Newsgroups: bit.listserv.scuba-l
Date:         Mon, 12 Oct 1992 11:54:59 PDT
From:         Dave Waller <dave@HPDSM0B.PTP.HP.COM>
Subject:      Re: To compute or not to compute
              12 Oct 92 11:54:24 PDT." <9210121815.AA05220@relay.hp.com>

Steven G. Lanaghen writes:

>         "Leaving the bottom" is pretty black and white, but is not the
>         key factor - it is the beginning of the direct ascent to the
>         surface.  As long as you are ascending (or holding a calculated
>         safety/decompression stop) you are off-gassing and this should not
>         be considered bottom time (on-gassing).

Whether ascvent time should be considered bottom time is a not something that
can be quantitatively described, but is subject to interpretation, hence the
confusion an conflict over its definition.

However, ongassing and offgassing are clearly defined terms, and as such, your
assertions about are incorrect. Whether a theoretical compartment is
ongassing or offgassing is solely dependent on the pressure gradient between
the compartment and the ambient pressure, and is totally independent of the
ascent/descent status of the diver.

This means that, while ascending, it is still possible (indeed, very likely)
that one or more compartments in the model are ongassing during all or a
portion of the ascent. This is particluarly true for slow compartments, which
are likely to have an inert gas loading that is below that of the partial
pressure of inter gas inthe breathing mixture for all but the shallowest
few feet of depth.

Because of this, it is possible to exceed the M0 for a marginal compartment
during a slow ascent, and put the diver in a mandatory decompression situation
while they may have otherwise avoided mandatory decompression with a faster
ascent.

At the same time, a too fast ascent can result in excessive bubbling in the
tissues, leading to DCS. What is "too fast" is dependent on the inert gas
loading in the tissue, and therefore is not a simple number to come up with.
The same applies for what is a "too slow" ascent -- i.e. the risk of exceeding
the allowable inert has loading for the theoretical compartments modeled is
unique to any given loading profile. As such, no fixed minimum ascent rate
can be given as a universal endpoint.

My point here was to clarify the misconception that ongassing cannot take
place during ascent -- indeed it can, and can possibly significant,
depending on how marginally close the compartment loading is to the
maximum allowable under the model in question.

All of these concerns can be easily alleviated by simply staying well below
the allowable maximums. This translates into keeping a conservative magin
below the published NDLs for tables and computers.

The safest way to do this, IMO, is to maintain a PERCENTAGE cushion rather
than an absolute one. This better accounts for the variation in halftimes
of the various controlling compartments at different depths. So, for
example, if your tables give you 100 minutes at 50 feet, and you are
keeping a 70% margin, dont stay longer than 70 minutes. If your tables
give you 30 minutes at 90, don't stay longer than about 20 minutes. I'd
further modify this rule the set a minimum margin of 5 minutes so when
0.3*(NDL)<5, use a margin of 5. Note that this method requires calculation
of these limits during dive planning, and cannot be applied during the dive
to the remaining NDL displayed by a computer.

If you have a computer that displays actual compartment loading, like the
Delphi, you can dispense with the approximation of applying the margin rule
to the NDLs displayed on the surface prior to the dive, and simply apply a
70% rule to the loading indicator.

Dave Waller
YMCA Instructor
Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU