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Watches made for normal use are not suitable for underwater use. Both mechanical and electronic watches contain a number of essential components that can rust. Rust damage is mainly found at the entry point of the adjustment and winding button or under the watch face. If the back of the watch has a sign indicating that the watch is to some degree waterproof, this means that special protection features have been added to the watch to protect it against the entry of moisture. In these watches the glass is contained within a plastic coating. The back cover and loose glass edging are also treated with a seal, as is the crown (adjustment knob) and any other buttons. These buttons are mounted on an axis that runs through a tube sticking out of the watch. The button contains a gasket that closes round the tube. In some models this closure is inside the rim of the watchcase. In practice the following markings are used:

  • 3 ATM / 30 meter (also water-resistant / water sealed): these watches are resistant to rain and splashes.
  • 5 ATM / 50 meter (also waterproof): these watches are suitable for use in the shower or for swimming, as long as there are no wide variations in temperature. They are, however, not suitable for snorkelling or diving with compressed air.
  • 10 ATM / 100 meter (also waterproof): these watches are suitable for most kinds of water sports, such as swimming, sailing and shallow water snorkelling. They are not suitable for high diving or diving with compressed air. Wide temperature variation should be avoided.
  • 20 ATM / 200 meter (also waterproof / diving proof): these watches are suitable for all kinds of water sports and diving.

Watches used for hobby or (semi) professional diving have to meet a test level of minimum 15 atm. These watchcases can resist high water pressure and changes of temperature. The glass is thicker, the watchcases are heavier and the closing gaskets are stronger. The back is closed with a screw seal. The crown, where a number of closures are found is much larger than in other watches and needs to be screwed tight after use to ensure better closure and to reduce the risk of damage from blows. To guarantee waterproofing in these watches, regular professional care is needed. Maintenance at least once a year is needed. 

Temperature changes and holidays

During the holiday period temperature changes are a treacherous factor that causes a lot of leaks in so-called waterproof watches. A ‘waterproof’ watchcase intended for daily showering can fill with water if the user goes swimming with it on holiday. The cause of this is as follows: When the watch is used in the shower, the water is approximately the same temperature as the watchcase. If the watch is worn in a tropical climate the temperature can rise to 40 to 50 degrees Celsius. If the watch suddenly comes into contact with swimming pool or seawater, with a temperature of 20 to 25 degrees Celsius, the watch case receives a severe shock from the sudden variation in temperature. A watchcase that is not made to withstand these conditions can then leak and fill with water.
Beware, salt water can have a very damaging effect on watches and jewellery. 

Changing the battery

Changing the battery in a waterproof watch involves re-waterproofing the watch. Not doing this can invalidate the terms of the warranty. In case of water damage to a waterproof watch within the warranty period, this will be evaluated by the Sanjoya repairs department to see whether this is the result of an unauthorised repair or incorrect battery replacement.