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Weighting and buoyancy in fresh & salt water December 26, 2008

Posted by Chris Sullivan in Technical Diving.
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PADI divers learn to adjust their weighting by making sure that they float at eye level with an empty BC and while holding a normal breath. This is supposed to be done with an almost empty tank (something I rarely see actually done) as an 80 cubic foot tank holds about 6 pounds of air. Alternatively, you can do the procedure with a full tank then add 5 or 6 pounds. I always check to see that the student isn’t kicking and that their BC is really empty.

Another thing that a student is taught, although not with a lot of emphasis in initial underwater training, is to control buoyancy by controlling the amount of air in the lungs. Tidal Volume, which is the amount a person normally breathes in an out is a mere 500ml, which displaces 1/2 kg (just over a pound) of water. Vital Capacity, the amount you can breathe in and out from very full to really empty, is 4.6l, displacing 4.6kg or about 10 pounds – which is quite a variation in buoyancy (just imagine dropping a 10 pound weight belt).

On the weight check, the top of the head is sticking out of the water. Up to eye level, the diver is displacing a volume of water equal to his own weight. By breathing out, if the diver reduces his volume equal to the displacement of the top of his head, he becomes neutrally buoyant. By breathing out some more, he starts to sink. (A properly weighted diver  will just sink by emptying his BC because he is carrying 5 extra pounds of weight of air in his full tank.)

The concept of displacement can be used to estimate the amount that weight would change from going from fresh to salt water. Assuming you have the same equipment configuration, including the thermal protection, if you are weighted corrected then you will approximately displace your total weight of water. In fresh water this is 62.4 pounds per cubic foot of volume. So if you weigh 170 pounds, your equipment weighs 70 pounds and you required 25 pounds of weight in fresh water, you and your equipment displaces about 265/62.4 cubic feet of water. Multiply this by 64 pounds (the density of sea water vs. the 62.4 for fresh water) and you will know the weight of sea water you displace. In this case the volume is just over 4 cubic feet so the extra density of sea water will mean that about 6 1/2 more pounds of weight will be required to achieve the same buoyancy.

This number isn’t going to vary all that much with equipment changes. Unless you’re going to be going down with 4 tanks, a scooter, and tons of other gear you’re not going to vary your  that much even going from basic 3mm Caribbean configuration to full northern dry suit. Maybe 40 pounds. That equates to only about 1 pound difference between the extra weight needed.

For exact amounts you can use Calchemy, which is a free program available on the web that’s useful for Scuba calculations (once you get the hang of it). I’ve written about this program a couple of times before for doing other Scuba related calculations. If you enter the following expression in the text box labelled Expression:

265*pound*(dens_sea_water-dens_water)/dens_water

and the word pound in the box labelled Result Unit, then click Evaluate, it returns the value 6.625 pounds. This is precisely the amount of extra weight needed for a 265 pound object that is neutrally buoyancy in fresh water to be neutrally buoyant in sea water. By substituting kilogram for pound in both places, the calculation converts to metric. Either way, if you leave out the number 265 then the result (for 1 pound or 1 kilogram) will be .025 or 1/40th. This means that from fresh to sea water, 1/40th of a pound of extra weight is needed for every pound of weight of the diver + equipment.

To go from sea water to fresh water you just substitute one for the other everywhere in the expression:

265*pound*(dens_water-dens_sea_water)/dens_sea_water

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Comments»

1. Help on Dry suit weighting??? - Scuba Forum - Scuba Diving Forums and Discussion Board - December 26, 2008

[…] I have written this up in a bit more detail in my blog. […]

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2. Scuba Diving Calculations on Demand « Chronicle of an older diver - January 17, 2009

[…] Salt water is more dense than fresh water so objects, including divers, will be more buoyant. Look here to figure out by how […]

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