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PDIS (Profile Dependent Intermediate Stop)
7. PDIS (PROFILE DEPENDENT INTERMEDIATE STOP)
7.1 Introduction to PDIS (Prole-Dependent Intermediate Stop)
The main purpose of a dive computer is to track your nitrogen uptake and recommend a
safe ascent procedure. Diving within the so called no-stop limits means that at the end of
the dive you can ascend directly to the surface, albeit at a safe ascent rate, while for dives
outside of the no-stop limit (so-called decompression dives), you must perform stops at
certain depths below the surface and allow time for excess nitrogen to be expelled from your
body before finishing the dive.
In both cases, it can be beneficial to stop for a few minutes at an intermediate depth between
the maximum attained depth during the dive and the surface or, in case of a decompression
dive, the first (deepest) decompression stop.
An intermediate stop of this kind is beneficial as soon as the ambient pressure at that depth
is low enough to ensure that your body is predominantly offgassing nitrogen, even if under a
very small pressure gradient. In such a situation, you can still cruise along the reef and enjoy
the dive, while your body gets a chance to slowly release nitrogen.
In recent times, so called “deep” stops have been introduced in some dive computers and
tables, defined as half the distance from the maximum reached depth and the surface (or
the lowest decompression stop). Spending 2 or 15 minutes at 30m/100ft would result in the
same deep stop of 15m/50ft.
With PDIS, as the name suggests, Subgear XP-3G 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 continuously 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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