FSW vs. FFW?

[Marek K]

So, why is it necessary to differentiate between feet of sea water (fsw) and feet of fresh water (ffw)? Or meters?

Yeah yeah, sea water is 2.5% denser than fresh water. But we don't ever measure diving depth using a tape measure or a fathom line... we measure it using a depth gauge or a computer, which in turn measures the water pressure.

A given displayed depth would actually be slightly shallower in salt water, and slightly deeper in fresh water. But the pressure effect on our bodies (nitrogen loading, in particular) should be same at a given pressure, no matter what the absolute tape-measure depth.

Yes? Or what am I missing?

So why do computers, for instance, adjust (or have the capability to be adjusted) between fresh and salt water?

--Marek

 

[loosebits]

It's a very annoying gimmick. FFW/FSW are not measures of depth but measures of pressure just like mm of mercury. As divers, we could care less what the actual distance to the surface is, the only thing we care about is the pressure we are being exposed to. And since all of our tables and the numbers we've memorized are in FSW (e.g. MOD of EAN32 is 111 FSW), it would be silly to set your computer to FFW when you are in fresh water.

Personally, I wish I could find a computer that gave me my pressure in atmospheres. When I am 33' deep in salt water, I want it to read 2 ATA. Think about how easy it would make all the calculations, no more multiplying and dividing by 33. What is the PO2 of EAN 40 at 3 ATA? Well, its 1.2.

 

[Gary D.]

Don't give them anymore money making ideas. Next will be the blue fin vs yellow fin calculation.

Gary D.

 

[ScubaMike14]

To be simplistic, the pressure that results from the weight of air in space, pushing against our bodies is called atmospheric pressure. As we descend in water this pressure increases. We all learned in Open Water class that when diving in Sea/Salt water, every 33 feet is equivalent to 1 atmostphere. And at the surface we are already at 1 atmosphere or ATA. So at a depth of 33 feet (FSW) we are at 2 atmospheres (ATA) We can use this formula for the math:

(FSW/33)+1=ATA Sea/Saltwater Formula

BUT when diving in Freshwater (FFW) we have to remember that it is about 3% less dense that Saltwater. To account for this, we measure each atmosphere (ATA) by 34 feet, rather than 33. So:

(FFW/34)+1=ATA

This correlation between depth and pressure is very dependant upon the type of water we are in, hence the reason for the setting on the computer. I think the big issue here becomes not so much how deep you dive, but how LONG you are at a given depth. Both are important, but the time seems to be the big factor here when your computer does it's calculations.

Mike Rushton
PADI DM 174448

 

[ScubaMike14]

It's a very annoying gimmick. FFW/FSW are not measures of depth but measures of pressure just like mm of mercury. As divers, we could care less what the actual distance to the surface is

Actually if you read my post you will see that it's no gimmick at all. And not to flame you or sound mean, but most divers care very much about how far it is to the surface.

Mike Rushton
PADI DM 174448

 

[Marek K]

This correlation between depth and pressure is very dependant upon the type of water we are in

Alles klar, Mike. But what I'm saying, and what I think Loosebits is agreeing with, is that what's important in diving is what pressure you're under. If you're under a pressure of, let's say, 1.0 ATA, then it's not really relevant whether you're 33 or 34 feet underwater...

but most divers care very much about how far it is to the surface

...unless divers just want the bragging rights of being at the deeper fresh-water depth.

Okay, it might make a difference when doing an emergency ascent, if you can't make it that last 2.5% of the way.

Hmmm... you'd be less buoyant in fresh water, and have that much further to go!

--Marek

 

[Walter]

Actually if you read my post you will see that it's no gimmick at all. And not to flame you or sound mean, but most divers care very much about how far it is to the surface.

Mike Rushton
PADI DM 174448

They may care, but they don't know. 33 fsw is a measurment of pressure, not a linear distance. On average (salinity varies), 33 fsw is slightly deeper than 33 linear feet from the surface. 34 ffw is slightly shallower than 34 linear feet from the surface. Also 33 feet does not equal 10 meters, but 33 fsw does equal 10 msw. There is not one additional atmosphere of pressure every 33 linear feet we descend into the ocean, but it's very close to that value. It's close enough that the definition of 33 fsw = 1 atmosphere.