Riding deco ceiling on ascent

[boulderjohn]

ETA: @kaylee_ann - have you had a chance to grab a copy of Powell's Deco For Divers yet? Highly recommended given your aspirations and the threads in which you seem to participate.

The problem with Powell's book is that when he wrote it, he was very much within the deeper stop mode of thought. He has since come to the conclusion that those stops are too deep, but that change of thinking is not reflected in the book.

 

[kaylee_ann]

All (non-saturation) deco dives work that way. It's just a question of where the line is between "fast" and "slow" -- more correctly, between "off-gas" and "on-gas". The "deep stop" debate you seem to be referencing is simply about where to position that line. Bubble-models typically place that line on the faster side of the spectrum (thus more tissue compartments continue to on-gas).

ETA: @kaylee_ann - have you had a chance to grab a copy of Powell's Deco For Divers yet? Highly recommended given your aspirations and the threads in which you seem to participate.

I’m talking about where the first stop is placed. My idea was if the first stop was just shallow enough for the slowest tissue to begin off gassing, and that’s where Suunto has their ceiling, if you went below their ceiling you’d increase deco. Maybe something isn’t connecting here.
I actually have that book, and have read it many times!

 

[kaylee_ann]

The problem with Powell's book is that when he wrote it, he was very much within the deeper stop mode of thought. He has since come to the conclusion that those stops are too deep, but that change of thinking is not reflected in the book.

Yeah- latest I’ve read (or watched, rather) on deco theory is a talk from Dr. Simon Mitchell. I retained the other information like gradient factors and all that from Deco for Divers, but didn’t agree with the deep stop part of it.

 

[inquis]

My idea was if the first stop was just shallow enough for the slowest tissue to begin off gassing, and that’s where Suunto has their ceiling

That seems like a recipe for disaster since it effectively eliminates deco stops almost entirely. The slowest tissue hasn't absorbed nearly as much as the other tissues, so its equilibrium pressure is very near the surface.

 

[100days-a-year]

I've said this more than once.
No need for books.
This place has links to salient papers on pubmed, esteemed mathematicians, esteemed software writers, esteemed decompression doctors and theorists and enough empirical data to draw conclusions from that are going to likely be more up to date and more scientifically rigorous than books that are oftentimes full of or containing outdated, disproved or useless decompression strategies.

 

[kaylee_ann]

That seems like a recipe for disaster since it effectively eliminates deco stops almost entirely. The slowest tissue hasn't absorbed nearly as much as the other tissues, so its equilibrium pressure is very near the surface.

I didn’t even think of thatlol
Eta I don’t even know why I said that. Because that wasn’t what I was saying in the original post…wtf idk I’m sick as a dog. Ignore me

 

[dmaziuk]

I’m talking about where the first stop is placed. My idea was if the first stop was just shallow enough for the slowest tissue to begin off gassing, and that’s where Suunto has their ceiling, if you went below their ceiling you’d increase deco.

No. The stop has to be shallow enough to create sufficient delta-pressure for the tissues to off-gas, but not so shallow as to exceed the M(agical)-value of delta-pressure in any of the compartments. I.e. it has to be as shallow as possibly safe: the opposite of just shallow enough to begin off-gassing. Obviously, the definition of "safe" varies between the models.

Whatever Suunto is doing is an artifact of their programming. My Cressi also doesn't seem to recalculate the deco obligation when I come up, unless I come up exactly to its prescribed stop depth and complete the stop exactly as it wants it. I suspect it's the artifact of simplified "folded" RGBM implementation rather than the bubble model itself, but since nobody's ever seen the code: who knows.

 

[BlueTrin]

I recently had a look at a friend's Suunto D5 user manual and was very surprised to see this statement:

NOTE: It is always recommended to keep close to the decompression ceiling when ascending.​

In fact, the depth -- labeled "Stop" on the main screen -- seems to be identically the ceiling value. Their guidance is to stay within a range of [-2 +10] ft of that changing value. In their words, "providing continuous decompression with optimum ascent time."

In contrast, Shearwater computers (and I actually think my very old Suunto Mosquito did as well) round the ceiling up to the next multiple of 10 ft / 3 m for the displayed Stop depth. The continuous ceiling is available, of course, and the Perdix can even be set to automatically display it in place of the NDL field. However, the Perdix manual has this note:

Please note that there is very limited information on the effects of following a continuous ceiling instead of stopping at stops and only moving up to the next stop when the stop has cleared​

The Teric manual, in contrast, does not have such a statement.

I'm curious then...
Is the lack of such a warning in the Teric manual and flat out recommendation by Suunto's newest release indicative of any recent research?
If you use a different Suunto computer for technical diving, does it also guide you toward a continuous ascent (as the D5 does) rather than traditional discrete stop depths?
If you use a Shearwater, do you ride the ceiling up on a routine basis? (I can easily see doing this in an emergency situation, likely with more aggressive GFs as well.)
Any sense of the runtime savings by such a practice?

Out of curiosity, how do you communicate your continuous ascent rate to your buddy?

Wave a “bye bye” while you ascend?

It’s probably not ideal for team diving.

 

[BlueTrin]

No. The stop has to be shallow enough to create sufficient delta-pressure for the tissues to off-gas, but not so shallow as to exceed the M(agical)-value of delta-pressure in any of the compartments. I.e. it has to be as shallow as possibly safe: the opposite of just shallow enough to begin off-gassing. Obviously, the definition of "safe" varies between the models.

Whatever Suunto is doing is an artifact of their programming. My Cressi also doesn't seem to recalculate the deco obligation when I come up, unless I come up exactly to its prescribed stop depth and complete the stop exactly as it wants it. I suspect it's the artifact of simplified "folded" RGBM implementation rather than the bubble model itself, but since nobody's ever seen the code: who knows.

What do you mean by “folded” out of curiosity?

 

[dmaziuk]

What do you mean by “folded” out of curiosity?

It's what Suunto calls the code running on low-end computers. Maybe also Cressi and Mares though I'm not sure about them. Nobody knows what it is but given the official explanation: these devices don't have the CPU oomph to run the full "iterative" RGBM, one could make an educated guess: lookups in pre-computed tables and simplified functions that work "close enough" within the device's intended range.