ATM

ATM (atmosphere, German Atmosphäre) is the unit commonly used in watchmaking for pressure resistance. 1 ATM physically equals mean atmospheric pressure at sea level (1.013 bar) and in watch practice roughly 1 bar or 10 metres of static water column. A watch rated "10 ATM" withstands a laboratory pressure equivalent to about 100 metres of water depth.

The figure is a laboratory value, not a real-world guarantee. This is the most frequently misunderstood point in retail conversation.

What the steps mean

Industry conventions, not strict standards:

• 3 ATM (30 m). Splash-resistant. Safe in rain, hand-washing. Not safe for swimming.
• 5 ATM (50 m). Suited to light swimming in still water. Not for diving or showering.
• 10 ATM (100 m). Safe for swimming, snorkelling, water sports.
• 20 ATM (200 m). Suited to scuba diving at recreational depths.
• 30 ATM (300 m) and above. Professional dive depths. Most dedicated dive watches sit here — Submariner, Sea-Dweller, Seamaster Diver, Tudor Pelagos.

The gap between rating and usable depth arises from dynamic pressure in real use. A dive into the pool, vigorous swim strokes, a shower stream — all produce local pressures significantly above the static value at the depth in question. A 3 ATM watch in the shower can fail because the shower stream reaches 4 to 5 bar locally.

ATM, bar, metres

The three units are used interchangeably but are not exactly equal:

• 1 ATM = 1.013 bar. ATM is the physically defined standard atmosphere.
• 1 bar = 10 metres of static water column. Bar is an SI-conformant unit, standard in Europe.
• 10 metres = 1 bar = 0.987 ATM. Practically identical, used interchangeably in watch marketing.

In Europe "Bar" appears more often on the dial than ATM; in the US and Japan, the metre indication dominates. The ISO 22810:2010 standard governs test procedures but does not mandate a unit.

Why the specification matters at sale

At our atelier in Munich we regularly encounter the classic case: a customer buys an elegant three-hander rated 3 ATM, wears it in the shower, and returns it three weeks later with condensation under the crystal. That is not a material defect. That is the gap between laboratory rating and real-world use.

In service and acquisition, what counts is not just the figure on the dial, but the current condition of the gaskets. A Submariner rated 30 ATM that has not been serviced for eight years almost certainly no longer holds 30 ATM — the gaskets have hardened and leak. Before selling a pre-owned watch we routinely pressure-test to rated depth or, where that is not possible, to a conservative level (5 bar), and disclose the result to the buyer.

ATM on vintage watches

Vintage watches with historical ATM ratings are to be treated conservatively in practice. A Datejust 1601 from the 1970s carried 10 ATM ex factory. After 50 years without documented service, that corresponds only to the theoretical construction maximum, not to the current condition. Vintage watches in principle are not worn in water without a current pressure test.