What does "FSW" stand for?

[padiscubapro]

My Uwatec bottom timer is metric and calibrated for metres fresh water.

The pressure is important, not the actual distance to the surface. While my bottom timer may say 10m while in the ocean, I know I am actually slightly shallower than 10m. If the gauge displays 10m, I'm at 2ata regardless of what the water tastes like.

not quite.... 10m is not an ata.. a standard ata is 1.01325 bar, so 2 atas is a bit deeper.. it should be a pressure reading of 2.0265 bar..

most guages calibrated for metric set the surface pressure as 1.0 bar (although some do self calibrate at the surface) so before you even descent there is a 1.3% ERROR on the depth already...

if it does every 10.13 meters (of the above water definition) is 1 ata

temperature also effects the "depth" the specific gravity of water changes based on temperature...

[Packhorse]

There are two diffrent messurements for an atmosphere, standard and technical. Technical being equal to 1kgcm2 or a pillar of water 10m high and 1cm2 (fresh water at 4deg C), while standard is 1.033. Then there is bar which is 1.0197 kgcm2
But then there is salt water which is diffrent again for diffrent locations around the world, and as pointed out before this all changes again with temperature.
So whats the point of debating the diffrent of Bar, Ata, fresh or salt water to the second decimal place with out taking into account temp or salinity?
All you need to realy know is 1 bar = 1 ata = 1kgcm2=10meters ( or 33ft if you really have to) fresh or salt. Chances are your depth gauge is out but more than 2 or 3% anyway.

[H2Andy]

All you need to realy know is 1 bar = 1 ata =
1kgcm2=10meters ( or 33ft if you really have to) fresh or salt.

yeah, that's close enough for government work.

but it helps to know more than just the simplified version, if for anything else,
to make you a more educated diver

[Rick Inman]

So, next question for discussion.

If I do a 30min dive to 130' in somewhat-salty Puget Sound, then a week later do the same dive at my local somewhat-fresh water lake Coeur d' Alene, my deco obligation is longer in the fresh water than in the salt.

Why? Oh, and BTW, in lake Pend Orille, 80 miles north, ATAs are even smaller.

[Thalassamania]

What's the altitude of the lake and how are you measuring your depth in each case?

[ZAquaman]

The pressure is important, not the actual distance to the surface. While my bottom timer may say 10m while in the ocean, I know I am actually slightly shallower than 10m. If the gauge displays 10m, I'm at 2ata regardless of what the water tastes like.

SeanQ sums up my understanding of this debate quite well, it's about relative pressure not depth. Salinity varies around the world, so even fsw will not be a constant.

I think that the altitude difference between lake PDA in North Idaho (about 2000 feet) and Puget Sound (sea level) the might be the bigger factor in deco time for Rick's question?

[Rick Inman]

What's the altitude of the lake and how are you measuring your depth in each case?

Right and yes.

2k'. ATAs shrink with altitude.

[Walter]

So, next question for discussion.

If I do a 30min dive to 130' in somewhat-salty Puget Sound, then a week later do the same dive at my local somewhat-fresh water lake Coeur d' Alene, my deco obligation is longer in the fresh water than in the salt.

Why? Oh, and BTW, in lake Pend Orille, 80 miles north, ATAs are even smaller.

Because the deco obligation at Puget sound is designed to get you back to 1 ATM safely. At Coeur d' Alene, your deco obligastion is to get you safely to less than 1 ATM. You aren't stopping at sea level pressure. ATM is not smaller, but at altitude, you are at less and 1 ATM.

[H2Andy]

ok, if you have to deploy a bag and do deco alone away from the boat,
and you are hanging at 15 feet, and a jellyfish gives you a sincere
sting to the lip, does anyone hear you scream?

[Thalassamania]

It depends on your gauge. For example, if you used a capillary gauge then everything would be the same (except your true depth at the lake would be less).