Info Deeply Safe Labs: A website for dive computer testing

[dmaziuk]

They seem to think that microbubbles from previous dives interfere with gas exchange at the alveolar level enough to at least partially invalidate the dissolved gas models, but as far as I can see have referenced no actual research to support that assertion. Dr. Mitchell, OTOH, did summarize a study in response #50 which appears to indicate that this is not an issue.

Moreover, that is one of Bruce Wienke's "gradient reduction" factors built into RGBM. There's plenty of us vacation divers using RGBM computers diving next DSAT and lately: ZH-L16 computer users. Empirically, they don't seem to get bent any more than we do. That strongly suggests that purely Haldanean models are quite sufficient to account for the phenomenon, if it exists at all.

 

[tursiops]

Moreover, that is one of Bruce Wienke's "gradient reduction" factors built into RGBM. There's plenty of us vacation divers using RGBM computers diving next DSAT and lately: ZH-L16 computer users. Empirically, they don't seem to get bent any more than we do. That strongly suggests that purely Haldanean models are quite sufficient to account for the phenomenon, if it exists at all.

Yeah, a century of not getting bent on "purely Haldanean models" is pretty convincing that no major problems exist.

 

[scubadada]

Moreover, that is one of Bruce Wienke's "gradient reduction" factors built into RGBM. There's plenty of us vacation divers using RGBM computers diving next DSAT and lately: ZH-L16 computer users. Empirically, they don't seem to get bent any more than we do. That strongly suggests that purely Haldanean models are quite sufficient to account for the phenomenon, if it exists at all.

DSAT and Buhlmann ZH-L16C with GF do behave differently on first and repetitive dives. For DSAT vs. GF high of 95, DSAT is generally more liberal on the first dive, perhaps especially if deeper. On the repetitive dive, Buhlmann tends to be more liberal, perhaps especially if shallower. I have always simply attributed this to some basic difference(s) between the algorithms. I have posted examples on SB and have not been offered any more specific explanation. Anyone have additional thoughts here?

Here are a couple of examples I have previously posted. Differences I see are often less than shown here.

DSAT vs GF high 95, examples

I have written previously about my experience diving DSAT and Buhlmann ZH-L16C with a GF high of 95. I have an example of what I have previously reported. This dive is a first dive of the day on the Castor in Boynton Beach. It was relatively deep with a max depth of 107 feet and an avg depth of...

scubaboard.com

 

[dmaziuk]

Follow the numbers. DSAT tissue compartment half-times are 5.0, 10.0, 20.0, 30.0, 40.0, 60.0, 80.0, 100.0, 120.0, 160.0, 200.0, 240.0, 360.0, and 480.0 minutes.

ZH-L16 TCs are 4.0 or 5.0 (pick one), 8.0, 12.5, 18.5, 27.0, 38.3, 54.3, 77.0, 109.0, 146.0, 187.0, 239.0, 305.0, 390.0, 498.0, and 635.0 minutes.

The devil is of course in the details, specifically what compartments get "loaded" on a particular dive profile, but assuming your controlling compartments on no-stop dives are in the 10 to 60 minutes range:

DSAT: 10.0, 20.0, 30.0, 40.0, 60.0
ZH-L: 12.5, 18.5, 27.0, 38.3, 54.3

You can see that Buhlmann's compartments, except for the 12.5 one, are slightly "faster". Or you could argue

DSAT: 10.0, 20.0, 30.0, 40.0, 60.0
ZH-L: 8.0, 12.5, 18.5, 27.0, 38.3

in which case they are really "faster". Either way this should result in faster off-gassing during SI and lower residual loading for the subsequent dive. And therefore: ZH-L being "more liberal" on repetitive dives.

That is of course the calculated theoretical loading. The $15 question is whether that accurately reflects the reality -- we know that DSAT works well for this kind of diving, I expect there's nowhere near as much "vacation" diving done using ZH-L computers yet for a meaningful comparison.

That said, if people are diving ZH-L on 93/93 or lower, that's adding conservatism to all dives including the "repetitive" ones and may well compensate for the lower computed residual loading. So maybe if you plan to do 6 dives/day for 6 days straight, you should use 85/75 instead of 90/90.

 

[elmo]

Follow the numbers. DSAT tissue compartment half-times are 5.0, 10.0, 20.0, 30.0, 40.0, 60.0, 80.0, 100.0, 120.0, 160.0, 200.0, 240.0, 360.0, and 480.0 minutes.

ZH-L16 TCs are 4.0 or 5.0 (pick one), 8.0, 12.5, 18.5, 27.0, 38.3, 54.3, 77.0, 109.0, 146.0, 187.0, 239.0, 305.0, 390.0, 498.0, and 635.0 minutes.

The devil is of course in the details, specifically what compartments get "loaded" on a particular dive profile, but assuming your controlling compartments on no-stop dives are in the 10 to 60 minutes range:

DSAT: 10.0, 20.0, 30.0, 40.0, 60.0
ZH-L: 12.5, 18.5, 27.0, 38.3, 54.3

You can see that Buhlmann's compartments, except for the 12.5 one, are slightly "faster". Or you could argue

DSAT: 10.0, 20.0, 30.0, 40.0, 60.0
ZH-L: 8.0, 12.5, 18.5, 27.0, 38.3

in which case they are really "faster". Either way this should result in faster off-gassing during SI and lower residual loading for the subsequent dive. And therefore: ZH-L being "more liberal" on repetitive dives.

That is of course the calculated theoretical loading. The $15 question is whether that accurately reflects the reality -- we know that DSAT works well for this kind of diving, I expect there's nowhere near as much "vacation" diving done using ZH-L computers yet for a meaningful comparison.

That said, if people are diving ZH-L on 93/93 or lower, that's adding conservatism to all dives including the "repetitive" ones and may well compensate for the lower computed residual loading. So maybe if you plan to do 6 dives/day for 6 days straight, you should use 85/75 instead of 90/90.

The algorithm compartment half-times might be part of it but I suspect the differences in how the calculation the allowable pressures are more significant. I understand the Bühlmann M value calculation. Are you aware of the DSAT equivalent?

 

[inquis]

Are you aware of the DSAT equivalent?

The DSAT calculation is similar. Its M-Values are higher than Bulhmann ZHL-16C for faster compartments and lower for slower compartments. [Baker, Understanding M-values] Graphically:

 

[scubadada]

The algorithm compartment half-times might be part of it but I suspect the differences in how the calculation the allowable pressures are more significant. I understand the Bühlmann M value calculation. Are you aware of the DSAT equivalent?

The DSAT calculation is similar. Its M-Values are higher than Bulhmann ZHL-16C for faster compartments and lower for slower compartments. [Baker, Understanding M-values] Graphically:
View attachment 835198

The Mo values for Buhlmann ZH-L16C and for DSAT are in the back of Mark Powell's Deco for Divers. The delta M for Buhlmann is also given, but not for DSAT.

 

[dmaziuk]

The algorithm compartment half-times might be part of it but I suspect the differences in how the calculation the allowable pressures are more significant. I understand the Bühlmann M value calculation. Are you aware of the DSAT equivalent?

M-values are not calculated on the fly, they don't change with repetitive dives.

The actual M-values are different but similar enough: check Erik Baker's Understanding M-Values. Yes, they do affect the NDL/deco schedule on each given dive, but the accumulation over repetitive dives is controlled by on/off-gassing rate that is defined as the half-times.

I don't believe DSAT adds any "repetitive dive" penalty as done by RGBM and as suggested by the OP. What they originally did instead was tell people to "sit every 6th day out". I have not seen the code though.

 

[inquis]

The delta M for Buhlmann is also given, but not for DSAT.

The slope is not given in Baker's paper either. A case can be made for assuming 1.0, as that seems to be the asymptotic value in other sets.

 

[dmaziuk]

The slope is not given in Baker's paper either. A case can be made for assuming 1.0, as that seems to be the asymptotic value in other sets (and on the conservative side).

My understanding is they just use M0 all the way down as the model was not designed for decompression diving in the first place.