Shearwater Petrel
Revision 1.1.0
43
Shearwater Research Inc.
This number can be used in several ways� First, it can be used to calculate an aggressive
ascent that still has some justification in decompression science. For example, if a diver were
to lose a significant portion of their gas and needed to get shallow fast, they could ascend until
they reached a gradient of 90, then stop until it dropped to 80, then ascend to 90 again, etc�
That would produce a Bühlmann-like profile with very little conservatism. In an emergency, that
may be an acceptable risk�
Another use might be to do a slower ascent on a dive to sightsee, but to stay in the
decompression zone by keeping the gradient above 0�
Another use would be to observe the rapidly
increasing gradient in the last 10 feet to the
surface and slow that ascent�
All of this is based on gradient theory
that may be completely false� There
is significant disagreement in the
decompression research community about
the nature and practice of decompression�
Any techniques described here should be
considered experimental, but the concepts
may be useful to the advanced diver�
The last selection is
@+5
� This feature was inspired by Dan Wible’s
CCR2000 computer (Thanks Dan!) It is the time-to-surface (TTS) if
you were to stay at the current depth for five more minutes. This can
be used as a measure of how much you are on-gassing or off-gassing�
DEPTH TIME STOP TIME
Change
Save
O
NDL Display @+5
.85 .86 .84
For example, on a dive on a wreck, you go to the bottom until you accumulate the desired
decompression and TTS� After ascending to the second deck, you notice that the @+5 and
TTS are the same� That means that you can spend 5 minutes exploring this deck without
incurring more decompression�
Once you get to the top deck, the current has picked up� The line runs from the top of the deck
to the surface which is a distance of 30 feet/10 m� You see that your @+5 is 11 minutes and
your TTS is 15 minutes� That means that you can stay down out of the current for 5 minutes
and burn off about 4 minutes of deco� You may decide to accept the 80% decompression
efficiency and stay out of the current.
When your TTS is 10 minutes, you see that your @+5 is 9 minutes� Since the decompression
is not very efficient now, you go up the line and spend the last 10 minutes in the current.
Summary of Contents for Petrel EXT
Page 1: ...Standalone EXT Models...