· By Buffy
Water Resistance
What 30m, 100m, and 200m actually mean, why depth ratings lie about swimming, and how gaskets age you out of the pool.
Depth ratings on watch dials are among the most misunderstood numbers in consumer products. 30 m does not mean you can dive to 30 m. Here is what the numbers mean and what actually keeps water out.
The rating table (ISO 22810 convention)
| Marking | Means | Actually OK for |
|---|---|---|
| 3 ATM / 30 m | splash resistant | rain, hand washing |
| 5 ATM / 50 m | brief immersion | swimming carefully, shower (debatable) |
| 10 ATM / 100 m | swimming/snorkel | pool, surface swimming |
| 20 ATM / 200 m | serious water | recreational scuba with a dive-rated crown |
| 250 m+ / “Diver’s” | ISO 6425 dive watch | diving, tested as equipment |
The ratings describe static pressure in a lab. Real swimming adds dynamic pressure (a dive into water, a hard stroke) many times the static value — which is why the “don’t swim with 3 ATM” rule exists.
How water gets in
- Gaskets (crystal, case back, crown, pushers) — rubber ages, flattens, and dies faster with heat, soap, chlorine, and crown-pulling.
- Crown and pushers — pulled/pushed underwater is the classic self-inflicted failure. Screw-down crowns (keyless works) help; locked crowns are the real rule.
- Case condition — a caseback gasket pinched at last service; a crystal seal that lost its groove.
What to do
- Pressure test after every service and annually if you swim; it’s cheap.
- Rinse salt water; never operate pushers submerged unless the manual says so.
- Warm-to-cold transitions (pool → AC) fog crystals via condensation on the inside — often the first sign of a tired seal.
- Vintage watches: assume zero water resistance regardless of marking.
For the machinery these seals protect, see the Anatomy series; for the maintenance calendar, Servicing.