Operations Manual Perdix
Manual DocRev B
42
Decompression information displayed by this computer, including NDL, stop depth,
stop time, and TTS are predictions. These values are continuously recalculated and
will change with changing conditions. The accuracy of this information is dependent
on several assumptions made by the decompression algorithm. It is important to
understand these assumptions to ensure accurate decompression information.
It is assumed that the diver’s ascent rate is 33ft/min. Ascending significantly faster
or slower than this will impact decompression obligations. It is also assumed that the
diver is carrying and plans to use every gas that is currently turned on. Leaving gasses
that are not expected to be used turned on will result in inaccurate time to surface,
decompression stop and decompression time information being displayed.
On ascent, it is assumed that the diver will perform decompression stops using the
gas with the highest PPO2 below the OC Deco PPO2 value (default 1.61). If there is a
better gas available, the current gas will be displayed in yellow, indicating that a gas
change is expected. The decompression prediction displayed always assumes that the
best gas will be used. Even if the switch to a better gas has not been completed yet,
decompression predictions will be displayed as if the switch is about to occur in the
next 5 seconds.
Divers can encounter longer than expected decompression stops as well as inaccurate
time to surface predictions if they fail to to switch to a better gas when prompted by
the computer.
Example: A diver on a decompression dive to 40m/131ft for 40 minutes with GF
settings of 45/85 has two gasses programmed into their computer and turned on:
21/00 & 99/00. The diver’s decompression schedule will be calculated based on
breathing 21% oxygen for the decent, bottom and ascent phases of the dive until the
diver ascends to 6m/20ft. At 6m/20ft the PPO2 of the 99/00 mix is 1.606 (less than
1.61), so it is the best decompression gas available.
Decompression information for the remaining stops will be calculated and displayed
assuming the diver is going to switch to this better gas. This dive profile indicates
these stops would be 8 minutes at 6m/20ft and 12 minutes at 3m/10ft. If the diver
never makes the switch to 99/00, the computer will not allow them to surface until
adequate off-gassing has occurred, but it will continue to assume the diver is about
to make the gas switch and the decompression times given will be grossly inaccurate.
The 6m/20ft stop will take 19 minutes to clear and the 3m/10ft stop will take 38
minutes to clear. That is a total time to surface difference of 37 minutes.
In a lost gas scenario or in the event a diver forgets to turn off a gas they are not
carrying before a dive, gasses can be turned off during the dive in the Dive Setup ->
Define Gas menu.
DECOMPRESSION INFORMATION ACCURACY
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