Deco Theory 101, 201, 301, and 401

[dmaziuk]

I will categorically block any user who tries to harvest data from the Subsurface Cloud.

So would I. I said donate, and if you did start something like that, you'd have to filter out everything that's not the dive profile and some pertinent details (gases, temperature maybe -- TBD) from the xml on upload. (Even if some of us don't mind exposing the deviceid of our computers.)

Where subsurface cloud comes in is you don't (in theory, not knowing where and how you store the data etc.) have to make them re-upload their files, you could just give them a consent form with "donate" button and do all the copying and filtering server-side.

 

[leadduck]

In the slides I wonder about the numbers of the ISS values:

The unit of integral supersaturation should be "bar * min" or "mbar * min", but not "mbar/min". I guess this is a typo only, but assuming "mbar*min" as the unit, then the values shown here of about 1000 are very small. 1000 mbar*min is just 1 bar supersaturation for 1 min, which is almost nothing.

I tried to calculate by myself and got 190000 mbar*min for GF53/53 (30min bottom time at 51m, 200min total runtime). I wonder what are the numbers shown in this graph.

 

[UWSojourner]

In the slides I wonder about the numbers of the ISS values:

View attachment 657401

The unit of integral supersaturation should be "bar * min" or "mbar * min", but not "mbar/min". I guess this is a typo only, but assuming "mbar*min" as the unit, then the values shown here of about 1000 are very small. 1000 mbar*min is just 1 bar supersaturation for 1 min, which is almost nothing.

I tried to calculate by myself and got 190000 mbar*min for GF53/53 (30min bottom time at 51m, 200min total runtime). I wonder what are the numbers shown in this graph.

Hi LD,

My guess is Dr. Mitchell took a snap of the charts and then labeled them more appropriately for his presentations. The originals do say mb-min and indicate the scale is in thousands. They're posted on multiple forums ... here's an example.

 

[Scuba Client]

This presentation, by @Dr Simon Mitchell, titled "What is optimal decompression" is, quite simply, awesome! It will take you from the very basics right through to the very advanced. It includes analysis of the current status (as of May, 2020) of the known science comparing deep stops and bubble models to shallower stops and dissolved gas models.

It is 2 hours long, including the Q&A session at the end, but there is great info that is covered in the Q&A as well, so if you have time to watch it, and you are not already an expert on decompression science, I think it is worth the time.


After watching it, I now have a couple of questions for Dr Mitchell. Dr Mitchell, if you are reading this, I hope you won't mind me asking these questions here in this venue.

1) You presented analysis of the Integral Supersaturation (IS) of various dive profiles. The analysis showed that the IS predicts outcomes that match the experimental data - which showed that (putting it in simple terms for the sake of discussion) shallower stops have less risk of DCS than using deeper stops.

If I understand the IS calculation process correctly, the IS is calculated by taking the IS of each individual compartment (of the 16 compartments defined by Buhlmann ZHL-16C) and summing them to result in 1 number, which is then compared against the IS from other dive profiles.

My question is: Why do you sum the 16 numbers? Why not take the Maximum, from among all the 16 compartments and compare that?

2) I corresponded with you 2 or 3 years ago, when you had said that you had settled on use of GF50/80 for your personal diving. I asked you then if there was any reason to think that GF80/80 would be more or less likely to result in DCS - because it seemed to me that using the same number for GFLo as you use for GFHi would be safer than using a number for GFLo that was lower. At the time, my takeaway from your response was that using 80/80 might be better than 50/80, but that that was a big departure from Best Practice up to that point, so you felt it was best to approach that slowly, rather than jumping straight to 80/80.

Through most of the presentation, it seemed like the data showed support for the notion that using the same number for GFLo and GFHi would be "less risky". The IS for any two ascent plans of the same total length would always (I think) be lower, when GFLo was higher.

But, a presentation I saw recently, by Alessandro Marroni, seemed to suggest that using a GFLo that is actually lower than your GFHi might be advantageous. E.g. GF50/80 might actually be safer than GF80/80.

Also, towards the end of your presentation, you said that your current settings are still GF50/70 or 50/80, depending on the dive.

My question here is: What are your current thoughts on using 70 or 80 for GFLo (i.e. the same as whatever GFHi value you are using)? Any change in your thinking since the time we spoke about this in the past?

Dr Richard Pyle contradicts his gradient factors. A very humble man in my opinion. Although I don't agree with his CCR quote of being a novice diver when training on CCRs. My take, you got it or you haven't. Assistant Professor Mitchell is trying to make a name for himself (not my quote), although I agree with most of his on/off-gassing theories.

 

[leadduck]

Hi LD,

My guess is Dr. Mitchell took a snap of the charts and then labeled them more appropriately for his presentations. The originals do say mb-min and indicate the scale is in thousands. They're posted on multiple forums ... here's an example.

Hi @UWSojourner, thank you very much for the link.

Now I still wondered why my calculation gave a smaller number and had a look at the last chart in your link:

My calculation did not produce any supersaturation in the slowest compartment (#16, th=635min). In the NEDU dive with 30min @ 51m, the nitrogen tissue pressure of #16 starts from 0.79bar and reaches 0.92bar after 30min:

6.1*0.79 - (6.1*0.79-0.79)*2^(-30/635) = 0.92 bar

For the GF53/53 ascent It never exceeds the ambient pressure and therefore never sees supersaturation ptis-pamb. Its ISS value should be 0, but in this plot ISS[16]= 60 bar*min, why?

 

[UWSojourner]

Hi @UWSojourner, thank you very much for the link.

Now I still wondered why my calculation gave a smaller number and had a look at the last chart in your link:


View attachment 658426


My calculation did not produce any supersaturation in the slowest compartment (#16, th=635min). In the NEDU dive with 30min @ 51m, the nitrogen tissue pressure of #16 starts from 0.79bar and reaches 0.92bar after 30min:

6.1*0.79 - (6.1*0.79-0.79)*2^(-30/635) = 0.92 bar

For the GF53/53 ascent It never exceeds the ambient pressure and therefore never sees supersaturation ptis-pamb. Its ISS value should be 0, but in this plot ISS[16]= 60 bar*min, why?

What value are you using for the pressure from other gases besides N2?

 

[leadduck]

What value are you using for the pressure from other gases besides N2?

None. Helium was not present, and regarding O2 ...

in

Parker, E. & Survanshi, S. & Thalmann, E. & Weathersby, Paul. (1996). Statistically Based Decompression Tables IX: Probabilistic Models of the Role of Oxygen in Human Decompression Sickness. 37.

they claim that their probabilistic DCS models based on N2 + He integral supersaturation only (ignoring O2) work well unless you use O2 fraction >40% in the decompression gases. So I didn't find it relevant for calculating ISS of an air dive.

 

[UWSojourner]

None. Helium was not present, and regarding O2 ...

in

Parker, E. & Survanshi, S. & Thalmann, E. & Weathersby, Paul. (1996). Statistically Based Decompression Tables IX: Probabilistic Models of the Role of Oxygen in Human Decompression Sickness. 37.

they claim that their probabilistic DCS models based on N2 + He integral supersaturation only (ignoring O2) work well unless you use O2 fraction >40% in the decompression gases. So I didn't find it relevant for calculating ISS of an air dive.

Then that's likely the difference.

 

[Dr Simon Mitchell]

Example:

One dive profile results in one compartment having an IS of 10 and all fifteen of the other compartments had an IS of 1 each. The sum - the IS, the way your calculating it - is 25.

A different profile (for the same depth, bottom time, and run time) results in having nine compartments with an IS of 2, and the remaining seven compartments having an IS of 1. That also yields an IS of 25.

The implication is that these two dive profiles are equally at risk for resulting in DCS.

My question is why don't we score the first profile as a 10 and the second one as a 2 (instead of scoring them both a 25)? Intuition (which can obviously be VERY wrong) suggests that the profile that had an IS of 10 in one compartment is more likely to result in bubble formation that could lead to DCS - as compared to the profile that had a maximum IS in any one compartment of 2. This would seem to be in line with how the Buhlmann algorithm works, where it always determines which compartment is the controlling compartment, but looking at the maximum over pressurization in each one individually. Why not look at IS the same way - identifying a controlling compartment for IS, instead of summing them all?

Hello,

Two apologies.

First, for being so late in responding to this. I have had a very busy few weeks clinically.

Second, for mislabelling the Y axis on the ISS graphs. They have been transposed between presentations so many times I had not noticed that the units were not properly defined any more.

Stuart, to your question.

You describe a somewhat hypothetical question that could not happen in reality - particularly your first dive profile. It would not be possible to have one tissue at 10 and all the others at 1 because in achieving 10 in one tissue would inevitably mean that other tissues with similar kinetics would be much closer, with a graduated change to differing values as the tissue half times diverged. Moreover comparing profiles with such divergent supersaturation patterns like the hypothetical you propose is not the intent of ISS. This is a bit similar to the argument Ross tried when suggesting that the ISS was crap because going on a ski trip at altitude would produce a total ISS similar to a dive. ISS comparisons are only useful for comparing profiles that are substantially equivalent in important respects (eg to the same depth for the same bottom time with the same total decompression time, but with different distribution of stop depths and times). It would not be a valid method of comparing two profiles that would produce the sort of wildly different patterns of supersaturation that you describe in your hypothetical.

How you actually use ISS, and the extent to which we can consider it a validated means of evaluating optimal decompression remain open to discussion, but it has been an interesting metric in comparing the profiles discussed in debates over deep vs less deep stop approaches.

Simon M

 

[stuartv]

Hello,

Two apologies.

First, for being so late in responding to this. I have had a very busy few weeks clinically.

Second, for mislabelling the Y axis on the ISS graphs. They have been transposed between presentations so many times I had not noticed that the units were not properly defined any more.

Stuart, to your question.

You describe a somewhat hypothetical question that could not happen in reality - particularly your first dive profile. It would not be possible to have one tissue at 10 and all the others at 1 because in achieving 10 in one tissue would inevitably mean that other tissues with similar kinetics would be much closer, with a graduated change to differing values as the tissue half times diverged. Moreover comparing profiles with such divergent supersaturation patterns like the hypothetical you propose is not the intent of ISS. This is a bit similar to the argument Ross tried when suggesting that the ISS was crap because going on a ski trip at altitude would produce a total ISS similar to a dive. ISS comparisons are only useful for comparing profiles that are substantially equivalent in important respects (eg to the same depth for the same bottom time with the same total decompression time, but with different distribution of stop depths and times). It would not be a valid method of comparing two profiles that would produce the sort of wildly different patterns of supersaturation that you describe in your hypothetical.

How you actually use ISS, and the extent to which we can consider it a validated means of evaluating optimal decompression remain open to discussion, but it has been an interesting metric in comparing the profiles discussed in debates over deep vs less deep stop approaches.

Simon M

Absolutely no worries on taking a bit of time to get back to this! I really appreciate that you took the time to come back and address it at all!

Thank you for the answer. That makes sense to me.