Salinity, density, Bermuda [and BS]

[sam1]

Over the Fourth I was in Bermuda and did two two-tank boat dives. The DMs were telling customers that the water in Bermuda is saltier than normal and to add weight to whatever one thought his/her standard configuration was. (I believed this on day one—or at least went along thinking that it might be true—but I did a little research before day two and some more since returning home.) I have learned that the relationship between the salinity of water and its density is more complicated than I thought (i.e., density is a non-linear function of pressure, temperature, and salinity). However, to make a long story short, from what I can tell from a NOAA surface salinity map, the salinity around Bermuda is about 36.6 (grams of salt per kilogram of sea water); average sea water salinity is about 35, and salinity throughout much of the Caribbean varies from 35 to 36. At the surface, sea water with a salinity of 35 (at a temperature of 25 C = 77 F) has a specific gravity of 1.023343 (2.3343 percent heavier than an equivalent amount of fresh water). A salinity of 36.6 has a specific gravity of 1.024553. Thus, water with a salinity of 36.6 is 0.12% (just a bit more than one tenth of one percent) denser than water with a salinity of 35. So a diver with a total weight of, say, 225 pounds in water with a salinity of 35 would need (only) about 0.25 pounds more lead in water with a salinity of 36.6.

All this is to say that I have come to believe that the salinity claim is designed to make sure divers have more lead than they need in order to speed up the diving process.

 

[spschneer]

Maybe they're comparing to either Caribbean (heavier exposure protection for 75 vs 85 degree water) or quarry/lakes (freshwater/saltwater)...I've heard some Caribbean divemasters say the same thing and assumed it's for the tourists that usually dive other conditions...(specifically fw vs sw).

As far as I can guess, the only two places that salinity/density "might" make a difference are the Dead Sea and Great Salt Lake (both have higher than seawater salinity due to geographic isolation/empoundment)...but since I've never been to either, I really don't know....

 

[Charlie99]

What type of weights did they have?

The density difference that I've noticed is undersized weights made out of old car batteries.

It sounds better to say "our water is more dense" than to say "we did a poor job of making weights, so add a few extra pounds".

 

[ClayJar]

All this is to say that I have come to believe that the salinity claim is designed to make sure divers have more lead than they need in order to speed up the diving process.

I dare say that is almost certainly correct.

I've also seen almost the inverse. A shop was telling everyone to "add 3-4 pounds" of lead between diving a spring and diving out in salt water. I did the math once on myself, and I calculated my buoyancy would change by *7* pounds from fresh to salt water. If all else remained unchanged and they were properly weighted in fresh water, they would be buoyant in salt water.

As a rule of thumb, perhaps it works for new student divers, as there's a good chance they're overweighted in the spring, especially if they relax more by the time they hit salt water. On the other hand, someone who is nicely relaxed in fresh water who takes their dialed in weighting and adds 3-4 pounds for salt water will end up being underweighted, which your observed "Bermuda rule" would work around.

(Frankly, I'd much rather they understand the why behind everything than to have to keep making up rules to cancel the effects of previous rules, but hey, fudge factors are easier, eh? :biggrin

 

[sam1]

What type of weights did they have?

Conventional (solid) lead weights. They performed like other weights with the same ostensible weight.

 

[spectrum]

of 35 would need (only) about 0.25 pounds more lead in water with a salinity of 36.6.

All this is to say that I have come to believe that the salinity claim is designed to make sure divers have more lead than they need in order to speed up the diving process.

We got the same line from Blue Water Divers when we dove in Bermuda.

They also boasted of their home made weights made from wreck & ammo salvage. They were notoriously under poured and anything but precise. I think as you suspect that the salinity line is another may of making sure divers are weighted so they can get down and stay down.

Pete