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Understanding Algorithms
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Charlie99
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Most dive computers are multicompartment dissolved gas algorithms/models. One specific multicompartment dissolved gas model is Buhlmann. These models are also sometimes called neo-Haldanian. The PADI/DSAT model is also a neo-Haldanian model, as is the Workmann/USN model. The Uwatec model is yet another multicompartment dissolved gas model, but some versions tweak the limits according to air consumption and temperature.
VPM is a dual phase model. It tracks dissolved gas in a multicompartment dissolved gas model, and then uses those compartment loadings and ambient pressure to estimate free gas volume (bubble volume). RGBM is another dual phase model, but most "RGBM" computers (including most Suunto models) are really multicompartment dissolved gas models where the model limits are reduced for repetitive dives in response to things like sawtooth profiles, short SI, no safety stop, or dives deeper than 100'.
Many computer manufacturers decide to pad the model limits a bit. Even a slight amount of padding of the limits will be more significant than the difference between Buhlmann, DSAT/PADI, and DCAP models.
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A few other random comments and observations ----
Small changes in model limits result in huge differences in NDL when shallow. In other words, one model may figure that you are at 98% of the medium speed compartment limit while another one with slightly different model limits has you at 95% of limit. At 30' this will result in very large difference in remaining NDL because you are right near the tail end of the exponential curve.
The DSAT/PADI model is unique in that it has only a surfacing limit for each compartment. Other models have both a limit and a change with depth for each compartment. The increasing compartment limit with increasing depth is what allows calculation of decompression stops. The Oceanic computers use an unpadded DSAT model for NDL, but then borrow some slope info from the USN model for decompression. The mandatory decompression limits of Oceanic computers are much more conservative than the unpadded USN/Workmann model.
Just like with tables, one is not required to push the limits. Having an NDL of zero minutes is the same as saying that the deco ceiling is 0'. Even when a deeper ceiling, such as 10' is being displayed by the computer, there is nothing that requires one to immediately pop up directly to the ceiling limit.
An intelligent computer user uses the computer to do the bookkeeping to keep track of decompression information, while the user sets the profile.
A couple of good articles to start with to learn about decompresion are the Deep Stops and Understanding M-values articles by Erik Baker. A good, solid understanding of the basic multicompartment dissolved gas model makes understanding of dual phase models such as VPM much easier.
Charlie Allen
VPM is a dual phase model. It tracks dissolved gas in a multicompartment dissolved gas model, and then uses those compartment loadings and ambient pressure to estimate free gas volume (bubble volume). RGBM is another dual phase model, but most "RGBM" computers (including most Suunto models) are really multicompartment dissolved gas models where the model limits are reduced for repetitive dives in response to things like sawtooth profiles, short SI, no safety stop, or dives deeper than 100'.
Many computer manufacturers decide to pad the model limits a bit. Even a slight amount of padding of the limits will be more significant than the difference between Buhlmann, DSAT/PADI, and DCAP models.
------------------------
A few other random comments and observations ----
Small changes in model limits result in huge differences in NDL when shallow. In other words, one model may figure that you are at 98% of the medium speed compartment limit while another one with slightly different model limits has you at 95% of limit. At 30' this will result in very large difference in remaining NDL because you are right near the tail end of the exponential curve.
The DSAT/PADI model is unique in that it has only a surfacing limit for each compartment. Other models have both a limit and a change with depth for each compartment. The increasing compartment limit with increasing depth is what allows calculation of decompression stops. The Oceanic computers use an unpadded DSAT model for NDL, but then borrow some slope info from the USN model for decompression. The mandatory decompression limits of Oceanic computers are much more conservative than the unpadded USN/Workmann model.
Just like with tables, one is not required to push the limits. Having an NDL of zero minutes is the same as saying that the deco ceiling is 0'. Even when a deeper ceiling, such as 10' is being displayed by the computer, there is nothing that requires one to immediately pop up directly to the ceiling limit.
An intelligent computer user uses the computer to do the bookkeeping to keep track of decompression information, while the user sets the profile.
A couple of good articles to start with to learn about decompresion are the Deep Stops and Understanding M-values articles by Erik Baker. A good, solid understanding of the basic multicompartment dissolved gas model makes understanding of dual phase models such as VPM much easier.
Charlie Allen
OP
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