fresh water to salt water depth conversion?

[OzeDiver]

my bottom timer/depth gauge is calibrated to fresh water so it says i am slightly deeper than i actually am (all of my diving is in salt water). Just out of curiosty is there anyway i can convert the depth? from 61 ffw to fsw?

thanks

 

[scubatoys]

Sure, it takes 33 feet of saltwater to make 1 atm of pressure, and 34 feet of fresh... so basically, you are one foot off for every 33 feet of depth...

So 3 feet off at 100 feet... pretty insignificant - so most people just ignore it.

In case you cared, the reason is because salt water is more dense due to the dissolved salts, so a cubic foot of fresh water weighs approx 62.4 lbs, while a cubic foot of salt water weights 64 lbs.

This is also the reason you need to add about 5 lbs of lead when moving from fresh to salt to maintain your buoyancy.


Quick edit: your answer is 59.2... just take your ffw depth divided by 34, then multiply by 33... Make sense?

 

[OzeDiver]

oh ok i didn't now the difference was so insignificant, thanks for the help anyhow

 

[Walter]

Good response, Larry.

And you're really measuring pressure, not linear depth anyway. The linear depth is slightly more than 33 feet in salt water and slightly less than 34 feet in fresh water at 1 ATM. Also, the salinity of the ocean is not uniform from place to place nor constant even in the same place, so the linear depth for 1 ATM varies anyway. FFW and FSW are units of pressure, not distance.