Riding GF99 instead of mandatory/safety stops

[old frogman]

It appears that you are now gradually concluding that staged decompression is the way to go.

While on GF, there is an interesting article by Kevin Gurr "Variable gradient model: An approach to create more efficient decompression". In Depth Magazine July 2, 2019.

The VGM was available on the later models VR3 and VRX dive computers.

https://www.bing.com/ck/a?!&&p=11ed...c2lvbnMtdXNpbmctZ3JhZGllbnQtZmFjdG9ycy8&ntb=1
https://www.bing.com/ck/a?!&&p=11ed...c2lvbnMtdXNpbmctZ3JhZGllbnQtZmFjdG9ycy8&ntb=1

 

[rjack321]

While on GF, there is an interesting article by Kevin Gurr "Variable gradient model: An approach to create more efficient decompression". In Depth Magazine July 2, 2019.

The VGM was available on the later models VR3 and VRX dive computers.

https://www.bing.com/ck/a?!&&p=11ed...c2lvbnMtdXNpbmctZ3JhZGllbnQtZmFjdG9ycy8&ntb=1
https://www.bing.com/ck/a?!&&p=11ed...c2lvbnMtdXNpbmctZ3JhZGllbnQtZmFjdG9ycy8&ntb=1

The problem here is that Kevin is using the GF low of 20 at all. If the low and the high are compressed into something like 55/70 or 60/80, then his whole model squashes and there's almost no practical need to adjust the GFs throughout the decompression.

 

[OctopusLover]

Paper was published in 2022:

Ceiling-controlled versus staged decompression: comparison between decompression duration and tissue tensions​

Conclusions
Ceiling-controlled decompression shortens the decompression duration at the cost of higher supersaturation in the faster tissues. While this increase in supersaturation does not lead to a breach of the limits of the decompression algorithm, one cannot a priori state that it does not lead to an increase in risk of decompression sickness. Computer simulations comparing dives using staged decompression and ceiling-controlled decompression and subsequent analysis of the inert gas tensions suggest that the two procedures might be similarly acceptable and thus the matter should be investigated further.

Oh! So someone has already done the numbers, will read today. At least this validates that the question I'm asking is actually valid.

It appears that you are now gradually concluding that staged decompression is the way to go.

No. You can approximate the area of a circle as the sum of a bunch of squares that fit inside, and the smaller the squares you use the closer you get. If the squares are infinitely small, the answer you get is actually correct. Similarly, what I was saying is that for the purpose of illustrating the point, don't go all the way to infinitely small deco stops, simply smaller stops than what's conventional and ask the question if that's not already better

 

[OctopusLover]

The study tests *exactly* what I was asking. The headline is simple:

Ceiling controlled approaches accelerate decompression but the effect of this on the risk of decompression sickness is unknown

The first and obvious limitation of the study is that it's based on simulations running Bulman rather than with people so as they say, we don't really know what the effects would actually be:

Countless dives, both in dedicated experiment scarried out in hyperbaric chambers and in the field, have provided the empirical data to establish a scale of such risk. The vast majority if not all of this data stems from staged decompression protocols and it cannot be extrapolated, a priori, to ceiling-controlled dives

But it's unquestionable that it would lead to faster decompression (hence my frustration that this wasn't obvious to everyone here). The authors say as much:

The reduction in decompression duration when followingthe ceiling is a logical fact

There are however, two important ideas being presented as to why or why not this faster deco schedule would come with increased or decreased risk. On one side, we don't know if holding tissues at the ceiling level (or some safer value below that) continuously is actually as safe as holding them for a short time like traditional deco does.

This would be equivalent to saying that the M-values havea time limit; they are tolerated only because in stageddecompression there is only a short exposure to the highestvalue. It would imply that the apparent inefficiency of thestaged decompression is actually an intrinsically valuable component in the process itself.

But now we get to what to me is most encouraging. I won't quote the entire thing, read the discussion part yourself. The main idea behind why that would be the case is bubble formation. But bubble formation models have largely been disproven. Not only that, the inherent sudden jumps in pressure created by sudden ascents (ascending to next stop) might itself trigger bubble formation.

So while we don't know, there's reason to suspect this strategy could actually not just be inherently shorter, but inherently safer for a given ceiling, which could allow the ceiling to be increased further for the same amount of risk, leading to even shorter total deco.

The study quotes 4-12% shorter times (more benefit the deeper the first stop), but that's assuming that

1. The above is incorrect and it is not in fact inherently safer.
2. Furthermore, they stop the ceiling strategy at 6 meters saying that it's not practical to continue ascending above 6 meters. Well, I know that not to be true because I do dynamic safety stops as a recreation diver as described here (hell that's what got me started thinking about this whole thing!), and routinely arrive at 5 meters then slowly ascend up to 2 by the end of the safety stop. This is made even easier if using a line or DSMB, at which point you could well be ascending inch by inch.
3. They explicitly are not using different gas mixes and acknowledge that the effect would be even larger when changing gases because you can switch to higher oxygen earlier

So all in all, the strategy works in theory, and the results presented are very conservative. I could see the improvement if riding it to the surface and at a more aggressive ceiling value being 2-3x as good as what they present.

 

[gmerick]

we don't know if holding tissues at the ceiling level (or some safer value below that) continuously is actually as safe as holding them for a short time like traditional deco does.

My take on it: Instead of 30/80, and only approaching that "80" briefly when ascending to a new deco stop, set 30/70, and ride the ceiling, or GF99=70. Doing so may wipe out any savings in ascent time.

 

[dmaziuk]

My take on it: Instead of 30/80, and only approaching that "80" briefly when ascending to a new deco stop, set 30/70, and ride the ceiling, or GF99=70. Doing so may wipe out any savings in ascent time.

Or you could follow Doolette's cue and set your GF Low to .83 of GF High: 66/80. I'll be surprised if the resulting TTS will be more than a couple of minutes different from "riding the ceiling" at 30/80.

Gradient Factors in a Post-Deep Stops World

World-recognized decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

indepthmag.com

 

[old frogman]

Oh! So someone has already done the numbers, will read today. At least this validates that the question I'm asking is actually valid.


No. You can approximate the area of a circle as the sum of a bunch of squares that fit inside, and the smaller the squares you use the closer you get. If the squares are infinitely small, the answer you get is actually correct. Similarly, what I was saying is that for the purpose of illustrating the point, don't go all the way to infinitely small deco stops, simply smaller stops than what's conventional and ask the question if that's not already better

A small stop is still a stop. My point being that you have gone from continuous decompression to small stops.

Now, in the real world of cold water, underwater currents, low visibility and 6ft (2m) surface swell how does your small stops all the way to the surface work?

 

[old frogman]

The problem here is that Kevin is using the GF low of 20 at all. If the low and the high are compressed into something like 55/70 or 60/80, then his whole model squashes and there's almost no practical need to adjust the GFs throughout the decompression.

From Kevin's article I perceive that if you were to do a wall dive e.g. 10 mins@ 300ft, then ascend for 10 mins @ 200ft, then ascend for 10 mins @100ft and finally ascend to surface conducting appropriate stops. The VGM algorithm would continuously be readjusting the GF throughout the dive, possibly, according to Kevin, providing the ultimate deco profile.

Please correct me if I am taking you out of context. From your comment above would a 55/70 or a 60/80 provide similar results as the VGM (or perhaps better). Or perhaps is it possible or practical to manually change the GF, let's say on a Shearwater dive computer throughout the same dive.

If however, you need to manually change the GF throughout the dive what GFs would you suggest.

 

[OctopusLover]

Or you could follow Doolette's cue and set your GF Low to .83 of GF High: 66/80. I'll be surprised if the resulting TTS will be more than a couple of minutes different from "riding the ceiling" at 30/80.

Gradient Factors in a Post-Deep Stops World

World-recognized decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

indepthmag.com

One of the tests in the paper is with 85/85. Same improvement.

 

[dmaziuk]

One of the tests in the paper is with 85/85. Same improvement.

If you're talking about the paper quoted in that threadlet, two: one with 2 minutes improvement (air) and one with 1 minute improvement (air + EAN). Not what I'd call "meaningful".

I have not looked at the paper you cited yet.