Seeking the Light
- Reaction score
- Lexington, SC
- # of dives
- 500 - 999
A ScubaBoard Staff Message...
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?