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4.9 PDIS (Profile Dependent Intermediate Stop)
4.9.1 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 TEC 3H 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.
PDIS (Profile Dependent Intermediate Stop)
