What is the ISO 6425?
ISO 6425 is a standard governing dive watches. ISO 6425 also provides minimum requirements for mechanical diver’s watches. It was first published in 1982 and recently updated in 2018. ISO 6425 sets the standard for reliability underwater, water-tightness, resistance at a water overpressure, and resistance to thermal shock.
What is ISO?
ISO is the International Organization for Standardization. They develop and publish international standards. ISO standards are based on global expert opinion, basically scientists and engineers, and are reviewed every five years.
What are the parameters?
- Reliability under water. The watches under test shall be immersed in water to a depth of 30 cm ± 2 cm for 50 hours at 18 °C to 25 °C and all the mechanisms shall still function correctly. The condensation test shall be carried out before and after this test to ensure that the result is related to the above test.
- Condensation test. The watch shall be placed on a heated plate at a temperature between 40 °C and 45 °C until the watch has reached the temperature of the heated plate (in practice, a heating time of 10 minutes to 20 minutes, depending on the type of watch, will be sufficient). A drop of water, at a temperature of 18 °C to 25 °C shall be placed on the glass of the watch. After about 1 minute, the glass shall be wiped with a dry rag. Any watch which has condensation on the interior surface of the glass shall be eliminated.
- Resistance of crowns and other setting devices to an external force. The watches under test shall be subjected to an overpressure in water of 125% of the rated pressure for 10 minutes and to an external force of 5 N perpendicular to the crown and pusher buttons (if any). The condensation test shall be carried out before and after this test to ensure that the result is related to the above test.
- Water-tightness and resistance at a water overpressure. The watches under test shall be immersed in water contained in a suitable vessel. Then an overpressure of 125% of the rated pressure shall be applied within 1 minute and maintained for 2 hours. Subsequently the overpressure shall be reduced to 0.3 bar within 1 minute and maintained at this pressure for 1 hour. The watches shall then be removed from the water and dried with a rag. No evidence of water intrusion or condensation is allowed.
- Resistance to thermal shock. Immersion of the watch in 30 cm ± 2 cm of water at the following temperatures for 10 minutes each, 40 °C, 5 °C and 40 °C again. The time of transition from one immersion to the other shall not exceed 1 min. No evidence of water intrusion or condensation is allowed.
- An optional test originating from the ISO 2281 tests (but not required for obtaining ISO 6425 approval) is exposing the watch to an overpressure of 2 bar. The watch shall show no air-flow exceeding 50 μg/min.
- Except the thermal shock resistance test all further ISO 6425 testing should be conducted at 18 °C to 25 °C temperature. Regarding pressure ISO 6425 defines: 1 bar = 105 Pa = 105 N/m2. The required 125% test pressure provides a safety margin against dynamic pressure increase events, water density variations (seawater is 2 to 5% denser than freshwater) and degradation of the seals.
- Movement induced dynamic pressure increase is sometimes the subject of urban myths and marketing arguments for diver’s watches with high water resistance ratings. When a diver makes a fast swimming movement of 10 m/s (32.8 ft/s) (the best competitive swimmers and finswimmers do not move their hands nor swim that fast) physics dictates that the diver generates a dynamic pressure of 0.5 bar or the equivalent of 5 meters of additional water depth.
- Besides water resistance standards to a minimum of 100 metres (330 ft) depth rating ISO 6425 also provides minimum requirements for mechanical diver’s watches (quartz and digital watches have slightly differing readability requirements) such as:
- The presence of a time-preselecting device, for example a unidirectional rotating bezel or a digital display. Such a device shall be protected against inadvertent rotation or wrong manipulation. If it is a rotating bezel, it shall have a minute scale going up to 60 min. The markings indicating every 5 min shall be clearly indicated. The markings on the dial, if existing, shall be coordinated with those of the preselecting device and shall be clearly visible. If the preselecting device is a digital display, it shall be clearly visible.
- The following items of the watch shall be legible at a distance of 25 cm (9.8 in) in the dark:
- time (the minute hand shall be clearly distinguishable from the hour hand);
- set time of the time-preselecting device;
- indication that the watch is running (This is usually indicated by a running second hand with a luminous tip or tail.);
- in the case of battery-powered watches, a battery end-of-life indication.
- The presence of an indication that the watch is running in total darkness. This is usually indicated by a running second hand with a luminous tip or tail.
- Magnetic resistance. This is tested by 3 expositions to a direct current magnetic field of 4,800 A/m. The watch must keep its accuracy to +/- 30 seconds/day as measured before the test despite the magnetic field.
- Shock resistance. This is tested by two shocks (one on the 9 o’clock side, and one to the crystal and perpendicular to the face). The shock is usually delivered by a hard plastic hammer mounted as a pendulum, so as to deliver a measured amount of energy, specifically, a 3 kg hammer with an impact velocity of 4.43 m/s. The change in rate allowed is +/- 60 seconds/day.
- Resistance to salty water. The watches under test shall be put in a 30 g/l NaCl (sodium chloride) solution and kept there for 24 hours at 18 °C to 25 °C. This test water solution has salinity comparable to normal seawater. After this test, the case and accessories shall be examined for any possible changes. Moving parts, particularly the rotating bezel, shall be checked for correct functioning.
- Resistance of attachments to an external force (strap/band solidity). This is tested by applying a force of 200 N (45 lbf) to each springbar (or attaching point) in opposite directions with no damage to the watch of attachment point. The bracelet of the watch being tested shall be closed.
Marking. Watches conforming to ISO 6425 are marked with the word DIVER’S WATCH L M or DIVER’S L M to distinguish diving watches from look a like watches that are not suitable for actual scuba diving. The letter L indicates the diving depth, in metres, guaranteed by the manufacturer.
Is it a dive watch if it does not meet ISO 6425?
Yes actually, it still is. Dive watches pre-date the ISO standard, but like anything in life, it really depends on the brand and how much you believe in it. I have absolutely no doubt a luxury watch from Rolex, Omega, Doxa, Blancpain, etc. are built to a quality that far exceeds their listed depth rating.
I also have no issue with well respected brands such as Citizen or Seiko’s lower budget watches, these are well respected brands, with or without the ISO standard. There are dozens of brands that do not claim to meet ISO standards and do not have the Diver’s mark on them. I still trust watches from Glycine, Phoibos and Orient, just to name a few.
How about the very low budget under $100 watches by Invicta, Casio, and Neymar? Well, I have personally tested these watches to a diving depth of 30 meters and they were fine.
If I was doing deep diving to 100 meters would I do it only with a budget diver? Probably not.
That said, I always dive with a computer, and when I was a young man and did decompression diving I would actually take a backup computer. I think a budget diver is fine for a backup, or even shallower dives. If it stops, you know when it stopped and you can ascend accordingly.
100M vs. 200M
The standard says 100 meters, why do some people say a dive watch needs to be water resistant to 200 meters?
Well, here’s the deal. If you’ve ever rebuilt a scuba regulator or tinkered with dive gear or anything with an o-ring or gasket you will know that they dry out and degrade over time. I have tried to find out how long it takes o-rings manufactured from different materials to degrade, but I have yet to uncover any good research on the subject.
The bottom line is this, when you purchase a new dive watch, the crystal gasket, crown and caseback o-rings are brand new. These are the parts of the watch that keep the water out. Over time they degrade and need to be replaced. As they degrade they are more likely to be permeated by water under pressure. The more degradation, the more likely they are to fail.
Now you could replace them every year or two, but I don’t know many people who do that, but it is definitely an option. Or, like me you can just consider anything water resistant to 200 meters to be considered a dive watch and anything with a shallower depth rating to be a snorkeling or swimming watch.
If you don’t dive and it’s not something you are likely to do I wouldn’t worry about it at all.
If, like me, you are a diver, I would stick with 200 meters. As I have reviewed here and here, there are plenty of dive watches to fit every budget. So you don’t necessarily need to spend a lot of money to acquire a 200 meter ISO standard dive watch.
Who Does the Testing?
Contrary to popular belief, ISO does not actually do any testing. ISO only publishes the standards. It is up to the manufacturer to do the testing to make sure the watch complies with the standards.
So this goes back to what I wrote up above, how much do you trust the brand. I would go so far as to say the trust the brand has built over time is probably more important than the Diver’s mark on the watch.
A manufacturer intent on providing consumers with a quality, durable, water resistant timepiece is going to do so. It also calls into question their quality control. If the manufacturer doesn’t have all of their processes and procedures in order it is going to leak out on the internet now that information is so readily available.
If I didn’t cover something in this post you are interested in, please drop me a line in my Contact form. I hope this clears up what the ISO dive watch standards are.