Making a Tool Watch: A Basic Guide to ISO, DIN, and Related Industry Standards

Though it’s typically easy enough for a collector to differentiate between various categories of watches, sussing out the technical definition of each of these categories is admittedly a more involved process. What precisely constitutes a dive watch, or a pilot’s watch? Is there a particular water resistance that must be met if a watch is meant to be used underwater, or a specific altitude or barometric pressure at which a pilot’s watch must be able to operate?

The International Organization for Standardization provides some help in answering these questions. ISO (which is not an acronym, but derived from the Greek isos, meaning “equal”) is an international body comprised of representatives from various national standards organizations whose purpose is to set international standards and guidelines for products and services. There are 162 member countries and funding is provided through a combination of organizations that manage specific products, subscriptions from member bodies, and the sale of standards.

Robert Maloubier diving with a dive watch of his design, the Blancpain Fifty Fathoms.

Chances are that if you actually SCUBA dive with your dive watch, you’ve at the very least heard of certain ISO standards in passing, the most well-known of which is likely the ISO 6425 (the German DIN 8306 is an equivalent). The 6425 standard provides water resistance specifications down to 100 meters, as well as minimum requirements for mechanical diver’s watches, such as:

  1. The presence of a unidirectional bezel with a scale going up to 60 minutes, the five-minute demarcations (or other timekeeping device) clearly marked, and a pre-selected marker to indicate a specific minute marking.
  2. Adequate readability/visibility at 25 cm (9.8 in.) in complete darkness.
  3. The presence of clearly distinguishable minute markings on the dial.
  4. The presence of an indication that the watch is running in total darkness. (generally indicted by a running second hand with a luminous tip or tail).
  5. Magnetic resistance, tested via three exposures to a direct magnetic field of 4,800 A/m, after which the watch must retain accuracy to +/- 30 seconds/day as measured prior to the test. This test is part of the ISO 764 standard, which governs anti-magnetic properties.
  6. Shock resistance test via two shocks (one to the nine o’clock side of the watch, and one to the crystal and perpendicular to the dial). The shock is delivered via a hard plastic hammer mounted on a pendulum, so as to deliver a measured amount of energy (a 3 kilogram hammer with an impact velocity of 4.43 m/s). The change in rate allowed is +/- 60 seconds/day. This test is part of the ISO 1413 standard, which governs shock resistance.
  7. Chemical resistance test via immersion in a 30 g/l NaCl solution for 24 hours to test rust resistance (the test water solution has a salinity comparable to that of seawater).
  8. Strap/band solidity test via applied force of 200 N (45 pounds) to each spring bar or attachment point in opposite directions with no damage to the watch or attachment point.
  9. The presence of an End of Life (EOL) indicator on battery-powered watches.

It should be stressed that participation in ISO testing is voluntary, and certain manufacturers completely forgo the testing in favor of their own stringent certification tests (we’re looking at you, Rolex). If a watch passes the ISO 6425 testing and meets the standard, it can officially read “Diver’s” on the dial (like the above Seiko SKX007) or case and feature the depth rating in meters. A manufacturer might also choose to test its watches to the ISO standard (or better) and simply not feature the word “Diver’s” on the dial or case, which is what Omega does with its dive watches.

So what does the depth indication on a dive watch actually mean? ISO tests for “water resistance at overpressure,” meaning that the watch in question is actually subjected to 25% more depth pressure than what’s indicated by the rated depth (a 200-meter rated watch, for example, is meant to perform accurately without condensation forming after exposure to 250 meters of depth). It’s also important to note that the overpressure test must be performed on each and every watch with an ISO 6425 rating (some ISO standards allow for testing to be done on a representative sample set of products). This means that if your watch is ISO 6245-rated, the actual watch on your wrist underwent testing.Water resistance testing is checked via a condensation test in which the watch is submerged beneath a foot of fresh water at room temperature for 50 hours, after which it is set on a heated plate that brings it up to 100 degrees Fahrenheit. A drop of water is placed on the crystal for one minute, and if condensation forms inside the crystal, the watch fails the test.

For watches intended for use in mixed-gas/saturation diving (in which helium is introduced into the breathing gas mixture), ISO introduces additional testing, which includes operation at a gas overpressure and a decompression simulation test via internal pressure. If the watch functions correctly after these two tests, it is marked with the words “DIVER’S WATCH xxxM FOR MIXED-GAS DIVING,” in which the letters xxx are replaced by the diving depth in meters guaranteed by the manufacturer. The composition of the gas mixture for which the watch is intended will also accompany the watch’s instructions.

The Citizen Promaster 1000M Professional Diver is rated for mixed-gas diving.

Of course, there exist standards for watches other than those for diving. DIN, the German Institute for Standardization, along with Sinn, EuroCopter, and the University of Aachen in Germany, recently developed the TeSTaF standard (“Technischer Standard Fliegeruhren,” or Technical Standard for Pilot’s Watches), which addresses pilots watches and their various feature sets.

Building on TeSTaF, DIN developed the 8330 Horology-Aviator standard, which calls for a watch that will operate at temperatures ranging from -15 degrees C to +55 degrees C, can pass a pressure change test that simulates the pressure changes inherent in aircraft takeoff, and withstand and resist liquid typically encountered in and around aircraft (fuel, lubricants, de-icing fluid, etc.). There are also requirements pertaining to the withstanding of centrifugal forces, vibrations, impact, and legibility of the dial for easy reading, as well as the minimization of crystal reflections.

The Sinn UTC Ti UTC IFR is DIN 8330-certified.

Other ISO and DIN standards pertain to shock resistance, anti-magnetic properties, gold alloy coverings on watch cases, movement types, and much more, though those for shock resistance and anti-magnetic properties are included as part of the ISO 6425 specification for dive watches. With the advent of the DIN 8330 standard in Germany, it will be interesting to see if ISO will develop its own equivalent standard for pilot’s watches in the future and perhaps set the course for standardization on these timepieces.

Editor’s note: clarification about the required demarcations on a diver’s bezel.

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Oren Hartov is the watches editor at Gear Patrol, a contributor to several other publications, and a graduate of the Berklee College of Music. He is a reserve paratrooper in the Israel Defense Forces and enjoys music, history, archaeology, militaria, scuba diving, languages and travel. He is of the opinion that Steely Dan’s “The Royal Scam” may in fact be a better record than “Aja,” but he’s not positive.