Riding GF99 instead of mandatory/safety stops

[L13]

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.

Small quibble: The bubble formation models have not been disproved. What has been shown empirically, is that deco algorithms emphasizing bubble formation kinetics under-perform versus deco algorithms that emphasize limiting supersaturation in slower TC tissues AT PREVENTING DCS.

There is no question that the bubble formation models are effective at predicting measured venous bubbles in the circulatory system. I suspect that the reason that the bubble formation deco algorithms under-perform is NOT because the model on which they are based is wrong about how bubbles form, but rather because the incidence of DCS is more sensitive to where and when bubbles form than to the bulk magnitude of bubbles present.

 

[OctopusLover]

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.

That's because the tests for 85/85 have 24 and 53 minutes of deco respectively, most of which are above 6 meters and thus not even using the ceiling strategy as I explained earlier.

Small quibble: The bubble formation models have not been disproved. What has been shown empirically, is that deco algorithms emphasizing bubble formation kinetics under-perform versus deco algorithms that emphasize limiting supersaturation in slower TC tissues AT PREVENTING DCS.

There is no question that the bubble formation models are effective at predicting measured venous bubbles in the circulatory system. I suspect that the reason that the bubble formation deco algorithms under-perform is NOT because the model on which they are based is wrong about how bubbles form, but rather because the incidence of DCS is more sensitive to where and when bubbles form than to the bulk magnitude of bubbles present.

Yes. And the ceiling strategy actually pushes even further into minimising slow tissue saturation while allowing higher fast tissue saturation, hence why I think it has merit irrespective of the fact it might be good even for bubble formation.

 

[rjack321]

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.

Kevin isn't doing a multi-level dive in his paper. He's adjusting the GF based on the controlling compartment throughout the decompression.
I would start with a GF low up in the 60-65 range in line with modern 'no deep stop' recommendations. If the GF high is at say 80% that really squashes the curve and these little tweaks in the 60-80 deco window are super tiny. Not unlike staying somewhere between 1 and 9ft below your ceiling as you offgas.

 

[old frogman]

Kevin isn't doing a multi-level dive in his paper. He's adjusting the GF based on the controlling compartment throughout the decompression.
I would start with a GF low up in the 60-65 range in line with modern 'no deep stop' recommendations. If the GF high is at say 80% that really squashes the curve and these little tweaks in the 60-80 deco window are super tiny. Not unlike staying somewhere between 1 and 9ft below your ceiling as you offgas.

Interesting, 60/80 plotted on a graph puts you reasonably close to the M-value line (off-gassing would be good). However, my concern is the dealing with medium/slow tissue on-gassing throughout the dive.

Although Kevin's paper is not about deep multi-level diving, it raises the question in my mind. Would VGM be more suitable for multi-level diving than GF.

The controlling compartment will change e.g. at the start of the ascent at 300ft it will be the fast tissue compartments controlling the ascent. As on-gassing occurs at the various depths i.e. 200ft for 10 mins and 100ft for 10 mins the medium/slow tissue compartments may then become the ascent control, requiring more time off-gassing in the shallows. Consequently, the GFhigh (80) may need adjusting to cater for this eventuality.

In conclusion, it would be interesting comparing GF with VGM on a deep multilevel dive.

 

[dmaziuk]

... Consequently, the GFhigh (80) may need adjusting to cater for this eventuality.

How would that work? "Adjusting" either GF mid-ascent will change the slope of the GF line, shifting the current M-value -- potentially above or below your current tissue loading. Shift it one way and you can be over the M-value and "bent". Change GF High the other way and it could shift you back into the no-stop territory and all the deco you've done up to that point was a plain waste of time.

Not in practice, of course: the range isn't that great IRL, but that's how it'd work in principle.

 

[rjack321]

Interesting, 60/80 plotted on a graph puts you reasonably close to the M-value line (off-gassing would be good). However, my concern is the dealing with medium/slow tissue on-gassing throughout the dive.

Although Kevin's paper is not about deep multi-level diving, it raises the question in my mind. Would VGM be more suitable for multi-level diving than GF.

The controlling compartment will change e.g. at the start of the ascent at 300ft it will be the fast tissue compartments controlling the ascent. As on-gassing occurs at the various depths i.e. 200ft for 10 mins and 100ft for 10 mins the medium/slow tissue compartments may then become the ascent control, requiring more time off-gassing in the shallows. Consequently, the GFhigh (80) may need adjusting to cater for this eventuality.

In conclusion, it would be interesting comparing GF with VGM on a deep multilevel dive.

After 10mins at 300ft it's a no stop ascent to 200ft under most model assumptions on CCR. After your now 20mins of bottom time (10@300ft and 10@200ft) you can just barely ascend to 100ft without a stop depending on the GF low you choose. At least on CCR sp1.2 and 8/80 dil. OC would hinge on what gas you decide for the 200ft time and the whole 'plan' makes less and less sense overall.

 

[old frogman]

How would that work? "Adjusting" either GF mid-ascent will change the slope of the GF line, shifting the current M-value -- potentially above or below your current tissue loading. Shift it one way and you can be over the M-value and "bent". Change GF High the other way and it could shift you back into the no-stop territory and all the deco you've done up to that point was a plain waste of time.

Not in practice, of course: the range isn't that great IRL, but that's how it'd work in principle.

OK, from your comments and those of rjack 321, my understanding is, do not change the GF in mid-ascent.

However, the Shearwater literature states that the GF can be manually changed during the dive, but to my knowledge it does not stipulate when during the dive.

If I have this wrong, please correct me.

It would be interesting to know if the VGM software automatically changes the GF at the start of the ascent or changes it throughout the whole dive including the ascent.

I know that VGM has gone milspec under the Avon brand and is available with the MCM 100 rebreather and the MDC 150 dive computer. Therefore, it does not appear to be obsolete as suggested by some authors.

Any gurus out there that can shed more light on the VGM software?

 

[tursiops]

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.

I guess you are not diving in the ocean with waves overhead! There is an open-ocean reason why folks often terminate their deco stops at 6m rather than 3m.