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PDIS (Profile Dependent Intermediate Stop)
4.9.2 How does PDIS work?
The mathematical decompression model in Aladin TEC 3H, called ZH-L8 ADT MB, tracks your
decompression status by dividing your body into 8 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.
The depth of the PDI stop is calculated as that at which the leading compartment for the decompression
calculation switches from ongassing to offgassing, 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, combined with the relatively high ambient pressure, inhibits bubble growth.
It should be noted that the two fastest compartments, with 5 and 10 minutes half times respectively,
are not considered for the determination of the PDI stop depth. This is due to the fact that these
compartments are only “leading” for very short dives, for which an intermediate stop is not required at all.
F
NOTE:
The PDI stop is not a mandatory stop and it is NOT a substitute for the 3-5 minute safety stop
at 5m/15ft.
WARNING
Even when performing a PDI stop, you still MUST perform a safety stop at 5m/15ft for 3 to 5
minutes. Performing a 3 to 5 minute stop at 5m/15ft at the end of any dive is still the best thing
you can do for yourself!
