexibility and limited scope problems of the model design of 3 fixed values. The end result would still be range bound, but just in a different way.

 

[PfcAJ]

Hmmm. Here is some thoughts on those ideas.


1/ If helium is no longer the culprit causing DCS, then what is causing us the DCS that is to be avoided ??? Are they now suggesting that DCS is no longer caused by excessive gas pressure stresses, but instead now its being caused by other (external) influences like cold, or physical activity stresses, or what?? Are they suggestion He now has non-linear kinetic rates with pressure changes?

When we pressed Simon and David on this issue, they respectfully declined to answer, or clarify this conundrum.


2/ Problem with GF in this situation, is that it's over riding the ZHL model. So you cannot be comparing random x/y GF values here. You need to go back to raw ZHL as a comparison point. Also see next answer re; iso-risk.



3/ ZHL has issues with long dives, because the ZHL formula (paired A/B coefficients matched to a fixed half time) is rather inflexible. When the formula is extrapolated to longer dives, then is tends to go off scale or out of context. Plus if we add GF to the result, it compounds the exaggeration as GF has its own set of issues with expanding times.

VPM-B, on the other hand, is far better at keeping context with longer dives, as the physics behind the models controlling ascent limitations (micro-bubbles), are not bound in the same way as ZHL is. VPM-B calculations are more dynamic and better able to match the dive conditions.

---------- Post added December 15th, 2015 at 05:02 AM ----------




Can it change ZHL Bühlmann into something new? Not really.


The ZHL calculation comprises of 16 (or 32) sets of 3 matched values (a halftime, with a matched A/B value). In order to modify ZHL successfully, one would need to redefine all three. That would result in a whole new algorithm and model. And when its finished, its still a range bound and inflexible design.


But, VPM is more than capable of handling any gas kinetic changes that might come our way, without any real modifications needed. Maybe if the science can come up with enough tested date points, we can have a VPM-C one day

Ross, I'm not nearly as polite or patient as Doolette and Mitchell. All the questions you're asking me now have been answered by them over on ccrx.

1- No? Wtf do you get that idea out of what I wrote? Helium can (and does) cause DCS, I don't see where I suggested otherwise or where anyone has suggested that.... Adding extra deco just because of helium appears to not be warranted.

2- Its a problem with the 'big two' deco algorithms. None of them are calibrated to provide pDCS based ascent plans. You are unable to tell me what the pDCS is using vpm+2, just as I am unable to tell you the pDCS for a 20/85 ascent. Similarly, pDCS for a 20/85 ascent following a 20min@300ft dive is totally different (greater) than the pDCS for a 20min@150ft dive. Along the same lines, pDCS for a 20/85 ascent for a 20min@300ft does not equal pDCS for 80mins@300ft. Same same with VPM. It makes comparing apples to apples very difficult.

3- When I compare buhlmann (default 20/85) with vpm +2, the times are almost always shorter with VPM. Doing LESS deco (vpm) isn't going to help reduce your dcs rate.

 

[shoredivr]

There's a great thread about this over on CCRExplorers with people like Simon Mitchell and David Doolette chiming in.

Interesting read.

More thoughts on helium perfusion and solubility