[PATCH] Support different salinity in planner

[Henrik Brautaset Aronsen]

Anton Lundin wrote:
> Yes, from a deco perspective the pressure is the relevant thing, but as
> stated earlier, a lot of the time our planning depth comes from
> salinity-independent measurements as done by sonar equipment.
>
> Of course we can always re-calculate our sonar-depth to a corresponding
> "salt-water-depth" but my opinion is that entering the salinity level
> is simpler.

Here's what Shearwater says:

«Why does the depth reading on my Petrel disagree with my other computer?

Depth is not measured directly. Dive computers measure pressure, and 
convert this to depth based on an assumed density of water.
Water density varies by type. The weight of salts dissolved in salt 
water make it heavier than fresh water. If two dive computers are using 
different densities of water, then their displayed depths will differ.

The water density is adjustable on the Predator. In the Advanced Config 
1 menu, the Salinity setting can be set from 1000 kg/m³ to 1040kg/m³. A 
cubic meter of pure water has a mass of 1000kg. So what we are setting 
is the mass of salts dissolved in the water.

Some guidelines:

Fresh Water = 1000kg/m³
EN13319 = 1020 kg/m³
Salt Water = 1025 to 1035 kg/m³
The EN13319 (European CE standard for dive computers) value is between 
fresh and salt and is the Predator default value. The EN13319 value 
corresponds to a 10m increase in depth for pressure increase of 1 bar.»

-H