the words "DIVER'S WATCH xxx M FOR MIXED-GAS DIVING". The letters xxx are replaced by the diving depth, in metres, guaranteed by the manufacturer. The composition of the gas mixture used for the test shall be given in the operating instructions accompanying the watch. Most manufacturers recommend divers to have their diving watch pressure tested by an authorized service and repair facility annually or every two to three years and have the seals replaced. Water resistance classification Watches are often classified by watch manufacturers by their degree of water resistance which, due to the absence of official classification standards, roughly translates to the following (1 metre ≈ 3.29 feet). These vagueries have since been superseded by ISO 22810:2010, in which "any watch on the market sold as water-resistant must satisfy ISO 22810 – regardless of the brand." See also IP Code Watch Diving watch Magnetic-resistant watch Shock-resistant watch Certified chronometer References External links Watch Engineering - How Does Water Resistance Work?... Certification marks Watches