Uwatec PDIS Manual page 3

With PDIS, as the name suggests, Galileo interprets your dive profile and suggests an
intermediate stop that is a function of your nitrogen uptake so far. The PDI stop will therefore
change through the course of the dive to reflect the continuosly changing situation in your
body. Along the same lines, PDIS will account for the accumulated nitrogen from previous
dives, hence PDIS is also repetitive-dive dependent. Conventional deep stops completely
ignore these facts.
The following figure quantifies the extent of PDIS and illustrates its dependence on cumulative
nitrogen uptake for two sample dive profiles. This figure also demonstrates the conceptual
difference between PDIS and the rather rudimental "deep" stops.
Specifically, the figure compares two dive profiles to a maximum depth of 40m/132ft
but otherwise very different. Profile 1 stays at 40m/132ft for 7 minutes, then ascends to
30m/100ft for 3 minutes, followed by 12 minutes at 20m/65ft. Profile 2 stays less than two
minutes at 40m/132ft, then ascends to 21m/69ft and stays there for 33 minutes. Both dive
profiles are no-stop dives to the limit of entering decompression. The solid line with triangles
represents the PDIS depth as displayed on the computer screen during the course of the
dive for profile 1, the dotted line with triangles represents the PDIS depth as displayed on
the computer screen during the course of profile 2. One can see that the displayed PDIS
depth increases as more nitrogen is accumulated in the body, but does so very differently
in the two dives due to the different exposure in the two profiles. The PDI stop is carried
out at 25 minutes for profile 1 and at 37 minutes for profile 2, followed by the safety stop
at 5m/15ft.
The solid line with open circles on the other hand represents the depth that would be
displayed by a computer following the conventional deep stop method, and it would be
the same for the two dive profiles. Deep stops completely ignore any facts about the dives
themselves aside for the max depth.
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