Water-tightness — sealing and pressure testing of watches
Why a waterproof watch is not waterproof forever
A watch sold as waterproof is only that until someone opens it. Every time a watchmaker removes the caseback—for example to change the battery or clean the movement—the seal is at risk, and it's the watchmaker's responsibility to recreate and verify it before the watch is returned to the customer.
§What makes a watch waterproof?
- 01Rubber or plastic gaskets at the jar base, glass and crown seal against water ingress
- 02The crown's type is important: a screw crown that is screwed against a gasket in the abutment typically seals better than a regular pull crown
- 03All sealing rings age, harden, or shrink over time and thus lose their ability to seal
- 04Bolts, glass and bottom must be tensioned or mounted correctly for the gaskets to be pressed together properly
§Marking — and what it means in practice
A watch's water resistance is typically stated in meters or ATM (atmospheres) based on a static laboratory test and not necessarily on real-world movement, temperature changes, and pressure shocks in water. Therefore, there is a widespread rule of thumb in the industry that the rating should be read with some safety margin.
| Marking | Common use in the trade |
|---|---|
| 30 m / 3 ATM | Splashes and rain — not recommended for swimming |
| 50 m / 5 ATM | Brief water contact — not suitable for swimming or diving |
| 100 m / 10 ATM | Swimming and snorkelling |
| 200 m or more | Diving — only with a dedicated diving watch certified according to ISO 6425. |
§Standards behind water-tightness
Internationally, ordinary water-resistant watches are typically tested according to the standard ISO 22810, while watches marketed as actual diving watches must meet the stricter requirements in ISO 6425. A watch that is simply marked with a depth in meters is therefore not automatically a diving watch in the standard's sense, even if the number looks large.
§Pressure testing in practice
After service the watchmaker should pressure test the watch — often through a dry test in a pressure chamber where the watch is exposed to simulated pressure while watching for signs of leakage such as a dent in the glass. Some workshops follow up with a gentle wet test. If the test shows a defect, the seals must be checked again before the watch is delivered.
Water-tightness is therefore not something you assume — it is something you test, document and communicate clearly to the customer every time the watch case has been opened.