Continuous Decompression AND Staged Decompression

[dmaziuk]

If you play around in multideco or decoplanner and force them to do 1ft stops the "much shorter" isn't even a minute.

Yeah, but they really don't make much sense in terms of the model. That's what I meant: the model says six half-times and you're full, whereas a computer will happily compute you your gas loading by millisecond after that. The practically usable step should be somewhere in-between: less that 3 m, more than 1 ft.

It's not clear to me how one would figure out what that is from looking at calculated GF continuously updated on the computer's screen.

In 1983 Buhlmann reckoned that in-water decompression steps smaller than 0.3 bar are "difficult to maintain", but he suggested continuous or 0.1 bar steps in-chamber (p. 29 of Decompression - Decompression Sickness, last paragraph of section 3.5.2). If you can do 1 msw in open water nowadays, that may actually be just what the doctor ordered.

 

[EFX]

If I understand the Schreiner equation correctly which determines inert gas pressure in the tissues, when you first arrive at a deco stop the pressure gradient is at its greatest. As you hang at the stop the gradient decreases non-linearly according to the Schreiner equation. When the gradient has dropped off to a point you can ascend at 30 ft/min to the next stop and thus increase the gradient back to more efficient offgassing. So, the amount of time spent at maximum offgassing is a fraction of the overall offgassing rate for the entire stop.

In contrast, a continuous ascent will maintain a maximum offgassing rate because you are maintaining a fixed higher pressure gradient as the depth changes and the ambient pressure decreases. The question is: Can the body maintain a fixed high rate of (safe) offgassing for the entire trip to the surface? The ascent rate for continuous deco would probably be lower than the 33 ft (10 m) / min rate between deco stops.

 

[FlutterTubes]

If I understand the Schreiner equation correctly which determines inert gas pressure in the tissues, when you first arrive at a deco stop the pressure gradient is at its greatest. As you hang at the stop the gradient decreases non-linearly according to the Schreiner equation. When the gradient has dropped off to a point you can ascend at 30 ft/min to the next stop and thus increase the gradient back to more efficient offgassing. So, the amount of time spent at maximum offgassing is a fraction of the overall offgassing rate for the entire stop.

In contrast, a continuous ascent will maintain a maximum offgassing rate because you are maintaining a fixed higher pressure gradient as the depth changes and the ambient pressure decreases. The question is: Can the body maintain a fixed high rate of (safe) offgassing for the entire trip to the surface? The ascent rate for continuous deco would probably be lower than the 33 ft (10 m) / min rate between deco stops.

Yes EFX you said it perfectly! I have been fiddeling around with this though of continuous decompression for a long time, and this is the first place where I have seen the idea mentioned.
Sure there are practical challenges with it when it comes to longer decompression times, but one can't deny that it's theoretically way superior and probably safer, because you would avoid the spikes at the start of every stop, and simultaneously optimize the dull periods at the end of every stop.
My thinking is, that the function of gas volume is f(x)=10/(x+10) where x is depth in meters and y is ambient volume of gas.
The rate of ascent would be inversely negatively proportional to the inclination of the function, with some modifier derived from the gradient factor.

IMG_20200728_020133

• FlutterTubes
• Aug 3, 2020

Couldn't this be solved quite easily with todays dive computers, where one could have a coloured area showing ones optimal depth in the water column, green in the center fading through yellow and red at the borders, which would gradually move up (illustrated my a marker that moves down) and actually giving the diver an easy way to ride the algorithm, and even visualizing how aggressive ones ascent speed?

 

[dmaziuk]

Yes EFX you said it perfectly! I have been fiddeling around with this though of continuous decompression for a long time, and this is the first place where I have seen the idea mentioned.

Buhlmann does, in Decomperssion - Decompression Sickness, mention steps smaller than 3 msw are hard to maintain in open water, and that chambers could easily use much smaller steps instead. IIRC the context is about why deco stops are spaced at 3 msw and not about continuous decompression, but that was published the same year the first dive computer went on sale, so...

 

[boulderjohn]

. IIRC the context is about why deco stops are spaced at 3 msw and not about continuous decompression, but that was published the same year the first dive computer went on sale, so...

The debate began in 1907 when john Haldane published a paper arguing that periodic decompression stops were superior to the "uniform compression" system (continuous slow ascent) previously advocated by people like Paul Bert (1878) and Hermann von Schrötter (1895). Haldane's position was accepted and has largely held sway ever since, although continuous ascent has recently been advocated by others. To my knowledge, there is no new research supporting this, and in some cases I get the idea that people advocating it think it is a new idea.

 

[dmaziuk]

By numbers, it could get interesting at leading tissue compartment switches, or gas switches: if there is an "optimal" overpressure gradient, you can maintain it during continuous decompression, but it will change at the switch. Of course, thanks to Herr Dr. Buhlman, you can have make the number of tissue compartments large enough so the switches will be smooth, and modern CPUs can handle it even in a dive computer form factor. That still leaves gas switches though.

 

[EFX]

Using my spreadsheet and setting it up for a 100 ft dive for 30 minutes using air, a 60 fpm descent rate, a 30 fpm ascent rate, and a GF of 60/85, and setting the stop time loop for 0.1 minutes with 10 ft stop increment and a 10 ft last stop depth, I got the following deco schedule:

Depth Time Ceil
30 0.1 15
20 2 9
10 9 0

for a total deco time (includes ascent time) of 14.4 minutes

Setting the dive up for 1 ft stop depths and a 1 ft final stop depth yielded a deco ascent schedule starting at 24 ft for 0.1 minute. The next stop depth was 1 ft shallower at 0.1 minute. All shallower stop depths through 12 ft were for 0.1 minute. Then the schedule changed to:

Depth Time Ceil
11 1 10
10 0.5 9
9 1 8
8 1 7
7 0.5 6
6 1 5
5 1 4
4 1 3
3 1 2
2 1 1
1 1 0

for a total deco time of 14.6 minutes.

As you can see, because of the increased change in pressure gradient at shallower depths the deco time is longer to maintain a ceiling shallower than the stop depth. A 0.1 minute is 6 seconds at deeper stop depths. Based on this data a more efficient ascent schedule may be following the actual ceiling for the deeper stops but above a shallower depth follow staged deco stops of larger stop depth increments.

 

[TrimixToo]

To me, the reason to stay with staged deco stops is very simple. It's what everyone else is doing. That means the vast majority of decompressions are done the same way. The continuous decompression model (I happen to own a VR3, which lives on as a backup in case one of my Shearwaters should ever fall) is a much less-traveled road, with less evidence to support its use as a safe ascent method. With enough research, someone could prove that it's better, or worse. I don't know whether it's ever been done. I'll go with the crowd on this, not because I'm a conformist, but because in this case there is safety in numbers. My memory is a fickle thing on a good day, but I seem to recall Simon Mitchell saying something similar some time ago. Maybe someone else remembers for sure.

Just my 2 PSI.

 

[shoredivr]

No, it's not that kind of deco. That's just taking it easy to reach your stops and by the time you get there it's cleared. It's not what I'm referring to.

I'm assuming nobody is familiar with the old VR3 here or perhaps they forgot about that feature.. I still have a manual of that thing, let me (try to) post a capture perhaps I refresh some memories

I am not familiar with the VR3