Diving with gradient factors for a new recreational diver

[dmaziuk]

Are you really that ignorant? Or just pretending?

Do you have a cite to back any of your dearly held beliefs? Or do you just believe in linear monotonical increases because Shearwater has straight lines in their infographic?

 

[L13]

Do you have any cites that contradict any of my statements? I never suggested it was linear, to my knowledge Shearwater has never suggested it is linear. I don't think anyone with half a clues has ever suggested it was linear. All the evidence for over a hundred years strongly suggests that the risk drops of rapidly as you reduce supersaturation below the limits. If you are informed on the subject I wouldn't need a cite for that statement, and if you aren't your should get informed before you debate on the subject. If you have a cite that contradicts any of this, please bring it up.

That leaves as the most likely conclusion that you are just pretending to be ignorant. You seem to like building straw men and knocking them down. Which kind of makes it pointless to discuss this with you, and is probably why everyone else has already dropped out of the conversation.

 

[crofrog]

For a 30 minute dive to 100ft on Nx32

What is the probability of DCS at a GF high of 100, 85,75 and 50?

 

[scubadada]

For a 30 minute dive to 100ft on Nx32

What is the probability of DCS at a GF high of 100, 85,75 and 50?

These are all deco dives. risk would be dictated by how you handle the deco and the surfacing GF you choose. I pad the 10 ft stop and surface with a GF of <80. Of course, the dives with a lowest GF highs would already be below this value.

 

[Angelo Farina]

For a 30 minute dive to 100ft on Nx32

What is the probability of DCS at a GF high of 100, 85,75 and 50?

What follows is my take on GFs, which of course can be wrong.
Surfacing with GF=100 means that the critical compartment contains the maximum allowed content of nitrogen according to the deco limit chosen.
In the case of US Navy limits, this means that you incur a residual risk of DCS in the order of 1 case every 1000 dives.
If now you reduce the GF to 50 (making a slower ascent and longer deco stops), it means that your critical compartment now contains half of the maximum allowed content of nitrogen.
At first approximation, I estimate that this reduces the risk of DCS to half, so 1 case every 2000 dives.
Probably the risk is reduced even more, but we do not have an established model relating the reduction of nitrogen content in the critical compartment to the reduction of risk of incurring in DCS.
So assuming a linear relationship is just reasonable and prudent.

 

[crofrog]

These are all deco dives. risk would be dictated by how you handle the deco and the surfacing GF you choose. I pad the 10 ft stop and surface with a GF of <80. Of course, the dives with a lowest GF highs would already be below this value.

View attachment 808785

All dives are deco dives, this is the very limit of what most people would consider a recreational profile, and is the max no-stop time on the NOAA 32% table.

Quantify the risk of the different GF factors, are you 100% less likely to get DCS with a GF high of 50 versus GF high of 100?

 

[crofrog]

So assuming a linear relationship is just reasonable and prudent.

Is it though? There are other objective hazards associated with being in the water.

• Decreased off-gassing efficiency from hypothermia
• Drowning
• Marine life
• Medical Events
• Boats hitting you
• Weather changing
• etc.

 

[Angelo Farina]

Is it though? There are other objective hazards associated with being in the water.

• Decreased off-gassing efficiency from hypothermia
• Drowning
• Marine life
• Medical Events
• Boats hitting you
• Weather changing
• etc.

???
I do not see any logical connection between the amount of nitrogen in the critical compartment and these risk factors.
The question I tried to answer was very specific: regarding the risk of DCS, which reduction do you expect when profiling your ascent to a GF lower than 100?
I provided one reasonable answer: a GF = 50 halves the risk.
Do you have a better GF=>risk relationship?

 

[Angelo Farina]

However I have just found a post by @scubadada, who found a strongly not linear relationship between risk of DCS and GFs, using a software called Saul Dive Planner:
Post in thread 'Conservatism' Conservatism

 

[goldfishtornado]

If the underlying theory of Buhlmann is even close to correct:

• Gradient Factor is the equivalent of "Normalized Tissue Supersaturation".
• Higher tissue supersaturation is more dangerous than lower tissue supersaturation.
• More time spent at high tissue supersaturation is more dangerous that less time.

These are exactly the assumptions that go into the common risk models. Expressed as a formula in the form P(DCS) = 1 - exp(- integral(sum(r_i), dt)), where r_i is the instantaneous risk function function for a given compartment very close to proportional to the normalized saturation.

See for example:
Weathersby PK, Homer LD, Flynn ET. On the likelihood of decompression sickness. J Appl Physiol Respir Environ Exerc Physiol. 1984 Sep;57(3):815-25. doi: 10.1152/jappl.1984.57.3.815

Thalmann ED, Parker EC, Survanshi SS, Weathersby PK. Improved probabilistic decompression model risk predictions using linear-exponential kinetics. Undersea Hyperb Med. 1997 Winter;24(4):255-74. PMID: 9444058.

E. C. Parker, S. S. Survanshi, P. B. Massell, and P. K. Weathersby Probabilistic models of the role of oxygen in human decompression sickness. Journal of Applied Physiology 1998 84:3, 1096-1102 doi:10.1152/jappl.1998.84.3.1096

Howle LE, Weber PW, Nichols JM. Bayesian approach to decompression sickness model parameter estimation. Comput Biol Med. 2017 Mar 1;82:3-11. doi: 10.1016/j.compbiomed.2017.01.006

Everything else in this theory operates on log curves, so it's just as likely that this one's a log curve too. That would mean first halving of the M-value results in, what, 4% reduction of risk? I'm sure it's worth it, better safer than safe, right?

Do you have a cite to back any of your dearly held beliefs?

I'm pretty sure you have the logarithm on the wrong side of the equation. See the formula above, or one of the references for a more exact specification. A reduction in the integrated time at a given gradient results in an exponential reduction in risk in these models.

What is the probability of DCS at a GF high of 100, 85,75 and 50?

There is some software and raw code floating around to implement the above probabilistic risk estimates. The parameter estimation is a bit tricky as noted in the papers above. You can calulate the profile for a given GF then estimate the risk for that profiles with one of the algoritms, e.g. NMRI98 or BVM.

In the case of US Navy limits, this means that you incur a residual risk of DCS in the order of 1 case every 1000 dives.

I think diving at the limits on the Navy Tables is a bit higher risk than that, (iirc there is a section in the manual that requires a chamber being available). In Gerth, Wayne A. and D J Doolette. “VVal-18 and VVal-18M Thalmann Algorithm Air Decompression Tables and Procedures.” (2007). The section "Comparative Analyses of Estimated DCS Risks and Total Stop Times of Tabulated Schedules," seems to indicate the schedules are not iso-risk but order low-single digit %.

I provided one reasonable answer: a GF = 50 halves the risk.
Do you have a better GF=>risk relationship?

I see you found the non-linearity, but of course the expoential expands linearly to first order as do most functions....

Digging into the sources a bit it looks like they regress across a decent number of dives under a reasonably wide range of conditions, and produce statistically significant DSC probablity estimates. Given that, you probably can infer something about the risk by calulating a given profile and then probogating them through the risk models.