Chanel J12 Marine © Chanel
Water-resistance means a watch is sealed against dust and water. It refers to the pressure the watch can withstand, measured in atmospheres or bars. Pressure is a force applied to a surface where 1 bar = 1 kg / sq. cm.
There are two types of pressure:
- Relative or hydrostatic pressure which is the pressure exerted by water.
- Absolute pressure which is water pressure + atmospheric pressure.
The water-resistance indicated on a watch usually corresponds to relative pressure. During diving, the pressure on the watch is absolute pressure. At sea level (zero altitude), atmospheric pressure equals 1 bar and increases by 1 bar for every 10 metres below sea level.
- 10 metres = 2 bars
- 20 metres = 3 bars
- 100 metres = 11 bars
Freshwater has a density of 1, meaning one litre of fresh water weighs 1 kg.
Saltwater has a density of 1.026 because of the salt it contains.
In reality, relative pressure increases by 0.98 bar in freshwater and by 1.007 bars in saltwater.