Stryker:
also is there a difference in calculating pressure in salt water and fresh water? I realize buoyancy is different, but wouldnt pressure stay the same?
If your gauge or computer calculates depth in feet sea water (FSW) and you plan a fresh water dive accordingly in FSW, it makes no difference as the depth gauge is technically measuring "pressure" rather than actual "depth". So, if your salt water calibrated gauge reads 150 feet sea water in a fresh water lake, a tape measue would show you being a few feet deeper than 150' under the surface but the pressure would be the same as if you were in 150 ft of salt water and the SAC, PPO2 and deco caclulations would be done according to the 150' sea water pressure you are actually under.
Where it gets potentially confusing is where your computer "helps" you by figuring the dive in feet fresh water (FFW) for a dive at a site with an altitude over 1000 ft. This is in my opinion a truly useless feature as knowing your exact depth underwater is not important, while knowing your exact pressure (as measured in FSW) is very important.
Given that most deco tables, PPO2's etc are based on FSW and 33 ft of water equalling one ATA, all of the dive calculations are normally done in FSW and that is the figure you want to know. Computers that display depth in FFW makes that conversion in their calculations, but it makes it technically harder for you if you are doing contingency planning.
I have had some interesting disagreements between computer and bottom timer in lakes at about 1000 ft altitude with one calling it a sea level dive and indicating I was at 150 ft (FSW) and the other saying it was an altitude dive and that I was at 156 ft (FFW). From a deco, PPO2 and SAC standpoint, the number that counted was the 150 ft FSW number.
The good news here though is that if you are at altitude and plan a dive to 150 ft (FSW) and your computer or bottom timer is reading in FFW, it is adding a little more conservatism as when it reads 150 ft FFW, you will in reality be under a little less pressure than you would be at 150 FSW and will use a little less gas and acquire a little less deco obligation than you are planning for. And a little extra conservatism is not a bad thing to have on an altitude dive where your theoretical depth can be substantially deeper than either the FFW or FSW depths. For example if the 150' (FSW)dive is at 5000 ft, the theoretical depth will be approx 180' and your NDL/deco, if using sea level tables, will need to be figured based on that depth.
In short, simplify your life and plan everything in FSW and then assume all depth readings are in FSW even when they are actually in FFW as any error will be on the side of caution.
