From: dave@hpdstma.ptp.hp.com (Dave Waller)
Subject: Re: Delphi -> Phoenix Trade-Up Report
Organization: Hewlett Packard
Date: Wed, 19 Aug 1992 17:16:17 GMT

In article <dan.53.714211481@basil.eng.monash.edu.au>, dan@basil.eng.monash.edu.au (Dan Grimm) writes:
|> In article <930551@hp-ptp.ptp.hp.com> davew@hp-ptp.ptp.hp.com (Dave Waller) writes:
|> >Subject: Re: Delphi -> Phoenix Trade-Up Report
|> >story@sgi.com (David (Duis) Story) writes:
|> >
|> >I've run into this several times myself (always with fully redundant air
|> >on my back, of course, but I digress). The Delphi/Phoenix uses a moving
|> 
|> Dave and David,
|> This seems to have been an interesting discussion any chance of having 
|> the original article posted to me?

Hmmm... not sure what you mean by "original article".

|> Now the real reason for this post:-
|> Having never used an air integrated dive computer (and no one I know diving
|> here in melbourne,australia, seems to have one),  what do they do when they
|> think you have exceeded air limits?

What the computer does is analyze the rate of change of tank PSI over a given
time window (the moving average). Based on this moving average of psi/min
consumption, it displays an estimate of the time remaining to reach 500 psi
on the tank, adjusting for the instantaneous depth.

For example, assume at 60_ft (18_m) you are consuming air at a rate of
about 100_psi/min (6.9_bar/min). You've been down there awhile, and your
tank pressure reads 1500_psi (~100_bar). The air display will tell you that
you have 10 minutes left at that depth of 60_ft (18_m).

However, if you swim up to 30_ft (9_m) and continue your dive, based upon your
consumption rate of 100_psi/min (6.9_bar/min) at 60_ft (18_m) it will
now tell you that you have about 15 minutes of air left, adjusting for the
reduced actual consumption due to the lower ambient pressure.

If you swam to 30_ft (9_m) because you were chasing a really cool ray with
your U/W video camera, and your breathing rate increased to 3x what it was
at 60_ft (18_m), it whill take awhile for the moving average to drop the
lower actual breathing rate from its calculations, thereby making the
15 minute estimate very inaccurate. However, if this load condition persists
at 30_ft (9_m), within a few minutes the display will drop from 15 minutes
to an estimate of about 3-4 minutes, because you are breathing 3x as fast.
Were you to quickly return to 60_ft (18_m) at this point, the estimate
would track your depth, and reduce to ~1 minute (better start heading for
the surface!).

|> The situation I am thinking of is Twin tank diving.
|> I guess that you have to 'trick' the dive computer into thinking that you
|> always have lots of air left.
|> Some of the people I dive with work this way: use 1/2, swap regs, use 1/2
|>  of other tank, [do deco time if needed] surface. This works fine for non
|> integrated computers.

This is a real problem for the air estimation calculations, and the only way
to maintain accuracy is to join the tanks through a manifold and breath
them down at the same time.

However, the air feature is still quite useful in under the scenario you
describe. If the tanks are of equal internal volume, then the switchover
will not interfere with the accuracy -- when the computer sees the pressure
go from 500_psi (35.5_bar) on switchover, it will simply base the estimated
air time from that new tank pressure.

Of course, of your twin tank has a completely separate regulator that the
computer is not hooked into, then the air remaining calculations cease to
have any mean at all once you switch tanks. In this case you'd need TWO
computers, one on each reg.

-- 
Dave Waller
YMCA Instructor