Deep Stops Updates (from May, 2020)

[stuartv]

This is a presentation by Dr Alessandro Marroni at DAN South Africa (or DAN Europe?)


It was posted in May, 2020.

It presents recent research on the subject of deep stops. The information seems to also be readily applicable to evaluation of choices in Gradient Factors.

Don't get off the bus at the first stop, either. Dr Marroni continues to present new and interesting information pretty much all the way to the end.

 

[stuartv]

One teaser, from late in the presentation: They did a small study comparing a group of divers all doing the same controlled dives. They used GF20/85 and GF50/75.

Dr Marroni acknowledges that it is debatable as to which set of GF you would consider to be more conservative.

The results of the bubble studies of the divers is that the (being very simplistic here) amount of bubbling is comparable between the 2 choices of GF. But, when viewed over time, the the peak of bubbles in the system comes later for GF20/85, and the amount of bubbles after 45 minutes and later is higher with GF20/85.

Dr Marroni suggests that, with that particular set of data, the results from GF50/75 look better, because the diver gets the bubbles out of their system more quickly.

 

[EFX]

Thanks @stuartv for posting this. Overall it was interesting especially the study between the GF of 20/85 vs. 50/75. I was a little disappointed in their selection of GF's for the study I just mentioned. In my way of thinking you should only adjust one factor at a time. As Dr. Marroni pondered the results, was the difference in bubble count due to mainly the GFLo or GFHi? He thinks it's probably GFHi. But why speculate? Two studies could have been done: (1) GF 20/85 vs. GF 50/85 comparing GF lows and, (2) GF 20/75 vs. GF 20/85 comparing GF highs. Not only can you determine whether the GF is better higher or lower, but you can also determine which GF factor is the most significant for bubble count and profile.

 

[stuartv]

@EFX

From what I gathered, it seemed that they chose those GFs to compare because they gave the same time to surface, for the same planned bottom time.

I took that as addressing the question: "If I spend X minutes at depth Y, and then spend Z minutes on my ascent to the surface, is it better to spread my Z minutes out so that I do my first stop deeper, or wait and do my first stop more shallow?"

If I am remembering correctly, that approach - of comparing ascent plans by keeping bottom time and total runtime equal - was also used in other studies I have read about.

And what is the alternative? What happens if you compare GF20/85 to GF50/85 (for the same depth and bottom time)?

The GF20/85 dive is certainly going to have a longer total runtime.

And then what? Let's say that the GF20/85 divers gets out with less bubbles. Does that mean that GFLo of 20 is better? Or does it mean that the longer time in the water is better? How do you know that using GF50/85, and extending the last 2 or 3 stops to make the total runtime as long as the GF20/85 dive wouldn't result in even LESS bubbles?

It seems like maybe the way they are doing these studies does indeed make good sense.

 

[EFX]

You need to keep the bottom time the same if you are comparing GF's which, as you describe will alter the ascent plan. I mentioned previously that you only want to change one factor at a time. What needs to be changed is an initial condition not an outcome. Changing bottom time, gas mix, GF's are initial conditions. Changing the total runtime is an outcome; that you don't want to mess with. So, as you say lowering GFLo will result in longer shallow stops and thus a longer runtime (which is an outcome).

Something tells me we should be working backwards on this. Start with the number and size of bubbles that produce DCS in x% of the divers. Once we know the combination of quantity and size (can we multiply those numbers together to get a bubble stress [BS] factor?), then we can fiddle with the GF's that produces that BS factor. Change first GFLo and then GFHi and see how each GF factor affects the BS.

 

[GF99/99]

Dr. Simon Mitchell has a good presentation as well

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