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From: "Doug Chapman" <doug@su*.ne*>
To: <billwolk@ea*.ne*>
Cc: <techdiver@aquanaut.com>
Subject: Re:Bottom Timers v.s. computers
Date: Tue, 1 Jul 1997 13:12:06 -0400
Bill,
I'm assuming what you mean by "beyond recreational diving on air" is the
use of other gases than air such as trimix. If you are using trimix, an air
or nitrox computer will serve little purpose since the decompression model
is incompatible with trimix. In this case you should get a high quality
bottom timer such as what is offered by Uwatec. There is one exception,
however, the old Beuchat (made by Uwatec, not Cochran) and the prior
generation Uwetec Aladdin Pro make excellent bottom timers for depths
exceeding 500ft. It still functions as a depth instrument after you bend it
while following your tables; however it may beep you to death. You can
unsolder the battery terminal and resolder it to reset it if you need it
for a dive before it clears. The newer Uwatec Aladdin Pro appears to work
reliably down to about 425ft; you may be able to disable the beeping with
the computer interface. I don't know, I don't personally have one. 

If you find an analog gauge for deep depths, you should have it calibrated.
My experience with used analog gauges (such as the SOS 500ft gauge) is they
tend to lose calibration and can be off by over 10% (+30ft on a 300ft dive
for example). A digital bottom timer is a better instrument as it typically
relies on a piezoelectric material, or in some cases a small bender
element, rather than a complex mechanism of a squashed bourdon tube, gears,
linkage, and needle (which also can be quite temperature dependent if it is
not compensated).

I have seen several problems with the DiveRite Bridge and Bridge II nitrox
computers, to the point I consider them unreliable.  Cochran computers look
pretty but they also have a bad reputation. I haven't heard much good or
bad about the Aladdin nitrox computer.  Personally from the perspective of
the kind of diving I do, a Nitrox computer offers little utility. I would
prefer to dive a set of tables for nitrox. I would look very closely at
your anticipated profiles, oxygen limits, and expected bottom times to
decide if a nitrox computer will help. They are also expensive.

In summary, for diving with trimix use a bottom timer and a wrist watch
such as a Casio G-shock. Avoid the Rolex Submariners that "have been deep
in Wakula." They don't add any value to your system, or to the watch for
that matter. For intermediate dives, consider a Uwetec Aladdin Pro air
computer which may serve as a bottom timer for table dives, or as a
computer for appropriate air dives. You can pick up a good used prior
generation Pro for about $250. Avoid the Bridge, Cochrans and analog
gauges. 

You asked for my opinion. I hope it helps.

Doug Chapman



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