Trích :
http://www.chronocentric.com/watches/wresist.shtml
Water resistance is specified in the depth of water measured in atmospheres, feet or meters. Here are common water resistance levels and the degree to which they protect your watch:
1 Atmosphere / 33 Feet / 10 Meters: A watch with this resistance level is protected against accidental exposure to water; for example, splashes, perspiration or accidental immersion. It should not be exposed to any water pressure.
3 Atmospheres / 100 Feet / 30 Meters: This level will easily withstand splashes or brief immersion in water. However, it is not sufficiently resistant for swimming.
5 Atmospheres / 165 Feet / 50 Meters: A watch with this level of resistance is wearable around household sinks, while playing sports and while swimming in shallow water. Do not wear it while scuba diving.
10 Atmospheres / 330 Feet / 100 Meters: This level of water resistance will allow a watch to be worn around household sinks, while playing sports and while swimming or poolside diving. Do not wear it while scuba diving.
15 Atmospheres / 500 Feet / 150 Meters: A watch with this resistance level is wearable around household sinks, while playing sports and while swimming or poolside diving. It is also suitable for snorkeling.
20 Atmospheres / 660 Feet / 200 Meters: This level of water resistance will allow a watch to be worn around household sinks, while playing sports and while swimming or poolside diving. It will also allow skin diving and scuba diving at depths not requiring helium gas.
The term "waterproof" was discontinued starting in the late 1960's. This change was brought about from several government organizations, including the Federal Trade Commission in the USA, who were investigating truthfulness and accuracy of product labeling and advertising.
"Waterproof" was considered to have misrepresented the products as more capable of preventing the entry of water under normal use circumstances than they were actually capable of. Specifically, diving-type watches never have been completely 'proof' of water entry under normal use and within the stated depth ratings. The seals that keep water out are not completely impervious and their effectiveness can be reduced over time with age, deterioration, and exposure to chemicals.
The term "water resistant" is now used to describe such watches. There are no technical differences between a waterproof watch and a water resistant watch--they use the exact same methods and technologies to keep water out. The difference is only in what term was considered to appropriate to describe it at the time it was made.