Rounding altitude settings on Cressi Leonardo

[NAM001]

Workman's model was tied to sea-level altitude and atmospheric pressure relative to 1 atm. It couldn't be easily adjusted for altitude: atmospheric pressure changes non-linearly with altitude. One of the key points of Buhlmann's work was diving in Swiss mountain lakes. So his formula works in absolute pressure and is suitable for altitude diving with nothing special to "handle". You just need to feed it the correct atmospheric pressure.

I agree with you I have read the same articles you got your comment from also. that change was made from a surface pressure of for calculations to absolute for calculations. You are right you have to input the right surface pressure to make it work right. The inputting of that pressure or lack of is the crux of the problem. The (for me) the main concern is how does that impact the GUI of the computer. garbage in garbage out. Like boulder john said most the ascent is of no real consequence but he last 30 feet or so is the snake in the woodpile. the proper altitude setting effects more than the dive it also controls the calculations of off gassing on the surface which controls the max time of repeated dives. It is a chain effect in which one fix causes problems in other areas of the problem. That problem further manifests it self in the lack of knowledge of the users to make the proper corrections to keep the dive safe as opposed to compounding the issue. If posted before that computer use and operations would make a great class for divers to take.

 

[CycleCat]

Quote snipped to the relevant parts I'm replying to:

The reason for the change is the lesser pressure on the surface. As a diver ascends, the diver must make sure the pressure of gases in the body is not too much greater than the surrounding ambient pressure. Decompression algorithms are designed to make sure the ascent allows enough off-gassing for this to be safe.
[SNIP]
As you ascend, there is still not a whole lot of difference for the first 2/3 of the ascent, but then the air pressure starts to become a bigger portion of ambient pressure. At 34 feet, the sea level ambient pressure is 2 atmospheres; at Lake Tahoe, it is 1.8 atmospheres (90%). At the surface, sea level ambient pressure is 1 atmosphere; at lake Tahoe it is 0.8 Atmospheres (80%).

In summary, when diving at altitude, your body on-gases pretty much the way it does at sea level, and it off-gasses about the same for most of the dive. When you get shallow, things change rapidly, and if you follow a sea level ascent profile, you will be in danger of DCS.

If I were diving at the Blue Hole in New Mexico (about 4,600 feet) with a computer that adjusted for altitude manually, I would set it for the altitude it calls for.

Thanks for a more thorough explanation of what I was getting at with my novice experience... The atmospheric setting makes more of a difference from 33' to my 15' safety stop, and from my 15' safety stop to the surface than at depth.

I hope to be diving at Blue Hole next weekend for my final cert dives. My Cressi computer's level 1 setting covers from 2296' to 4921' and that is where it will be set.... 300 feet inside the max altitude for that setting.

 

[boulderjohn]

I hope to be diving at Blue Hole next weekend for my final cert dives. My Cressi computer's level 1 setting covers from 2296' to 4921' and that is where it will be set.... 300 feet inside the max altitude for that setting.

Good plan.

I hope to be in the area not long after that, now that New Mexico is allowing foreigners like me into the state again. I won't be diving the Blue Hole, but I will be getting gas fills at Stella's.

 

[dmaziuk]

Buhlmann adjusted the algorithm after Swiss divers had problems at altitude. His adjustments were used successfully by Swiss divers at Lake Titicaca. I would not say, though, that there has been a whole lot of true research on those adjustments at very high altitude, and I would hesitate to trust them (or any other algorithm) at a very high altitude.

AFAIK the M-values in ZH-L16 were derived from fitting a curve to empirical results. One problem with that is you don't know if your fitted curve is still valid outside your empirical range. So just from the basic principles they are questionable at either end: altitudes above those that were tested and depths/exposure times greater than those tested.

PS. of course what Cressi RGBM does is a whole 'nother question.

 

[boulderjohn]

PS. of course what Cressi RGBM does is a whole 'nother question.

Bruce Weinke wrote a book about diving at altitude, and he lived in New Mexico; however, all his work with RGBM is based on his theory of what the titles says--reduced bubble gradients. He has published precious little actual research in support of it. In the very important DAN workshop on deep stops, he presented a whole lot of data in support of deep stops and RGBM, but it was all numbers, with no publication, no explanation, etc. Dr. Neal Pollock asked him flat out where he got those numbers, and the tap dance of the non-reply is very telling.