Modern research/thoughts on Ascent Rates

[inquis]

"Decompression stress is defined as the amount of inert gas dissolved in various tissues throughout the body. During ascent, bubbles increase in size and are released by tissues into the veins.

No argument there. As the OP is contending, however, a given bubble doubles in volume when depth is changed from 500 to 233 ft. It also doubles going from 33 ft to 0. Substitute whatever size increase you wish for "doubles", and it remains that there can be a larger depth decrease the deeper you start while yielding the same size bubble.

In other words, the rate of bubble expansion is identical when going faster/deeper. Our imperfect off-gassing mechanism is able to "keep up" with that expansion rate when shallow, why can it not keep up with it when deeper?

I also see a distinction between ascent rate and deco stops. The latter is dependent on the ratio of tissue pressure to ambient. Empirically, we know you have to stop before that gets too big (e.g., Haldane's 2x). I'm not aware that any of that deco research investigated the ascent RATE while the ratio was below the threshold, but I'm happy to be corrected on that.

Back to the original hypothesis under discussion, the time taken to go from a tissue:ambient ratio of 1.0 to 2.0 (i.e., "reach the next stop") is identical between faster/deeper and slower/shallower. I understand your point that our off-gassing mechanism has a fixed "throughput", but when the available time is the same and the additional bubble volume is the same, what causes the system performance to be different?

 

[EFX]

I think you are missing that deco stops are the primary method for adjusting ascent rate over a given pressure gradient. Include the stops and you will find a very different curve.

Using my spreadsheet I compared two dives, both to 250 ft for 20 min, at a descent rate of 60 fpm, using a GF of 60/85 on trimix 20/40. One dive used an ascent rate of 30 fpm for the entire dive. The second dive used an ascent rate of 120 fpm up to 100 ft which was the first stop and then 30 fpm between the stops and to the surface. It made no difference in the profiles. Both dives had the same stops, stop times, and total deco time.

For what it's worth here are the controlling TC's using Buhlmann 16 starting at the level segment at 250 ft (TC1 - TC16 = fastest to slowest):
1,3,3,4,4,4,4,4,4,4,4,5,5,6,6,6,6,7,7,9,9,11,11.

Since I'm not a technical diver I have nothing from experience to give you.

 

[inquis]

In theory, the fastest total ascent time would come from a continuous ascent that rides the supersaturation limit.

You're talking about minimizing the time between first stop and the surface, which is only part of the ascent. The OP's concern is what happens BEFORE the supersaturation limit is reached. As that greatly influences where the first stop occurs, I'd wager the OP's approach of faster/deeper results in a shorter total ascent time (bottom to surface).

 

[inquis]

It made no difference in the profiles. Both dives had the same stops, stop times, and total deco time.

I cannot duplicate this in Subsurface. If I do 99 fpm ascent below 50% average depth (which is up to about 100 ft), there is a significant difference in total run time. 139 mins (fast initial ascent) vs 161 mins. The faster initial ascent had a deeper first stop (90 ft vs 80 ft), but it was reached 3 mins earlier.

 

[lowwall]

You're talking about minimizing the time between first stop and the surface, which is only part of the ascent. The OP's concern is what happens BEFORE the supersaturation limit is reached. As that greatly influences where the first stop occurs, I'd wager the OP's approach of faster/deeper results in a shorter total ascent time (bottom to surface).

No, I'm talking about minimizing the total time of ascent from the bottom from a purely theoretical perspective. The initial ascent rate would - again, theoretically - be infinite up to the point of maximum acceptable supersaturation. After that you'd continuously ascend - whether it's at a rate of single centimeters or tens of meters per minute - by riding the supersaturation limit all the way to the surface. The smoothness of the ascent will depend on the number and saturation rates of the tissue compartments in the deco algorithm

You won't be able to reproduce this using a deco planner because they all include limit assumptions, but the math is straightforward if you want to DIY it

 

[inquis]

The initial ascent rate would - again, theoretically - be infinite up to the point of maximum acceptable supersaturation. After that you'd continuously ascend - whether it's at a rate of single centimeters or tens of meters per minute - by riding the supersaturation limit all the way to the surface.

OK, I agree that's the shortest total time. FWIW, @EFX did the DIY version from the first stop onward (quantized to 1 ft deco intervals):

It appears that it makes no difference in total deco time between a staged and continuous deco schedule. The overriding consideration is one of safety and precedence in the diving community.

 

[Manatee Diver]

I can't find it anymore, I seem to remember reading a document by GI3 when he was running WKPP, that they used a 1ft per minute ascent rate after they cleared their oxygen deco at 20ft. As that is the time you will have the greatest pressure gradients.

 

[Lorenzoid]

I can't find it anymore, I seem to remember reading a document by GI3 when he was running WKPP, that they used a 1ft per minute ascent rate after they cleared their oxygen deco at 20ft. As that is the time you will have the greatest pressure gradients.

It lives on in GUE protocol.

 

[Akimbo]

I seem to remember reading a document by GI3 when he was running WKPP, that they used a 1ft per minute ascent rate after they cleared their oxygen deco at 20ft.

I would guess that the precedent for this is the US Navy Treatment Tables 5&6, of course the O2 stops are deeper in a chamber:

You might look at the 1'/min rate from 60 and from 30' and think it could be faster. That is true from an outgassing perspective but there is a dual purpose. It gives the inside tender and medical personnel time to observe the patient for symptoms and stop ascent.

I have seen treatments when the diver's symptoms returned, or might have returned, and the table 5 was extended to a table 6 or table 6a.

 

[EFX]

I can't find it anymore, I seem to remember reading a document by GI3 when he was running WKPP, that they used a 1ft per minute ascent rate after they cleared their oxygen deco at 20ft. As that is the time you will have the greatest pressure gradients.

Wow! 1 fpm. That seems agonizingly slow and agonizingly unnecessary. I don't dive with pure O2 but I think once you're cleared to go to the surface you stay on O2 all the way up. Since you're not taking on any inert gas what's the point of making such a slow ascent?

Edit: I posted this before seeing Akimbo's treatment table No. 5 posted above. For a recovering DCS patient this makes sense but for a normal non-DCS ascent, why?