Gradient Factors - What is Everyone Using?

[JMarc]

Longer stops mean that you are leaving the stop with all compartments closer to saturation. Those which were supersaturated are less so. Those which weren't yet at saturation are more saturated.

With normal profiles that means that you are trading off less load in faster compartments for more load in slower one. It seems to be that a serious study is needed to know at which point the trade-off becomes less profitable. Especially that my guess is that we know the MValues of slow compartments with more incertainty for purely measure theory reasons (and we already know that both the MValues and the load are subject to lot of individual and circumstantial variations).

I'm far from being a specialist in the matter, I'd like to have references to sources more in depth than the usual one targeting divers ("Deco for divers" for instance)

 

[boulderjohn]

I'm far from being a specialist in the matter, I'd like to have references to sources more in depth than the usual one targeting divers ("Deco for divers" for instance)

Deco for Divers was written by a deep stop enthusiast (Mark Powell) at the peak of his deep stop enthusiasm. Since the second edition, he has revised his stance and decided that some stops are too deep. He has since done some research (unpublished) which includes a test which to my knowledge had not been done previously. He not only tested people for venous gas emboli (VGE) after surfacing, he continued to test them at intervals after that. As I recall, the people who had done the shallower stops had VGE decrease over time as expected, but the deeper stop divers did not, possibly indicating that VGE were still being formed on the surface.

 

[Wibble]

Mark Powell is not a deep stop enthusiast**. Eight years ago he was using 50:80 and adjusted the GF-hi as appropriate and, as far as I know, he still uses that, maybe with slightly lower GF-hi in line with current thinking.

I took several courses with him and he wasn’t at that time a proponent of deep stops but discussed it during his courses. VPM (bubble models) had fallen from favour around then and their consequential deep stops were skewered by the navy research.

Bulhmann + GF is pretty much the only game in town nowadays for decompression diving. Similarly, gradient factors around 50:75 are most common (e.g. in the range of 50:70 to 50:80)


** Obviously I cannot speak for Mark but in my interactions with him he never pushed deep stops although discussed them in his classes, generally along the lines of "Why would you want to on-gas at a deep stop?"

 

[exodusmaximus]

Those are equally conservative for no-stop (NDL) dives.

If that is truly DAN's current guidance, they are woefully out of touch with that of the researchers in the field. 30/70 is viewed as LESS conservative than 50/70 for mandatory-stop (deco) dives. See the videos and articles linked previously in this thread for more details.

Yes and no. It's fast loading vs slow loading. The main difference in in 50 vs 30 is the depth of forst stop - it all depends on your dive (how deep, how long, what deco gases are you using) the default gradient these days, for many dive plan models, is 50/75

Let's take a 55m dive on air with 20 min bottom time and deco using nitrox 40 (for simplicity and availability) and pure 02.

At 30/70 you will make your first stop at 27m with total dive time around 62 m (depending on the last stop 02)

At 50/75 your first stop is at 24m (switch to nitrox) for total dive time of 57 min (only thing changing is the gradient) - it doesn't look like 50/70 is more conservative (50/70 kept the same first stop but added 2 more minutes)

Basically the differe between 30/70 and 50/ 70 is omitting the first stop at 27m (30/70 plan) for 2 min.

 

[exodusmaximus]

Mark Powell is not a deep stop enthusiast**. Eight years ago he was using 50:80 and adjusted the GF-hi as appropriate and, as far as I know, he still uses that, maybe with slightly lower GF-hi in line with current thinking.

I took several courses with him and he wasn’t at that time a proponent of deep stops but discussed it during his courses. VPM (bubble models) had fallen from favour around then and their consequential deep stops were skewered by the navy research.

Bulhmann + GF is pretty much the only game in town nowadays for decompression diving. Similarly, gradient factors around 50:75 are most common (e.g. in the range of 50:70 to 50:80)


** Obviously I cannot speak for Mark but in my interactions with him he never pushed deep stops although discussed them in his classes, generally along the lines of "Why would you want to on-gas at a deep stop?"

Only difference between 50/70 and 30/70 is that 50/70 will have one deeper stop less (24m (50/70) vs 27m (30/70) for let's say 55m dive). We are now into debate of fast vs. slow tissue loading and how best to offload it. That's something we just can't fully predict - for me, longer times always equal more safety, we decrease the depth of the first stop to, in many cases, conserve our primary gas used at depth (so the dive plan looks safer).

 

[inquis]

Let's take a 55m dive on air

Lol, I'd prefer not to, thanks though. Although you quoted me, you didn't address what I said: none of the leading researchers in the field dive 30/70, and they view 30/70 as less conservative than 50/70. If you have evidence otherwise, please share.

 

[exodusmaximus]

Lol, I'd prefer not to, thanks though. Although you quoted me, you didn't address what I said: non of the leading researchers in the field dive 30/70, and they view 30/70 as less conservative than 50/70. If you have evidence otherwise, please share.

Doesn't 30/70 add a deeper stop over 50/70?

 

[exodusmaximus]

Lol, I'd prefer not to, thanks though. Although you quoted me, you didn't address what I said: non of the leading researchers in the field dive 30/70, and they view 30/70 as less conservative than 50/70. If you have evidence otherwise, please share.

Same holds for trimix 21/35...

 

[L13]

Doesn't 30/70 add a deeper stop over 50/70?

Yes, which makes 30/70 less conservative than 50/70, according to all the research for over a decade. Deep stops increase the risk of DCS because slow tissues continue to on-gas during the deeper stops.

 

[boulderjohn]

Doesn't 30/70 add a deeper stop over 50/70?

You are echoing the thinking from 15-20 years ago. This article is about 4 years old now. It summarizes more recent thinking.