Fresh vs Salt

[LakerPride]

I have a few odd questions, I have done almost all of my diving in the ocean but now that I live in Sault Sainte Marie I want to dive the plethora of wrecks around me. One odd question when referring to depth we usually use fsw, as in feet sea water, now that it's fresh water do I record it as ffw?

Second I know the density of sea water is more than fresh, what are the considerations with planning my dives using dive tables?

I am planning a dive that would be at the edge of rec limits, 130 (ffw) and I want to know what the physiological differences will be.

Last for those more experienced freshwater divers do you notice a difference in gas consumption in fresh v salt

 

[Teller]

I have a few odd questions, I have done almost all of my diving in the ocean but now that I live in Sault Sainte Marie I want to dive the plethora of wrecks around me. One odd question when referring to depth we usually use fsw, as in feet sea water, now that it's fresh water do I record it as ffw?

Second I know the density of sea water is more than fresh, what are the considerations with planning my dives using dive tables?

I am planning a dive that would be at the edge of rec limits, 130 (ffw) and I want to know what the physiological differences will be.

Last for those more experienced freshwater divers do you notice a difference in gas consumption in fresh v salt

As far as planning keep in mind 1 ATA in salt water is 33 feet and 1 ATA in fresh water is 34 feet. In all my years diving I have never put any thought in to using the same table in fresh water and sea water. I have not died yet. My SAC is the same no matter where I dive.

 

[spectrum]

lakerPride,

If you stop and think about it depth gauge does not measure depth. It measures pressure and is scaled for depth. Likewise all of the gas calculations are really depth based. In other words there is no need to make allowances unless you're trying to measure for something. If that's the case the depth gauge is probably not the best tool anyhow.

As for going from the ocean to fresh you need to allow for the difference in specific gravity of the water and it's that simple IF your gear remains identical. Take your entire dry diver weight; you, suit, gear, tank weighs...... and divide by 40. Round down and remove that much lead. More here.

Pete

 

[LakerPride]

Awesome thanks!

 

[bleeb]

If you stop and think about it depth gauge does not measure depth. It measures pressure and is scaled for depth. Likewise all of the gas calculations are really depth based. In other words there is no need to make allowances unless you're trying to measure for something. If that's the case the depth gauge is probably not the best tool anyhow.

Just to elaborate a bit, most analog gauges have their printed scales calibrated for salt water, and I think pretty well all dive tables are as well, so you're fine as long as tables and gauges are using the same units of measure, as they almost certainly are. Most computers also seem to use salt water calibration, although some have the option to switch to displaying fresh water-based depths for greater tape measure accuracy, but the ingassing/offgassing models they're running are pressure based, and so don't actually care which you choose to display.

 

[TMHeimer]

I believe 100 fsw = 103 ffw. Same pressure, same thing. But an analog gauge in 103 ffw will read 100. So you must keep in mind you're actually 3 feet deeper.

 

[emoreira]

My dive computer only switches to fresh water when it´s in altitude (higher than 2000 feet), as it is supposed that altitude lakes are fresh water. Diving in fresh water near sea level involves an error as the dive computer remains setted for salt water. However, this error remains within the boundaries of the safety margin and the measurement error.

 

[Saxatilis]

I switch between salt and fresh all the time. Within recreational limits, I notice practically no difference. I dive with 2 computers and decided to test the difference between salt and fresh. I did a 80 ft. dive in a fresh water quarry with one computer set to salt and one to fresh. The difference was negligible. I suspect that the only reason they were different at all is because each computer uses a slightly different algorithm. HOWEVER, if you plan on tech diving I strongly recommend that you make sure that whatever software you use to generate tables must be on the correct setting (salt or fresh). Rec diving has a far greater margin of error than tech so even if the difference is negligible it still might change how you would decompress. Better safe than sorry!

 

[tadawson]

Consider that ongassing/offgassing rates are determined by pressure, not physical depth, this should not matter at all! If the gauge reads 130 feet, be it salt or fresh, then the pressure you are in is the same in either case, no matter the physical difference in actual depth, and your deco, air consumtion, etc. will be the same. If you can adjust for salinity on the computer, then in 130' of fresh water, you will be under *less* pressure than in salt, and things will change accordingly.

It all depends if you want to consider your 130' limit by pressure or yardstick . . . and if you care about the 3.93 physical feet of difference to get the same pressure depth . . . and not much more . . .

- Tim

 

[LakerPride]

These responses have answered my questions, thanks everyone. The reason I posted this is because I have dove to 130' before in the ocean but I will be making a few dives this summer in Lake Superior on progressively deeper wrecks eventually reaching 180-200' so I wondered about the off/on gassing and if using my current programs for salt water would affect my deco. I just bought Deco for Diver and plan on devouring it in the two months between now and the diving season.