Safety stops when monitoring SurGF

[Brett Hatch]

To properly science this, we would need something directly measurable for personalized decompression, such as a high quality consumer submersible doppler, deployed into the wild and gathering realtime data for thousands of people's dives. Then gather all that in-water data together and look to see if there is something, anything in there that reliably and predictably links a measurable value to DCS occurrences. Someone please invest $50 million to make it happen

This line of reasoning does not strike me as correct. Like, I take your overall point that Buhlmann's algorithm is "just a model," and therefore can't be trusted to keep us safe. That point has some merit: it is a simplified model of our bodies, that is not capable of making perfect predictions about the world, since it has inherent flaws.

But, the idea that the model can only be useful if it is backed up by some empiric, scientifically measurable (and measured) quantity about the world, and in particular, a very large number of real-life in-water doppler measurements, seems a little absurd. Like, the thing about Buhlmann's model is that we have a great, great number of measurements about its effectiveness: many divers use it to make many dives every day, for the last several decades. When divers dive under the model's guidance, they're pretty safe.

So we know more about the effectiveness of the model, than we do about the explanatory power of any particular mechanism of its effectiveness. And SurfGF is nothing new, really, it's the same exact Buhlmann model, with the equation solved for a different variable.

I'm not trained in tech diving, and am just reading this thread because it's interesting. So, people who know better should correct me if I'm wrong here.

 

[boulderjohn]

Yes, unfortunately actual personalized decompression is very much still a black art, which is clearly evident in so many anecdotal stories of "but I did my PADI safety stops and still got bent"

You speak as if getting DCS during an NDL dive is little more than a roll of the dice, but the rate of DCS on standard NDL dives is miniscule. We can't be sure of the numbers because we can't be sure how many dives are actually done, but best estimates are in the range of a few thousandths of a percent. DCS on an NDL dive does happen, but it is exceedingly rare.

Go ahead and do the PADI stop if that's what your team agreed to, because there are several safety features of doing so--notably crowd control and surface hazards. But it is probably no more important for DCS than simply ascending and surfacing slowly.

As Tursiops noted earlier and you ignored, the safety stop is based on research. Research beyond what he noted includes a study of saturation divers that indicates that the M-values of all tissues in the body are such that they can safely ascend to the surface when fully saturated at pressures shallower than 20 feet. That means that during a safety stop, any of the faster tissues that need to off-gas to a safe level will be able to do so, and the slower tissues that are still on-gassing cannot on-gas beyond a safe level.

The safety stop has been field tested many millions of times since it was created, and it was found to be effective. Yes, there are rare cases where an NDL dive ending in a proper safety stop still results in DCS, but no one knows why. I have my theories that I have expressed elsewhere.

So for "NDL" diving sure, ascend at 3 metres/10 feet per minute or less above half your max depth, and take some extra time to regroup, tidy up, and communicate somewhere between 6 and 3 metres.

So you basically say that the thoroughly tested safety stop is no more than a wild guess, and then you advocate an ascent strategy that has never been scientifically tested, has only a tiny portion of adherents, and for which there is no theoretical basis other than a belief that the shallower part of the ascent should be slower than the deeper part.

 

[NDonahue]

As Tursiops noted earlier and you ignored, the safety stop is based on research.

it’s important to put the safety stop research in context. The original Pilmanis research, and the later Uguccioni research both compared the effect of safety stops relative to the common dive practices of the time (1974-1994).

The later research confirms that a 3 min safety reduces bubble formation using a cohort of divers who were planning and following dive tables.

What we don’t know scientifically is the degree to which the use of dynamic data and dive computers to achieve a given SurfGF obviates the need to do a safety stop.

Given the extreme rarity of DCI among NDL divers, doing this research would require a very large sample. In the end, the research probably isn’t worth doing. Contemporary NDL divers are conditioned to do a safety stop, and any divers who substitute a given SurfGF as their safety requirement are likely not at significant marginal risk.

 

[iointerrupt]

The thing about Buhlmann being just a model, with it's imperfect match to reality, is that if it's going to fail you, it's going to fail you at the high end of SurfGFs.

On any given dive, I don't trust the model enough to surface with a SurfGF of 99%. I'll do whatever it takes to avoid surfacing under those conditions.

But conversely, I'm very confident I can directly surface with a SurfGF of 1%, and be perfectly fine, every single time.

The model is surely imperfect when pushed to the limit, but the further away from the M-values we are, the lower and lower the actual risk becomes.

And that's where I think this conversation belongs, somewhere between 1% and 99%, we lose complete trust, where does that happen? When I no longer have complete trust in the model, I do the safety stop. But when I do have complete trust, I get out of the water.

 

[EFX]

The thing about Buhlmann being just a model, with it's imperfect match to reality, is that if it's going to fail you, it's going to fail you at the high end of SurfGFs.

On any given dive, I don't trust the model enough to surface with a SurfGF of 99%. I'll do whatever it takes to avoid surfacing under those conditions.

But conversely, I'm very confident I can directly surface with a SurfGF of 1%, and be perfectly fine, every single time.

The model is surely imperfect when pushed to the limit, but the further away from the M-values we are, the lower and lower the actual risk becomes.

And that's where I think this conversation belongs, somewhere between 1% and 99%, we lose complete trust, where does that happen? When I no longer have complete trust in the model, I do the safety stop. But when I do have complete trust, I get out of the water.

You say you'll do whatever it takes to avoid surfacing with a SurGF of 99%. That's easy! Just set your GFHi to some percentage less than 99%. The Buhlmann algorithm with GF's is designed to calculate an NDL based on GFHi. That is, as long as you ascend with more than 0 minutes remaining of NDL, you will not exceed your GFHi when you reach the surface. The same applies to deco dives. The only two scenarios I can think of to obtain a SurGF of 99 or greater is (1) you set your GFHi to 99 and you ascended with 1 minute or less time remaining of NDL or (2) you went into deco and you blew through the stops on your way to the surface in which case SurGF could be greater than 100. For additional information on the calculation of GF99 and SurGF see my posts #24 and #35 in the thread "Using GF99 and SurGF".