Redistribution of decompression stops to optimise integral supersaturation

[David Carron]

Hello all,

I had a bit of spare time on my hands and I just got through generating a whole slew of ascent profiles over a range of depths and dive durations. For each dive profile I calculate a total decompression time using ZHL16C in 80/80 then redistribute that time across all possible stops to find the profile with the lowest possible integral supersaturation.

One of the most interesting (to me, in any case) things about the generated profiles is that whilst a very general pattern that emerges is to push the deeper stops shallower, the minimum integral supersaturation is systematically found when the shallowest stop is executed at 6m instead of 3m.

I'm certainly not going to be actually diving any of these ascents (quite apart from anything else, the optimisation is way too intense to be credible in real-time on my dive computer), but I absolutely could switch to doing my last stop at 6m.

Is anybody aware of any proper research on the optimal depth for the last stop?

David.

 

[JMarc]

Interesting. To lift a doubt -- if your last gaz is pure O2, the result is obvious -- what kind of gaz were you using?

 

[David Carron]

I had a bit of spare time on my hands and I just got through generating a whole slew of ascent profiles over a range of depths and dive durations. For each dive profile I calculate a total decompression time using ZHL16C in 80/80 then redistribute that time across all possible stops to find the profile with the lowest possible integral supersaturation.

One of the most interesting (to me, in any case) things about the generated profiles is that whilst a very general pattern that emerges is to push the deeper stops shallower, the minimum integral supersaturation is systematically found when the shallowest stop is executed at 6m instead of 3m.

Interesting. To lift a doubt -- if your last gaz is pure O2, the result is obvious -- what kind of gaz were you using?

These are all air dives. I wasn't particularly interested in the actual values or even necessarily realistic settings; just the general patterns that emerge.

 

[harvalen]

What impact does it have on your surfacing supersaturation? Your supersaturation will be lower at 6 m than at 3 m, but the off-gassing would also be slower. Would probably make sense to include X amount of time at the surface in the integral supersaturation.

 

[LFMarm]

If you are diving air, either I misunderstand what you did or there is a mistake. Deco on air (and any other EAN leaner than pure O2) at 3m is more efficient than at 6m while it’s the same on pure O2.

 

[David Carron]

What impact does it have on your surfacing supersaturation? Your supersaturation will be lower at 6 m than at 3 m, but the off-gassing would also be slower. Would probably make sense to include X amount of time at the surface in the integral supersaturation.

The integral supersaturation calculation terminates when the instantaneous supersaturation falls to zero, so that typically includes several hours of post-dive surface time.

For an integral supersaturation optimised ascent, the surfacing or maximum instantaneous GFs are typically above the programmed value; one of the many reasons that I'm not particularly interested in the specific profiles.

 

[inquis]

Does your definition of integral super saturation just integrating the saturation across time? If so, then the 20 ft final stop makes sense to me because it's a lower saturation than at 10 ft but time is only increased by a sqrt(ish) factor.

My recollection is that the better metric for "exposure" is the product of tissue pressure and the sqrt(time).

Can you elaborate on your process?

 

[harvalen]

The integral supersaturation calculation terminates when the instantaneous supersaturation falls to zero, so that typically includes several hours of post-dive surface time.

For an integral supersaturation optimised ascent, the surfacing or maximum instantaneous GFs are typically above the programmed value; one of the many reasons that I'm not particularly interested in the specific profiles.

That 6 m is more efficient with these parameters is probably just a side effect of 6 m still being shallow enough to not on-gass much and the lower supersaturation, compared to 3 m, has a lower impact than the higher instantaneous supersaturation when surfacing.

Ignoring instantaneous supersaturation, the most efficient strategy to optimize integral supersaturation would likely be to teleport to the surface, but that would violate the model and also ignore the problem with all the bubbles being formed..

 

[bardass]

to keep up

 

[dmaziuk]

That 6 m is more efficient with these parameters is probably just a side effect of 6 m still being shallow enough to not on-gass much and the lower supersaturation, compared to 3 m, has a lower impact than the higher instantaneous supersaturation when surfacing.

There is no meaningful on-gassing at 6 msw. The difference between 3 and 6 msw is extra .3 atm driving the off-gassing -- and presumably adding to supersaturation, so the question is what's more "better": the faster off-gassing driven by higher delta-P or the lower integral supersaturation from the lower delta-P.