salt v fresh weighting

[bryanmc57]

I'm just trying to reason something out. Maybe someone can explain this to me.

Why is it when we figure out weight change from fresh to salt we take the weight of the diver and gear in fresh water (including ballast) and add 3% of the total weight (in ballast) for salt water. It seems to me if a diver is X amount buoyant in fresh water (requiring xx lbs of ballast) shouldn't we be able to calculate additional weight of ballast needed for salt water from the anount of ballast needed in fresh water without using the total weight of the diver and gear?

Maybe I'm just overthinking this?

 

[BKP]

You have to consider the total amount of salt water (and dense salt) you are displacing. It's not just your ballast displacing water, but you, your gear, as well as your weights.

If average ocean salinity is 32 parts per thousand (and fresh actually has a salinity of somewhere between 1 and 5ppt), than the average increase in density is somewhere between 2.5 and 3.0 percent. So, if you and your gear weigh 200 lbs., you need approximately 5 to 6 more lbs. of ballast to offset the additional 5 to 6 total lbs. of displacement.

 

[ronbeau]

You have to consider the total amount of salt water (and dense salt) you are displacing. It's not just your ballast displacing water, but you, your gear, as well as your weights.

If average ocean salinity is 32 parts per thousand (and fresh actually has a salinity of somewhere between 1 and 5ppt), than the average increase in density is somewhere between 2.5 and 3.0 percent. So, if you and your gear weigh 200 lbs., you need approximately 5 to 6 more lbs. of ballast to offset the additional 5 to 6 total lbs. of displacement.

Excellent explanation.

 

[Colliam7]

You have to consider the total amount of salt water (and dense salt) you are displacing. It's not just your ballast displacing water, but you, your gear, as well as your weights.

As ronbeau said, excellent explanation! If we are neutrally buoyant in fresh water with whatever weight we have added, then the amount of water we are displacing can be reasonably inferred by our total weight. To initially estimate the additional weight we may need in salt water, all we have to do is account for the difference in the weight (and therefore buoyancy) of salt vs fresh water, which is about 2.56% [(64-62.4)/62.4)], or roughly 3%.

 

[bryanmc57]

That is a great explanation... now if I could just get it to sink in. I understand the density difference , I'm just stuck on.... If we know an objects buoyancy in fresh water, shouldn't there be a way to calculate it's buoyancy in salt water without using it's total weight?

 

[Charlie99]

I'm just stuck on.... If we know an objects buoyancy in fresh water, shouldn't there be a way to calculate it's buoyancy in salt water without using it's total weight?

Not really. the objects total weight is one of the two forces working on the object.

The buoyancy of an object is simply the difference between the weight of the water it displaces when submerged and the weight of the object.

To say the same thing yet another way ------
an object is pushed upward with a buoyant force equal to that of the weight of the liquid it displaces and is pulled downward by a force equal to its weight. The difference between these two forces is the objects net buoyancy.

Yet another way to look at it is to imagine two objects that are neutrally buoyant in fresh water. One is a tiny little thing, the other is much larger. But both are neutrally buoyant in freshwater. When you take the two objects over to saltwater, the much larger volume of the bigger object means that the denser saltwater will give it much more additional lift than the tiny object. To figure out how much more lift, you need to know the weight of each object (or alternatively, the volume of each object .... but that is directly related to total weight). Just knowing that the objects are both neutrally buoyant in freshwater is not enough to tell you how much more buoyancy they will have in saltwater.