A dive watch is defined as a watch that will withstand submersion under water at depths of 100 meters or more and has a system to measure or control the time the watch is submersed. The water resistance rating of a watch is determined by the gaskets that form a watertight seal between the back of the watch and case of the watch. The type and strength of sealant used to secure the crystal to the case and the material from which the case is manufactured are also part of the water resistance rating. Be careful of any watch claiming to be waterproof, as they do not exist. Water resistance is the correct terminology.
Dive watches are regulated by ISO 6425, published by the International Organization for Standardization (ISO) which establishes the standards and features that a watch must possess in order to be considered a dive watch. Testing of the water resistance rating of a dive watch required by ISO 6425 is entirely different from testing of non-dive watches, which are regulated by ISO 2281. The ISO 2281 standard does not qualify a watch as a dive watch and is not discussed because of that fact. Every watch labeled as a dive watch by the manufacturer must be fully tested and pass the standards of ISO 6425.
Dive watches are tested in static water at 125% of the water resistance rating depth of the watch. For example, a watch with a water resistance rating of 200 meters will be water resistant to a depth of 250 meters in static or still water when stationary. In order to qualify as a scuba dive watch, each watch must meet eight requirements established by ISO 6425 and two additional requirements for mixed-gas saturation diving.
ISO 6425 Tests
• Resistance to Thermal Shock
• Resistance of Crowns and Other Setting Devices to External Forces
• Water Tightness and Resistance to Excess Water Pressure
• Reliability under Water
ISO 6425 Minimum Requirements for Mechanical Watches
Please note that the requirements for digital and quartz watches have slightly different readability requirements. The minimum requirements for mechanical watches are identified below:
• Presence of a Timing Device
• Visibility Distance in Total Darkness
• Indicator that the Watch is Running in Total Darkness
• Shock Resistance
• Magnetic Resistance
• Salt Water Resistance
• Resistance of Attachments to Outside Forces
• Salinity of Solution Compared to Normal Sea Water
ISO 6425 Standards
To pass the tests and requirements of the ISO 6425 standards, dive watches must meet the following standards:
- Resistance to Thermal Shock – Immersion of the watch at 30cm +/- 2cm at temperatures of 40C, 5C and 40C degrees with no more than 1 minute between immersions. No evidence of condensation or water leakage can be present.
- Resistance of Crowns and Other Settings Devices to External Forces – Immersion to 125% of water resistance rating/10 bar for ten minutes with an external force of 5 N perpendicular to the crown and other push buttons. Condensation test required both before and after this test to insure results.
- Water Tightness and Resistance to Excess Water Pressure – Immersion in a suitable vessel while overpressure of 125% of water resistance rating is applied within one minute of immersion and maintained for two hours. Pressure reduced to 0.3 bar for one minute and maintained for one hour. No evidence of condensation or intrusion of water can be present.
- Condensation Test – Watch placed on a heated plate to 40C to 45C until it reaches the temperature of the plate. A drop of water with a temperature of 18C to 25C (about 64F to 77F) is placed on the crystal for one minute. No evidence of condensation on the interior of the crystal can be present.
- Reliability Under Water – Immersion in water to a depth of 30cm +/- 2cm at temperature of 18C to 25C for fifty hours. All mechanisms must function properly. Condensation test required both before and after this test to insure results.
ISO 6425 Minimum Standards for Mechanical Watches
To meet the tests and requirements of ISO 6425, mechanical dive watches must meet the following standards:
- Presence of a Timing Device – A unidirectional rotating bezel with a 60 minute scale with visible markers in five minute intervals, coordinated with a luminous minute hand that is clearly visible.
- Visibility Distance in Total Darkness – The minute hand must be clearly distinguished from the hour hand, set time buttons and second hand to verify functioning of the watch must all be clearly visible and distinguishable to a distance of 25cm (about 10 inches).
- Shock Resistance – Two shocks are administered to the 9 o'clock position and the other to the crystal perpendicular to the face by a hard plastic hammer delivering 3 kg of energy with an impact speed of 4.43 m/s. Allowable rate of change in accuracy is +/- 60 seconds per day.
- Magnetic Resistance – Three exposures to a DC magnetic field of 4800 A/m. Allowable rate of change of accuracy is +/- 30 seconds per day while exposed.
- Salt Water Resistance – Submerged in a 30 g/l sodium chloride solution (comparable to sea water) for 24 hours at 18C – 25C. Case and accessories can show no evidence of change and all moving parts must function properly.
- Resistance of Attachments to Outside Forces – Watch and band must be closed. Force of 200 N to each attachment point in opposing directions. No evidence of damage at the attachment is allowed.
- Markings – Back of the watch is marked "Diver's Watch LM", where L is the depth in M meters of water resistance to distinguish it from knock-offs unsuitable for scuba diving. "Diver's Watch LM for Mixed-Gas Diving" marking indicates the dive watch is safe for saturation diving.
Ratings and Activity Suitability
Water resistance ratings and the water activities for which they are suitable are listed below:
- 100 Meters – Swimming, sailing, snorkeling and recreational surfing
- 200 Meters – Serious surface water sports, professional marine activity and scuba diving at depths not requiring helium gas.
- 300 Meters – Saturation diving (helium enriched environment). Watches qualified for helium mixed-gas diving have additional identifying markings.
The standard for measuring static water resistance is normally stated in meters. Some watch manufacturers' measure water resistance rating in feet, bars and atmospheres (ATM). Bars and atmospheres (ATM's) are actually measurements of pressure, rather than depth. Meters and ATM are the most common water resistance measurement. Under water the pressure increases by one atmosphere for every 10 meters of depth. A watch with a rating of 20 ATM's would be equal to a static water resistance rating of 200 meters.
If you are considering purchasing a certified dive watch for scuba and other deep water diving, you should familiarize yourself with the ISO 6425 standard so that you find the watch that meets your water sports expectations.
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