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OSTC's running hwOS 
ceiling computation
Posted by: jlm ()
Date: April 11, 2019 09:13AM

Question for Ralph.

I have problems understanding the value hwOS (current 3.01 version) displays for the ceiling during the deco phase of a dive. Obviously, I find these things while working on the OSTC4 code.

While on deco, I expect the ceiling to be smaller than the current stop depth, but when the ceiling is 3m shallower than the current stop depth, we should be able to ascend to the next (shallower stop. At the very moment we arrive at the shallower stop, the ceiling and new current stop depth are equal (for some time, ie. very short for short stops, but rather long for long stops). In other words: I expect the ceiling and current stop depth to progress in sync.

But it is not like that on hwOS. See for example some screen dumps in the GF factor brochure. Picture 11 shows ceiling 8.2m while we are still at a stop at 15m (and have 2 minutes of deco to go there). I simply do not understand this, and it feels wrong. Simply, when the ceiling is 8.2m we would be able to ascend to the stop at 9m, without violating GFhigh.


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Re: ceiling computation
Posted by: jlm ()
Date: April 11, 2019 12:22PM

I think I just answered my own question smiling smiley

I just re-implemented the OSTC4's ceiling algorithm. While the old algorithm does work, and gives results that I did understand, it seems not efficient ... and my new algorithm gives very similar results to hwOS. That was not my intention, so it was a bit of a surprise, when I ran it.

The ceiling represents an ascend relative to GFhigh, and the "normal" deco stops do not do that. In other words, when following the ceiling, you follow GFhigh through all deco stops, so you violate the deeper stops, and you end up at the surface at GFhigh (like you would when following the deco stops).

Not sure I really would want to dive a full deco schedule on GFhigh smiling smiley


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Re: ceiling computation
Posted by: Ralph ()
Date: April 11, 2019 07:01PM

Hi Jan,

you're right, the ceiling is calculated with GF-high. Within the envelop the Buhlmann model was validated in, it's ok to use e.g. a 80/80 setting, thus staying at 80% of the Buhlmann limit throughout the whole ascent. As long as GF high = GF low, you will see the stop being set at the next multiple of 3 meters of the ceiling depth. As soon as the ceiling rises to current stop - 3 meters, the stop depth will step up to the ceiling depth and the game repeats. When you see more than 3 meters distance between the ceiling and the stop depth, it's because the stop depth is calculated at GF low or some value between GF low and GF high, pushing the stop depth further down.

There is another effect with the GF low you also discovered yet: as the tissues take up gas, the depth of the first stop goes more and more down. And as the depth of the first stop sets the depth reference for the GF slope = (GF high - GF low) / (depth of first stop), the GF that is applied at a certain depth is continuously changing while deco builds up. If the stop becomes very big, i.e. shallow first stop, small GF low, high GF high, that can trigger the effect that stops can be left out because while going up a certain distance the GF-moderated allowed supersaturation rises more than the tissue supersaturation rises by itself.

I have reworked all the deco code a lot during the last months, making it more clearer and faster. The latest p2_deco.c file is not yet in the repository. Please establish PM contact with me via Matthias so i can send you the latest code.


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