Depth to ATA

[H2Andy]

okie guys... here's a question

in my earlier post, i said that if you were at the same absolute depth in both
fresh and saltwater, your gauge would actually show you shallower in the fresh
water, due to less pressure per same volume of water above you.

is this correct? i keep thinking about it and getting a headache

 

[Saturation]

Not to confuse matters further, salinity varies per region, per ocean, per depth and possibly per unit time. So best not to get too worked up over its absolute value. Its one reason some manufacturers perfer fresh water calibration over salt, at least fresh water is more 'standardized' and the minor error read in your salt water dive will be consistent throughout the dive.

http://www.windows.ucar.edu/tour/link=/earth/Water/salinity.html&edu=high

For practical matters in real world sport diving, the difference between ffw and fsw is moot. In a dive to 1000 fsw, the error will be in about 27 ffw, for 100 fsw, the error is ~ 3 ffw.

 

[Walter]

in my earlier post, i said that if you were at the same absolute depth in both
fresh and saltwater, your gauge would actually show you shallower in the fresh
water, due to less pressure per same volume of water above you.

is this correct?

Correct.

 

[H2Andy]

thanks, Walter

 

[GoBlue!]

Not to confuse matters further, salinity varies per region, per ocean, per depth and possibly per unit time. So best not to get too worked up over its absolute value.

Agreed. But if a deco model is developed & translated such that the user inputs depths that assume 1 atm=33fsw, the argument is that no matter what salinity you are diving in (including freshwater), you should use the same conversion when planning/analyzing your dive. That is, one's ambient pressure (ata) should always be calculated as gauge pressure/33+1....no matter what the salinity.

Is this not correct? (I realize we're splitting hairs....it's more the concept I'm going for.)

in my earlier post, i said that if you were at the same absolute depth in both fresh and saltwater, your gauge would actually show you shallower in the fresh water, due to less pressure per same volume of water above you.

Yep. And if I were to calculate my RMV, I'd use 1atm=33ft, whether saltwater or freshwater.....because RMV is dependent on ambient pressure, and if your gauge is calibrated for fsw, I will always use the fsw-to-atm conversion. That brings us full circle to the original post.

Jim

 

[caveseeker7]

??? Fresh & seawater have nothing to do with metric or imperial. Did you mean something else?

I was trying to say that some computers allow you to select the ambient pressure to be displayed in either fresh- or seawater calibration.

Just like some computers allow you to select the depth to be displayed in either feet or meters. Or the time in 12 or 24 hours.
As in: Go into the menu and change the setting.

 

[BigJetDriver]

---Not to confuse matters further, salinity varies per region, per ocean, per depth and possibly per unit time. So best not to get too worked up over its absolute value.---

For practical matters in real world sport diving, the difference between ffw and fsw is moot. In a dive to 1000 fsw, the error will be in about 27 ffw, for 100 fsw, the error is ~ 3 ffw.

Folks,

As Doc Sat points out, it is best not to get wrapped around the axle about this. For our purposes, it makes very little difference.

A lot of our calculations are, in fact, based on "accepted averages" and, guess what? They work very well.

In aviation, we work with an average for the weight of a gallon of jet fuel. For our average range of temperatures and types of fuel, it works very well. For situations that are outside the normal range, we have our charts.

In diving, we use the average:

Depth divided by 33 plus one equals pressure absolute (fsw)

and:

Depth divided by 34 plus one equals pressure absolute (ffw).

These will be within the average range for any conceivable diving that 99.99% of us will ever do.

Cheers!

 

[H2Andy]

yes, we understand, BJD69. but sometimes looking into the details is fun.

 

[Saturation]

Enjoy ...

http://ioc.unesco.org/oceanteacher/resourcekit/M3/Converters/SeaWaterEquationOfState/Sea%20Water%20Equation%20of%20State%20Calculator.htm

 

[BigJetDriver]

yes, we understand, BJD69. but sometimes looking into the details is fun.

Yo, Andy! You got a point there, Judge!