Fresh vs salt water depth

[OneDragons]

I know that salt and fresh water has a different weight so a given depth of water will yield a different depth depending on if you are in fresh or salt.

The thing I don't understand is why does it matter.

If your computer reads a given depth it is actually showing a set pressure, this pressure is what acts on you as a diver and is the important element. If you are under the same pressure in salt as fresh then yes you will be at a different depth but since the depth itself is irrelevant then surely it doesnt matter.

If the computer is calibrated to the decompression algoritm correctly then it could tell you you are 12.2 cats in depth and it would make no real difference.

So my main question is why do some get hung up about whether the computer is calibrated in fresh water or salt?

[PeteM]

I know that salt and fresh water has a different weight so a given depth of water will yield a different depth depending on if you are in fresh or salt.

The thing I don't understand is why does it matter.

It doesn't. We use Bar as our unit of pressure, the merkins use PSI, others pascals, another you might come across is MSW, or less commonly MFW (Metres of Sea Water/Metres of Fresh Water) - they are equally good measurements. We only care about the pressure which is why depth gauges are calibrated for one or the other, most computers work in MSW and those that don't are only applying a fiddle factor to get to MFW

[Nigel Hewitt]

If the computer is calibrated to the decompression algoritm correctly then it could tell you you are 12.2 cats in depth and it would make no real difference.

So my main question is why do some get hung up about whether the computer is calibrated in fresh water or salt?
Exactly. However if you tell it you want your stops in even numbers of cats it won't match my multiples of three meters of salty water. We'll both decompress nicely but not together.

[ChristianG]

Exactly. However if you tell it you want your stops in even numbers of cats it won't match my multiples of three meters of salty water. We'll both decompress nicely but not together.
Cats?

Oh, silly me, of course you mean the diving variety. :eek: :D

Speaking a bit more seriously, salt water pressure also depends on the salt water that is being considered. The extreme is the Dead Sea (but you wouldn't dive there anyway - as the name implies) and other very confined salt water seas such as the Black Sea. There, 30 msw is not going to be the same as 30 msw in, say, the Pacific. Oh, and in the Black Sea the sluggishness, or not, of the Danube is going to also make a difference.

[Edward]

I did a fresh water dive back in 93 my computer recorded 10m, but when the water (and the crud) was drained out of the Reservoir it measured 6m.

Edward

[Richard Whitcombe]

A depth gauge is just a pressure gauge. Nothing more than that. It just multiplies out using a conversion fact to display a "depth".

The depth it reads is unlikely to be the REAL physical distance between a diver and the surface as water density varies. For decompression calculations it doesnt matter- its working on pressures not imaginary depths so you dont need to set the computer for one or the other.

[BigBlueTech]

I know that salt and fresh water has a different weight so a given depth of water will yield a different depth depending on if you are in fresh or salt.

The thing I don't understand is why does it matter.

If your computer reads a given depth it is actually showing a set pressure, this pressure is what acts on you as a diver and is the important element. If you are under the same pressure in salt as fresh then yes you will be at a different depth but since the depth itself is irrelevant then surely it doesnt matter.

If the computer is calibrated to the decompression algoritm correctly then it could tell you you are 12.2 cats in depth and it would make no real difference.

So my main question is why do some get hung up about whether the computer is calibrated in fresh water or salt?

While some computers will show you if you are in salt or fresh most just measure the density of the water. If you went diving in custard it would read different as well because the density of the medium that applies pressure on your tissues is increased.

So regardless of actual depth, what you need to remember is the computer monitors the pressure in relation to your body. The different is small 1kg of weight is equal to 1l in fresh where 1kg of weight is equal to 0.97 in salt (more weight needed to dive in salt)

[OneDragons]

So in short anyone getting all het up about the computer being calibrated in salt vs fresh water is indeed worrying about nothing as I thought.

Thanks

[ChristianG]

So in short anyone getting all het up about the computer being calibrated in salt vs fresh water is indeed worrying about nothing as I thought.
Probably,

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perhaps.

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possibly. :D :D :D

I suppose it depends on how argumentative anyone-at-all is feeling. There seems, or may be, the odd hole in some of the explanations proposed. ;)

[BigBlueTech]

So in short anyone getting all het up about the computer being calibrated in salt vs fresh water is indeed worrying about nothing as I thought.

Thanks

As someone who technical dives in fresh and salt i have found no information that has caused me any concern beyond the fact i was 70m in a lake in the middle of a jungle in thailand.

[HEO2N2]

I dive with a Uwatec Tec 2G in gauge mode and also the Uwatec Bottom Timer. The bottom timer is clibrated to Freshwater, the Tec 2G saltwater.

Even on dives to 50 - 60m there is so little difference as to mean I dont worry about it.

[Richard Whitcombe]

To put some numbers on this assuming sea water average density of 1025kg/l or similar at 40m this would produce only a 1m difference in depth reading vs a gauge calibrate to fresh water.

The tolerances of the gauge itself and other factors are bigger than this effect so in the real world its not an issue.

The red sea is saltier but even then not that much of a difference.