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I don't know about adjustments for altitude, but apparently at least
some of Cochran's computers scale their depth reading based on whether
the diver is in fresh or salt water. This is mentioned at...
http://www.unlv.edu/SCUBA/idcr/reviews/cochran.html
...and I *think* I heard a Cochran rep mention it back at Tek95. My
recollection is that whether or not this occurs is user-selectable,
and that in salt water the assumption is sea water (so it would be a
little off in the Great Salt Lake or Dead Sea, for example).
Obviously, compensating depth for salinity would have the same issues
as compensating for altitude, when it came to comparisons with a standard
table or a non-compensating gauge.
- Todd
> From owner-techdiver@terra.net Thu Aug 15 20:02:59 1996
> Date: Thu, 15 Aug 1996 12:07:31 -1000 (HST)
> From: Richard Pyle <deepreef@bi*.bi*.Ha*.Or*>
> To: "JHEIMANN.US.ORACLE.COM" <JHEIMANN@us*.or*.co*>
> Cc: gmiiii@in*.co*, techdiver@terra.net
> Subject: Re: High altitude decompression
>
> > This is NOT true for at least some computers and digital gauges (e.g., the
> > Aladin Pro, and maybe also on the Uwatec digital depth gauge I normally
use).
> > My understanding is that these sense the decrease in atmospheric pressure
at
> > altitude and compensate for it, effectively resetting the pressure for
depth =
> > 0 from 1 Bar to < 1 Bar. This means that the absolute pressure at an
> > indicated depth of 20 ffw is less than that at sea level.
>
>
> Are you sure about that? It seems to me that these computers should be
> taking a pre-dive ambient pressure reading so that they know what
> pressure to decompress the diver "to". I can't imagine they adjust the
> numbers that they read on the depth scale (= the numbers that they
> indicate as decompression ceilings) just to give the diver a better idea
> of the linear feet bewteen him.herself and the surface. Maybe they do -
> the only advantage would be that the computer wouldn't read "0" at 4 feet
> (or whatever 1 bar would be). But my understanding is that, because many
> divers use analog gauges, and therefore follow decompression stops
> according to ambient pressrues as it would be in saltwater at sea level,
> it would be dangerous for a computer to depart from that standard.
> Personally, I think we should all measure our depth in terms of atm or
> bar, rather feet or meters, because all of the physiological stuff is
> keyed to pressure, not linear feet from surface.
>
> Sorry is I added to any confusion.
>
> Aloha,
> Rich
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