Make water salinity editable

[Willem Ferguson]

I have code to make the salinity editable on the Information tab. See 
attached two screenshots. The motivation is twofold:

1) Frequently the salinity is not provided when logging a dive. There 
can be two reasons:

         a) Either the dive computer does not report on salinity or 
libdivecomputer does not know how to extract that information.

         b) The dive is entered by hand.

2) To make the salinity datum more immediately user friendly. Most users 
do not know the density of the water they dive and, in the end, it is 
treated as one of the main categories of water: fresh/sea/EN13319. For 
any user it is not intuitive to provide a water density. My current code 
is backwards-compatible and does not affect the dive or dc structures or 
the way any of the calculations are done.

However, there are few issues to be decided on.

1) The density of sea water is not standardised. My Galileo reports a 
sea water density of 1025 g/l because this is what the libdivecomputer 
parser provides. However, in the rest of the software, as far as I can 
see, sea water density is 1030 g/l because that is the value of 
SEAWATER_SALINITY in units.h. We should probably move towards more 
consistency in this. As far as I can see it does not affect the 
computations at all but it is not ideal, especially with data where no 
explicit water density measurements were taken but where the dives only 
indicates a category.

2) I am undecided about the utility of EN13319. This is a European 
standard for the certification of dive computers, using a water density 
of 1.0198 for calibrating dc depth sensors. This standard density is 
used to make the calibration of depth sensors across dive computers 
comparable. I have not found out why the chose this specific density. 
The utility of the EN13319 water density is questionable for divers that 
log dives after a dive. The numeric value is pretty close to the density 
of sea water (some authorities quote the density of sea water as low as 
1.023, less than a half a percent different from that of EN13319. I do 
not think that EN13319 is a halfway-value between sea water and fresh 
water. Although the use of EN13319 is likely to be confusing to divers 
and it clutters the UI, I actually have no strong opinion on this.

Kind regards,

willem







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Please refer to 
http://upnet.up.ac.za/services/it/documentation/docs/004167.pdf 
<http://upnet.up.ac.za/services/it/documentation/docs/004167.pdf> for
full 
details.
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