From 6m to Surface after Deco

[Remy B.]

Do they? I hadn't heard that...

Seen it my self, different divers, different dives.

 

[kensuf]

It shouldn't impact the slow tissues because I'm taking the 10' (3m) stop and splitting the time between 15' (4.5m) and 10' (3m) instead of spending all of the 10' (3m) stop at 20' (6m).

The impact on bubbles is that it should reduce risk compared to spending all your time at 20' (6m) because the pressure gradient from 10' (3m) to the surface is less than from 20' (6m) to the surface.

Spending all your oxygen deco time at 20' is so 90s.

 

[Remy B.]

But then you are stopping at 9m and go all the way to 4.5m meaning you are not stopping every 3 m, I can understand if you stop at 12-9-6-3. it seems a step to fast

 

[Remy B.]

this a consistent problem of everybody following what works for them, and you end up with an assortment of different methods in which apparently there has not been enough research. so far it works but nobody knows how how much risk there is really involved.

the Nedu study give some results aldo VPM had proven successful from 60m in up

Radio deco from UTD was as well a sworn method and proven not that good aldo some people had been very successful using it.

But thru scientific studies and tests, it had been proven to some extend what is more optimal.

 

[Kevrumbo]

It shouldn't impact the slow tissues because I'm taking the 10' (3m) stop and splitting the time between 15' (4.5m) and 10' (3m) instead of spending all of the 10' (3m) stop at 20' (6m).

The impact on bubbles is that it should reduce risk compared to spending all your time at 20' (6m) because the pressure gradient from 10' (3m) to the surface is less than from 20' (6m) to the surface.

Spending all your oxygen deco time at 20' is so 90s.

No. Here is where depth/ambient pressure while using O2 comes into play, and staying at the deepest recommended depth of 6m/20ft for a normal Oxygen deco stop.

(Look at it in terms of Boyle's Law bubble mechanics: pressure and volume inversely proportional relationship, and bubble size proportional to the cube root of its volume).

Suppose you surface with a large Doppler Score of VGE bubbles, for whatever reason.

Going back down to:

3m/10' ~bubble size reduces to 91% of the pathological size at the surface;
6m/20' ~bubble size reduces to 85% of the pathological size at the surface;
9m/30' ~reduces to 80% (max recommended depth for O2 IWR with a ppO2 of 2.0).
And for reference of note:
18m/60' ~reduces to 70% (Recompression Chamber Table 6);
50m/165' -reduces to 55% or nearly half of the pathological size at the surface (Recompression Chamber Table 6A breathing Eanx50 or Heliox 50/50).

So within the give & take of O2/CNS Toxicity exposure time, staying at 6m/20' on O2 for the entire shallow stop profile -at the highest recommended O2 partial pressure AND ambient pressure of 1.6 ATA- is better to keep those bubbles small and off-gassing efficiently as long as possible as prescribed by the deco profile, and then a slow 1mpm/3fpm surfacing ascent to gradually reduce ambient pressure and decompression stress on those supersaturated slow tissues.

 

[LiteHedded]

Sounds like old AG stuff to me

Wouldn't going up ten feet increase the bubble size and surface area therefore making it offgas more efficiently out of the bubble?

 

[Kevrumbo]

Sounds like old AG stuff to me

Wouldn't going up ten feet increase the bubble size and surface area therefore making it offgas more efficiently out of the bubble?

Yeah sure!

Increasing bubble size reduces surface tension and pressure inside the bubble making it more susceptible for inert gas to diffuse in -a nasty growing Boyle expansion pathology from depth. Shouldn't be an issue though at the final O2 shallow deco stop unless you manage to violate all earlier intermediate deco stop tissue M-values for some reason -or happen to perform overly long DeepStops -in which case you may have to extend your 6m O2 stop for some time (or perform IWR, or have a Recompression Chamber ready standing-by). . .

Anyway, you're breathing the highest non-inert containing gas possible -Oxygen- at the shallow stop depths, and so the argument goes, you have the highest delta concentration gradient possible for inert gas elimination in the lungs alveolar bed, whether you do the deco stop at 6m or shallower at 3m. But you're slightly better off keeping any other VGE & tissue bubbles "squeezed" and smaller at the deeper depth of 6m for the entire remaining prescribed deco profile. (Also in open ocean, it may be easier to hold a deco stop at 6m than shallower in the swells & surge at 3m depth).

 

[LiteHedded]

I do everything at 20. Or move to 15 or 10 if the mood strikes. Seems fine either way.

At wakulla they do an hour from 30 to the surface

 

[Nasser]

For what it's worth, I use the GF99 info on my Shearwater to dictate my final ascent. If I've set, say, GF85 as my surfacing tension, I'll limit my final ascent to a lower GF...let's say 75%. I'll ascend slowly from the final stop until I reach 75%, pause until it drops 2-3% and rise again. This creates a series of 'micro stops' towards the surface... a very fine curve...as the stops get longer as the surface nears. It's a slow crawl, where in the final couple of meters you're rising in 10cm increments.

This is cool - did you come up with that approach yourself? I'm going to start applying it to my dives.

I trained with BSAC for a while before I started tech and we would do 1 min from 6m to the surface on NDL dives. When I started tech my ,instructors also taught me to adopt similar approaches on the last 6 meters and would also hang on the surface for a bit prior to climbing back onboard.. and also continue to breath the rich mix for 5-10 minutes while on board.

Using the GF99 on the Petrel to help throttle down the ascent is a great tip.

 

[Michael Guerrero]

this a consistent problem

Not sure I agree it's a problem. "So far" covers probably 60 years of successful outcomes and doing the time, regardless of 20' or 15' or 10' or whatever, works. Dissecting the minutia of it seems overkill.