The Dry Facts About The ISO 6425 Divers’ Watch Standard

What does it mean, and what kind of tests are involved?

May 30, 2022

comments 26

Although incredibly boring, ISO standards are highly informative. Unfortunately, to be that informative, they also have to be voluminous and extensive, which doesn’t always make for the most pleasurable reading. Today, we’ll try to explain the critical elements of the ISO 6425 standard for divers’ watches in everyday language.

I first want to stress that an ISO 6425 certified divers’ watch is different than a watch that simply complies with the technical requirements of ISO 6425. The ISO 6425 standard goes significantly beyond technical requirements and specifications. Besides that, it’s essential to know that ISO itself does not certify. ISO produces the rules, but to certify a divers’ watch, you’ll need an independent body accredited by ISO’s Committee on Conformity Assessment (CASCO) to carry out the certification. Many countries have local certification bodies. Examples are Lloyd’s and TÜV, as well as over 1,000 others found through this link. A watch brand stating that a divers’ watch is ISO 6425 certified should indicate which body has carried out the certification, for example, “ISO 6425 certified by TÜV”.

The facts about the ISO 6425:2018 standard

ISO published the first 6425 standard for divers’ watches in 1982. After 1984 and 1996, currently, it’s in its fourth edition, formally known as ISO 6425:2018. As you may have guessed, ISO introduced this revision in 2018. For the sake of simplicity, I’ll use just “ISO 6425” for the rest of this article. Where to start, then? I’ll go through the requirements in the same order as ISO uses in the 6425 standard. This is for two reasons. First, it’s easier for me to write, and second, much of the necessary testing has to follow a specific order.

I intend to explain the critical elements of ISO 6425, so I’ll skip some (mainly theoretical and administrative) items. And divers’ watches meeting the ISO 6425 requirements should also meet horological ISO requirements for magnetic resistance, shock resistance, accuracy, and so on. Those requirements, however, are most likely a story for another day, so I won’t go into them here. I might just make note of the applicable ISO standards at some point. Here we go!

Water resistance of at least 100 meters

What’s the most essential feature of a divers’ watch? Exactly — that it should function underwater and shouldn’t fill up. Therefore, ISO requires that the watch must have a minimum of 100 meters of water resistance. That requirement doesn’t seem too challenging to meet, as we see many watches that aren’t even divers’ watches with a 100-meter depth rating.

A diving time indicator

This feature is more interesting than it seems at first glance. Especially when thinking about analog divers’ watches, we immediately imagine a uni-directional rotating minute bezel. And we’re right in doing so because ISO 6425 requires that a divers’ watch is “equipped with a diving time indicator (e.g. rotating bezel, digital display, or other).” It also has to be “protected against inadvertent handling,” so the unidirectional bezel perfectly fits this job. Another requirement here — “clear markings indicating every 5 minutes” — normally doesn’t cause a problem either.

A resolution of one minute or better over at least 60 minutes

ISO 6425 states, “This device shall allow the reading of the diving time with a resolution of 1 minute or better over at least 60 minutes.” There is, however, some discrepancy in interpretation. Many well-known dive bezels do not have 60 one-minute markings. Many have minute markings for the first 15 or 20 minutes but rely on five-minute markings thereafter. Some say that the bezel should have these one-minute markings all around the bezel to meet the ISO requirements. Others say that the bezel should have at least 60 clicks in order to position the luminescent dot on the bezel at every minute. What’s your opinion? You can tell us in the comments below. Anyway, watches with both types of bezels apparently comply with the ISO 6425 requirements, such as the Tissot Seastar 2000 Professional you see below.

Another well-known example of a bezel without a full 60-minute indication is the Rolex Submariner. Then again, Rolex doesn’t promise compliance with ISO 6425. The newer Sea-Dweller and Deepsea have minutes all around the bezel, though. The bezel on the Oris Divers Sixty-Five also does, although the brand doesn’t mention compliance with ISO either. This could be because it does not meet other ISO requirements.

Testing

Next up is testing — both type testing and 100% single-watch testing. Type testing of a watch, known as homologation, is done to determine if a watch model, in general, is able to meet the ISO 6425 requirements. These tests — I’ll come to what type of tests they are in a minute — have to be carried out in a specific order. And every one of the test samples must pass every test. Here is a brief summary of these tests in chronological order.

Type testing — visibility and magnetic resistance

Visibility: In light, the diving time indicator has to be legible with a minimum of 50 lux. In the darkness, after exposure to light for 180 minutes (in accordance with ISO 17514) and checked at a distance of 25cm, the time, the diving time, five-minute markings, running indicator, and end-of-life indication (in case of battery operation) should be clearly visible. The minute indicator should be clearly distinguishable from the hour indicator (I can imagine a different shape or color of the hands). The reading of the diving time is allowed an uncertainty of ±2.5 minutes, hence the need for luminous five-minute markings.

Magnetic resistance: The watch shall be tested in accordance with ISO 764 and shall comply with its requirements. To put it succinctly, a watch must resist exposure to a direct current magnetic field of 4800 A/m. It must then keep its accuracy to ±30 seconds/day as measured before the test to comply with ISO 764.

Type testing — temperature and corrosion

Temperature cycling: The sample watch has to be exposed to an air temperature of -20ºC. It shall then be placed at room temperature for 30 minutes. It should then be brought to an air temperature of +60ºC for 60 minutes, after which it should be submerged in water of +2ºC for another hour. After being removed from the water and wiped down, the watch must then undergo a condensation test according to ISO 22810. The test is considered successful if the watch still functions normally after this process.

In order not to become as boring as the actual ISO standard is, I’m going to skip over a few of the tests here. Further requirements are to determine corrosion resistance by a salt spray test, as well as shock resistance properties, including being hit twice by a 3kg hammer with an impact velocity of 4.43 m/s and even a free-fall test from one meter onto a horizontal hardwood surface.

On to the good stuff — water resistance

Slowly but surely, we’re getting to where we are here for — water resistance! But not before the watch has undergone a condensation test according to yet another standard, ISO 22810:2010. Then, for type testing, the watch shall be submitted to the following procedure:

Immerse in water at a depth of 30 cm
Operate in water all of the mechanisms that are allowed to be used underwater (i.e. the bezel, not the crown and the pushers)
Keep immersed for 24 hours
Operate in water all of the mechanisms that are allowed to be used underwater once more
Keep immersed for another 24 hours

The watch shall then be removed from the water, wiped, and undergo the condensation test once again. After all these steps, the watch then should function normally.

Resistance of the crown and other setting devices

Still underwater, but now with an overpressure of 25% above the indicated depth rating of the watch, a force of half a kilogram has to be generated to the crown and push-pieces. Of course, it’s allowed to have the crown and push-pieces locked or screwed down if they have that feature. This force has to be applied perpendicular to the crown and push-pieces for a duration of 10 minutes. Once again, the watch shall be removed from the water, wiped, undergo the condensation test, and should function normally.

For the sake of readability, I’m shuffling the items a bit here. Besides the resistance of the crown, the bracelet of the watch has to be tested as well. While closed, it has to withstand a force of 20 kilograms, pulled in both directions from the watch.

On with water resistance, but now getting serious

From the tests mentioned above, we already know that the watch doesn’t fill up when submerged, not even with an overpressure of 25% while putting force on the crown and pushers. But does it last? Scuba diving doesn’t mean dipping a watch in and out of the water, after all. A diver resides underwater for a longer period of time and doesn’t want his diver’s watch to fail during that period. So, here is the test sequence that will simulate a dive to make sure the diver’s watch will be fit for the job. ISO calls it “resistance at a water overpressure”.

Immerse the watch in water (duh…)
Apply an overpressure of 25% over the indicated rating of the watch. For example, if a watch is rated to 200 meters (20 bar), the pressure applied would be 25 bar. This pressure has to be applied within 10 minutes of submersion and shall be maintained for two hours.
Then reduce the overpressure to 0.3 bar (mimicking a depth of three meters) within 60 seconds and maintain that for one hour.

The watch must function normally during and after the test. And should not fail the condensation test according to ISO 22810:2010 test after it has been wiped.

100% single-watch testing

This is the hard part for watch manufacturers and the reason that hardly any diver’s watches are certified officially certified. The above testing sequence should not only be carried out on a sample watch. Rather, the manufacturer must test every single watch that is claimed to be ISO 6425 certified. For brands, regardless of whether they produce a larger or smaller amount of watches, this is pretty much impossible and/or would be extremely expensive. I don’t think anyone would like to pay such a premium for their watch just to have it officially certified by an independent body. A brand promising to produce the watch in accordance with the ISO 6425 requirements should be sufficient, assuming, of course, that the brand upholds that promise.

Conclusion

The ISO 6425 standard definitely defines some well-considered essential technical specifications and features. The 100% single-watch testing requirement, however, makes it more or less impractical to actually certify divers’ watches according to the standard. But I don’t think that is necessarily a bad thing, and neither does it make the standard itself useless. We see many brands following the requirements when designing their diver’s watches anyway. For consumers, a watch indicating that it meets the requirements gives at least the insurance that it’s suitable for scuba diving.

Annex

The ISO 6425:2018 standard doesn’t stop here, though. In Annex-A of ISO 6425, additional requirements for divers’ watches suitable for saturation diving are explained. In saturation diving, a watch is subjected to the pressure of a gas mixture, which acts differently than water pressure. Consequently, it’s recommended to subject these watches to extra tests. I won’t include those in this article, though.

How important are the above requirements to you when you choose a dive watch? Do you specifically select your watch with ISO 6425 as a criterion? Please let us know your thoughts in the comments below. And if you like, follow or contact me on IG @gerardnijenbrinks

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The Dry Facts About The ISO 6425 Divers’ Watch Standard

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RR
WatchesSwiss says:

May 30, 2022 at 11:32

I’ve owned more than a few dive watches in my life. I grew up living near the ocean, lakes, rivers, and swimming pools, and have participated in most water sports (except scuba) in my younger days. I think it’s safe to say that my dive watches have never gone deeper than 8 meters. None of them have ever leaked. The majority of dive watches are overkill for what most of use them for…

Reply
1. RR
  WatchesSwiss says:
  
  May 30, 2022 at 13:30
  
  ….most of us use them for…
  
  Reply
  1. Arnold says:
    
    May 30, 2022 at 18:48
    
    Agree.
    
    It is nice to know they can keep water out and take a beating, and it is nice that the decent ones are legible day & night. But one really should use proper dive computers to stay safe when scubadiving……
    
    The zone between surface and 10meters below is rather troublesome really, a change of 1 bar pressure in 10 meters pops a lot of ears…..
    
    For an interesting opinion about dive watches: https://www.hodinkee.com/articles/a-commercial-diver-confronts-the-realities-of-the-professional-dive-watch
    
    Reply
    1. RR
      WatchesSwiss says:
      
      May 30, 2022 at 19:16
      
      I spent my younger years surfing, water skiing, and swimming in the pool. I practically lived in the water. Never tried scuba with tanks and gear. Too scary for me.
      
      Reply
      1. Arnold says:
        
        May 30, 2022 at 21:31
        
        I used to evaluate my activities every year or so, and had a rule – to start something new, I had to abandon something else.
        
        One important reason for abandoning diving was all the gear, transport, maintenance and so on. Didn’t consider it risky provided gear was maintained and procedures followed, and I always was careful about that.
        
        Tropical waters with a shortie? Fine. European waters? Too much gear, transport and preparations.
        
        Reply
2. Warky says:
  
  May 30, 2022 at 22:36
  
  I have taken my 300m Panerai in the pool on holiday.
  
  None of my others would be risked by being taken on holiday?
  
  Reply
  1. LC
    Chow says:
    
    May 31, 2022 at 02:52
    
    You don’t swim with any other watches or you don’t travel with any other watches?
    
    Reply
    1. Warky says:
      
      May 31, 2022 at 06:51
      
      I don’t take on holiday (or swim with) any of the others.
      
      Luminor with a rubber, leather and canvas strap is my holiday watch.
      
      Reply
      1. LC
        Chow says:
        
        Jun 01, 2022 at 00:52
        
        I understand. For me it depends on how long I’ll be traveling for and where I’m going. I haven’t been on your side of the pond since the china virus hit us. Things have changed. I never felt uncomfortable there wearing a Rolex or AP. But I guess things have changed with the surge of crime now. And I can’t legally carry a gun overseas. So I guess this summer if I cross the pond I’ll just bring a Panerai. Is the watch theft as bad as I hear in Europe?
        
        Reply
        
        Warky says:
        
        Jun 01, 2022 at 09:22
        
        In my part of the U.K. it is practically non existent. I can wear any of my watches without fear whenever and wherever I want locally.
        
        When I visit London though I don’t even wear my Panerai!
        
        Reply
        
        LC
        Chow says:
        
        Jun 01, 2022 at 13:55
        
        Fish, chips, cup o tea, bad food, worse weather, Mary fkn poppins, London!
        
        It’s that bad there?
        
        Reply
        
        Anthony.S says:
        
        Jun 01, 2022 at 23:24
        
        No it’s not that bad here at all. No different to any major city. I live in Manchester, work often in London and never need to watch what I have on my wrist in either city.
        
        I’ve also travelled extensively and one thing is always clear – try not to look like a tourist (don’t look lost, gawp or obviously check maps) and don’t overtly flash things around that you don’t want taken off you.
        
        Most thieves don’t recognise much past a Rolex unless it’s particularly “blingy”
        
        Reply
jh
James H says:

May 30, 2022 at 13:09

OK, so it seems a bit of a dry topic, but surely a deeper dive is required here? I get the slight ambivalence and skepticism toward the Standard when the ISO require every watch produced to be tested. My understanding of comparable safety standards is that a production model must pass the tests, then a subset or random selection be inspected periodically thereafter. These are then assumed analogous to the production models that ‘theoretically’ conform to the Standard.

But, all that said, some manufacturers/watches are tested or meet the Standard and it would have been good if the article told us what they were. The photos accompanying the article hint at some who bother to meet the Standard (I understand this is denoted by the use of ‘Divers’ or ‘Professional’ on the watch dial/case), but who conforms to testing? Name and shame please! Also, the testing of shock and magnetic resistance is pretty important, so it would be worth elucidating as it makes true dive watches very tough. Basically, the watch market is flooded with ersatz dive watches that wouldn’t hold up to a wet weekend. Some of them are very expensive. The ISO tries to protect and inform consumers (and yes, there are still a lot of divers who wear a dive watch), as does Fratello, so surely an opportunity here for more depth and investigation?

Reply
1. Gerard says:
  
  May 30, 2022 at 14:24
  
  Thanks for your reply James,
  
  We’re never afraid of some naming and shaming here at Fratello, as you probably noticed, and I sure tried to get this kind of information above water. That was more difficult than it seems. I’ve contacted many large and smaller brands to ask if, and how, they had their ISO 6425 watches tested and certified.
  
  Certified almost none had. Omega has their Ultra Deep certified according to ISO 6425 by Metas, Sinn has – ISO equal DIN standard – certification done by DNV GL, and Tissot told me to use an ISO certified laboratory within the Swatch Group. Many others, for instance, Citizen, Certina, Helm, etc. inform to do the testing according to ISO 6425 in-house, but do not have it independently certified (so you can’t tell for sure what and how they exactly test). And some, Seiko for instance, are not willing to disclose information on if and/or how they test and/or certify but assure their watches to be ISO 6425 compliant.
  
  There will be a second part to this article soon, that could shine a bit more light on this matter.
  
  Reply
  1. Arnold says:
    
    May 30, 2022 at 19:31
    
    Great article 🙂
    
    I’m a fan of good standards/certifications if found, but one should probably be rather familiar with the standard and certification in order to assess it’s value as customer. Anything that enlightens is great, thus thank you 🙂
    
    Have been through a few corporate ISO certifications “as victim”, have gone through a lot of vettings to meet quality standards, have made comprehensive procedures approved by DNV GL, and I have worked through many many meters of ISO certified procedures to verify potential breaches.
    
    A PROPER standard and certification that documents the quality or telling a user/customer what to expect from a product, service or organisation is great. On the other hand, a poorly made standard and certification masks the reality brilliantly and is worthless at best – downright lethal and dangerous at worst.
    
    Let’s assume that a company is within any legal/finacial/operational requirement and certifies the operational procedures. In that certified procedure there is a really REALLY stupid way of doing things (many of the geniuses writing ISO++ procedures does not have a clue about the real thing, all they know is how to write a procedure that is approved).
    
    Now, the really REALLY stupid operation has to be carried out in the very same silly way every time. If someone does the operation the right way without changing the procedure first, they would be in breach of the procedure and can be held liable – or fired.
    
    The idea behind ISO standards and certification is great, the reality not always so….
    
    Reply
  2. Simon Page says:
    
    Oct 19, 2024 at 13:33
    
    Is there a second part to this now? Great article.
    
    Reply
    1. Gerard says:
      
      Oct 19, 2024 at 14:58
      
      Thank you Simon,
      
      There’s not exactly a first and second part, but the following article was published in combination with this one: https://www.fratellowatches.com/iso-6425-certification-for-divers-watches-is-it-useless/
      
      Reply
Ahmet Krom says:

May 30, 2022 at 15:39

Dive watches are a bit of anachronism noadays. In fact in my 35 years of diving, dive time was not measured by a watch but a atch computer. Of course I had a dive watch and that as used as a) decoration b) backup for the computer should it fail. To my mind a dive atch has to be a computer backup type that measures depth and has rapid ascent descent alarms dive memory etc. Otherwise if you only have time then you also need dive tables and that is a very approach as we have seen because your dive time fluctuates according to your depth during the dive.
So in conclusion atrue dive watch must some dive computer functions plus it must also be a reasonable price. any tach over over 1500-2000$ is just a total waste of money and is showing off unnecessarily.

Reply
1. Gerard says:
  
  May 30, 2022 at 15:45
  
  Thanks for sharing your thoughts, Ahmet.
  
  Reply
Gerard says:

May 30, 2022 at 16:30

Do you mean it should be 0.3 bar ‘overpressure’ Martin? Which it is indeed, as indicated in the first line of that text.

Reply
CJ
clockjockey says:

May 30, 2022 at 21:01

Dear Gerard,
Great article. I have couple of comments as someone who has developed ISO, ANSI, EN, … standards:
1. Compliance with the standard, as written, would require single-minute marks THROUGHOUT the 60-minute bezel.
2. The standard is poorly written since ‘protection from inadvertent handling” is open to interpretation. I would argue that a unidirectional-rotating bezel is insufficient because it can still be nudged by accident—albeit only in one direction. A locking feature to the bezel, a prescribed minimum force for bezel rotation, or a geometric-guarding specification for the bezel edges provided by the crystal and case profiles (or similar) would provide the necessary design guidance for watchmakers.
3. As with many standards, this is not necessarily a safety issue but only a compliance issue.
4. This brings to everyone’s attention the flaws of standards development: not all standards are well-written, good, necessary, or developed with pure motivations. This standard is not well written (not complete) and perhaps written under the influence of some watchmakers not wanting their timepieces to fail meeting the standard.
Regardless, this was a nice summary of the ISO standard which is expensive to obtain for many people.
Thank you.

Reply
1. Gerard says:
  
  May 30, 2022 at 22:40
  
  Thank you very much for your valuable insights Clockjockey!
  
  I’d like to explain my findings on point 1. of your reply. ISO 6425 dictates about the diving bezel: “This device shall allow the reading of the diving time with a resolution of 1 minute or better over at least 60 minutes.”
  
  The bezel shall allow the reading, it’s not utterly clear in the ISO 6425 standard that reading should be on the bezel, or made available by the bezel. The latter would mean that just a (luminous) point or arrow on that bezel is enough. However, if you are right and the bezel would require single-minute marks throughout the 60-minutes, for instance, the Tissot Seastar would not comply. While Tissot ensures us that the watch is ISO 6425 compliant AND certified by an independent body.
  
  I totally agree with your point 2. that the standard is poorly written – as well – concerning a rotating bezel. It’s not clear if a uni-directional is sufficient to ‘protect from inadvertent handling’. Although I didn’t pen it down that way, I agree with you that it probably isn’t.
  
  Thanks again for your comments.
  
  Reply
  1. CJ
    clockjockey says:
    
    May 31, 2022 at 06:22
    
    Thank you, Gerard, for your reply.
    Again, a great article!
    
    Reply
LC
Chow says:

May 30, 2022 at 23:19

OMEGA makes the greatest watches in the world. Period, end of story.

Reply
1. Gerard says:
  
  May 31, 2022 at 09:08
  
  I agree Chow, some of the greatest 😉
  
  Reply
rgibson214 says:

Oct 25, 2022 at 07:00

I doubt that many luxury brands meet the the shock, antimagnetic, and condensation tests required for 6425 certification which is why the manufacturer claim “ISO compliance.”

The watch is placed on a support that holds it while a 6.6 pound plastic-headed club is mechanically released from a height of one meter. First it hits the side of the watch, on the “9”, and then the watch is reset and hit on its face with a shock equal to about 5,000 Gs. That seems like a lot, but the weight of the club, the speed and the distance really do calculate to about 5000 Gs. It’s been compared to the force of a strong golf swing, so you can imagine whacking it on the railing of the boat. To meet the ISO standard, the watch has to keep time just as before, within plus or minus 60 seconds per day after the test.

The ISO 764 requirement is for a watch that can maintain normal accuracy within plus or minus 30 seconds per day after exposure to a magnetic field of 4,800 amperes travelling along three different axes for 1 minute each. Good for any electric eels you might encounter while diving.

Spring bars are tested against an outside force of 200 newtons in each direction – about 45 pounds of force for each spring bar. That’s enough to get your attention and let you slip the watch free before the tiger shark circles around again.

After a control condensation test, the watch is submerged in hot water (104 degrees F) for 10 minutes, then cool water (41 degrees F) for 10 minutes, then hot water for 10 again, with less than a minute in between temperature changes. It’s then tested for condensation again.

Reply