FSW - What is this telling me?

[gcarter]

I have a Uwatec bottom timer. It's calibrated for fresh water. (presumably because Switzerland is land locked?).
If I'm diving in the ocean it will read 103' (FFW) even though I am only at 100' (Actual depth,FSW)

Specifying what your computer/gauge is using makes it a bit more accurate although the practical difference is minimal.

Many/most? computers assume a dive at an altitude of zero feet is salt water. Dive a freshwater lake at sea level,or close to sea level, and the computer will assume you are in the ocean and give (slightly) inaccurate depths.
None of this has any impact on NDL calculations as that only depends on pressure.

Now there's something meaty for me to ask another stupid question about

Are you saying that the presure is different enough at the same depth in fresh and salt to warrant the distinction? I read elsewhere that it only creates a 3% variance at 100 feet. I would suspect that variability between dive computers with their individual margins of error would render that variation meaningless.

 

[DuboisP]

I have a Uwatec bottom timer. It's calibrated for fresh water. (presumably because Switzerland is land locked?). If I'm diving in the ocean it will read 103' (FFW) even though I am only at 100' (Actual depth,FSW)

User Guide :
The depth measurement is calibrated for fresh water according to international standards"
"Depth accuracy: 0.1% ±0.1 m (±0,4 ft)"
International standards ? de facto rule ? written rule ? seems to be a European rule, according the user guide.
Why is my Suunto calibrated for salt water ?
Does the maker can choice ?
the "EN 13319 Diving accessories - Depth gauges and combined depth and time measuring devices" cannot be downloaded for free !

 

[Thalassamania]

To be technical about it, 100 feet is a distance, 100 FSW or 100 FFW is pressure, like 760 mm Hg.

 

[PhuketPaul]

User Guide :
The depth measurement is calibrated for fresh water according to international standards"
"Depth accuracy: 0.1% ±0.1 m (±0,4 ft)"
International standards ? de facto rule ? written rule ? seems to be a European rule, according the user guide.
Why is my Suunto calibrated for salt water ?
Does the maker can choice ?
the "EN 13319 Diving accessories - Depth gauges and combined depth and time measuring devices" cannot be downloaded for free !

I think the International Standards it refers to are the standards for the pressure due to the depth of fresh water, not the fact that it relates pressure to depth of fresh water instead of sea water and not the accuracy of the depth gauge.

I have a Uwatec (Galileo Sol) and it can be switched between Salt to Fresh, but for diving the actual pressure is more important than the linear depth as this is what decides NDL, air use etc

 

[Tortuga68]

User Guide :
The depth measurement is calibrated for fresh water according to international standards"
"Depth accuracy: 0.1% ±0.1 m (±0,4 ft)"
International standards ? de facto rule ? written rule ? seems to be a European rule, according the user guide

I believe they're referring to SI (International System of Units) standards

 

[DMDavid]

If I am diving a quarry, it's a pretty safe bet it is fresh, and if I talk about reef diving, it is a pretty safe bet it is salt.

There are a few saltwater quarries in the Florida Keys, one is called the Horseshoe.

The main reason I add ffw/fsw is because the reader may not know that my dive is in salt or fresh water.
For example: I dove down to 10fsw at the Blue Heron Bridge.



FB-Florida Scuba Diver

 

[knowone]

I just wish, my cuff was
still only reading mmHg

 

[AfterDark]

I don't type or like to, it's one less key I need to find and hit.

 

[spectrum]

I see a lot of posts where rather than just say, for example 90 feet, someone will say 90 FSW. Other that telling me that it is in salt water, is there any other reason to use this convention?

It may seem like a silly question, but I honestly don't understand the reason for the convention.

The primary reason is that it is what the instrument is calibrated to. While we speak of depth the means of detection is pressure. You know from dive weights 101 that salt water has a specific gravity of about 1.028. This means that in fresh water you are about 3% deeper for a given reading.

In terms of gas management it all comes out in the wash since consumption, absorption and decompression are all influenced by the pressure, not distance. The only real problem would be getting a precision distance reading from the surface without applying a correction factor. If that's the case you have the wrong instrument in the first place since in many cases instrument tolerance will have comparable significance.

Other things such as conveying water type and so fort are ancillary.

Pete

 

[knowone]

Mostly dependant on the audience