The same holds for tables, but you'd need to make sure your depth gauge is converting pressure to depth the same way your table is doing it. If your table is using salt water depths and your depth gauge is using salt water depht, it shouldn't matter if you dive fresh water since the depth in both cases doesn't represent depth but pressure.
Is that making sense for anybody else?
Let's make things a bit weird to make it more intuitive. Let's say salt water, at 33 ft, is 2 atm, and let's say fresh water, at 66 ft, is 2 atm. While the difference is not that large, we concede that there is indeed a difference, so enlarging said difference will only enlarge any error we are attempting to isolate.
Thus:
A pressure gauge in salt water at a pressure at 2 atms will output a depth of 33 ft, when the actual depth is 33 ft.
A pressure gauge in fresh water at a pressure of 2 atms will output a depth of 33 ft, when the actual depth is 66 ft.
The dive table will receive an input of 33 ft regardless of the actual real depth of the diver.
The dive table will translate 33 ft to 2 atms.
Everything would be fine! Except the diver may be a little more cold thinking they are at 33 ft! Lol.
However, as you mentioned, the actual difference is so small as to be a moot point. Our instruments just aren't that accurate, but the least accurate by far (in this scenario) are the dive tables, since you have to figure everything based on your deepest depth during the entire dive. But what are scuba forums for if we can't examine something statistically insignificant?
To throw a real wrench into everything, I don't really think Dive Tables are formulated based on salt water! As far as I know, they are based on evidence gathered from real life dives on real life divers. Basically, gather a bunch of volunteers together, put them at different depths for different times, and record the results. Apply some spiffy data analysis, remove outliers, then figure out how many standard deviations or where at in the bell curve you are comfortable drawing the line for all scuba divers in general.
(I guess I should note that if your gauge is calibrated for fresh water, in the above example we would still be safe. Here's why:
A pressure gauge in salt water at a pressure at 2 atms will output a depth of 66 ft, when the actual depth is 33 ft.
A pressure gauge in fresh water at a pressure of 2 atms will output a depth of 66 ft, when the actual depth is 66 ft.
The dive table will receive an input of 66 ft regardless of the actual real depth of the diver.
The dive table will translate 66 ft to 3 atms.
Because the table thinks we are at a greater pressure than we really are, our dives will be cut shorter on time, in effect giving us less residual nitrogen, with a larger safety margin than necessary.)
Salt water is heavier than fresh water. Fresh water is lighter than salt water. But the atmospheric pressure is the same? Unless I am just thinking too much, it seems that the depth of each ATM of pressure would be different. Example: if 33ft of salt water is 2ATM, then why isn't 33ft of fresh water 1.8ATM(est)?
