Fresh water vs salt water weighting
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Schwim
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I think it's about 2-3% (If I'm wrong someone will chime in). I just moved from fresh to salt and needed to add 4lbs to an otherwise unweighted SS BP/W setup.
To ballpark it:
The difference in density between salt water and fresh water is about .025%, so the fresh water you and all your stuff will displace is going to weigh about .025% less than the same amount of salt water. (In order to get completely submerged you will have to displace the same amount of water, which is why you need more weight to submerge in salt water.) All you have to remember when making the adjustment from salt to fresh (or fresh to salt) water is that the adjustment has to be made on the TOTAL weight of EVERYTHING you are taking into the water - you AND all your stuff.
Therefore, if you already know how much weight you need with the particular combination of equipment and stuff that you are diving in salt water and are going to be diving in fresh water (obviously none of this works if you are also going from a 3mm suit to a 7mm suit, etc.) - take ALL of that stuff (including the weight you carry in salt water), and weigh it in the air. Add your body weight. (You can put it all on and stand on a scale and do it all at once.)
Multiply the total weight by .025. If you are going from salt to fresh water, subtract that much from the weight you were carrying in salt water. If you are going from fresh water to salt water, add that much weight.
If the combined weight of everything is 200 pounds, you would subtract 5 pounds; if the total weight in air is 270 pounds, you would subtract 6.75 pounds, and that should put you in the range for fine tuning.
The difference in density between salt water and fresh water is about .025%, so the fresh water you and all your stuff will displace is going to weigh about .025% less than the same amount of salt water. (In order to get completely submerged you will have to displace the same amount of water, which is why you need more weight to submerge in salt water.) All you have to remember when making the adjustment from salt to fresh (or fresh to salt) water is that the adjustment has to be made on the TOTAL weight of EVERYTHING you are taking into the water - you AND all your stuff.
Therefore, if you already know how much weight you need with the particular combination of equipment and stuff that you are diving in salt water and are going to be diving in fresh water (obviously none of this works if you are also going from a 3mm suit to a 7mm suit, etc.) - take ALL of that stuff (including the weight you carry in salt water), and weigh it in the air. Add your body weight. (You can put it all on and stand on a scale and do it all at once.)
Multiply the total weight by .025. If you are going from salt to fresh water, subtract that much from the weight you were carrying in salt water. If you are going from fresh water to salt water, add that much weight.
If the combined weight of everything is 200 pounds, you would subtract 5 pounds; if the total weight in air is 270 pounds, you would subtract 6.75 pounds, and that should put you in the range for fine tuning.
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