Fresh vs Salt water

[John Yaskowich]

On my vacation dive just completed I forgot to change my Mares 2 from fresh to salt setting. How much difference will that make in my depth reading? (max depth showed fresh 68ft) Is there a way to retroactively change it? Barring that, what conversion factor should I apply for my log?

Thanks

[CompuDude]

In salt water, 33' = 1 ATA. In fresh water, 34' = 1 ATA. Your computer actually reads pressure (ATA), so if your computer is set to fresh water and you're diving salt, it will read 1' deeper than you actually are for every 33'. So your 68' dive was actually more like 66'. The error actually made your dive safer... the other direction would be more problematic (in theory... in reality it probably wouldn't matter).

Considering your depth is only off by 3' at 100', it's not really something to worry too much about. Many computers ship defaulted to fresh water, since that's the more conservative setting, technically, and few people ever even notice to change it.

If you really want a precise conversion factor, divide your ffw depth by 34, then multiply by 33. Thing is, considering ocean salinity varies so much from location to location anyway, there's really no point to bothering trying to be precise. 33' is technically an average that may or may not precisely align with where you were diving, and considering the difference is small anyway there's not much point to sweating over a few feet, since it's all probably well within a rounding error anyway.

[Rainer]

Great response.

In summary, you were fine!

[John Yaskowich]

Originally Posted by CompuDude

In salt water, 33' = 1 ATA. In fresh water, 34' = 1 ATA.

Thank you. I was worried that there was a bigger difference than 1ft/33. I will just leave things be and know I was "about" 67 ft deep on a wonderful dive (with Scuba Fun in St Maarten).

[PTAaron]

Aren't analog gauges going to be affected the same way by the difference in pressure between the different water densities? They also simply measure the pressue...

[CODMAN]

Originally Posted by PTAaron

Aren't analog gauges going to be affected the same way by the difference in pressure between the different water densities? They also simply measure the pressue...

Yup they are!

[fisheater]

And don't forget that as that storm front moves through, the atmospheric pressure will change the reading, too.

Then, of course, the problem of the surge and waves needs to be taken into account. If you stay in the same place, but the surface varies due to wave action, your gauge will show a pressure change. But, then, there will be a lag between the wave passing overhead and the pressure change reaching the gauge. And the lag as the gauge reacts to the pressure change.

Oh vey! You have NO IDEA what your depth was!

[Defman]

Originally Posted by CompuDude

In salt water, 33' = 1 ATA. In fresh water, 34' = 1 ATA. Your computer actually reads pressure (ATA), so if your computer is set to fresh water and you're diving salt, it will read 1' deeper than you actually are for every 33'. So your 68' dive was actually more like 66'. The error actually made your dive safer... the other direction would be more problematic (in theory... in reality it probably wouldn't matter).

Considering your depth is only off by 3' at 100', it's not really something to worry too much about. Many computers ship defaulted to fresh water, since that's the more conservative setting, technically, and few people ever even notice to change it.

If you really want a precise conversion factor, divide your ffw depth by 34, then multiply by 33. Thing is, considering ocean salinity varies so much from location to location anyway, there's really no point to bothering trying to be precise. 33' is technically an average that may or may not precisely align with where you were diving, and considering the difference is small anyway there's not much point to sweating over a few feet, since it's all probably well within a rounding error anyway.

Right, but what time does the 2nd train get to Chicago????