# [PATCH] Support different salinity in planner

Rick Walsh rickmwalsh at gmail.com
Thu Nov 13 12:25:49 PST 2014

```On 13 Nov 2014 20:21, "Robert Helling" <helling at atdotde.de> wrote:
>
>
> On 12.11.2014, at 23:33, Anton Lundin <glance at acc.umu.se> wrote:
>
> Anton,
>
>> Depth is often mentioned in a length unit, but what we care about is
>> pressure. When diving in fresh water the pressure is lower than the same
>> depth in salt water. This adds support for using different salinities in
>> planning.
>
>
> of course I agree that the relation between depth and ambient pressure
depends on the density of water and thus on the salinity.
>
> Of course, I don’t know about you, but I myself don’t take a yard stick
to the water but rather used depth gauges (e.g. my dive computer) that
pretend to measure depth but in reality measure ambient pressure (and don’t
take into account the salinity). So what we call “depth” in diving is, most
of the time, actually not really depth but “effective depth” being the
pressure equivalent depth assuming some standard diving liquid.
>
> What I am trying to say is: It’s true that it is an error of not taking
it into account, but when running a deco schedule against a depth gauge do
do this error twice in opposite directions and thus it cancels out. Your
patch eliminates this error once, but strictly speaking, the resulting deco
profiles should then be measured with a yard strick rather than a pressure
based depth gauge, otherwise they are wrong (although probably only by a
tiny amount that does not really make a difference). But then the error in
the first place was probably not significant. But as a plus, it makes the
user believe we take everything into account… ;-)
>
> Best
> Robert

Just putting this in perspective.  The font of all knowledge tells us that
the density of seawater varies around the world but is approximately 1.027
t/m3 (cf 1.00 t/m3 for pure water).

This means that at 40 m true depth in fresh water, a gauge assuming
seawater would read 41 m.  If you really care about that difference, I hope
you are always in trim and that you always hold your computer level with
your body. And then we should probably account for gravity (varies around
the world) and atmospheric pressure.

>
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> Robert C. Helling     Elite Master Course Theoretical and Mathematical
Physics
>                       Scientific Coordinator
>                       Ludwig Maximilians Universitaet Muenchen, Dept.
Physik
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