How Does Pdis Work - Uwatec ALADIN SPORT MATRIX User Manual

cumulative nitrogen uptake for 2 sample dive profiles. This figure also demonstrates the
conceptual difference between PDIS and the rather rudimentary "deep" stops.
Specifically, the figure compares 2 dive profiles to a maximum depth of 40m/132ft that
are 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 2
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 represents the PDIS depth as displayed on the computer screen during the
course of the dive for profile 1, the broken line 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 2 dives due to the different exposure in the 2 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 line made up of small solid dots, 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 2 dive profiles. Deep stops completely ignore any facts about the dives
themselves aside from max depth.
10m
10m
20m
20m
30m
30m
40m
40m
3.10.2

How does PDIS work?

The mathematical decompression model
in the Aladin Sport, called ZH-L16 ADT
MB PMG, tracks your decompression
status by dividing your body into 16 so-
called compartments and mathematically
following the uptake and release of
nitrogen in each with the appropriate laws
of physics. The various compartments
simulate parts of your body such as central
nervous system, muscles, bones, skin and
so on.
32
8min 16min
8min 16min
PDIS Profile 1 PDIS Profile 1 PDIS Profile 2 PDIS Profile 2
Deep Stop
Deep Stop
Dive Profile 2
Dive Profile 2
Dive Profile 1
Dive Profile 1
24min 32min 40min
24min 32min 40min
DIVE TIME
PDI Stop
PDI STOP
The depth of the PDI stop is calculated as
that at which the leading compartment for
the decompression calculation switches
from on-gassing to off-gassing, and the
diver is advised to perform a 2-minute
stop above the displayed depth (this is the
opposite of a decompression stop, where
you are asked to remain just beneath the
displayed depth). During this intermediate
stop, the body is not taking up any more
nitrogen in the leading compartment, but
rather releasing nitrogen (though under
a very small pressure gradient). This,
48min
48min
Safety Stop
SAFETY STOP
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