In 2024, the German company Sinn is introducing a new version of its deep-diving “Hydro” watch family - the Sinn U50 Hydro collection with a 41mm width.
The new collection features various case finishes and a multitude of strap or bracelet options (with the price difference between the versions being nearly $1,000). For a long time, Sinn has been one of the few (and sometimes the only) watch companies producing oil-filled diver's watches. This was the main feature of the popular Sinn UX Hydro collection, which now includes the new Sinn U50 Hydro.
Careful comparison of the Sinn U50 and UX shows that, while the models are very similar in shape, there is more than just a size difference between the 41mm U50 Hydro and the 44mm UX Hydro. The watches also have two different movements. The Sinn UX Hydro watches are harder to find than the new U50 Hydro models, as many are already discontinued. Choosing a winner between these two models will be a matter of personal taste, considering they are also similar in price (with the UX Hydro being slightly more expensive, but not by much).
I will return to the differences in movements a bit later, but first, I want to talk about what makes these diver's watches interesting compared to many other diving watches. The cases are filled with a special oil (Sinn uses its own blend), which fills the dial and most of the movement compartment. The purpose of the oil is to eliminate the empty space that could compress with increased atmospheric pressure on the watches.
Of course, other watches have shown that with a very massive case, it isn't necessary to use oil-filled watches for diving to such depths, but the Hydro technology allows for an extremely high level of depth resistance without an overly large case. With a width of 41mm and a thickness of 11.8mm, the Sinn U50 Hydro watches have water resistance up to 5,000 meters. The case weight without the strap is only 72 grams. Other watches with this level of water resistance without oil-filled interiors are much larger (think of Rolex and Omega offerings in this area).
An interesting question: does the larger Sinn UX model with a width of 44mm and thickness of 13.5mm have a greater diving depth than the U50? The answer is both “yes” and “no.” Both models are rated for a diving depth of up to 5,000 meters. However, the UX case is rated for an even greater depth - 12,000 meters. So, what does this mean? The reason the depth rating for each of the watches cannot be below 5,000 meters is that at such pressure, the case housing the quartz crystal of the movement breaks down on its own. In oil-filled watches, quartz, not mechanical movements, must be used because the oil makes mechanical movement operation impossible. Thus, technically speaking, if you want to dive to crazy depths, a standard quartz movement will break at 5,000 meters, while a mechanical movement could theoretically work deeper if protected by a sturdy case.
However, to protect a mechanical movement at such depth, the case would need to be so large that it would be unwearable on most wrists. Theoretically, a quartz movement designed for high pressure in an oil-filled case could become the deepest diving watch, but such an opportunity does not yet exist. Thus, we have a strange situation where there is no clear winner in the question of the deepest watch from a technical or practical standpoint.
Such wild depths are purely the realm of adventure fiction since no human can dive to such depths and survive, and being in a pressurized underwater vessel at such a depth does not require special watches. This means if you like to boast with macho serious sturdy watches and want to wear them regularly, the convenient size of oil-filled diver watches is likely to outperform similar mechanical ones.
Another interesting advantage of oil-filled watches is clarity. The presence of oil eliminates the visual sense of dial depth. It optically brings the dial image to the surface, where it meets the sapphire crystal, giving the dial an extremely clear view, but still easily readable at a wide angle. This is difficult to describe unless you hold the watch in your hands, but the visual effect is very cool, and that's why many people love to buy Sinn UX watches and, possibly now, the U50 Hydro.
And lastly, a very important note about the oil-filled case system used by Sinn: Older oil-filling technologies required a bubble in the oil. The bubble was necessary to give the oil space to expand under pressure. Otherwise, the expanding volume of oil could rupture the case, which completely negates the purpose of the watch. Some people even liked the bubble so much that they made it a fashion statement.
U-Boat is a watch brand that produces a series of watches called Dark Moon, which is presented in a dizzying number of versions and styles and includes oil-filled cases with visible moving bubbles. Only in the case of U-Boat's Dark Moon watches, the oil is purely cosmetic, and the bubble is something Sinn would certainly not be proud of. Sinn developed a case back with a slightly expanding membrane, eliminating the need for an oil bubble. When the oil expands under pressure, the case back slightly protrudes - like a person who has eaten a big meal. When the pressure subsides, the case back returns to its normal position. This system is used in Sinn UX watches as well as the U50 Hydro model, so there is no need for bubbles, and the dials result in looking surprisingly clean and clear.
Inside the Sinn U50 is the Swiss quartz caliber Ronda 715 Li (time plus date). The Sinn UX houses the Swiss caliber ETA Thermoline/Flatline 955.652. Both movements operate for about 10 years on lithium-ion batteries, and both are roughly the same size. The difference is that the older ETA 955.652 movements have thermal compensation and are thus noticeably more accurate. The accuracy difference is about 10 seconds per year for the ETA Thermoline and about 10 seconds per month for the more standard quartz Ronda 715. So why did Sinn decide to use the simpler Ronda model for the U50? I don't know, but I have a guess. Sinn was more or less the only company using the ETA 955.652, and overall, I know that ETA has stopped producing many of its thermocompensated movements.
For example, Breitling had to start producing its own analog-digital quartz movements with thermal compensation for the new Aerospace collection, as ETA reportedly ceased production of the movements it previously used. Expensive and very niche, thermocompensated quartz movements may have a future, but currently, ETA, part of the Swatch Group, has decided to discontinue their production. This decision limits the options for brands like Sinn, and thus for the U50 Hydro, another movement had to be ordered from another Swiss company.
On WatchBuys.com, where Sinn watches are sold in the United States, there are currently 15 different versions of the Sinn U50 Hydro available. This should serve as an example of how many versions Sinn offers. The most basic version of the U50 has a fully German submarine steel case and a textile strap (essentially a double-thick nylon strap, which is quite nice).
One step up is the same watch but made from Tegimented steel (a special bead-blasted treatment that makes the surface extremely scratch-resistant). Sinn also offers the U50 with a black rotating bezel (U50 Hydro SDR) or completely black U50 (U50 Hydro S). Tegimented steel and black Tegimented steel bracelets are also available. Not exclusive to the U50 model, in 2024 Sinn also introduced a new delpoyant clasp for its straps, which includes micro-adjustment, unlike the much less useful diver's extension that was used before. It is available on the U50 and other watches in the Sinn catalog.
Even without an ultra-precise quartz movement, the U50 Hydro collection is a splendid continuation of the Sinn UX Hydro in a more wearable size and for a new generation of enthusiasts. These tool-style watches are charming and beautiful because they do their job well, as well as due to the many convenient and useful features that Sinn includes, starting with excellent luminescence and clarity, crown placement at 4 o'clock (so it doesn't dig into the wrist), and exceptional comfort on any of the strap or bracelet options.
I'm glad that more and more high-end quartz watches like the U50 Hydro are coming to market, which also have a special dial look unlike most diving watches available. While these watches are not for everyone, they are an incredible tool produced by a renowned company with few real competitors. I would wear such a watch very often.
In US dollars, prices for the Sinn U50 Hydro range from $2,460 to $3,400. More detailed information can be found on the Sinn website.