Question regarding Baker's Decolessons

[EFX]

Im still trying to comprehend the different approaches to the M gradient calculations. I have read it, not just here but elsewhere, but I don't fully understand it. It seems to stem from a difference in the way Buhlmann and was it Workman or someone else based their calculations. That is what I vaguely recall.

Would it be fair to say that one method is applicable to calculating for sea level pressure diving only. Whereas the other one can be used for calculations for sea level diving and will also produce results for diving at altitude? Or am I miss interpreting it.

Workman referenced his tissue pressures to sea level pressure which he arbitrarily set as 0 gauge pressure. Buhlmann used 0 absolute pressure. In reference to my post above regarding the two different m-value calculations, they can be done in gauge or absolute pressures as long as the surface pressure is not above sea level.

Both Baker in his program and the values in my spreadsheet work with Buhlmann a and b coefficients which use absolute pressures. Therefore, the GF or MV (Baker's so-called max m-value in his sample program output) in my ss will be calculated using absolute pressures. I have re-calculated current GF on the ss according to the method outlined in my previous posts in this thread and have verified that this is the correct approach for displaying current GF. I will post a major revision soon.

The math can be daunting. If anyone is having trouble with the math look at figure 3 in Baker's paper "Understanding M-values". The graph says it all. Baker's 91% is the % M-value on the graph. The GF calculation is % M-value gradient. I hope this helps.

 

[rsingler]

If anyone is having trouble with the math look at figure 3 in Baker's paper "Understanding M-values". The graph says it all. Baker's 91% is the % M-value on the graph. The GF calculation is % M-value gradient. I hope this helps.

Like this?
I portrayed (in this example) ~80% of M-value yielding ~50% of GF. Or is it relative to the chosen GFHi?

 

[EFX]

At the red line where you show 50% is the current GF (the % m-value gradient) , the base of which is ambient pressure, displayed by the yellow block in the graph. The 80% would be the % m-value displayed by the blue block whose base is 0 pressure. Baker's 91% m-value and GFHi of 75% correspond to the surfacing m-values whose points lie along the surface pressure dashed vertical line in the graph.

In the graph above it looks like the dive ends (where the black stair step line intersects with the surface pressure dashed line) at a GFHi of 20% (the % m-value gradient) and a % m-value of 60%. It appears, looking at the graph, that this dive is a reverse GF of 45/20. That is, GFlo = 45% and GFHi = 20%.

 

[dmaziuk]

Like this?
I portrayed (in this example) ~80% of M-value yielding ~50% of GF. Or is it relative to the chosen GFHi?

You should lose the M-value line and look at just the e.g. surfacing value in a single compartment. It's simpler that way.

ZH-L16 a and b coefficients for 5-minute TC are 1.2599 and 0.505 resp. Using conversion formula from Baker's paper, its surfacing M-value (M0) is 2.96 atm. Assume ambient pressure of 1 atm at the surface. Your "acceptably safe" surfacing inspired gas pressure aka "loading" in that TC is anywhere between 1 and 2.96; you can take the actual computed value, compare it to one or the other, express the result in percents or logs, or dead king's shoes, and call it "surfacing gradient factor 99 and a quarter in fucsia polka dots". Or whatever. (Note that the empirically-derived base model does not claim that e.g. 2.1 is any "safer" than 2.96, it just claims that either is "safe enough".)

A GFHi of 0.8 will change that a coefficient above from 1.2599 to 1.2599 * 0.8. This will be used to calculate the depth of your last deco stop, should you have one, or (indirectly) your no-stop time. (Note that on a no-stop dive, if you don't ride up all the way to 0 NDL and/or make a safety stop on the way up, you'll never hit that calculated value upon surfacing.)

M-values go up with pressure (depth) so then you extend the above into "M-value line". The lines are different for each tissue compartment, so you then extend it into "M-value surface" for all 16 TCs. It'll look something like this:

Then you can have fun plotting that ascent "staricase" in 3D on top.

 

[EFX]

That's an impressive graph. What it shows quite clearly is that for deeper depths faster TC's (lower half-times), can tolerate higher m-values.

 

[dmaziuk]

That's an impressive graph. What it shows quite clearly is that for deeper depths faster TC's (lower half-times), can tolerate higher m-values.

Which is what makes deep stops attractive on the "math" side: deeper stops effectively remove "the bump" and make it look all nice and uniform. Never mind that "the bump" comes from empirical studies and is what real goats (and humans) can really tolerate.

 

[UWSojourner]

Which is what makes deep stops attractive on the "math" side: deeper stops effectively remove "the bump" and make it look all nice and uniform. Never mind that "the bump" comes from empirical studies and is what real goats (and humans) can really tolerate.

On this topic, the problem with VPM's treatment of increasing gas loads in slower compartments was discussed here.

 

[dmaziuk]

On this topic, the problem with VPM's treatment of increasing gas loads in slower compartments was discussed here.

That's the difference between Baker's GFs and bubble models: ZH-L + GF is still ZH-L and will account for slower tissues on-gassing and will pad the shallow stops. Unless you push GFHi over 100. In bubble models going over empirically-derived M0 in slower tissues does not cause DCS symptoms because "miracle happens here".

 

[EFX]

never mind. deleted.