From: brad.radius.com (brad may)
Newsgroups: rec.scuba
Subject: Re: How about the Oceanic Datamax?
Date: 26 Aug 92 17:06:55 GMT
Reply-To: brad.radius.com (brad may)

A consideration in choosing a dive computer which I haven't 
seen discussed here is how closely your computer's models 
approximate those of your dive buddies' computers.

I own a Datamax Sport and like it a lot for my typical 
California diving.  I recently went on a liveaboard trip to 
Saba (Netherlands Antilles) with a group of divers who pre-
dominantly use Orca Delphis or SkinnyDippers.

I admit to being a plebe about the model my Datamax uses, and 
unfortunately the user's manual is very deficient in technical
detail.  The "Modified Haldanean" model has been futzed with 
such that no outgassing is allowed through the two shortest 
compartments, which I actually liked because it makes the 
model very conservative for "bad" profiles like repeated 
bounce dives, reversed profiles (shallow first, going deeper)
or reversed series of dive profiles.  But I wasn't prepared 
for how conservative it could be on single deep dives com-
parted to, say, a Delphi.

Example: First dive of day, no residual N2 from previous day.
We are doing a live boat dive a Shark Shoals (about 110'min. 
depth by memory).  The boat split into two dive groups, my 
buddy and I are first buddy team in first group in the water.

After ~4 minutes on the bottom (120-130') my Datamax 
goes into decompression mode; check with buddy--Delphi shows 
several minutes bottom time remaining.  I descend to videotape 
a school of amberjack; after <2 mins. of filming I look at my 
Datamax. Damn! Datamax stops displaying deco information 
below 130', but shows 141' depth.  I work my way back up to 
my buddy; we check computers.  His shows ~3 min no-deco time 
remaining, mine now requires a deco stop at 20'!!  We futz 
for ~1 minute and start ascent  (we're the 1st buddy team 
to start up).  His Delphi shows ~2 min no-deco time, mine now 
requires a deco stop at 30'!!

I ascend very quickly at first to minimize further N2 absorp-
tion (Datamax uses progressively slower max ascent rate, like 
most computers--60, 45, 30 fpm), 30' deco stop is cleared as 
I pass ~35', but I have a *17 min. stop at 20'*! Hanging on a 
line very close to 20' to clear N2 as fast as possible, I 
clear 20' stop and ascend to wait out *23 min deco stop* at 
10'.

By this point, by buddy has been on the surface for a long 
time, watching me hang, the rest of the first group is back 
on the boat, all the second group has completed its dive and 
is waiting on the surface for me to surface, so the boat can 
come back and make a pick up.  The captain is already about 
15 min. behind schedule to leave for the next site, and I'm 
passing hand signals to the divemaster for how long I have 
left to hang.

To summarize, whereas I stayed higher than my buddy the 
whole dive except for the amberjack excursion (and to wait 
out deco stops), he was never very close to his no-deco 
limit and I had *40 minutes* of deco time.  Apperently no 
one else went into decompression, in spite of several people 
being well below my max. depth.  I had the distinct feeling 
of being held captive underwater by my computer!

My advice (in addition to very good advice already posted): 
study the models used by computers you are considering, think 
about how they will work under the kinds of diving you anti-
cipate, and how they compare to your prospective buddies' 
computer models.

bmay@radius.com (Brad May)
Newsgroups: rec.scuba