Water and Diving

Terms that explain how watches resist water and what a dive watch actually needs.

ATM and Bar: ATM and bar are the pressure units used for water resistance. One ATM equals about one bar, roughly the static pressure of 10 metres of water, so a 20 ATM watch is rated to 200m. That figure is a laboratory pressure rating, not an actual diving depth.
Depth Rating: Depth rating is the water resistance figure printed on a watch case, given in metres or ATM. It reflects a static pressure test carried out in a lab, not the depth to which you can safely dive in real water.
Dive Watch (Diver): A dive watch has at least 100 metres of water resistance (200 is common for extra margin), a screw-down crown, a unidirectional rotating bezel and strong lume. Professional-grade models meet the ISO 6425 standard, which sets a 100 m minimum, and carry the marking DIVER'S WATCH followed by the rated depth on the dial.
Diver's Extension: A diver extension is a folding or sliding section built into a dive watch clasp. It lets the bracelet open up by a set amount so the watch fits over a thick wetsuit sleeve, then closes back to its normal length on dry land.
Helium Escape Valve: A helium escape valve is a small valve that lets helium atoms vent out of the case during decompression after a saturation dive. As ambient pressure drops, trapped helium would otherwise push against the crystal, so the valve releases it. It only matters to professional saturation divers and is unnecessary on most watches.
ISO 6425: ISO 6425 is the international standard a watch must meet to be marked a diver's watch. It tests water resistance of at least 100 metres, a timing bezel, legibility in the dark, and resistance to magnetism, shock and salt water. Many Seiko Prospex diver references are built to meet it.
Unidirectional Bezel: A unidirectional bezel is a diving bezel that turns one way only, conventionally anticlockwise. You align its zero marker to the minute hand to time the dive. Because it rotates in just one direction, an accidental knock can only make the elapsed time read longer, never shorter, so the diver believes less time remains and surfaces sooner. ISO 6425 requires the timing bezel to resist accidental movement, which in practice means unidirectional rotation.
Water Resistance: The water resistance figure on a dial is a laboratory pressure rating, not a real diving depth. 30 metres covers hand washing and rain, 50 metres covers showering and shallow swimming, and 100 metres is the ISO 6425 floor for a certified dive watch. No watch is truly waterproof; it is only water resistant.