? on Nitrox tables/calculations

[DNK]

Wasn't sure where to post this, but thought that this looked the best.
I am presently preparing for my Nitrox Course and while reading through the book and doing examples of ABT, MAX depth, O2 exposure, etc. I realized that all the tables and calculations were based upon FSW(feet sea water). I normally due fresh water diving. I realize that the AIR tables have enough safety margine to acount for the different densities of fresh verses salt water. But I would assume that when dealing with O2 exposure even a marginal difference would be significant.
Can anybody help to explain to me the following.
1] Is there a difference between sea water and fresh water when calculating ABT, MAX and O2Exposure?
2] If so why do the tables only reference FSW.


I think I just answered my own question. Since SW is denser than FW, using the SW tables for a FW dive results in an added margin of safty?

Now the only question is, is there anyway to actually calculate the difference?

Sorry about the stream of thought post.
BTW I do use a computer but would like to know more about the physics and the reasoning behind the calculations.

thanks

 

[The Kraken]

Take the calculated difference in pressure at a given depth between sea water and fresh water, subtract it from the depth sea water and use that in conjunction with your dive table as your depth.

 

[Rick Murchison]

Measure with a micrometer, mark it with chalk, cut it with an ax.
How tall are you? How thick?
When your depth gauge reads 60 feet, how deep are you?
Your "depth" gauge is really a pressure gauge calibrated in FSW.
Fresh water is less dense than salt water...
Throw all that in the hopper and what do you get?
Unless you're at altitude (above 1000'/300M or so), the tables work fine for fresh water, whether you're diving Nitrox or air.
---
Yes, you can calculate the difference if you really want to, but first you'd have to have a depth gauge calibrated for fresh water. But since the whole purpose of calculating the difference in the first place would be to convert your depth back to its equivalent in salt water, and your depth gauge already does that, you're sorta running around the barn to get back where you started aren't you?
IOW, when you are at an actual depth of 34FFW, your "Salt water equivalent depth" is 33' - and your depth gauge will read 33'. So unless you're measuring depth with a string, the conversion's already made for you.
Rick

 

[The Kraken]

The difference is miniscule, I must admit.

 

[jonnythan]

When you dive in fresh water, you aren't going to know how deep you are in ffw (feet fresh water). When your gauge says 33 feet, you might in reality be 34 feet below the surface, but since your gauge is calibrated to fsw it doesn't matter. When diving fresh water your gauge will always be inaccurate in terms of linear feet, but that's OK because linear feet don't matter in diving. What matters is actual pressure, and in that sense your gauge will be accurate.

 

[Randy43068]

Measure with a micrometer, mark it with chalk, cut it with an ax.
How tall are you? How thick?
When your depth gauge reads 60 feet, how deep are you?
Your "depth" gauge is really a pressure gauge calibrated in FSW.
Fresh water is less dense than salt water...
Throw all that in the hopper and what do you get?
Unless you're at altitude (above 1000'/300M or so), the tables work fine for fresh water, whether you're diving Nitrox or air.
---
Yes, you can calculate the difference if you really want to, but first you'd have to have a depth gauge calibrated for fresh water. But since the whole purpose of calculating the difference in the first place would be to convert your depth back to its equivalent in salt water, and your depth gauge already does that, you're sorta running around the barn to get back where you started aren't you?
IOW, when you are at an actual depth of 34FFW, your "Salt water equivalent depth" is 33' - and your depth gauge will read 33'. So unless you're measuring depth with a string, the conversion's already made for you.
Rick

are a smart guy. I like reading your posts.

Randy

 

[DA Aquamaster]

I have encountered the rare situation (right around 1000' elevation) where the bottom timer figured depth in ft fresh water and the computer figured it in ft sea water. In one case, the bottom timer read 156 ft (FFW) when the computer read 150 ft. (FSW)

If you are using a depth gauge, it is really measuring absolute pressure not actual water depth and it will be measuring that pressure in FSW anyway, so it will take care of the conversion for you.

For some unknown reason equipment makers seem to think that having a computer or electronic depth gauge reads in FFW conveys some sort of advantage. It does not as most people really do not care exactly how many feet they are under water, they are instead more concerned about pressure (whether they realize this or not). So FFW readings mostly just cause confusion.

Computers don't care, they make the conversion anyway. And fortunately given that the tables are figured using FSW, if your computer, bottom timer, sounding weight, tape measure or whatever reads in FFW rather than FSW, it is going to give you a more conservative (deeper) depth reading as the tables are based on FSW depth which will always be numerically less in value than the FFW depth at an identical water pressure.

So don't worry about it.

Altitude diving and theoretical depth at altitudes over about 1000' elevation however are things to be concerned about and depth and table corrections will need to be considered if you are using tables rather than an altitude diving capable computer.

A capillary depth gauge can come in handy in altitude diving as it bases it's readings on the atmospheric pressure at the surface so it will automatically determine and show theoretical depth.