Freshwater VS. SaltWater ???

[Scuba-Quebec]

I made about 65 dives in freshwater until now but I never dived in cold saltwater before....is there a simple way for me to calculate how much weight should I bring with me to be sure not missing some? I'm usually wearing 12lbs lead with my full suit in freshwater....

Thanks everyone..

 

[mccabejc]

You can start by adding about 6 lbs, and tweak it from there. I've heard people say anywhere from 4-8 lbs, but I think if you calculate it you'll get something closer to 6.

 

[SquattingRadishDM]

1 litre freshwater = 1kg
1 litre saltwater = 1.03 kg

You will displace a 3% heavier volume of water in saltwater and therefore you should theoretically be 3% more bouyant. I would suggest you take an extra pound and that should be enough.

-Jack

PS remember to rinse you gear well after diving in saltwater!

 

[SquattingRadishDM]

oops just read post above mine by mccabejc, is my reasoning correct about the 3% incrase in bouyancy?

 

[AzAtty]

For the precise calculation, look at this article.

Otherwise, look at this thread, or just add between 4 and 7lbs to your freshwater weight.

 

[Charlie99]

oops just read post above mine by mccabejc, is my reasoning correct about the 3% incrase in bouyancy?

Yes you reasoning is correct, although salinity and therefore density varies a bit. Another common figure is 2.5% denser than freshwater.

Don't forget to apply the 3% or 2.5% correction to ALL of the weight, including you, your gear, tanks, lead, etc.

A typical 180 diver with 60 pounds of gear will need a weight change of about 6 pounds for saltwater.

This assumes of course, that everything, including the type of tank and thickness of wetsuit remains the same.

 

[av8er23]

that is a confusing formula on that link that azatty posted

 

[eponym]

that is a confusing formula on that link that azatty posted

Well, Sigma, the summation symbol, can throw a chill into the stout-hearted but higher-math-challenged (the differently-numerated?).

I'd say the math is correct. I'd also say the conclusion can be deduced directly from Archimedes' Principle. You and your gear are buoyed upward by a force equal to the weight of the water you and your gear displace. Imagine putting the water you and your gear displaced into a big bag--salt water will weigh about three percent more than fresh, so your buoyancy will differ by three percent. Unfortunately, the math doesn't tell you what your volume is, and that's a hard thing to measure outside of a lab tank.

If your volume fully geared up is three cubic feet (average-sized human plus tank), that means an increase for salt water of .03 times 3 time 64 or about six pounds. Hence the six-pound rule of thumb.

Practically: take four extra two-pound weights down with you. Descend a little and swim around to work the air out of your wetsuit and gear. Find a clear (sand, rock) spot about 16 feet deep. This is the optimal depth because you want to be weighted correctly for your 15-foot safety stop, where your wetsuit is more buoyant than it will be deeper down.

Station your buddy close to you. Hover just off the bottom. If the weights are on a weight belt, take it off and hold firmly to one end. Lay the belt on the bottom very slowly, so one weight after another rests on the bottom instead of holding you down. (If it's loose weights you're carrying, drop them one by one.) Do this slowly. Vent air from your BC as necessary while you do this, so you stay just off the bottom.

When you can still hover there but all the air is out of your BC, any weights still resting on the sand are not needed to keep you down with your current air level. To this minimum weight for you and your current exposure suit and gear, you should (I do) add enough weight to account for the fact that you will breathe out some or most of the weight of the air in your tank.

An AL80 holds about six pounds of air when fully charged (3000 psi). If you drained it fully, you'd need six pounds extra weight to insure you didn't become a buoyant missle shooting uncontrollably to the surface. The 500 psi you'll surface with weighs about one pound.

I blew a safety stop on my eighth dive with my new drysuit. I hadn't added weight to account for losing tank air, and when the tank got below 500 psi during my safety stop I started up without wanting to. Good lesson to learn, wrong way to learn it.

-Bryan


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