Different Weight for Different Salt Water?

[Homunculus]

The salinity (salt content) of ocean water varies.
The oceans are about 3.5% salt. The saltiest water is in the Red Sea and in the Persian Gulf, which have a salinity of about 4.0 % (due to very high evaporation rates and low fresh water influx). The least salty seas are in the polar regions, where both melting polar ice and a lot of rain dilute the salinity.
well, dead sea is ~33% but i don't believe anyone dives there

 

[String]

Even inside the Med its noticeable to me.

Far eastern Mediterranean just up from the red sea i generally need 1kg or so more lead than in the western med off Spain. Couldnt compare to the florida keys as they used aluminium tanks which threw all my calculations off.

Most of the variation ive noticed is just operators use different brand tanks which have different buoyancy characteristics. A faber differs from a heiser which differs from a luxfer and so on.

 

[BigTuna]

The Salt content of sea water can vary from about 1.020 to 1.035

Here's the perspective I take away from your kind responses.

Another post says the Red Sea and Persian Gulf are around 1.04. Putting it all together, salt water density can range from maybe 1.02 to 1.04 times fresh water. That's significant.

Let's assume I need 6 pounds of lead to be perfectly balanced with an empty tank at the end of a dive in 1.03 salt water. That's 2 pounds for each percent of salt. So if I were to go the Red Sea, I'd need as much as 2 pounds more for its extreme salinity. Or if I were to go to a much less salty place (Hawaii?) I'd need as much as two pounds less.

Those are maxima, so for starters I might only need a pound more for the Red Sea, or a pound less for Hawaii. That's in the range of adding or subtracting a piece of gear, such as a good light, for the dive.

I wonder if I have my ballast tuned to this level of precision. Probably not. It's time to check again....

 

[Rick Murchison]

Let's assume I need 6 pounds of lead to be perfectly balanced with an empty tank at the end of a dive in 1.03 salt water. That's 2 pounds for each percent of salt. So if I were to go the Red Sea, I'd need as much as 2 pounds more for its extreme salinity. Or if I were to go to a much less salty place (Hawaii?) I'd need as much as two pounds less.

Whoa!
What weight you need for a particular salinity only comes in play as a starting point for adding or subtracting weight. It isn't used in the "difference" calculations.
What does matter is your total weight, and therefore how much water you must displace.
For example, in the case above, if you weigh a total of 300 pounds with all your gear on (when neutrally buoyant you, your gear and your weights total 300 pounds and therefore displace 300 pounds of water), then a 1% change in water density will mean a 3 pound change in the weight you need (since you'll displace the same volume of water you'll be displacing 297 or 303 - depending on whether the 1% is a decrease or increase in water density - pounds now, and so to be neutral you must weigh that much). That is, if you start with 6 pounds and you raise the water density by 1% then you'll need 9; if you start with needing 10 pounds you'll need 13; if you start needing 2 you'll need 5.
Also understand that % salinity doesn't translate directly into a % density change. Sea water, for example, has an average salinity of about 3¼ - 3½ percent, and a specific gravity of about 1.025 (about a 2½% increase over fresh water).
Rick

 

[BigTuna]

Whoa!

Whoops!
I'm blushing. Thanks, Rick.

 

[DiverBeth]

Not sure how much of a difference it makes, but when I went to Hawaii I definitely noticed the difference in the "saltiness" of the water compared to the FL keys (where I dive few times a year), both in taste and weight used. The water in Hawaii didn't burn my eyes as much and tasted less salty. Oh, and I used a few pounds less.
Just my observation.

Oh good then I wasn't crazy when I needed a couple of extra pounds when diving in Cozumel after diving in Hawaii with the same gear.

 

[muddiver]

Think of it this way...

The Mediterranean, the Gulf of Mexico, the Sea of Cortez, Red Sea, Dead Sea are all prety shallow in respect to the rest of the ocean around the world. The ocean around Hawaii is pretty deep in comparison.

So, the shallow seas near the equator not only have more evaporation due to their proxsimity to the sun, but the evaperation can be close to or equil the in flow of new water, and is magnified by the depth of the sea to surface area. Thus the higher salt content in the water as opposed to the Pacific or Atlantic.

As Rick stated though the buoyancy of a diver and the required weight to sink the diver is based on the total density of the water in question. That includes the salt content, and the water temperature, because density of water varies with temperature.

 

[spectrum]

Were the dive ops lead weights homemade? The variation in lead weights is bigger than the change needed to account for differences in salinity.

This is particularly true when the lead is scavenged from things like old car batteries.

This sounds like the spiel we got in Bermuda a year ago from Blue Water divers. The weights were home pours from salvage. I suspect the advise was more to the fact the weights were imprecise than the water denser.

I set my weights as I do here in Maine and I was fine.

Pete

 

[ronpriskie]

As Bethjd said, I too needed a couple of extra # in Cozumel. A DM who was on trip with us (from CT) said the constant current tended to push you towards the surface. I don't know if this is true, but I know I needed more # than other salty water dives I've made in the same gear config.

 

[Dive-aholic]

I wouldn't think there would be a difference in Bermuda. muddiver's post a few up is the way I've always understood it. I dive the Sea of Cortez on a regular basis and usually have to add a couple of extra pounds there. The water also does taste a little saltier. It's the shallowness and probably that these systems are all "coves" to the world's oceans.