Converting weights from fresh to salt water

[Scotttyd]

Quest from a newbie, is there a generally accepted way to convert what weight amount you wear in fresh water to salt water diving, assuming you are wearing the same gear? I know the salinty varies from Atlantic ocean to Pacific ocean, etc. Specifically I am asking about the carribbean and NC coastal diving.

 

[AzAtty]

Going from salt to fresh, you'll probably subtract between 4 and 7 pounds of lead from your weight belt. The accepted method of determining how much weight to add or subtract is to perform a buoyancy check each time you switch. Your body weight and body composition will make that figure fluctuate.

Or, you could determine the volume of salt water you displace when neutrally buoyant, determine the difference in the weight of salt water displaced vis a vis fresh water, and subtract that amount from your weight belt.

It's simpler to do the buoyancy check.

 

[mike_s]

basically salt water is more buoyant, but only by a few pounds.

so you will most likely need a few pounds less lead for freshwater with the same gear (tank/BC/wetsuit, etc)

If you forgot to take off those few extra pounds though you'd be ok also. You'd just have to trim yourself out a little.


Best way to test this is to goto your local quarry or other freshwater dive spot with a tank with 500psi in it. This is the ending pressure of your tank at the end of a dive. you want to be completely neutral at this point.

So... get all your gear on and get in the water right at the dock. put regulator in mouth of course... then let all the air out of your BC slowly. If you start to sink below the waterline, you're overweighted. if you stay with your eyes about at the waterline you're about right (with no air in BC).

if you're overweighted, remove about 2 pounds and try again. repeat until your BC is empty and your eyes are at about the waterline. you will go up and down a little based on breathing in. But this way you're exactly neutral or at your 'peak buoyancy'.

hope that helps.

 

[Scotttyd]

basically salt water is more buoyant, but only by a few pounds.

so you will most likely need a few pounds less lead for freshwater with the same gear (tank/BC/wetsuit, etc)

If you forgot to take off those few extra pounds though you'd be ok also. You'd just have to trim yourself out a little.


Best way to test this is to goto your local quarry or other freshwater dive spot with a tank with 500psi in it. This is the ending pressure of your tank at the end of a dive. you want to be completely neutral at this point.

So... get all your gear on and get in the water right at the dock. put regulator in mouth of course... then let all the air out of your BC slowly. If you start to sink below the waterline, you're overweighted. if you stay with your eyes about at the waterline you're about right (with no air in BC).

if you're overweighted, remove about 2 pounds and try again. repeat until your BC is empty and your eyes are at about the waterline. you will go up and down a little based on breathing in. But this way you're exactly neutral or at your 'peak buoyancy'.

hope that helps.

Thanks for the info, although I do know how to perform a buoyancy check, I was just curious as to if there was a commonly accepted percentage different. IE. In saltwater you need 30% more weight,

 

[dkktsunami]

Approximation for average sized recreational diver would be 6 lbs more weight in salt water assuming the same gear is used.

 

[gcbryan]

Thanks for the info, although I do know how to perform a buoyancy check, I was just curious as to if there was a commonly accepted percentage different. IE. In saltwater you need 30% more weight,

There is, salt water is .025 more dense than fresh water so you multiply .025 times your total weight including body weight with all of your gear on and that is the additional weight needed when switching from fresh to salt water.

If you and all your gear weigh 250 lbs then you would add 6.25 lbs (.025*250lb=6.25 lbs).

 

[MyDiveLog]

Salt water weighs 64.1 lbs per cubic foot, while freshwater weights 62.4 lbs per cubic foot. This means, when going from fresh water to salt water, the water will be 2.7% more bouyant ((64.1 / 62.4) - 1). Take your body weight, add about 30# for your gear, and add your weights, multiply by .027, and that's how much more weight you should need.

Going from salt water to fresh is the same process in reverse. The water will be 2.7% less bouyant (1 - 62.4/64.1), so you need to take off about the same amount of weight.

 

[rawls]

I know the salinty varies from Atlantic ocean to Pacific ocean, etc

The best way to determine weight requirements is to get in the water and do a buoyancy check...That is really the only way you are going to know for sure.

 

[Walter]

There is, salt water is .025 more dense than fresh water so you multiply .025 times your total weight including body weight with all of your gear on and that is the additional weight needed when switching from fresh to salt water.

If you and all your gear weigh 250 lbs then you would add 6.25 lbs (.025*250lb=6.25 lbs).

This is correct.

I know the salinty varies from Atlantic ocean to Pacific ocean, etc.

The difference can be ignored unless you want to dive in the Dead Sea or Great Salt Lake.

 

[spectrum]

The best way to determine weight requirements is to get in the water and do a buoyancy check...That is really the only way you are going to know for sure.

Not really. With a few noatable exceptions (see Walter's post)physics will convert a good freshwater weight requirement to salt or the opposite. It's all about weight, displacement and specific gravity of the medium equaling buoyancy. The "same configuartion" requirement applies of course.