The American Watchmakers Institute has an article from a watchmaker in New Zealand and he wrote the following about waterproof and watches today"

The basic figures for the water resistancy ratings of watches are derived from ISO(International Standards Organixation) 2281 document.

Most watches will have some form of water resistancy rating printed on the dial(the watch face)or on the back of the case. Usually in the form of "Water Resistant X meters" where X is usually 30,50,100,150 or 200. Additionally the word "Divers" may be seen-this represents a higher level of water resistancy which requires different tests as outlined in the ISOspecification for Divers grade watches. Where no rating is given it is assumed that the watch is NOT suitable for immersion in water and no guarantee should be given as to the water resistance of the watch. Unless specified as a "Divers" grade watch, the watch should not be used as such.

  With regards to the pharse "Water proof"-This is an outdated expression which is no longer in use in the industry as it implies that a watch is completely impervious to any form of leakage regardless of conditions, this is not the case, as given extreme enough conditions(e.g. extereme ocean depths) there is virtually no way of making what is essentially a container such as a watch case completely watertight. The expression "Water resistant" is now used instead which can be taken to mean that it only resists water and given appropriate conditions this resistance may be overcome and the watch may leak.

The words "Water Resistant" and the "depth" rating do not directly rate to each other. To be eligible to be certified as "Water resistant" a watch must pass a combination of 5 of 6 tests. These test are as follows:

1) Resistance to air overpressure- A watch should not leak when subjected to a pressure of 2 bar at a rate of more than 50 micrograms of air per minute when put into a pressurized dry chamber.

2) Condensation Test- The watch is heated to a temperture of 40 degreesC to 45degreesC then a drop of water of 18degrees to 25dgrees is placed on the glass. After approximately 1 minute no condensation should have formed om the inside of the watch glass.

3)Resistance when immersed in water at a depth of 10cm-The watch should shoe no signs of leakage when left in 10cm of water for 1 hour.

4)Resistance of operative parts- The watch should not leak when a force of 5 Newtons is applied to the crown or other operating buttons at an angle perpendicular to the casing while the watch is immersed in 10cm of water for 5 minutes.

5) Resistance to different temperatures- At a depth of 10cm in water the watch should be heated successively to 40 degressC for 5 minutes, then 20 degreesC for 5 minutes and then again to 40degreesC for 5 minutes. The watch should show no signs of leakage at any stage during the test.