Fresh - Salt water and computer calculations...

[Manogr]

Since my new mosquito (and maybe every or most computers) don't have a choice between salt and fresh water dives can I assume that there are not differences (for the computer calculations) if we dive in fresh or salt water ?

Manogr

 

[Ber Rabbit]

My Orca Pilot asks for the salinity % of the water. Naturally I put in 0% for fresh water but since the DM never tells me what the salinity % is when I go on vacation I just use the default setting for salt water. I don't have my manual with me and I can't remember why this computer asks for that information.
Ber

 

[donacheson]

Originally posted by Manogr
Since my new mosquito (and maybe every or most computers) don't have a choice between salt and fresh water dives can I assume that there are not differences (for the computer calculations) if we dive in fresh or salt water ?

Manogr

Since dive computers determine depth by measuring pressure, fresh vs. salt water isn't an issue for the diver.

The only difference one could see would be in a comparison of a depth-sounder's measurement (based on the speed of sound in water) and that of a dive computer. Even then, since the speed of sound in water varies about 0.25% per degree Celcius, an uncertainty of +/-5C in the average temperature of the water column underneath the depth sounder is roughly equivalent to the difference between fresh and salt water densities and hence depths at the same pressure.

 

[jetblast00]

Originally posted by donacheson


Since dive computers determine depth by measuring pressure, fresh vs. salt water isn't an issue for the diver.

umm... an additional 14.7 psi of ambiant pressure occurs at 33ft in salt water and 34 ft in fresh.

 

[The Chairman]

they are really formulated using pressure... Depth gauges, including digital are sealed pressure gauges that have been calibrated to read in fsw (and in rare occaisions, ffw).

 

[QKRTHNU]

I know this is an old thread, I just wanted to bump it back up since I'm computer shopping.

I'm guessing this is not an issue unless you're diving somewhere like the dead sea where pressure would be higher than your average ocean water.

Fresh water dives based on salt water calculations would simply be conservative, nothing wrong with that.

But I'm wondering about this comment:

Since dive computers determine depth by measuring pressure, fresh vs. salt water isn't an issue for the diver.

If this is true, then the depth guage wouldn't be accurate in fresh water correct? It would read shallower than you actually are, although not by much (It would read 99ftw while @ 102ftw)

 

[Charlie99]

QKRTHNU once bubbled...
I know this is an old thread, I just wanted to bump it back up since I'm computer shopping.

Fresh water dives based on salt water calculations would simply be conservative, nothing wrong with that.

FSW is a measure of pressure, not linear distance. The N2 absorption of your tissues is determined by the ambient pressure, which is what the dive computer is measuring. Whether it displays the pressure in fsw, ffw, msw, bars or pascals is irrelevant. Whether this pressure is from fresh water, salt water, or super salty dead sea water is irrelevant to deco calculations.

The starting surface level air pressure, DOES matter, which is why most computers have altitude corrections.

Charlie

 

[QKRTHNU]

Doh! Don't know what I was thinking when I said it would be conservative. Pressure is Pressure. Thanks.

I am correct that the actual (distance) depth reading would be inacurate right? Not that it matters at all for deco.

 

[wreckchick]

Forgive me if this is the single stupidest question ever, but I always assumed that the depth on my gauge was the actual number of feet I was below the surface. Is this not correct?

Rachel

 

[Big-t-2538]

biscuit7 once bubbled...
Forgive me if this is the single stupidest question ever, but I always assumed that the depth on my gauge was the actual number of feet I was below the surface. Is this not correct?

Rachel

Well, it is and it isn't depending on what metric the depth gauge was based on. You will be within a foot if you're diving shallower than 80ft....and within 1 meter if you are european.

As for N2 absorption, if you have a computer, that pressure is what is important for the ongassing / offgassing of nitrogen. The pressure is the pressure, the only error would be in the depth reading displayed.