Gradient Factor Presets by Manufacturer/Computer

[NAM001]

It's in Baker's "understanding M-values". As well as using higher-that-actual inert gas fraction (as in diving nitrox w/ computer set to air) and modifying half-times for asymmetrical: slower off-gassing. As I understand it, Thalmann slows down off-gassing at some point, and bubble models do too when they think the bubble's "too much", i.e. that last one amounts to ditching the pure dissolved-phase model" not Buhlmann aymore.

With out digging into you post more,,,,,I have to take your word for that. My point is that there are many ways levels of achieving conservatism that has been implemented over the years. The only thing that is common appears to be the minimizing the tissue levels at near surface prior to making the highest ambient pressure change of going from a few meters to the surface. 30 ft t the surface is a 2:1 change in it self. that alone justifies the safety stop so you go from say 1.5 ambient :1 at the surface. One thing for sure the majority of many new divers will say Buhlmann who ????

 

[CandiveOz]

I believe in freedom from choice:

Your comment is a bit Owellian doublespeak. Before we git into our differences, lets summarize what we agree on. Simplicity. Your computer is very simple to operate and it serves your type of diving. Great, I like simple things too, that is why I am pushing the KISS principal in this thread. You believe in longer safety stops and understand why a diver would obviously be penalized for fast ascents, short surface intervals etc. Its all to keep the diver safe and reduce the risk of DCS. If we agree on all this, why do we have a difference?

No. The purpose was to add conservatism and specifically add it to protect fast tissues more than the base model does. I wouldn't be "happy" with 10/99 because current thinking is fast tissues don't need quite that much protection after all, so all that extra deco time and gas would be wasted. But not because it's "pushing the limit of the model" -- it doesn't. It counts in all the extra gas loading accrued during stops, it calculates the next deco stop by Buhlmann formula with your added conservatism factor.

This was the intent of Erik Baker, and the deep deco protocol did protect the fast tissues...but at the expense of the intermediate and and slow tissues which were still taking on gas. And even if the model lengthened the shallow stops, I guess they are not long enough to reduce the risk of DCS. I used 10/99 as an extreme example and I am surprised at your rational. Its not because the profile is too conservative but because the profile is too risky. Simon Mitchell and Bruce Partridge are not making their GF-lo's shallower because deep stops are too conservative and a waste of deco time. Its because deep deco stops are too risky! The GF-hi of 99 is too risky. Here is what Erik Baker says from his paper Understanding M -values

Even Erik Baker recognizes the inadequacies of M-values. He even uses the words "push the limits." Extreme gradient factor pairs push the limits and this is what I was trying identify in my Preset table (the red block).

off the top of my head I don't know why some (all?) vendors won't let you set GF Lo above GF Hi. I don't see what's wrong with spending more time at shallow stops,

As I indicated before, shallower stops didn't fit in with Baker's deeper stop argument.

But let me conclude that this does not invalidate "deeper stops", slow ascents and bubble models etc. Certainly we want to reduce gas intake in the intermediate and slower compartments and minimise bubble formation in the fast compartments but its a bit of a balancing act. The lesson that we learned is that you don't take it to the extreme.

Now microbubble management may not be as important in deep tech diving (due to the slow tissue uptake risk) as compared to recreational diving where we play in the shallow end of the pool and where the greater pressure changes occur. Maybe at these shallower depths, we don't want to shake the soda can too much before opening; meaning slow ascents and pauses may be a good thing. This article may be of interest.

 

[dmaziuk]

Your comment is a bit Owellian doublespeak.

Devo, actually.

This was the intent of Erik Baker, and the deep deco protocol did protect the fast tissues...but at the expense of the intermediate and and slow tissues which were still taking on gas. And even if the model lengthened the shallow stops, I guess they are not long enough to reduce the risk of DCS.

You guess wrong: "expense of the intermediate and slow tissues" happens only if you push gradient factors over 100%. The "deep stops" getting bashed are the profiles that do just that.

With GFs under 100% all off-gassing takes place between the ambient pressure line and Buhlman''s original M-value line. The only question is how close to the ambient pressure line you're keeping it, at GF Lo 10 you're likely too close to ambient to get much gas moving. That's what makes it inefficient.

The thing about math is, 2 plus 2 always makes 4 no matter which way you add them. If your math does not calculate shallow stops long enough to protect slow and intermediate tissues at GF 10/90, then it also does not calculate them long enough at GF 100/100. And vice versa.

 

[CandiveOz]

Certainly GFs of say, for example, 45/110 pushed beyond the limit of the model but you again failed to explain why divers are making their GF-lo's shallower; well within the model. And again you fail to understand the limitations of a mathematical model trying to replicate human physiology. And before you state another mathematical expression; two wrongs don't make a right.

 

[CandiveOz]

The following link is an excellent article on deep stops during decompression diving.
Evolving Thought on Deep Decompression Stops

@boulderjohn has clearly explained and consolidated the current studies regarding this issue. I personally think its required reading for all divers.

Its relevance here is the Fraedrich Study. I am currently waiting to obtain the complete paper to review personally but from @bouldersjohn article it appears appropriate gradient factor settings are much shallower and narrower than than previously thought.

"For Bühlmann ZHL-16C, GF low settings lower than 55 led to first stops that were considered to be too deep. The study mentioned that a GF Low of 70 was also acceptable, so it explicitly approved of a range of 55-70 but did not specify an upper limit. It also said a GF high of 70 or below was within the preferred US Navy limits."

 

[jstownse]

I don't have anything useful to add, just that I found the discussion interesting and civil. I appreciate the effort CandiveOz.

 

[CandiveOz]

Thank you @jstownse , you are very kind. Once I get the complete Fraedrich Study, I'll provide an update.

cheers