Fresh Water or Salt Water?

helopilot2be · Oct 22, 2009

Can anyone tell me why it matters to my new dive computer whether I am diving fresh water or salt water? Will the salinity change the readings or is it something else. And if it really matters what do I set it to when diving in Brackish water? Like the Chesapeake Bay.
Thanks in advance I'm new to diving with a computer
Chris

zaberman1 · Oct 22, 2009

1 ata in salt water is 33ft and 1 ata in freshwater is about 34 ft.

muddiver · Oct 22, 2009

Unless you know it's sea water, set it to fresh water mode. It may read one foot deeper per ata, but it won't hurt you to be shallower than the computer thinks.

sdorn · Oct 22, 2009

Do all computers make this distinction, or do some just assume fresh water as the more conservative measure? I don't remember seeing any way to set fresh or salt water on my computer.

Gombessa · Oct 22, 2009

Chris, in layman's terms, what zaber and muddiver are saying is that your computer calculates your depth by measuring pressure. Saltwater is denser than freshwater, so you're computer needs to know whether you're diving salt or fresh in order to translate the pressure it's measuring into an accurate display of depth.

sdorn - I think some computers assume fresh and some assume saltwater. Some even know without you having to tell it. My bottom timer reads only in ffw (feet fresh water), it's up to me to convert to saltwater depth (which I never do, since the difference is only about two feet at 66ft depth, it's more or less immaterial to your dive).

zaberman1 · Oct 22, 2009

some computers (like DiveRite's Nitek series) have an option to switch between the two, some computers use the ffw mode for conservatism...

M_Bipartitus · Oct 22, 2009

some computers base the fresh/salt mode on the pressure when they turn on. If the pressure when it turns on is lower than some specific value, the computer assumes the water is fresh, and may set the computer to altitude mode (but that's a separate thread).

divingguru · Oct 22, 2009

For most intensive purposes it will be absolutely no difference to you or your dive profile. One good thing is that you won't have to clean it in fresh water after the dive!

helopilot2be · Oct 22, 2009

Thanks. I was just curious why my computer wants me to set that parameter.

Bubbletrubble · Oct 22, 2009

Selecting "fresh water" or salt water" will affect the depth in feet that is displayed on the screen during the dive and saved for download. As has been pointed out, dive computers sense pressure and then mathematically convert this number into fsw (feet salt water) or ffw (feet fresh water) for display purposes. The different densities of salt water and fresh water cause their pressure-depth conversion equations to be slightly different. A computer calibrated for fresh water will display a 3% depth error in salt water (refer to the above post by zaberman1).

So will this difference affect the way you dive? Practically speaking, no. I'm quite certain, however, that dive computers use the depth conversion in feet to determine the shallow safety stop (10 - 20 fsw or ffw) and any deep stops (if your computer does them at all).

The take home message is: If your dive computer has a special setting for salt or fresh water, by all means use it. If not, it's no big deal.

Happy diving...

Fresh Water or Salt Water?

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