Ascent rate and decompression question

[gzscuba]

So I've got a question about decompression theory.

In another thread, it was mentioned that coming from a deep depth, you could ascend relatively quickly to a moderate depth, ascend at a moderate speed to a shallow depth, and then ascend to the surface at a really slow rate. Not that I actually plan on using this technique, but I'd just like to try to understand the science behind it.

So lets say that I'm diving at a relatively deep recreational depth and time. Some of my faster equilibrating body compartments will be fully saturated with N2, and some of the slower equilibrating compartments will be only partially saturated. The longer the half-time for the compartment, the lower the N2 concentration. So let's say that I start to ascend.

I can ascend fairly quickly to a moderate depth, since my saturated fast-equilibrating compartments will release N2 rather quickly. Eventually, I'll come to a point where my slower-equilibrating compartments are reaching rather high concentrations of N2. At that point, I need to slow my ascent rate to allow these slower-equlibrating compartments to release enough gas to prevent DCS.

I may need to further reduce my ascent rate at a shallower depth again to allow other compartments time to release enough gas to prevent DCS.

Sorry for the longish post. Please let me know if anything here is incorrect.

 

[Amanda]

If I recall well, I think that's about right (even if words in english sound strange to me )

 

[MonkSeal]

One more thing: relative depression is bigger between 10m and surface then between 20m and 10m. That's why you can ascent quicker at deeper depth.

 

[MikeFerrara]

In theory it isn't that simple. In practice I think it isn't that complicated.

It might be hard to get into a detailed discussion of ascent rate without relating it to a specific decompression model. For discussion purposses I think a gas absorbtion model is interesting to look at. I would guess you could illustrate the same with a bubble model but I'm not a familiar with the math.

If you get a chance, play with one of the decompression softwares like GAP or Dplan. One of the pieces of info they give you is the max depth at which you are decompressing (the depth at which ambient presure = presure in the leading compartment). I'll check my terminology and edit the post later if need be. Wasting time below this depth simply adds to your obligation. I shoot for a constant ascent speed of about 30 ft/ min. Any further adjustments are made with stop durations. This may have more to do with planning and control issues than optimum decompression. the final result may be a faster ascent (shorter stop times) deep and a slower ascent (longer stop times) shallow but the adjustment is done with stop times not with actual ascent rates. The acception is my final ascent from my last stop to the surface. I do this part of the ascent very slowly with a pause at the surface if I can.

 

[WreckWriter]

gzscuba once bubbled...
So lets say that I'm diving at a relatively deep recreational depth and time.

I think this primarily applies to actual deep diving, not recreational depths. The difference in allowable ascent rates for recreational depths is not very dramatic.

Stick with 30 FPM for recreational.

WW

 

[gzscuba]

Hmmm, doesn't look like I can find source code for decoplanner, but the gap website does have some fortran code on it. It's been a while since I've stared at fortran.

Oh, if it makes the discussion more interesting, Wreckwriter, feel free to think of the dive as a technical dive instead. Like I said, I'm not planning on actually applying this in my diving. That would be...:boom: bad.

Thanks,

 

[WreckWriter]

gzscuba once bubbled...
Oh, if it makes the discussion more interesting, Wreckwriter, feel free to think of the dive as a technical dive instead. Like I said, I'm not planning on actually applying this in my diving. That would be...:boom: bad.

Thanks,

Well, if you're interested in the software's ascent rates the default ascent rate for V-Planner is 60 FPM from 1000-100' and 30 FPM for 100-surface. In practice most people I know use a considerable slower rate from 20-surface.

Since you mentioned recreational I don't think it would really be a benefit to try to use an increased ascent rate for 30' or less then goe to the standard rate (30 FPM). Easier and safer to just use 30 under those circumstances.

WW

 

[The Iceni]

I make no pretence about knowing very much about the algorithms used by the various decompression models and have a general understanding, which I will try to explain in the very simplest of terms, but as Mike Ferrara says, in reality it is far more complex than this.

All dives are decompression dives and some bubbles will form in the veins during the ascent from any dive. These are normally filtered by the lungs

1) During the bottom phase of a dive ambient pressure forces inert gas into solution by diffusion along the pressure gradient from lungs to blood to tissue fluid. On the ascent this gas must come out of solution.

2) If the ascent is too rapid there is not enough time for the gas to diffuse from the tissues to the blood and then into the lungs, so it comes out of solution where it is. As when the top is taken of a carbonated drink, bubbles form within the tissues (and arteries) as well as in the venous blood. These grow as pressure is further reduced and also grow by ongassing after surfacing, to cause DCI.

3) Thus, when decompressing the body must not be subjected to too rapid a reduction in the rate of pressure change.

Now looking at sea dives;-

1) Pressure increases linearly with depth at 1 bar per 10 metres of salt water.

2) At a fixed rate of ascent the rate of pressure change will not be constant but it will increase towards the shallows. It will be the greatest as the diver approaches the surface;- At 10 M per minute (33 f/sec), the ascent from 50 M to 40 M will take 1 minute, while the pressure will change from 6 bar to 5 bar, which is a reduction of 16% per minute. However, from 20 M to 10 M this reduction is 33% per minute and from 10 M to the surface it is 50% per minute and so on in an exponential manner.

3) In order to keep bubble formation to within acceptable limits it is obviously desirable to limit the rate of pressure change. This means the diver must ascent at a slower and slower rate as he approaches the surface. This is effectively what is produced by traditional tables where the rate of ascent is controlled by stops zig-zagging around the "ideal" line, with stop duration getting progressively ever longer.

4) Thus, even for so called no-stop dives, it is good practice to slow the ascent close to the surface. Hence the "safety stop" and, as Mike also says,

The exception is my final ascent from my last stop to the surface. I do this part of the ascent very slowly with a pause at the surface if I can.

Now you know why!

It is nowhere nearly as simple as this in practice.

When tissue bubbles form they block the capilliaries and stop effective offgassing from the affected tissues, thus technical divers try to reduce, all bubble formation. Hence deep stops are now routinely used to slow down the rate of ascent much earlier which allows more rapid deco at the shallow depths. This is most famously employed in the Reduced Gradient Bubble Model.

Another risk factor for DCI, frequently mentioned by Dr Deco, is the increase in miconuclei (small bubble) formation by the effects of isometric work at both at depth and during all stages of the ascent, which includes climbing the ladder and carrying heavy kit.

So, to my mind ascent rate is important but it is only one factor.

 

[Doppler]

Hi gzscuba:

Yes, variable ascent rates through 'normal' OSS depths are common in many algorithms, but it's not really as simply stated as all that.

In a neo-haldanian model -- which it looks like you are familiar with -- the major consideration regarding bubble growth is change in ambient pressure -- driven by Boyle effects. Also the controlling tissue group during a long ascent may change several times and may at one point be a fast compartment and then a slower one and then a faster one again... the math is rather complex... but perhaps simpler than the physiology (hey, I was a physics major, give me a break!) However, in most modern theorem, bubble growth is not only due to the effect described by Boyle... it is also driven by diffusion across the bubble wall from dissolved gas in body tissue. Therefore variable ascent rates are generally not considered Kosher... see earlier posting regarding V-planner which uses the VPM algorithm... well, a variant of it to be precise. However, as far as the algorithm is concerned, you can do a ballistic acsent to the point that off-gassing begins without affecting your deco... in fact you may even shorten it. Of course you will likely also be bent.

Possibly more important to consider is that during the past 10 or 15 years, the acceptable ascent rate for recreational diving has dropped considerably... 75 fpm - 60 fpm - 45 fpm and now 30 fpm. Since several studies have pointed out that many recreational divers ascend at speeds in excess of 120 fpm -- counter indicated at any depth!!! -- you might simply spread the word that between 45 and 30 works well! Variable ascents can be confusing and possibly dangerous... keep it simple.


DD