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The Millennium Watch Book
Bell & Ross - Champion of the Abyss

Bell & Ross Champion of the Abyss

The Hydromax has reached record-breaking depth ratings of 11,100 metres by tackling the problem of deep-sea pressure at source — but its greatest achievement is managing to do so without sacrificing wearability

The arrival of Bell & Ross in the field of dive watches was wholly unexpected, as was the level of performance it achieved, especially given the stage the firm was at back then. But the fact remains: it is this brand that holds the record for the most watertight standard watch. Inevitably, the claim comes with a few qualifiers. First of all, it stands out a little in this The Millennium Watch Book, since it was released before the year 2000. Its longevity and importance nonetheless give it special status. In addition, a handful of other models have been capable of similar performance, and as a bonus. The other watches in question weren’t really much more than prototypes (or in some cases were still very much so): they were indeed so huge, thick and heavy, along with such a bulging crystal, that they could not reasonably be worn. By contrast, the incredible thing about the Hydromax is that it has all the characteristics of a recreational dive watch: it’s mass-produced, wearable, small, relatively slim, free of any helium valve, 39mm in diameter and 12mm thick. It also came at a launch price of €2,300 — and in addition, has been tested to 1,110 bar in a hyberbaric chamber by French marine research institute Ifremer. This bantam-weight floors hulks three times its size using an incomparable and extraordinarily ingenious technique.


The problem with dive watches is that they’re full of air, and air is compressed when pressure is exerted on it. As the internal volume of the watch case shrinks, it drags the parts that protect the case with it. The brainwave behind the Hydromax was to replace this air with a liquid. One of the properties of all liquids is that they can’t be compressed; their volume doesn’t change when they are crushed. For instance, seawater isn’t any heavier or thicker on the seabed than on the surface. But how could it possibly be a good idea to fill a watch with its sworn enemy, the very substance that poses a threat to its materials, integrity and most importantly the free movement of its mechanical parts? Surely the idea is, so to speak, dead in the water? Think again.


The inside of the timepiece’s steel case is immersed in a fluoride-based mineral oil known as Hydroil, exclusive to the brand. Pressure evens out naturally between liquids, but there are other considerations for one in a watch. Hydroil doesn’t oxidise the copper, brass, or steel components in the mouvement and doesn’t interfere with the battery, but that still leaves one big problem: the hands need to move without having to push the liquid out of their way as they rotate. Anyone who’s tried to walk under water will know what a huge effort that would be. In this respect, Hydroil has another useful characteristic: it’s structured into layers and offers very little horizontal resistance. For hands to move up or down would be incredibly difficult, using up all the calibre’s power; but they face no friction or adverse viscosity when moving forward in their own plane — thus enabling the Hydromax to have both a date and the sweep seconds hand required for ISO 6425 certification. And of course the oil in question is perfectly transparent.

Champion of the Abyss

Hydromax © Bell & Ross


While Hydroil is incompressible, it is nonetheless sensitive to temperature variations, which alter its volume. The difference between temperatures at the bottom of the sea (virtually 0°C) and a sun-baked beach in summer is 50 degrees. To compensate for this, the caseback (or at least the central section) is made from flexible rubber. Since the caseback rests against the skin, this part is not exposed and the watch remains fully functional. The other advantage of the watch being filled with liquid is that this eliminates the effect of refraction. When light reaches an immersed watch, it’s usually refracted, resulting in the infamous mirror effect — but there’s virtually no refraction when light passes from water to Hydroil. 

The Flip Side

Bell & Ross had installed a quartz movement in the Hydromax’s 39mm-diameter steel case, thus keeping the number of moving parts to a minimum and avoid the dispersal of residue from metal friction that’s inevitable in a mechanical movement. The watch does however have its drawbacks. Properly filling a watch with this highly specialised liquid is no easy task and requires very stringent manufacturing control, and the many accounts of problems in this respect suggest that the exercise has its limits. Moreover, handling this model for after-sales is proving to be uniquely difficult in terms of procedure and complexity. You can’t have it all.

This year GMT Magazine and WorldTempus have embarked on the ambitious project of summarising the divers watch since 2000 in The Millennium Watch Book - Divers watch, a big, beautifully laid out coffee table book. This article is an extract. The Millennium Watch Book - Divers watch is available in both French and English here:


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