30 m doesn't mean 30 m.
The truth about water resistance.

01 · The problemWhy the numbers lie.

The international standard ISO 22810 defines water-resistance tests for watches under static conditions: the watch is submerged in a pressurized tank of still water, without sudden movement. A "30 m" watch passes this test. But in real life, water is never still.

Swimming, diving, accidentally knocking your wrist against water — every dynamic action generates pressure spikes far greater than the nominal depth. A simple pool-edge dive can generate instantaneous peaks of 5-10 ATM. A sharp wrist movement while swimming can reach 2-3 ATM peaks. Moving water is aggressive.

The practical upshot is that the stated rating should be halved — at minimum. A 3 ATM watch is not a swimming watch: it resists splashes and rain. 10 ATM is the minimum for carefree swimming. See the table.

02 · The ATM tableWhat you can actually do.

ATM (atmosphere) and BAR are practically synonymous: 1 ATM ≈ 1 BAR ≈ 10 metres of water under static pressure. But remember: the rating is static, real use is dynamic. Here's what practice says:

Note: the BAR markings on modern watches (3 BAR, 5 BAR, 10 BAR, 20 BAR) correspond exactly to ATM. The double marking 30 m / 3 ATM you often see is the same thing expressed in two different units.

03 · The crownThe weak point of every watch.

The crown (the side button used to wind and set the time) is the most vulnerable water-entry point on any watch. The crown tube — the channel that runs through the case and lets the crown move — is sealed by gaskets, but remains structurally the most critical point.

This is why ISO 6425-certified dive watches must have a screw-down crown: it threads into the case, creating a second mechanical seal like a jar lid. Never operate the crown underwater, even on high-rated watches: setting the date in the pool or at sea is one of the most common and expensive mistakes. Always check the crown is fully screwed down before entering water.

04 · The gasketsTime destroys them.

The gaskets (O-rings) that seal the crown, caseback and crystal are almost always made of rubber, neoprene or Teflon. They degrade over time under heat, UV rays, pool chlorine and the chemicals we use daily — perfume, cream, sunscreen. A watch rated 10 ATM that has gone three years without maintenance might barely hold 3-5 ATM. When to have the seal checked:

• Every 2-3 years for watches used regularly in water (swimming, water sports).
• Every year for ISO 6425-certified dive watches.
• Immediately after a drop, a strong impact, or if the crown feels different (harder, softer) from when the watch was new.
• Before summer if you wear the watch at the beach — cheaper to check now than regret it later.

05 · WatchScopeThe timegrapher as early warning.

WatchScope doesn't test water resistance — that requires a pneumatic tool at a workshop. But it can give you an important early warning: if you've worn the watch in water and in the following days you notice an anomalous rate or a drop in amplitude, moisture may have entered the movement. What to watch for:

• Amplitude that collapses (from 260° to 190° within a few days): moisture inside the movement forms condensation on lubricated surfaces, increasing friction on the balance wheel. Act before rust damages the components.
• Very variable or suddenly high rate: moisture can alter the behaviour of the hairspring — effect similar to magnetization, but caused by physical condensation rather than magnetic fields.
• Before-and-after test: run a quick test before going to the beach and one after you get home. If parameters have changed significantly, take the watch to a watchmaker only — never open it yourself, to avoid introducing more humidity.