The ISO 6425 standard, in its October 2018 version, requires a dive watch to withstand at least 100 meters of depth. It does not require it to calculate your decompression.
This dividing line explains almost everything. The vocabulary used by stores blurs it, since under the same name you can find analog watches that only display the time and watch-format computers that manage your decompression stops.
Here we put each item in its place, with the text of the standard and the recommendations of DAN. No homemade tests, no ranking of models.
Dive watch or computer: what is the concrete difference?
A dive watch measures elapsed time and resists pressure. A dive computer continuously measures your depth and duration, then calculates the nitrogen theoretically dissolved in your tissues, your no-stop limit, any decompression stops, and your ascent rate. The watch provides a data point. The computer derives an instruction from it.
| Criterion | Dive watch (ISO 6425) | Dive computer |
|---|---|---|
| Measurement of dive time | Yes, by a device protected against handling errors | Yes, automatic |
| Depth | Not required by the standard | Yes, by pressure sensor |
| No-stop limit and decompression stops | No, to be calculated with tables | Yes, according to the manufacturer’s algorithm |
| Ascent rate | To be estimated with a depth gauge | Yes, with alarm |
| Tracking of successive dives | No | Yes, as long as you keep the same device |
| Power source | Mechanical or battery depending on the model | Battery or rechargeable battery to monitor |
The wrist format therefore says nothing about the function.
Many models sold today are computers in the shape of a watch. They have a strap, display the time on the surface, and include a pressure sensor with a decompression algorithm. Conversely, a mechanical dive watch has no depth sensor. But many product sheets mix the two terms, and a seller may call “dive watch” both a device that calculates decompression stops and an analog watch that calculates none. Look at the manual. If it mentions algorithm, decompression stops, or gases, you have a computer.
What does the ISO 6425 standard guarantee for a dive watch?
The ISO 6425:2018 standard, fourth edition, covers dive watches and those intended for saturation diving. It targets watches designed to withstand immersion of at least 100 meters, equipped with a secure system for measuring dive time and readable in the dark. Nothing about decompression.
The word “secure” matters. On most dive watches, it takes the form of a bezel that only turns in one direction. If you accidentally bump it, it can only shorten the time that seems available to you, never make you believe you have more margin. Readability in the dark follows the same concern, since you must be able to read the start time in dark or murky water, and for a night dive a good dive light takes over. Yet, the official summary of the standard mentions neither displayed depth, nor nitrogen, nor ascent rate.
A compliant watch measures time well, and stops there.
Also beware of catalog shortcuts. A watch marked “water-resistant” with a depth engraved on the back does not necessarily fall under ISO 6425, which also requires the time measurement device and readability in the dark. Look for the mention of the standard in the manufacturer’s documentation.
What does a dive computer calculate that a watch does not?
A computer tracks your depth, bottom time, ascent rate, and decompression status, the list given by DAN in Alert Diver in 2010. With these measurements, its algorithm estimates accumulated nitrogen and tells you how long you can stay or which decompression stops to make.
The calculation remains a model. Each manufacturer uses their own algorithm, and two computers worn on the same dive may display different limits. DAN therefore advises a backup computer as cautious as the main one. Many models offer a conservatism setting, but the device knows neither your age nor your condition that day. It only knows what it has measured.
And this memory is what gives it all its value on successive dives, since it keeps the day’s history and adjusts the limits of the next dive taking into account the residual nitrogen.
The downside is simple. If you change devices during your stay, the new one starts from zero while your body retains the nitrogen from previous dives. DAN recommends in this case to wait at least 12 hours after a single dive without decompression stop, or 18 hours after multiple dives, before starting again with a new device. As for your gas autonomy, it is read on the pressure gauge, and our guide on the dive tank, its pressure and volume details the link between the two.
Is a watch enough as a backup instrument?
Yes, if you combine it with a depth gauge, a pressure gauge, and dive tables, and if you know how to use them. This is the fallback solution described by DAN when the computer fails. Alone, the watch only gives you the time, and the same article reminds that a computer failure generally requires stopping the dive.
This fallback plan assumes that you have followed your profile throughout the dive.
For the ascent, DAN gives a simple reference. At 30 feet per minute, about 9 meters, you ascend one foot (30 centimeters) every two seconds. With one hand on the tip and your eyes on the watch and depth gauge, you can keep this pace without calculation. However, this reference says nothing about your decompression status. Following your buddy’s computer to control the ascent rate remains acceptable according to DAN, but not for your stops, since it has not measured your profile.
A dive slate is enough to note the maximum depth and the start time. Provided the buddy and the guide know the procedure, which is set during the briefing and not underwater.
What to choose for your first instrument purchase?
For a first purchase, the computer meets the real safety need, namely to track your depth, limits, and ascent. The dive watch comes next, as a backup instrument or for the love of the object. The choice of model depends on your practice, and our comparison of dive computers details the criteria.
If you rent your equipment on trips, the question changes a bit. Before a stay like diving holidays in Egypt, ask the center if they provide a computer to each diver and for how many dives. Keep the same device throughout the stay. Otherwise, you fall back into the problem of lost history, with the waiting times set by DAN. If you eventually aim for mixed gas diving, like trimix, the computer will need to manage multiple gases, which no watch does.
In short, the watch never replaces the computer at any point during the dive.
Thus, reversing the purchase order costs you margin on each dive done with only the watch on your wrist, and sometimes a canceled dive due to lack of equipment meeting the center’s requirements. The computer first, the watch when the budget allows.
FAQ
Is a watch waterproof to 100 meters a dive watch?
Not necessarily. The ISO 6425 standard also requires a secure system for measuring dive time and readability in darkness. Look for the mention of the standard in the manufacturer’s documentation.
Can you dive with just a simple dive watch?
A watch, a depth gauge, a pressure gauge, and dive tables form the backup configuration described by DAN. It requires knowing how to plan and follow your profile using the tables. In clubs, the required equipment depends on the center and your level, so ask when booking.
Should you wear two dive computers?
DAN presents the second computer as a backup solution. It must be used on all dives in the series and offer a level of caution comparable to the main one, otherwise its calculations are useless in case of failure.
How long to wait before diving with a new computer?
DAN recommends at least 12 hours after a single dive without decompression stop and at least 18 hours after multiple dives. The new device ignores the nitrogen accumulated before its activation.
Sources
- ISO 6425:2018 Horology, Divers’ watches (International Organization for Standardization, 2018)
- Your Computer Fails: Now What? (Divers Alert Network, Alert Diver, 2010)
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