The points about the salinity are true. What does complicate an exact comparison ("why X kg more instead of only Y kg according to maths?") is probably that other factors (equipment, tank, dive profile) will typically change when on travel...
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I am pretty sure @victorzamora means 3,5 % of total weight (3,5 kg if diver is 100kg), and that this will have to be added as lead. Not 3,5 % of your total lead weight.So, yeah, you'll need ~3.5% of your total weight in lead in the Red Sea than in freshwater whereas you'll only need ~2.5% for typical ocean diving.
Speedos??? I thought it's considered much better to wear nothing than wearing speedos, at least on the west side of the Atlantic.Please note that the wearing nothing part of the explanation is only for accademic purposes. Out of respect for fellow divers and local culture I would add a speedo to that test.
no thats not true it is1% of the divers total weight not 1% of the previous amount of lead..Hence the difference between normal sea and the Red sea should be just 1%, or between fresh water and Red Sea 3.5%. BTW I hear similar stories about the Mediterranean sea.
The OP claims that this 1% or 3.5% difference is 4kg which can't be true unless the OP dives with 400kg of lead in normal sea or 115kg in fresh water. Doesn't make sense to me...
For a "typical" 5-10kg weight belt the difference should be hardly noticeable (100-300gr or so)...
It is a bit late here and I am tired so my maths might be a bit off...
I am pretty sure @victorzamora means 3,5 % of total weight (3,5 kg if diver is 100kg), and that this will have to be added as lead. Not 3,5 % of your total lead weight.