DCS hit during final stop? Has it ever happened?

[Scuba Client]

I have just finished reading the excellent book "Deco for divers" by Mark Powell, and also reading a fair amouint of recent info on the controversy around deeps stops and trying to find an optimum ascent profile etc


On thing that really stuck with me is:

Draw a square (of any size) now draw a line through the middle, splitting it into two different but idential halves, of those two area's:

The one of the left is eqivalent to the change in gas volume by dscending from the surface to 10m depth.

The one of the right is equivalent to the change in gas volume by descending from 10m to an INFINITE depth!!

ie, the volume change in the first 10meters is the same as the total volume change from there (10M) to the bottom of the mariana trench !!



So, given that perhaps surprising knowlege, and the general trend that the larger the inert gas bubbles that result the more serious any symptoms of DCS are likely to be, is it possible to "get bent" ie to have observable physical symptoms of any kind during the final stop, lets say at between 6 and 3m depth?

(lets assume realistic maximum depth/time profiles for open circuit mixed gas tech diving, but not saturation diving, CCR or similar)

I'd be interested to hear anyones thoughts or links to any objective data on the subject?

Stick with the proven formula's of decompression theory and forget about the hype, unless those advocates are ready to put their life in jeopardy. If you're only doing a bounce dive, most gasses work to a certain depth.

 

[RyanT]

@Dsix36?

 

[MaxTorque]

Can confirm, you can get bent in the water. In my case I noticed symptoms after leaving the 9m stop. I went back down to 15m and repeated a whole bunch of time.

How long had you been at 9m on your stop, and what was your approx dive profile in this case?

 

[rjack321]

How long had you been at 9m on your stop, and what was your approx dive profile in this case?

Well it was 12 years ago but

Dive 1, 170ft for 20mins, 30mins of deco on 50% and O2
Dive 2, 200ft for 18-20mins there was 15-16mins of deco on 50% completed so far and i was bent at the start of the 9m stop.

If you're getting bent in the water, the bubble forms at one depth and becomes obviously symptomatic as you ascend. There was zero doubt I was bent almost immediately. Hence the importance of descending to crush the bubble and open any tissues to blood flow again

 

[Wibble]

PFOs will cause DCS whilst underwater simply because the effectiveness of clearing dissolved gas is impaired.

One fact of decompression science is it is not 100% accurate. Algorithms follow a statistical model and line drawn where there's a certain number of standard deviations away from suffering DCS.


Regarding deep vs shallow changes; that's why your last stop is at 6m (ish) and is why it's so long.

 

[MaxTorque]

Agreed! This is the biggest issue with DeComp Science, that the varriables are, er, too varriable!

What set me off on this line of questioning was if there was an empirical evidence (rather than theoretical or model based) for the appearance of DCS below say 10m.

This i found interesting because the gas compression is an inverse exponential against depth, but gas consumption is broadly linear with depth.

For example:

at 5m, where ambient pressure is 1.5 bar(abs), gas volume is 66% of Surface Volume, and OC consumption is 150% of Surface Consumption

at 10m, where ambient pressure is 2 bar (abs) so gas volume is 50% of SV, and OC consumption is 200% of SC

at 20m, where ambient pressure is 3 bar (abs) so gas volume is 33% of SV, and OC consumption is 300% of SC


Assuming no surface hung/dropped air supply is available, then obviously the maximum amount of deco time on any given remaining gas is going to be longer the shallower you are.

At our 5m point above we are using 50% more air than on the boat, but our bubbles are (nominally) 44% smaller
At our 10m point, we are using 100% more air, and our bubbles are 50% smaller
At our 20m point, we are using 200% more air, and our bubbles are 66% smaller


So in a theoretical emergency situation, what is the best depth to stop at and use up ALL your remaining gas? The answer is as shallow as possible whilst juuust avoiding any symptoms of DCS !! Hence my initial question

 

[MaxTorque]

If we take our consumption curve, and our volume curve, and divided one by the other, an rescale, we can get a curve that is some sort of "Relative Effectiveness of Deco" (see attached)

Ok, it's arbitrary and pretty meaningless in anything other than a pure mathematical sense, but it shows how effective it is, in terms of maximising total deco time and minimising bubble volume vs stop depth, with the surface as our reference point.

So, at 3m, our deco is 59% as effective

at 6m, our deco is 39% as effective

at 10m, just 25% effective


What does all this mean? No idea, but i thought it was an interesting avenue of thought

 

[boulderjohn]

Stick with the proven formula's of decompression theory and forget about the hype,

What are the "proven formulas"?

 

[Akimbo]

What are the "proven formulas"?

Stay on deck = never get bent

 

[GJC]

What are the "proven formulas"?

Haldane proved his tables using goats.

DCIEM and DSAT were proven using dopler testing on divers.

Most other tables and formulas have proven safe by having a very small number of divers being bent after years of being used