Water-resistance: 100m under water equals 11kg of pressure! ¶
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.
Here are some examples of water watches: