- The case back – this is how the case back is attached to the watch.
Snap-on case backs are pressure sealed and are considered the least waterproof. The slightest nick in the case or a deformation of a seal (which will occur over time) will allow water to enter the case. In general, these watches have a maximum water resistance of 30m/99ft – which allows contact with water, such as washing hands, but not immersion.
The second level of water resistance is the case backs, which are secured with screws. The fact that the case back is screwed on provides a much tighter seal than with a snap-on back, but deformation of the seal still allows water to enter. In general, these watches have a water resistance of up to 100m/330ft – allowing for light immersion such as swimming in a pool.
The screwed case backs are threaded and screwed into the case itself. This creates a double seal, using both the thread and the gasket as a seal coupled to the deep water pressure. Typically, dive watches with a water resistance of more than 100m/330ft are fitted with this type of back.
- Crown – the most important factor in ensuring water resistance.
The weakest link in a watch for water penetration is the hole in the crown stem. The crown stem is attached to the movement through a hole in the edge of the case. As the crown is constantly moved to different positions, wound and turned to correct the time, the seal is constantly compressed, rubbed and stressed. The slightest variation in the shape of the gasket or if the crown is not pushed in fully will allow water to enter the watch through the hole in the stem.
Screw-in crowns are threaded and screwed to a corresponding threaded tube in the case. The crown has a gasket that is compressed and seals the opening when the crown is tightened – thus ensuring water resistance. A screw-in crown is an essential feature for any watch you intend to swim with. In fact, we do not recommend swimming with a watch that does not have a screw-in crown. Regardless of whether the watch has a screw-down crown and chronograph pushers, the crowns and pushers should never be pushed, adjusted or operated while the watch is submerged in water – unless otherwise specified by the manufacturer. An additional advantage of the screw-in crown is that the crown is somewhat better protected against accidental shocks.
- The seals
“O-rings” are made of rubber, nylon or Teflon and form seals where the crystal, case back and crown meet the watch case. If the watch is a chronograph, the chronograph pushers will also have seals to protect them from dust and dirt. Check with your manufacturer if you are unsure whether your watch pushers can be operated in wet conditions.
Seals begin to erode and deteriorate over time, reducing the water resistance of a watch. It is important to test the water resistance of your watch once a year. Any competent watchmaker should have the basic equipment necessary to test the watch – the cost of doing so should be minimal.
Real life and water resistance
When a watch is tested by the manufacturer, this is usually done in a laboratory under optimal conditions, for example with a new gasket, standing still in a pressurised water tank and with still or no water movement. However, in reality, the results are completely different. Here are some scenarios:
Water temperatures in a hot bath or shower will affect the shape of the seals. Especially if the watch is removed from a hot temperature and immediately immersed in cold water – such as when moving from a hot bath to a swimming pool.
Sudden and rapid changes in pressure – such as diving (even shallow) into a swimming pool, the force of dipping the arm into the water while swimming, will stress the seals for a fraction of a second. If the gaskets do not meet the specifications, they may break and cause water to enter the watch.
As the watch ages, the gaskets begin to erode and do not maintain the same levels of water resistance.
Water resistance or water resistance
The FTC (Federal Trade Commission), which monitors the truthfulness of advertising, deemed the term “Waterproof” inappropriate. According to the FTC, a watch can never be truly 100% waterproof, as the seals deteriorate with time and exposure, reducing the specified depth of water resistance. In the words of the FTC: “The word proof connotes a measure of absolute protection that unfortunately does not exist with respect to watches, especially over extended periods of time.” The FTC felt that the term Water Resistant was more appropriate.
Pressure-based water resistance units
Meters/feet: This is the most common way to measure the water resistance of a watch. 100 metres is just over 328 feet.
ATM: This is the atmospheric pressure. At sea level, the ATM index is 1, which corresponds to 10 metres.
Bar: The bar is a metric unit of pressure named from the Greek word Baros meaning weight. It is not commonly shown on a watch as metres/feet and ATM are more widely understood by the general public. 1 bar is equal to 100,000 pascals, 14.5 PSI (pounds per square inch) or 0.98 ATM.
Helium exhaust valve
The helium escape/emergency valve is used only by deep diving expeditions when a diver is operating from a diver bell. When the bell is lowered, the pressure begins to rise and helium is added to the breathing mixture. Oxygen becomes toxic at higher pressure, for example, pure oxygen becomes toxic at 6 metres. Normal air would become toxic at the pressures at which technical and saturation divers usually work. They usually use a three-gas mixture (oxygen, nitrogen and helium), called Trimix. Helium is an important part of the mix because it has no narcotic effect on the body, thus reducing the risk of nitrogen narcosis. In addition, helium has a lower density than normal air, which means that it is also easier to inhale underwater.
The problem is that helium is one of the smallest molecules and will seep into the watch through the seals until the air pressure in the watch equals that of the diver’s bell. When the plunger bell rises to the surface and decompresses, the helium molecules inside the watch expand, and if there is no valve, the pressure in the watch will cause the glass to shatter. To prevent this, Omega and Rolex have developed their own helium escape valve system that allows the helium to escape. Omega uses a second crown that screws onto all Seamaster watches, except for the Aqua Terra which does not have one and the Ploprof which uses the automatic helium escape valve design common to Rolex dive watches. Many brands use the escape valve in one form or another. In general, the escape valve is found on watches with a water resistance of 300m or more.
Interpreting depth ratings
Even if a watch is classified as water resistant to 30m, this does not mean that it can be submerged to that depth. The depth rating displayed by the manufacturer is theoretical in nature and can only be achieved in the perfectly optimal environment of a laboratory – which is impossible to replicate in real life. It is also important to consider that watches such as the Rolex Datejust and Datejust II models can reach a depth of 100 metres and therefore be used for swimming, but they were not designed as full-fledged diving watches and therefore cannot survive prolonged experiences at more than 50 metres. You should also consider the chemicals that may be present in the water and the possibility of them reacting to the materials used in the construction of the watch.
Water resistance guide
No rating (dust/moisture proof) – 30m/99ft
Does not allow contact with water
30m/99ft – 50m/165ft
Allows contact with water such as hand washing and rain
50m/165ft – 100m/330ft
Allows for light swimming in the pool and swimming to the bottom of the pool.
100m/330ft – 200m/660ft
Allows you to swim and snorkel
200m/660ft – 500m/1650ft
Allows for impact water sports such as surfing and scuba diving.
Suitable for technical diving
Obviously, the higher the classification, the more suitable the watch is for deep diving.
IMPORTANT: We strongly recommend purchasing a watch with a screw-in crown if you intend to wear the watch when in contact with water.
Have your watch tested in water once a year.
Never open, wind or operate the crown while in water.
Never press the buttons of a chronograph watch while in water – unless otherwise specified by the manufacturer.
Do not subject your watch to extreme temperature changes.
Do not subject your watch to sudden and rapid changes in air pressure.
Do not allow your watch to come into contact with corrosive chemicals, such as abrasive soaps and highly chlorinated water.
Make sure the crown is always pushed in, and if you have a screw-in crown, make sure it is always tight. Double check before immersing in water.
What about a bath or shower?
You should not wear your watch in the toilet if you are in the habit of washing daily, even if it is a dive watch. The warm air in the room causes the watch’s seals to expand, the thermal shock that occurs when you step out into the cooler air of the rest of the building causes the seals to contract and a small amount of moisture to enter the case. After some time between maintenance, the watch hands and dial may rust, even though the watch is designed to withstand 300m or more.