ATM stands for physical atmosphere and was the unit for pressure in Germany and Austria until 1977. The SI-compliant unit for pressure has been the bar since 01/01/1978. 1 ATM corresponds to 1.01325 bar.
ATM was derived from the mean air pressure that prevails at sea level and corresponds to the value 1 ATM. This pressure is caused by the weight of the earth’s atmosphere. If you now dive into the water, then for every 10 meters of diving depth, there is also about 1 ATM or 1 bar hydrostatic water pressure in addition to the atmospheric air pressure of 1 ATM or 1 bar. At a water depth of 50 meters, a watch is then exposed to an overpressure of 5 ATM or 5 bar.
What does 5 ATM mean?
5 ATM means that a watch can withstand a pressure of 5 bar or a water column of 50 meters. In practice, this corresponds to the pressure of a flowing water jet when washing hands, showering or bathing. 5 ATM does not mean that a watch is waterproof to a depth of 50 meters, but that it can withstand the pressure of a water column of 50 meters. Because when swimming, the swimming movements can cause pressure peaks that are much higher than 5 bar. Due to these pressure peaks, only watches with the designation 10 ATM or higher are suitable for swimming.
Water resistance of watches: categories and legal regulations
In addition to 5 ATM, there are also the categories 3 ATM, 10 ATM, 20 ATM, etc. We will now take a closer look at what this information means, what you have to consider and what legal regulations exist for specifying the water resistance of watches.
Water resistance classification according to ISO 2281 and DIN 8310 September 2010
The standards DIN 8310 (time measurement technology – water resistance of watches – requirements and tests) and ISO 2281 regulate the criteria for the constructions and test pressures that watches have to withstand in order to be suitable for a certain water resistance.
If there is only “water resistant” on the bottom of a watch, this means that the watch is resistant to sweat, drops of water, rain, etc. in daily use. To get this designation, the watch must survive 30 minutes in 1 meter water depth and 90 seconds in 20 meter water depth without water penetrating into the case.
If there is an indication in bar or atm after the “water resistant”, then the test conditions have been tightened and the watches have a higher water resistance or water pressure resistance.
These criteria are regulated by the standards DIN 8310 and ISO 2281:
- 3 bar ≈ 3 atm (≈ 30 m): watches with the designation 3 ATM withstand a test pressure of 3 bar for a limited period and are insensitive to water splashes when washing hands or light rain.
- 5 bar ≈ 5 atm (≈ 50 m): Watches with the specification 5 ATM withstand a test pressure of 5 bar, are suitable for daily use and can be kept on when washing hands, showering and bathing.
- 10 bar ≈ 10 atm (≈ 100 m): watches marked 10 ATM withstand a test pressure of 10 bar and are suitable for swimming and snorkeling.
- 20 bar ≈ 20 atm (≈ 200 m): watches marked 20 ATM withstand a test pressure of 20 bar and can be kept at a shallow depth for snorkeling and free diving. For greater water depths, watches should be tested for 30 bar (or more) water resistance.
As a reminder: The indication of 50 meters on a watch with the designation “water-resistant 5 ATM” does not mean that the watch is suitable for diving up to 50 meters, but that it can withstand the pressure of a water column of 50 meters.
For professional diving watches that can be used for scuba diving, the maximum diving depth is very often indicated in meters (m) or feet (ft) directly on the dial.
Waterproofness information should be checked regularly
Since the water resistance of watches deteriorates over time due to the natural wear of the seals as well as sweat or acids and generally by wearing the watch, the information on the case back only applies to new watches.
Also, strong temperature fluctuations e.g. in winter, can cause that the different materials that make up a watch case can expand differently.
If you want to wear your watch e.g. while swimming or diving and want to be sure that the watch still has the specified water resistance, we recommend that you have your watch checked for water resistance regularly. You are welcome to contact our watchmaker who has the appropriate test equipment.
A regular check of the seals prevents water damage and the associated expensive repairs.