difference between 2 deco models

[miglanc]

Hi. Do you know the difference between ZHL16B and ZHL16C deco models or maybe tables? I was wondering why the first model shows longer deco when using trimix 21/35 (max depth 45m/147ft, bottom time 20 minutes, deco gases: O2 and EAN50) then air. But when I changed to ZHL16C outcome was different. Trimix needs shorter deco then air. It was counted using GUE Decompression Planner.

 

[gkndivebum]

As I understand it, ZHL-16B is designed for generating tables, and ZHL-16C is slightly more conservative and is designed for
implementation in dive computers. The A/B coefficients are slightly different between the two.

As an aside, the coefficients for He are from ZHL-16A, which I understand was for "theoretical" use only.

Clarification appreciated if I've gotten this wrong.

Best -

gkn

 

[Dr Deco]

Hello Readers:

I do not know the reason for the difference. :06: We await some reader to jump in on this one.

Dr Deco :doctor:

 

[Diver0001]

This might help:

Buhlmann has three versions of the ZHL16 algorithm, each of which
has slightly differing a/b coefficients. ZHL16a is for theoretical
use only. ZHL16b is for generating tables. ZHL16c has slightly
more conservatism built in, because it was intended for use in
dive computers (UWATEC's Aladin and many other late-model dive computers
use a variant of this). I originally used ZHL16c, but have added ZHL16b as
a user-configurable option. I first chose "c" because it is
supposedly better at funky multilevel profiles, but mostly because I have
a wrist-mounted version of this program for personal use. Generally, you
will notice little difference between the "b" an "c" options, except
for middle-length dives to middling depths, where the "b" version will
be slightly less conservative.http://zplan.com/download/README

Translated in layman's terms he says that the M values are different between the various models. "A" being useless for practical purposes. "B" being some kind of baseline and "C" being more conservative.

The actual differences in detail can be seen in this table:

A B C
106.4 same same
97.3 same same
83.2 same same
73.8 same same
66.8 same same
62.3 same 60.8
58.5 57.4 55.6
55.2 54.1 52.3
52.3 51.7 50.1
49.9 same 48.5
48.2 same 47.2
46.8 same 46.1
45.6 same 45.1
44.5 44.1 44.1
43.5 same 43.1
42.6 same 42.4
41.8 same same
source: Understanding M-values; Erik C. Baker, P.E.

R..

 

[pt40fathoms]

What is the difference between the ZHL-16-C, and the recently introduced RGBM? Both are being used in dive computers today.

 

[pt40fathoms]

Attached is the PDF file entitled Understanding M-Values by Eric C. Baker, P.E.

 

[Diver0001]

What is the difference between the ZHL-16-C, and the recently introduced RGBM? Both are being used in dive computers today.

The ZHL16 etc are based on a mathematical algorithm. RGBM is based on witchcraft and good ideas....

ok, just joking.

It's a different starting point all together Haldanian models (I'm sure Dr. Deco will improve on my explanation) are based on a number of theoretical 1/2-time "compartments" which sort of "hold" a nitrogen load. You fill them and empty them like 16 little cups of water (nitrogen) according to a hard and fast set of rules involving time, depth and nothing else.

RGBM doesn't have cups. It has bubbles. The calculations in RGBM are intended to keep the bubbles from growing. It's not easy to do that because there are a gazillion things that will influence bubble growth. The most important, however seems to be the presence of some kind of bubble seed called a micro-nucleus and somethink called a "gradient factor" that I can't explain very well but seems to have to do with bubble size and Boyle's law. I think the gradient factor attempts to describe when a bubble will start to grow according to Boyle's law. In any event, RGBM pays attention to a lot more variables (in theory) than a Haldanian algorithm. Things like ascent speed, deep stops (more efficient off gassing occurs before bubbles grow because bubbles are harder to get rid of), even temperature and of course depth and time.

That's sort of the deco-for-dummy's version and I left out a lot of details but I think it touches on what I see (at least) as the main differences.

R..

 

[SDAnderson]

Pretty close to my understanding, Diver0001. Someone also explained it to me like this:

• Buhlmann models use "tissue compartments" to manage the growth of bubbles. In essence, the tables assume some bubble formation but strive to keep the size of those bubbles small enough that your body can filter them out or that they harmlessly dissipate.
• RGBM models us "pressure gradients" to manage the formation of bubbles. In essence, the tables assume that the best way to prevent the formation of large bubbles is to prevent the formation of small bubbles, which should also have the added benefit of reducing the sub-clinical stress imposed by all those little bubbles.

Then again, we both may be about to get spanked by those wiser than us...

 

[Jai Bar]

are the number of tissue compartments, the M-Values assigned for each compartment and also some models have a slightly different way of calculating the M-Values.
The same applies, of course, for the various Buhlmann models.

 

[jagfish]

[*]RGBM models use "pressure gradients" to manage the formation of bubbles.

I thought I read in the Encyclopedia of diving that it was rather accepted that silent bubbles form in almost any dive?

If so, would the RGBM model be fightig an uphill battle?

JAG