Buoyancy I’m confused

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Did you trim the upper part of the farmer John off to form vest and pants and leave the vest off? Otherwise your torso has 5 mm + 5 mm on it, so 10 mm. So let’s say that’s 8 mm average for 16 pounds. Add 4 pounds for empty Al80 buoyancy. 20 pounds. Your 9 pound backplate not as dense as lead, so 8 pounds lead equivalent for buoyancy, so back to 12 pounds on the belt.
 
But it does overlap some
 
I’m envious that you have a pool to work these things out. I can use a general access pool only for swimming, so my weighting estimates for different suits are based off of guesses and my dive logs.
 
We don't have specific data for each of our suit sizes and thicknesses. If someone is curious, they might be able to place the suit in a mesh bag and add weights until it sinks. If you make sure to eliminate trapped air pockets, it should give a good estimate of the suit buoyancy. A brand new suit will have more buoyancy than one that is "broken in".

If the pool is warm enough, a diver who is curious, might also try to determine their weighting needs without a wetsuit or any gear at all. There is a good bit of variability in the buoyancy between individual divers.

There was also a question of how to use the freshwater weighting and translate that to saltwater. Since salt water weighs about 64 lbs per cuft and freshwater around 62.4, the difference is 1.6 / 62.4 = about 2.5%.

So you weigh (or estimate) ALL your gear (including your freshwater weights and yourself) and multiply that by about 2.5%.

So for a 200 lb diver with 10 lbs of lead and 60 lbs of gear, they would need 0.025 * (200 + 10 + 60) = about 7.

Around 7 lbs extra lead would be used in saltwater.

Edit: I just looked up another reference and (in the metric system) freshwater has a density of 1 and sea water about 1.025 (g/ml). So that also shows that the difference is around 0.025 or 2.5 %.

 
M-squared? Now you're just being a math showoff
I'm not showing off. MM is MxM which is the same as M². However, Mathematics is one of my degrees.

Your 9 pound backplate not as dense as lead, so 8 pounds lead equivalent for buoyancy,
Are you serious? 9# of steel = 9# of feathers = 9# of lead. In the water the feathers would probably have some positive buoyancy due to trapped air. Density has NOTHING to do with it when it comes to steel and lead.

Cheers -
 
How much contribution to buoyancy does a plastic container with a gallon of water weighing about 8 pounds contribute to a diver? Hint: It’s a lot less than 8 pounds.
 
My math was not correct. The 9 pound steel backplate will weigh about 7.9 pounds underwater (sg 8.00), while 9 pounds of lead weight will weigh about 8.2 pounds underwater (sg 11.35). The difference is just about 0.3 pounds, not one pound. 9 pounds of aluminum will weigh 5.7 pounds underwater (sg 2.73), and 9 pounds of feathers without air trapping will weigh 2.33 pounds underwater (just non mineralized protein, sg 1.35). 9 pounds of water underwater will weigh nothing (sg 1.00).
 
Just use the spreadsheet provided by @rsingler - heck I’m a Math Phd and this stuff is way too complicated for me. Give me complex topology problems any time.
 
You can see your predicted suit buoyancy on the Suit Tab, and the breakdown of components on the Calcs Tab.

Optimal Buoyancy Computer

Also keep in mind that advertised neoprene thickness does not always match reality. Better to fold a section lightly in half, measure, subtract non neoprene coating, and divide by 2. Finally, early buoyancy will be greater than late buoyancy after trapped air in the lining is squeezed out.
 
https://www.shearwater.com/products/teric/

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