There are several features that help make a watch water-resistant. The most important is the gaskets, or 0 rings-made of rubber, nylon or Teflon which form watertight seals at the joints where the crystal, case back and crown meet the watch case. if the watch is a chronograph, the chronograph pushers will also have gaskets.
In addition, water-resistant watch cases are lined with a sealant, applied in the form of a quick-hardening liquid, which helps keep water out.
The thickness and material of the case is also a big factor in determining whether a watch can safely be worn underwater. The case must be sturdy enough to withstand pressure without caving in. In general, this means a steel or titanium case or a steel case plated with gold, manufacturers say. Solid gold cases can be water resistant provided they are sufficiently thick.
A screw-in case back, as opposed to one that pushes in, also contributes to a watch's water resistance. A screw-in crown, a feature of many divers' watches, helps prevent water getting into the case through the watch-stem hole. When it is screwed down it forms a water tight seal much like the seal between a jar and its lid.
Definition of Water Resistance
The different levels of water resistance as expressed in meters are only theoretical. They refer to the depth at which a watch will keep out water if both watch and the water are perfectly motionless. These conditions, of course, are never met in the real swimmer's or diver's world. In real life, the movement of the wearer's arm through the water increases the pressure on the watch dramatically; so it can't be worn to the depths indicated by lab testing machines.
The following usage recommendations are suggested by most watch manufacturers.
Water-resistant to 30 meters (100 feet). Will withstand splashes of water or rain but should not be worn while swimming or diving.
Water-tested to 50 meters (165 feet). Suitable for light water activities
Water-tested to 100 meters (330 feet). Suitable for swimming and snorkeling.
Water-tested to 150 meters (500 feet). Suitable for snorkeling.
Water-tested to 200 meters (660 feet). Suitable for skin diving.
Diver's 150 meters (500 feet). Meets ISO standards and is suitable for scuba diving.
Diver's 200 meters (660 feet). Meets ISO standards and is suitable for scuba diving.
Please note that it is not recommend go swimming or diving with your watch unless it has a screw-down crown (also known as screw-lock or screw-in crown) and is water-resistant to at least 100 meters.
Care for a Water Resistant Watch
It is not recommended to wear your water resistant watch in a shower, sauna or hot tub. The extreme heat causes the metal parts to expand at a different rate than the rubber gaskets. This creates small openings that can allow water droplets to penetrate the watch. Sudden temperature changes are especially harsh. Take care not to jump into a cold pool after wearing your watch in the hot tub.
After swimming or diving in salt water, immediately rinse the watch in a stream of fresh water. If your watch has a rotating bezel, turn the bezel several times while rinsing it. This will prevent salt buildup and corrosion of the bezel ring.
Some chemicals can corrode the gaskets and make it vulnerable. Heavily chlorinated water can cause problems, as can spray-on perfumes and hairsprays that work their way into the watch's seams and damage the gaskets. (They can also damage your watch's finish.)
Leather straps can be made to be water resistant too. Generally however, leather straps are more easily damaged by frequent exposure to water. So if you are going to wear your watch while swimming -- think of buying one with a metal bracelet or a rubber or nylon diver strap.
Finally please note this is a general guide and you should refer to the watch manufacturers recommendation in your user manual.
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