Scubapro Aladin 2G Mode D'emploi page 122

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7. PDIS (Profile Dependent Intermediate Stop)
7. PDIS (PROFILE
DEPENDENT
INTERMEDIATE STOP)
7.1 Introduction to PDIS
(Profile-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,
Aladin 2G 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
44
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.
SCUBAPRO ALADIN 2G