Water resistance ratings as they are published and indicated on watches are very misleading. Most watches are rated at 3 bar, or 3 atmospheres, corresponding to 30 metres water resistance. So you may think, wow, 30 metres depth! No problem about taking my watch when I go swimming, or keeping it on in the shower or while washing the dishes. But in actual fact, even though a watch rated at 3 bar/30 metres is tested at that pressure, it’s tested using air and not water, and in practice, a watch with this rating is resistant only to drops of rain and occasional splashes. Certainly not for swimming or showing or doing the dishes, and absolutely not for diving at any depths, lower or higher than 30 metres.

Watches rated at 3 bar/30 metres are tested with air, not water. They are put into the machine, and air pressure is increased to 3 bar, and held there for a short time for stabilization. The actual measurement is performed over the course of a minute. In the closed chamber, if air seeps into the case, this will cause a change in the pressure difference between outside and inside the watch, and this will cause a slight deformation of the case. The machine measures this to an accuracy of a hundredth of a millimetre. A microscopic deformation of the case indicates a leak. The watch is removed, and if it didn’t pass, new gaskets are put in, and it’s retested.

So the 3 bar/30 metre rating refers to a watch with fresh gaskets, at a standard temperature, withstanding an air pressure of 3 bar for a short time. In real life, as soon as you dive into the water, there is an abrupt temperature change that will cause a slight change in case dimensions due to contraction – the case contracts more than the gasket. As you swim, the watch is subjected to micro-stresses, and the pressure of the water itself is constantly changing due to your own movements. So there is a high risk of water seeping in. Other problems may be caused by the chlorine in a swimming pool and the salt in seawater, both of which may accelerate the ageing of the gaskets. During showering, the watch is subjected to rapid changes in temperature which cause differential expansion. And soap reduces the water’s surface tension and enables it to seep in more easily.

Brief summary of water resistance ratings

There are two sets of ratings applied to watches, ISO 2281 for water-resistant watches, and ISO 6425 for diver’s watches. Diver’s watches are fundamentally different from non-diving watches in that each watch marked as compliant to ISO 6425 has to be tested in water, at 125% of the rated pressure to provide a safety margin. So a diver’s watch with 300 metres water resistance will remain watertight at 375 metres.

Water resistant 3 bar/3 atm/30 metres: Resistant to splashes and rain. Not suitable for showering, bathing, swimming, snorkelling, diving.

Water resistant 5 bar/5 atm/50 metres: Can be used for light surface water sports. Not suitable for snorkelling or diving. Some recommendations say that a watch with 50 metres water resistance can be used for swimming. Other more conservative recommendations say that you should not shower or swim with your watch unless it is rated 100 metres.

Water resistant 10 bar/10 atm/100 atm: Can be used for surfing, swimming, snorkeling, sailing and water sports. Not suitable for diving

Water resistant 20 bar/20 atm/200 metres: Can be used for swimming, intense surface water sports and skin diving. Not suitable for scuba diving.

Diver’s 100/200/300 metres: Suitable for for scuba diving, not suitable for saturation diving

Diver’s 300+ metres: Suitable for saturation diving, using a helium-enriched gas

Features important for watch resistance

Caseback. The least water-resistant type of caseback is the snap-on type, applied simply by pressure using a hand-press. Watches with this sort of caseback will be rated at 30 metres. Then there are casebacks attached by a series of screws, typically in watches with 100 metres water resistance. Watches with higher depth ratings generally have screw-in casebacks, so that both the gasket and the thread contribute to creating a perfect seal.
Crown. In a non-diver watch, when the crown is in its closed, winding position, the crown-stem hole is sealed by gaskets. But the constant compression, decompression and wear of the gaskets causes them to deteriorate, along with physiological ageing. That’s why it’s a good idea to have a watch serviced regularly – some say once a year, for most it should be at least once every few years – to replace gaskets. Screw-down crowns provide a more reliable seal, and should be a feature of any watch used in the water, for example for swimming.
Pushers. In most watches, chronograph pushers should never be operated under water. Some chronograph watches have screw-down pushers for this reason.