Freshwater, Saltwater, Depth gagues/computers and diving

[elan]

Ahh yeah of course. Using the computer and an extra gague in different modes will of course be an issue. Should also be plainly visible and ring a bell if you have two units showing different depth tho

I think what he meant was using a personal computer like PC or a smart phone like iPhone on the surface which run software like v- planner to plan the dive. Not a dive computer. And then execute the dive using a gauge. In this case it is not very obvious.

 

[Tigerman]

Ah yeah. If you use freshwater tables and saltwater gague(s) youll definetly have some discrepancies.
Guess thats the situations that big fat warning in the computer/gague manuals are for..

 

[Rich Keller]

The depth gauge reads pressure not depth, only the face of the gauge shows you that reading in feet of sea water. If you are in fresh water and your gauge reads 60' you will be a little deeper but the effect of gas absorption on the body will be the same. The only reason to have a fresh water gauge is if you need to know your exact depth in fresh water. The exception to that would be high altitude diving where the surface pressure is less then 14.7 but the tables for that are different.

 

[DivemasterDennis]

This is really a non-issue. While the density difference will affect the weight you need, other differences are so negligible as to not move you outside the safety margin of error built into any computer (or the rdp).
I dive often in fresh water and at altitude. The altitude adjusment tables are critical to follow in dive planning, but salt vs. fresh? Don't lose any sleep over it.
DivemasterDennis

 

[jtivat]

This is really a non-issue. While the density difference will affect the weight you need, other differences are so negligible as to not move you outside the safety margin of error built into any computer (or the rdp).
I dive often in fresh water and at altitude. The altitude adjusment tables are critical to follow in dive planning, but salt vs. fresh? Don't lose any sleep over it.
DivemasterDennis

I am going to have to disagree with that. On a local wreck I dive the difference in a gauge calibrated to salt water being used in fresh can mean about 10 minutes of deco time which I personally would rather not skip.

 

[Easdem]

The linear feet of liquid above your head really don't matter. What matter's is ambient pressure. In SW 2ATA is at 33 linear feet and the same is at 34 in fresh water. The tables and computer algorithms take into account the ambient pressure when making calculations, so it really makes no difference if your depth gauge says 33 and you actually have 34 feet of water over your head.

If you went diving in a liquid twice as dense as salt water, 2 ATA would be at 16.5 feet and your depth gauge would read 33 feet, your NDL's and such would still be accurate...it would just be hard as hell to swim.

 

[Easdem]

I am going to have to disagree with that. On a local wreck I dive the difference in a gauge calibrated to salt water being used in fresh can mean about 10 minutes of deco time which I personally would rather not skip.

ahhh...No. That would only happen if you used a freshwater gauge with SW tables.

 

[jtivat]

ahhh...No. That would only happen if you used a freshwater gauge with SW tables.

I am not sure why I do this but I will try again.

Fresh water is less dense than Salt water right?
So now with that in mind if my SPG is calibrated to Salt water but you are in fresh water you will really be about 3% deeper than the gauge reads as it is looking a pressure not depth.
So on a dive my gauge could read 300’ but I would really be at 309’
Now plan that dive on your choice of dive planner and see what happens one at the 300’ you think you are at and one at the 309’ you are really at.

 

[Easdem]

I am not sure why I do this but I will try again.

As an Instructor, I trust you understand the concept, perhaps I misspoke. Let see if we can figure out where we went wrong.

Fresh water is less dense than Salt water right?

YES!!!

So now with that in mind if my SPG is calibrated to Salt water but you are in fresh water you will really be about 3% deeper than the gauge reads as it is looking a pressure not depth. So on a dive my gauge could read 300’ but I would really be at 309’

YES!!!!!!!!!!! There would be 309 feet of fresh water above your head.

Now plan that dive on your choice of dive planner and see what happens one at the 300’ you think you are at and one at the 309’ you are really at.

Not in this case, if you were comparing 300’ of salt water to 309’ of salt water that would be true. The important part is the PRESSURE. Nitrogen does not care how many linear feet of liquid are above your head, it only cares about the PRESSURE around so he can dissolve in your blood and cause mayhem. 309 FFW has the same pressure effects as 300 FSW. Imagine if the “depth gauge” read in ATA’s only… at 300fsw it would read the same as 309ffw- So technically you 309 linear feet from the surface, but the pressure will be the same as 300fsw…so the NDL’s will be the same…Get it?

If not ask one of your LDS's senior instructors to explain it to you.