Perform an online search for “definition of a diver’s watch” or similar and you will find dozens of pages referencing ISO-6425. The International Standard Organisation document 6425 details the essential requirements of a watch that can be used for diving. In their own words …

This document specifies requirements and test methods for divers’ watches and for saturation divers’ watches for use in deep diving (see Annex A which deals with watches for saturation diving).

It applies to divers’ watches designed to withstand diving in water at depths of at least 100 m and equipped with a secured measuring system to indicate the diving time, which is visible in darkness.

Moreover, it indicates the marking which the manufacturer is authorized to apply to them…..

iso.org

However – it’s a voluntary certification – agreed it is seen by many as the benchmark of quality assurance for diver’s watches. But it’s not mandatory – there are many supreme quality, highly regarded dive watches on the market that do not hold IS0-6425.

Some of the key testing points of ISO 6425 include (please note – these are just summary bullet points – if you want the precise details, please refer to the ISO.org 6425 document – see external references at end)

  • Reliability under water – the watch is submersed for 50 hours in 30cm of water (temp 18-25°C)
  • Condensation test – this is conducted before and after the Reliability under water test. The watch is heated to 40-45°C for 10-20mins. A single drop of water (temp 18-25°C) is dropped on to the watch glass. This is then cleared off ater 1 minute. If the watch shows any signs of condensation inside the case – it is failed
  • Water Resistance – a more thorough test than the first – this one checks the watch will not fail due to water ingress under pressure. The watch is subjected to an overpressure test equivalent to 125% of the watch’s stated rating eg – a pressure equivalent to 250m for a 200m WR watch. The watch is kept at this pressure for 2 hours – after this time the pressure is reduced to 0.3Bar (equivalent to approx 30m) and kept at this pressure for 1 hour.
  • WR-100m or more – any watch striving for ISo-6425 must be water resistant to a minimum depth of 100m – a vast majority are 200m or more
  • Thermal Shock – the watch is subjected to rapid changes in surrounding water temperature. The water must be pre-heated / cooled to 40°C (jacuzzi hot) and 5°C (fridge cold). The watch is moved – taking less than 1 minute per transition – from HOT to COLD to HOT. Each time checking for water ingress.
  • Anti-Magnetic – ISO have a standard covering the affects of magnetism on a watch – ISO 764. All ISO 6425 watches must also conform with ISO 764. The watch is exposed to a magnetic field of 4,800 A/m from various angles. Following this exposure it must maintain an accuracy of no less than +/- 30 seconds per day
  • Shock Resistant – ISO have a standard covering the affects of shock on a watch – ISO 1413. All ISO 6425 watches must also conform with ISO 1413. The tests undertaken are supposed to simulate the shock experienced were the watch to be dropped from 1m on to a hardwood surface. ISO don’t literally drop the watches, instead it is struck with a plastic hammer at the 9 o’clock position and on the face at a strike velocity of 4.43 m/s
  • Dive Timer – the watch must also feature a dive time mechanism. For most dive watches this is the distinctive out bezel. The bezel must only rotate in one direction and be immune to accidental rotation. The numbers must be clear and legible and must be marked at 5 minute intervals.
  • Visibility in the Dark – this is a personal favourite of mine and something that first attracted me to dive watches. The three primary hands (H:M:S) must be clearly visible from 25cm away in the dark

“Diver’s Watch 200m”

It’s sometimes quite hard to know whether a watch does or doesn’t have ISO-6425. If it’s a popular brand like Seiko or Citizen check the wording on the watch dial. Look for these specific words – “DIVER’S WATCH 200m” or “DIVER’S 200m”

An example of a clearly identified ISO-6425 conforming watch from Seiko

Watches that conform to ISO 6425 are marked “Diver’s watch XXXm” or “Diver’s XXXm”. Watches that fail the test cannot add this wording to the watch dial.

What if it’s not ISO-6425

There are several brands of luxury / high quality watches that don’t put their products through ISO-6425. Such brands are confident in their own tests (some believe they are more thorough than ISO) and gladly provide the customer with a warranty of many years – including issues related to water resistancy.

Brands such as Rolex, Tudor, Panerai, Omega, Bremont etc do NOT add the wording “DIVER’S XXXm” to their watches. They may not have been officially certified to ISO-6425 – but their in-house tests meet or possibly exceed those of the International Standard Organisation.

Rolex performing in-house water resistancy test (courtesy Rolex.org)
Rolex Comex models – high pressure water resistancy tests (image courtesy Rolex.org)

Compliant, Exceed & Surpass

Manufacturer’s sometimes quote a 6425 reference in their marketing to reinforce their own in-house tests. Terms such a “our testing exceeds that of ISO 6425” and “tested to ISO 6425 standards” or “our tests surpass ISO-6425”.

What should Divers look for in a used watch

The chances are a diver (scuba) is going to have a dive-computer with them. It’s most likely the diver is going to rely on their dive-computer for depth and time-keeping matters. However, an analogue diver’s watch with dive time bezel is a great back-up time-keeper. When buying a USED diver’s watch there are a few things to consider / check

  • Has the back been taken off – maybe for repair or battery replacement. This will most likely be the case for a quartz (battery) watch. Was the water resistancy checked following battery change? Did the jeweller check the o’ring seals
  • Uni-directional Bezel action – check that the bezel only turns in one direction – anticlockwise. If it turns both ways you might knock it when diving and adjust the start time giving incorrect dive time stats.
  • Watch Strap security – there are many dive watches on eBay right now with non-original wrist straps. I have personally lost a watch when sailing as I didn’t check the security of the watch posts in the lugs. Firmly twist the strap in several directions checking that the pins do not pop out from the lugs.
  • Check the original spec – if you can get hold of the model number check online to see if the watch was rated water resistant for swimming or water resistant for scuba diving. If it says 200m or more – then it should be fine for scuba diving – assuming the water resistancy has never been compromised
  • Consider a Kinetic / Eco-Drive / Solar model – one way of avoiding a watch that’s has had it’s back removed several times (normally the quartz / automatic models) is to get one that generally won’t have been tinkered with. Watches like Seiko Kinetic, Citizen Eco-Drive and any model of Solar watch hopefully has never had the case opened. I’ve bought several Citizen Eco-Drive & Seiko Kinetic over the years that have always given good water resistancy – touch wood.

References & more reading

  • Watchtime.com – excellent article with a detailed summary of ISO-6425
  • ISO.org – standard 0S-6425:2018
  • WikiPedia – definition of ISO-6425 Diver’s Watch standard
  • WikiPedia – definition of ISO-764 – anti-magnetic watches
  • WikiPedia – definition of ISO-1413 – shock resistant watches
From my collection. Citizen and Seiko ISO-6425 certified – Bremont Supermarine 500 not so – but tested to exceed ISO-6425 expectations