For 2024, Germany’s Sinn introduces a new version of its “Hydro” deep diving watch family with the 41mm wide Sinn U50 Hydro timepiece collection. Within the new collection are a variety of case finishing options and many strap or bracelet attachments to choose from (with a price delta of nearly $1,000 among the various versions). For quite a while, Sinn has been one of the very few (sometimes, the only) watch companies to produce a diver’s watch filled with oil. This was the mainstay feature of the popular Sinn UX Hydro collection, which is now expanded with the new Sinn U50 Hydro collection.

A close comparison of the Sinn U50 and UX shows that while the models are very similar in form, there is more than just a size difference between the 41mm-wide U50 Hydro and the 44 mm-wide UX Hydro. The watches have two different movements as well. While Sinn UX Hydro watches are harder to get than the new U50 Hydro models as many of them are no longer produced. Choosing a winner between the two models will be a matter of personal taste given that they are also similar in price (with the UX Hydro being a bit more expensive, but not by much).

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I will return to the movement difference in a moment, but first I want to talk about what makes these diver’s watches interesting compared to the plethora of other diving-style watches available. The cases are filled with a special oil (Sinn uses its own special blend) that fills the dial and much of the movement compartment. The goal of the oil is to eliminate empty space that can be compressed as the watch increases its exposure to atmospheric pressure. Other watches have, of course, demonstrated that with an extremely beefy case, you don’t necessarily need to have an oil-filled watch to go that deep, but the Hydro technology does allow for an extremely high level of depth-resistance without having a case that is too large. Accordingly, the Sinn U50 Hydro timepieces are water resistant to 5,000 meters being 41mm wide and 11.8mm thick. Without a strap, the cases weigh only 72 grams. Other watches with water-resistant ratings at that level without oil-filled interiors are much larger (think of Rolex and Omega’s offerings in this space as an example).

An interesting question is whether the larger 44mm wide and 13.5mm thick Sinn UX has a deeper depth resistance than the U50. The answer is both “yes” and “no.” Both of the watches are rated to a depth resistance of 5,000 meters. But the UX case is rated to an even deeper 12,000 meters. So, what gives? The reason the depth rating for either of the watches cannot be lower than about 5,000 meters is that the housing that contains the quartz crystal in the movement collapses on itself at this pressure. Oil-filled watches must use quartz versus mechanical movements because exposure to oil would make the operation of a mechanical movement impossible. Thus, technically speaking if you want to go crazy deep a standard quartz movement will break at 5,000 meters, whereas a mechanical movement could theoretically work deeper provided it is protected in tough housing. That said, to protect a mechanical movement that deep, the case needs to be so large it verges on not being wearable for most wrists. In theory, a quartz movement designed to withstand high pressure in an oil-filled case would be the deepest diving watch possible – though such a moment does not yet exist. That leaves us with a strange situation where there is no clear winner on the deepest watch technically or practically.

Such wild depths are purely the realm of adventure fantasy as no human being could ever dive that deep and survive, and being in a pressurized submersible that deep underwater doesn’t tend to require a special timepiece. What this means is that if you like the macho bragging rights of a seriously durable timepiece and want to wear it on a regular basis, the comfortable size of an oil-filled diver’s watch will likely beat that of a similarly performing mechanical one.

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There is another interesting benefit to having a watch with an oil-filled case: legibility. The presence of the oil removes much of the visual sense of depth in the dial. It optically brings the image of the dial to the surface where it touches the sapphire crystal and gives the dial an extremely clear look, but also one that can be easily read at extreme angles. It is a bit difficult to describe unless you are handling the watch yourself in person, but the visual effect is very cool and often why many people like buying Sinn UX and probably now U50 Hydro watches.

A final and very important note about the oil-filled case system that Sinn uses: Older oil-filled watch technologies required a bubble to be in the oil. The purpose of the bubble was so that, under pressure, the oil would have room to expand into. Otherwise, the expanding volume of the oil would break open the case – totally defeating the purpose of an oil-filled watch. Some people even liked the bubble enough to make a fashion statement out of it. U-Boat is a watch brand that has a series of watches called the Dark Moon, which come in a dizzying array of versions and styles and includes oil-filled cases with prominent bubbles moving around in them. Only in the case of U-Boat’s Dark Moon watches the oil is purely cosmetic and the bubble is something that Sinn certainly wouldn’t be proud of. Sinn engineered a caseback with a slightly expanding membrane that eliminates the need for the bubble in the oil. When the oil expands under pressure, the caseback protrudes a little bit – just like someone who ate a large meal. When the pressure subsides, the caseback returns to normal. This system exists on the Sinn UX as well as the U50 Hydro models – so no bubble is necessary and the dials appear wonderfully clear and clean as a result.

The movement inside of the Sinn U50 is the Swiss Ronda quartz caliber 715 Li (time plus date). Inside the Sinn UX is the Swiss ETA Thermoline/Flatline caliber 955.652. Both movements have roughly 10 years of charge using lithium-ion batteries, and both are roughly the same size. The difference is that the older ETA 955.652 movements are thermo-compensated and thus markedly more accurate. We are talking a difference of about 10 seconds of accuracy per year with the ETA Thermoline, and about 10 seconds per month with the more standard quartz Ronda 715. Why, then, did Sinn decide to use the more basic Ronda for the U50 watches? I don’t really know but I have a good guess. Sinn was more or less the only company using the ETA 955.652 and in general, I know that ETA has been stopping production of a lot of its thermo-compensated movements. Breitling for instance had to begin production of its own analog-digital thermo-compensated quartz movements for the new Aerospace collection because ETA reportededly stopped production on the movements it used to use. Expensive and very niche, while there may be a future for thermo-compensated quartz movements, for now, the Swatch Group’s ETA movement-making company has decided not to make any (or many) of them. That business decision limits what brands like Sinn can get and thus it needed to order a different movement from a different Swiss company in order to outfit the U50 Hydro.

On where Sinn watches are available in the United States, 15 different versions of the Sinn U50 Hydro are currently available. That should offer an example of how many versions Sinn already offers. The most basic version of the U50 has an all-German submarine steel case and textile strap (basically, a double-thick nylon strap that is pretty nice). A step up from there is the same watch but with Tegimented steel (a special sandblasting-style treatment that leaves the surface extremely scratch-resistant). Sinn also offers the U50 with a black-colored rotating bezel (U50 Hydro SDR), or the U50 in all black (the U50 Hydro S). Matching Tegimented steel and black-colored Tegimented steel bracelets are also options. While not exclusive to U50 model watches, for 2024 Sinn has also introduced a new delpoyant clasp for its straps that includes a micro-adjust as opposed to a far less useful diver’s extension that was used before it. It is available on U50 and other watches in the Sinn catalog.

Even without the extra-high-precision quartz movement, the U50 Hydro collection is a wonderful follow-up to the Sinn UX Hydro, in a much more wearable size and for a new generation of enthusiasts. These tool-style watches are charming and beautiful because how well they do their job and all the many convenience and utility features that Sinn includes ranging from the excellent lume and legibility to the 4 o’clock placement of the crown (so it doesn’t just into your wrist) and the exceptional wearing comfort on any of the strap or bracelet options. I am glad to see more high-end specced quartz watches like the U50 Hydro become available, which also benefit from a special dial viewing experience that is unlike most diver’s watches on the market. While not for everyone, these are incredible tool watches produced by a rarified company that has few real competitors. I would wear one of these very frequently. In US dollars, prices for the Sinn U50 Hydro watches range from $2,460 USD to $3,400 USD. Learn more at the Sinn watches website.

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