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Water Difference a Dive Watch Makes: Guide to ISO 6425

September 10, 2021

Since 1946, the ISO (International Organization for Standardization) has set the benchmark for their own stringent certification tests. It is these ISO standards that see many dive watches tested for 6425 certification. Although participation is voluntary, if a specific watch undergoes and meets certain ISO 6425 tests and conditions, the company can officially brand them a diver watch, displaying the appropriate rated depth. 

Utilizing my obsession with time and extensive experience, I will set about uncovering and defining the murky truth surrounding the mystery of IS0 6425 certification. So let’s get straight to it.

What Does ISO Mean On Diver's Watches?

Let’s drain the swamp and clear the waters. Despite being around for many years (since 1996), most watch fiends are not up to speed with ISO tests. Although modified in 2018 to include mixed gas diving, Berner’s Illustrated Professional Dictionary of Horology state a diving watch is a “watch designed to withstand immersion to a depth of at least 100 m and to satisfy requirements specified in ISO standard 6425.”

Sounds straightforward enough. Nonetheless, dive watches must undergo a series of rigorous tests before they are awarded special certification. If not, it just ain't a dive watch.

What Is an ISO Certified Dive Watch?

The ISO plainly expresses a suitable dive watch is one providing water resistance to at least 100 meters. In addition, a pre-selected device gives the user the ability to choose any time period up to 60 minutes. 

It can be either a digital or unidirectional rotating bezel timing function offering protection from incorrect manipulation. The latter requires visible on the dial. Furthermore, there must be absolute clarity as far as the time is concerned, while the minutes and hour hand features should be easily recognizable. Clearly, this is something the ISO tends to favor. 

Setting the time on the device has to be clear with the three main hour, minute, and second hands visible and running. Regarding analog models, this can be done thanks to a luminous tip on the second hand that should be displayed. Battery-powered watches must be equipped with a low-battery indicator while all timepieces must be seen from around 11 inches in total darkness. 

Breitling watch face

The Salt Water Test

For diving watches that want the full IS0 6425 rating, the ISO requires companies to immerse them in a seawater solution made of sodium chloride for up to 24 hours. 

With salinity comparable to seawater, it must have a constant temperature between 18-25 degrees Centigrade (64F-77F) in order to identify rust resistance. It is examined for oxidation and moving parts to identify if it has a clear working function. Talk about rubbing salt in the wounds!

Reliability Underwater Test

The “reliability underwater” test is where it is subjected to dunking like a chocolate biscuit in tea for approximately 12 inches into a pure water solution. Lasting around two days, the timepiece is again inspected thoroughly for perfect function. However, there are two main disadvantages for those with mechanical watches.

Moisture and pressure play integral roles in functionality underwater. Normally, the body experiences 1 ATM (Atmospheric Pressure Measurement) when there's no water exposure. Comparable to an everyday watch with a dynamic pressure reading of 1 ATM, the further you want to plunge, the more force is exerted in the water. 

Pressure can augment every 30 feet you travel underwater. Sought-after diver's watches such as the Omega Seamaster Planet Ocean can take up to 60 bars of pressure while the Rolex Oyster Perpetual can plunge to a staggering 3,900 meters. They say the tough get going and the going get tough...none more so than the robust water overpressure test where watches endure 25% more pressure than the normal depth rating.

So, 200 meter watches intended for deep sea exploration would have to survive first-rate performance at minimum depths of 250 meters. Scuba diving enthusiasts, listen up. If you specifically own an ISO 6425 rated watch, it would have passed the overpressure examination with flying colors. It's reassuring to know, especially if you are considering resurfacing in a hurry. 

Man underwater making hand signs

Shock Resistance Test - Part One

The shock-resistance standard is performed in such a way that it emulates dropping your prized possession from a height onto a hard surface. This covers shock resistance involving two shocks from one meter to the watch located at the 9 o’clock area and around the top of the watch case.

Delivering a shock equivalent to around 5,000 Gs, it is similar to a croquet or golf swing. This pendulum-like movement tests the watch's strength to the maximum, delivered with full force at more than 4.4 meters per second. Ouch. After the impact, the watch must be kept at the correct accuracy of around +/-60 seconds on a daily basis as soon as the initial test has finished. 

Resistance Test - Part Two

Another key role on the road to ISO 6425 relates to the spring bar. The inside strap must undergo an external force of 200 neutrons or near 50 pounds. The strap must be shut to conduct this first test. 

Known as the heated plate condensation test, the ISO looks for no leakage, especially relating to the crown and other setting devices. A minimum of 5 newtons of internal pressure is applied to the crown. Guidelines express that there’s no necessity for a screw-down crown, meaning any type is passable. 

Thermal Shock Test

The thermal shock resistance examination tests the actual watch whereby it's supposed to be submerged for ten minute intervals in different temperatures.

First, the watch must be placed in hot water of around 104 degrees followed by cool water of around 40 degrees and then hot water. The time scale between immersions should be no longer than sixty seconds. When these three submersions are concluded, the hot plate test is conducted.

Anti Magnetic Test

Every ISO 6425 watch must undergo tests, exposing it to a magnetic field of 4,800 A/m from a myriad of positions. After this has taken place, the watch has to show evidence of keeping time within +/- 30 seconds per day.

Overpressure Test

The pressure is on. None more so than with the final test. Equalling 125% of the depth rating, the heated plate condensation test is done again to ensure that the watch shows no signs of moisture. The watch must then be taken to a pressure tester where it’s immersed for one minute.

After two hours, it’s removed, while depressurization occurs taking levels down to 3 meters. Finally, the pressure remains constant for another 60 minutes when the watch is taken out, dried and the test is conducted once more. 

Inner part of a wristwatch

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Tick Tock...The Final Word

Officially certified ISO 6425 guidelines adhere to international standards and are as strenuous as one of Lionel Messi’s workouts. Compared to the less strict ISO 2281 non-dive watches testing water-resistant watches, this is by no means a walk in the park. 

Despite most manufacturers not being governed by anything on the watch to signify it passes ISO 6425, only the elite can be branded with the official “Divers” mark. So the next time you are on the hunt for divers watches with a 200 meters or 300 meter rating, watch out for whether or not it’s an ISO 6425 diver or water-resistant watch.

The ISO is, without a doubt, the Rolls Royce of marks. If you are in the market for a diver's watch, here’s hoping your next purchase can withstand the pressure!

Simon Lazarus
As an avid watch fiend, Simon Lazarus isn’t any ordinary watch collector. Since the dawn of time (well almost), his obsession with timepieces has seen him establish a bijou collection across his extensive travels with his wife and two darling daughters including in the US and Dubai. With an extensive back catalogue of published work on timepieces for numerous sites globally, watches are moments in history. He certainly makes time to capture the true essence of watchmaking with an eye for impeccable detail showcased in his plethora of watch articles across numerous sites. With a well formed selection focusing on Swiss made as well as emerging microbrands that will no doubt gain in value, time is almost as precious as his two young princesses!
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