Why no accurate computers?

[Mod63]

And here is a pretty comprehensive comparison of different dive tables.

Decompression Diving

And if you are ever close to Stuttgart when Albi is doing on of his workshops it's absolutely worth it.

 

[100days-a-year]

My solution. Simple. Product liability lawyers and proprietary algorithms hadn't found the industry yet.

 

[dmaziuk]

Based on my understanding that it was based on the limitations of computer processing, I have been predicting that computers would make that switch as computer processors improved.

There is no compute difference between A, B, and C sets, they only differ in M-value (technically, the 'a' constant) in some tissue compartments.

I have not seen helium numbers for "C" set, maybe they exist in some edition of Tauchmedizin. Heinrichs-Weikamp chose this: Multi Deco vs Baltic - Matching and use and called it "C", but the actual He numbers are listed as "B" in every other source I have seen. So maybe helium numbers were never adjusted for "C" like nitrogen ones.

And to echo other posters, from all I read the "B" set was designed for cutting tables where there is extra conservatism in planning the dive for max. depth but not actually spending all your time there. With computers tracking the actual depth, that extra conservatism needs to be added to M-values and that's what the "C" set is: a bit more conservative than "B" from 27-minute compartment on.

(Note that for typical no-stop runtimes there is no appreciable difference between "B" and "C" because saturating 27-minute and slower compartments takes a while.)

 

[dmaziuk]

PS. the other issue is nobody is diving 100/100. The actual difference in numbers between 'B' and 'C' sets is on the order of 0.05 with the 38.3-minute compartment being the greatest (at a glance): 0.56 vs. 0.5043 resp. That's about 90%. It'd take a little math to translate that into GF but off the top of my head I'd expect you won't go over the 'C' numbers diving 'B' set on GF85.

 

[scubadada]

PS. the other issue is nobody is diving 100/100. The actual difference in numbers between 'B' and 'C' sets is on the order of 0.05 with the 38.3-minute compartment being the greatest (at a glance): 0.56 vs. 0.5043 resp. That's about 90%. It'd take a little math to translate that into GF but off the top of my head I'd expect you won't go over the 'C' numbers diving 'B' set on GF85.

I have heard, though cannot verify the accuracy, that diving B at a GF high of 75, 85, 95, would be about 77-78, 87-88, 97-98 with C. It is interesting that the highest Ratio preset is 93/93

 

[RayfromTX]

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[dmaziuk]

I have heard, though cannot verify the accuracy, that diving B at a GF high of 75, 85, 95, would be about 77-78, 87-88, 97-98 with C. It is interesting that the highest Ratio preset is 93/93

In general the GF that applies at a stop is somewhere between GF Lo and GF Hi. To see what GF will be applied at a given stop, you first have come up with a profile that has that stop, see where the first stop is and apply GF Lo there, draw the GF line from there to surface (GF Hi), and see where the stop in question falls on that line.

No-stop dives are easier because the only GF that applies is GF Hi, however, now you have to look at the controlling tissue compartments. ZH-L16's coefficients are the same in 'A', 'B', and 'C' sets for 4- to 18.5-minute compartments (1st 4). The differences start from 27-minute TC. I.e. in order to actually trigger those differences your profile has to load those middle compartments enough so they are controlling. I'm not sure what profile that'd take: a very long very shallow one?

On top of it all GF applies to Buhlmann's 'a' and 'b' coefficients whereas to humans % of M-value makes more sense. That requires a little math to convert from one to another, but that's just icing on the cake.

 

[scubadada]

Hi @dmaziuk

I was talking about no stop dives, so just the surfacing GF. I'm under the impression that just the a values were changed between A, B, and C. It seems this would change the Mo, surfacing M value, but not the deltaM, the slope of the line. Seems like this would shift the M-line to the right, more conservative, for the compartments that are affected. I'll have to think about what it take to make the 27 or 38.3 min compartment controlling, perhaps pay more attention to my tissue loading graph. Loading and unloading works identically in both directions.

 

[dmaziuk]

Hi @dmaziuk

I was talking about no stop dives, so just the surfacing GF. I'm under the impression that just the a values were changed between A, B, and C. It seems this would change the Mo, surfacing M value, but not the deltaM, the slope of the line. Seems like this would shift the M-line to the right, more conservative, for the compartments that are affected. I'll have to think about what it take to make the 27 or 38.3 min compartment controlling, perhaps pay more attention to my tissue loading graph. Loading and unloading works identically in both directions.

Right:
- Buhlmann's coefficients are kind of an inverse of M0 and dM because his formula is written to give you the next stop depth. The change in 'a' coefficient changes M0.
- On a no-stop dive the slope is not modified because GF Lo never applies. Effectively GF Lo = GF Hi and the slope of the line is the same as the original 100/100. So GF Hi just shifts the line left or right.

(Buhlman's formula is a convenient shortcut when you know you're going to have deco stops: it tells you where to stop without any additional calculations. On a no-stop dive it's not that useful, but only as long as you don't overstay the NDL. And in terms of programming there isn't much difference between doing it the Buhlmann's way or the "old way".)

 

[EFX]

I was talking about no stop dives, so just the surfacing GF. I'm under the impression that just the a values were changed between A, B, and C. It seems this would change the Mo, surfacing M value, but not the deltaM, the slope of the line.

You are correct. The a and b coefficients are calculated as follows:

a = Mo - S(Ps) ;Ps is the absolute pressure at the surface and S is the slope
b = 1 / S

Here are the Mo values for tables A, B, and C. The order is tissue compartment 1 through 16 left to right. The higher the value the more nitrogen the tissue can withstand upon surfacing. As you can see these values only affect the a coefficients per the equation above.

b16nMo_a = Array(97.3, 83.2, 73.8, 66.8, 62.3, 58.5, 55.2, 52.3, 49.9, 48.2, 46.8, 45.6, 44.5, 43.5, 42.6, 41.8)
b16nMo_b = Array(97.3, 83.2, 73.8, 66.8, 62.3, 57.4, 54.1, 51.7, 49.9, 48.2, 46.8, 45.6, 44.1, 43.5, 42.6, 41.8)
b16nMo_c = Array(97.3, 83.2, 73.8, 66.8, 60.8, 55.6, 52.3, 50.1, 48.5, 47.2, 46.1, 45.1, 44.1, 43.1, 42.4, 41.8)