Why S-Curve

I have being getting this question a little since some people don't quite understand. So I wanted to explain it a little and open the floor for questions....

We use a deco bottle to both reduce the requirements of rock bottom and to accelerate the decompression by 50%. The way we do this is by elevating the FO2 in the bottle, so that when we first switch to the deco bottle we have the highest tolerable PPO2 of 1.6, and then slowly reduce it over the 5 stops until it reaches a PPO2 of 0.8, and then we can switch again or do a dissolved profile. This will give us an average PPO2 of 1.2 over the 5 stops. In order to take advantage of the oxygen window, we will need to do an S-Curve shape for the 5 stops, so we will emphasize the time at the higher PPO2 of 1.6 and 1.4 and then reduce the time to ascend to the lower PPO2 while still allowing the top of the curve for dissolved gas profile. The advantages we get from an elevated oxygen include off-setting the inert gasses and lessoning the on-gas in the slower tissues of the inert gases during the deco ascent as well as the bubble mechanics of keeping the bubble smaller. These include the Surface Tension plus the ambient partial pressure keeping the internal pressure when relating to the ongas from tissue to the bubble and the O2 gradient between bubble and blood. We are also taking advantage of what is known as the Oxygen Window – when we use oxygen in the shallowest part of the deco we are treating the slow tissues with the best gradient between ambient and dissolved gas.

Questions?

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Permalink Reply by Brian Wiederspan on January 14, 2009 at 10:03am

I don't remember the dive (Harpoon?), but Doug and I ran a linear (pragmatic) profile once on a standard 140-150ish dive just for kicks and while I didn't feel like crap, I certainly didn't feel as good as I do when using ratio. Psychological? Maybe, but it was enough for me to stay with the method we're using.

Sounds like you're comfortable with what you're doing, which is perfectly acceptable. I simply prefer to keep things consistent for every dive I do and not have to make adjustments from dive to dive. Could I? Sure. This way simply works best for me.

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Permalink Reply by Brian Wiederspan on January 14, 2009 at 6:09pm

I was referring back to a statement you made earlier about how you understood leaving the 30' stop time the same for 2 deco bottle dives, but why do that for single deco bottle dives. Maybe I misunderstood what you were saying.

My point was, I would prefer to do linear/pragmatic deco for all dives, or shaped/ratio deco for all dives, and since I feel better with a shaped deco, that's what I personally use.

For me, using linear on a single deco bottle dive and a shaped on a multiple deco bottle dive gets confusing. And, where do you draw the line? Is it all single deco bottle dives, even on more aggressive 170ish dives? I'd much rather use a shaped deco on a 170', EAN50 deco dive than a linear, whereas on a 140' dive, I wouldn't be as opposed to a linear profile if that's what the team was doing.

That's all I was referring to about consistency.

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Permalink Reply by Mark Gottfried on January 15, 2009 at 10:13am

Hi Rainer,

I'm jumping little late into this discussion, but hopefully I can clarify your last paragraph.

If you only using one bottle (50% ) on a typical T1 dive (UTD Tech 2) for demonstration purposes you can look at it as if you were doing deco on back gas. In other words the shallower stops accommodate slow tissues using dissolved gas (gradient) strategy, hence need to be in the shape of the exponential curve. What that means is that the 30' is longer than the 40', the 20' stop is longer than the 30'.

In two bottles dive (UTD Trimix 1 and up) the 30' serves primary a physiological purpose to give the lungs a bit of a break before spiking the ppO2 again at 20' with Oxygen. As an added bonus longer time at 30' gives the team a chance to stow away the 50% and it does also keep it consistent with the above paragraph. Overall win - win.

Hope this helps.

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Permalink Reply by Kendall on March 3, 2009 at 10:09am

What work has been done to empirically support the S-Curve?. Has anyone compared post dive bubbling with a Doppler to compare results from S-Curve profiles versus a Fibonacci-like progression employed in RGBM, VPM, Spencer, DCIEM and other models?

I understand the intution, so no need to go through all that again. What I'm asking is whether there is anything other than intuition?

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Permalink Reply by Richard on March 9, 2009 at 11:26am

Rainer said:

That's what I was asking (did that get missed?). Is this SIMPLY a building block approach, or is there a deco advantage? You seem to be suggesting that the only reason to do more time at 30' than 40' is consistency. Thanks.

Consistency and you don't want to have massive jumps in the profile either. 1min @ 30ft and 10min @ 20ft would not be a smooth transition. 2min @ 30 and 10min @ 20 would be about as big a jump (1:5) as you'd want. With 1 deco gas there is nothing magic happening at 20ft. You could almost certainly succeed with a 3,5,10 up from 30ft (assuming decent surface conditions).