Freshwater to Saltwater Conversion Formula???

[Redfish1978]

Hi,

I'm stuck with freediving the freshwater lakes here in Southern Illinois for the time being. I use a Suunto D3 watch which I assume is calibrated for seawater depth pressure. I'm curious if anyone knows the conversion formula I can use to convert the displayed depth in seawater on the watch to the actual depth in freshwater that I'm diving in.

From charts the depth is listed as 86ft, but the watch is showing 71ft.

Thanks,

Mark

 

[Bill]

Just by density I'll take a guess at adding 3% to your readings.

 

[apneaboy]

Yeah. A 30 metre dive you get 1 metre extra. A 60 metre dive you get 2 metre extra.

The D3 would measure 30 metres in fresh water when actually the distance would be 31 metres.

Working in olden days money a 71 ft dive on the computer would be just over 73 ft. Its not all a computer reading error to get you to the 86 ft area you need to get searching for the deep bit, good luck

Having said that also some of those computers measure a little bit out, i have seen them measure 3-6 ft out at that sort of depth, if you touched the bottom with your non computer hand it wouldn't read the proper depth, and you could do with leaving the computer at the maximum depth for a couple of seconds so it registers. Again you might lose a few 0.1's off the depth for just momentarily sticking your hand down only.

Not that i'm that bothered about all those extra cm's myself

 

[osusim]

It's also depend from water temperature. Suunto promised 3% only at 20C.
In a past we calibrated one D3 in Hemmoor (fresh water) and this one have shown almost correct depth till 50m (about 4C).

 

[apneaboy]

I suppose the density changes again in less than 4 degrees as water starts to crystalise

 

[Bill]

Yeah. Water has some magical properties. I googled a temperature/density chart and it's weird. I read an article that used it as an argument in the design, evolution and religion debate.

 

[whucash]

I didn't know that type of water matters..
Normally I dive in lakes, cold lakes and reading your comments the temperature and its, lets say ingredients could change the reading even 1 meter..
I think this 'thing' is connected to all depth gauges..the model doesn't matter?

 

[apneaboy]

Most computers are calibrated initially to Salt Water for safety.

One massive change you will notice getting into the sea is that you need more weight, the more salty the sea the more weight you need. The Red Sea is a pretty salty sea where as the Med isn't as salty.

You can really tell the difference in speed of between salt and fresh, its liks soup the Red Sea compared to fresh water, it takes a while to get used to the difference.

 

[Redfish1978]

Thanks for the replies. I was looking for a more technical answer and I found it in the Navy Dive Manual. I will share:

(For columns of water at 15 degrees C):
-Saltwater weighs 64.0 lbs per cubic ft.
-Freshwater weighs 62.4 lbs per cubic ft.
-That makes saltwater 1.0256 times as dense as freshwater
-A saltwater calibrated depth gauge reading of 71 ft in freshwater would more likely indicate a depth of 72.8 ft. For a 200ft freshwater lake depth gauge reading, an actual depth of 205ft is more likely.

Summary: Doesn't make nearly as big of a gauge reading difference as I had initially thought. -But I thought it would have because of the clearly obvious buoyancy differences between salt and fresh water.....

Source: Navy Diving Manual Rev. 6
Table on page 2-31
Download: SEA 00C3 Diving Publications and Technical Documentation
(10th item down on list)

 

[Bill]

OK. If I'd known that you wanted a technical answer to four decimal places, I wouldn't have rounded it off. I admit to disagreeing with the navy to the tune of 0.44% but if you want to get picky, then the gauge that reads 71 (if interpreted properly) means between 71.0 and 71.9999 which corrects to 72.8205 and 73.8461.
I'll stick with my 1.03 correction factor.

 

[apneaboy]

OK. If I'd known that you wanted a technical answer to four decimal places, I wouldn't have rounded it off. I admit to disagreeing with the navy to the tune of 0.44% but if you want to get picky, then the gauge that reads 71 (if interpreted properly) means between 71.0 and 71.9999 which corrects to 72.8205 and 73.8461.
I'll stick with my 1.03 correction factor.

rofl


205.12FT SURELY?

Saltwater densities change, but anyway.

The less technical answer is you are not quite getting to the bottom of a pond, its not your computers fault you are so shallow

 

[samdive]

I keep getting asked this question and the answer I always give is

HOW LONG IS A PIECE OF STRING

If you're diving competitively, or pushing PBs - you should be diving on a line. Measure the line, go to the bottom of it and hey ho, that's your depth - fresh or salt!!

 

[rxcnc2]

HOW LONG IS A PIECE OF STRING

If you think about it, a piece of string is not particularly accurate once you soak it, stretch it, and add some current. I know that competition lines are supposed to be measured wet and stretched... but think again.