Cave Diver: An Expedition with the Sinn U2 S Dive Watch

We took the Sinn U2 S dive watch deep into the craggy darkness of a water cave. Does it come out alive? Find out in this comprehensive watch test from the archives, with original photos by Nik Schölzel.

Caves are dangerous places. Spelunking technology has advanced, but the risks associated with cave exploration remain. Before our team of explorers can navigate Falkenstein cave, the longest cave in Germany’s Swabian Alps, each of us must put on protective gear: a helmet with a watertight headlamp, rip-resistant coveralls, thick gloves and sturdy boots. The cavern we plan to explore is a water cave through which the Elsach River flows, so we need to dress to prevent hypothermia as well; a diving suit and neoprene footwear are necessities. Diving through “siphons,” parts of the cave that are completely filled with water, will be on today’s agenda, too, so each of us will need diving equipment: a mask, a breathing regulator and a heavy tank of air. Needless to say, we also want to put our test watch, the Sinn U2 S, through these ordeals.

Sinn U2-S - cave explorers
The cave’s entrance narrows quickly.

We chose the Sinn U2 S watch for this excursion mainly for its robust and scratch-resistant case; a caver’s watch often rubs against rough stone, and not only when he must crawl through low and narrow passageways. To create this rugged case, Sinn uses submarine steel, the same steel alloy that’s used on the outer hull of submarines. Then the steel is “Tegimented,” i.e., hardened using Sinn’s special engineering process, and coated with a layer of hard black material. The resulting surface is nearly as hard as sapphire. (For comparison, the 316L steel used for watch cases has a hardness of 220 Vickers; submarine steel is at least 300 Vickers; hardened submarine steel is 1,500 Vickers; and a coating of hard material brings the hardness up to 2,000 Vickers, i.e., the same hardness as sapphire.) But the case is this hard only on the surface; underneath, the hardness decreases with depth. Sinn hardens the steel case of the U2 S before coating it with the layer of black material to prevent an “eggshell effect” from occurring. If a hard coating is applied to a comparatively soft material, a strong blow can cause the hard outer layer to break inward like the shell of an egg: the softer underlying material yields to the impact. But the Sinn’s steel case is less likely to break than similar scratch-resistant ceramic cases because the underlying steel case has been hardened. In addition, the Sinn has a nonreflective coating, which has a hardness of 1,800 Vickers and is therefore highly scratch resistant. The case of the U2 S is thus very well prepared for our descent.

As we approach the entrance to the Falkenstein cave, the sun is shining on a picturesque scene: a mountain brook rushes along next to a hiking trail that winds through sparse forest toward the cliff face. We wriggle into our diving suits, slip on our red coveralls, pull on our helmets and strap tanks of air to our backs. I strap on the Sinn U2 S, which just barely fits around my diving suit’s 5-mm-thick neoprene sleeves with the divers’ extension folded out.

Sinn U2-S on wrist
The red accents on the dial are a perfect match for our red coveralls.

The watch is well protected against water and moisture. The case is water-resistant to 2,000 meters. The seals are made of green Viton, a synthetic rubber and fluoropolymer elastomer that has a longer lifespan than the black nitrile rubber often used for O-rings in watches. Viton lets just one-quarter as much gas and humidity diffuse into the watch’s interior and is resistant to a wider variety of chemicals. Water resistance is increased further by Sinn’s dehumidifying technology: the case is filled with a protective inert gas that has large molecules, which makes it even more difficult for water vapor to penetrate. Another element of Sinn’s dehumidifying system: copper sulfate capsules inside the case, which bind any residual moisture. The extra effort pays off: we see no fogging on the underside of the crystal – neither in the mild springtime air nor in the cave, where the watch will undergo a variety of conditions, including a dip in 48-degree water.

We phone a friend to say that we are getting ready to enter the cave. He promises to call rescuers if he doesn’t hear from us again in six hours. There’s no signal reception in a cave so cellphones and radios are useless. A spelunker is totally cut off from the outside world. If rock breaks from the ceiling and blocks our return route, our only hope of rescue would be from someone outside the cave who knew about our expedition and the approximate time we planned to resurface.

The Elsach River, which we had been following upstream, emerges from the impressive entrance to the cave. We turn the rotatable bezel of the U2 S to the minutes hand so we can measure the time we’ll spend inside. The bezel clicks into place in single-minute steps. This task is accomplished easily, as are all other settings. Fine tuning is convenient and speedy thanks to rapid resetting for the date, quick setting in hourly increments for the second time zone, and a stop-seconds function to halt the seconds hand while the hour hand and minutes hands are repositioned.

Now it’s time to venture into the darkness. We soon find ourselves on all fours, crawling through cold water at the first narrow spot in the cave. We have to keep our heads down to avoid banging our helmets on the low ceiling. Daylight fades and then disappears completely after we round two or three curves. The cone of light from our headlamps plays across the wet stone, and we try to shine our lights into the chest-deep water to see some of the big boulders that we will have to scramble over or slide down. Water penetrates our neoprene suits. We feel uncomfortably cold whenever we stop moving, for instance during our numerous stops to shoot photos, because we’re lying almost entirely in water. We’re also carrying heavy photographic equipment: a tripod, lamps and a bulky underwater camera. Nonetheless, our spirits are high. We all feel strong and confident. And we concentrate on treading carefully. Sometimes we have to climb over fallen rubble or squeeze through narrow passages.

Sinn U2-S - cave explorer in water
We donned diving gear to traverse a “siphon,” a low and narrow flooded passage in the cave.

The Sinn U2 S frequently makes contact with the rocky walls and sometimes these encounters are anything but gentle. The dial’s legibility remains very good, although I don’t need the ample coating of luminous material because my headlamp brightly illuminates the watch whenever I lower my gaze to check the time. The black and red of the watch matches my coveralls. This version of the U2 looks more like a “tool watch,” a piece of expedition equipment, than does the U2 model that has an uncoated steel case. After more than three hours in the cave, we encounter our first siphon. Only about 20 inches separate the floor from the ceiling here. This low crawlspace is completely filled with water or, to be more precise, water comes rushing toward us because the cave is a subterranean riverbed. We could not have gone any farther without diving equipment.

Fortunately, a cable has been installed at this challenging spot, so we should be able to use it to pull ourselves through to the other side. But before we attempt this maneuver, we put on our diving masks, open our tanks of air and insert the breathing apparatus into our mouths. The passageway is so low that we can’t keep the air tanks on our backs; we must pull them along beside us. Our neoprene suits make us so buoyant that the flowing water lifts us up and our backs and helmets scrape along the siphon’s ceiling. This produces a very disconcerting sound, as though someone were struggling to survive.

One after another, we dive through the siphon. The passageway is only a few yards long, but it takes courage to overcome our nervousness and plunge ahead under water, into the unknown. We’re also well aware that this siphon leads us deeper into the cave, making it more difficult to rescue anyone who becomes injured. But a few minutes later, we’re all safe and sound on the other side. Our guide congratulates each of us with a handshake: we’ve successfully traversed our first siphon!

Sinn U2-S - back
After the ordeal, the ETA 2893-2 keeps time as if nothing had happened.

It’s not much farther to get to one of the cave’s large halls, but my watch tells us that we’ve already been underground for nearly four hours. The photo shoots took much of our time. If we descend any farther we will have to navigate at least 22 siphons. The cave stretches onward after that, but that’s as far as anyone has explored so far. We, too, have had enough for today. Everyone wants to return to daylight. But first we have to dive through the siphon again, which is not quite so daunting the second time around, partly because we’ve experienced it before and partly because we know that each scrape and tug brings us closer to the cave’s entrance.

We progress along the return route without taking any long breaks to shoot photos. Our constant motion helps keep us from feeling cold. Fortunately, we don’t have to test the U2 S throughout its entire temperature range. Sinn guarantees that this watch will continue to work at temperatures between -45 and +80 degrees C (-49 and +176 degrees F). This wide range is achieved by using synthetic oil in tandem with manufacturing tolerances that take into account the watch materials’ tendency to expand or contract as temperatures rise or fall.

We repeatedly stop to admire unusual stone formations. The U2 S is almost impossible to feel through the sleeve of my neoprene suit. But I know that it is quite comfortable, despite its ample 44-mm diameter, because I have been wearing it for several days before our cave excursion. Nearly 5½ hours after entering the cave, we breathe a collective sigh of relief as we emerge into daylight and congratulate one another.

Sinn U2-S - Cave explorers groupshot
Many boulders had fallen long ago from the cave’s ceiling.

Now we finally find a use for the tripod that we’ve been carrying with us the whole way. We set it up and pose for a group photo. Then we make a quick phone call to our contact person to say that all’s well, thus making sure that the rescue service doesn’t receive a false alarm. It was a wise decision not to venture beyond the siphon to the large halls. We change into dry clothes and head for the local tavern, where we eat, drink, relax and review the day’s experiences.

The moment of truth for our test watch comes the next day. How well did the Sinn U2 S survive this ultimate endurance test? We see no scratches on the case with the naked eye. That’s surprising because I didn’t treat this watch very kindly during our expedition. Even though I was distracted by our surroundings, I knew that I smacked the watch against the rocky walls more than once.

The clasp didn’t fare quite as well. With its protruding safety buttons and smooth finishes, the rather thick folding clasp struck rock almost every time I reached for a handhold. Fine scratches are visible and we can see the underlying metallic material, even though the scratches aren’t very deep. The outer black layer seems to be rather thin. We know that stone is a harsh adversary and Sinn states that the colored coating is scratch resistant but not indestructible. Nonetheless, we’re a bit disappointed.

Would a thicker coating on the clasp have fared better? Would the scratches have been less prominent on the version of the clasp that was Tegimented but not coated with hard material? Both questions are difficult to answer. The case, at least, suffered no visible damage, and you can replace the scratched clasp with a new one. And we don’t expect extreme contact with rocky walls to occur in everyday life.

Sinn U2-S - buckle
Despite Tegimenting and coating with hard material, the clasp suffered some minor scratches.

All in all, the U2 S coped very well with the rigors of our subterranean excursion, but we did notice one drawback that also occurs in daily life: the silicone strap attracts dust and then looks as if it has discolored. Fortunately, the strap can be restored to pristine cleanliness simply by rinsing it in water.

The movement, the ETA 2893-2, which is the two-time-zone version of the 2892, performed quite well on our timing machine. With an average gain of 5.7 seconds per day, our test watch’s rate was quite satisfactory. The readings in the various positions weren’t spread too far: the greatest difference was just 7 seconds.

If you unscrew the fully threaded back, you’ll be pleased to see that the movement has a precise Glucydur balance and looks quite handsome, with rhodium-plated bridges and rotor, Geneva waves, cloud-pattern embellishment and several blued screws. The movement has been used for decades and has proven itself in countless watches. This is only a standard caliber, so connoisseurs cannot expect to find special features such as regulator-free fine adjustment.

The Sinn U2 S model that we tested sells for $3,450. This is quite a bit more than Sinn’s entry-level model at $1,090. But bear in mind exactly how much technology is packed into the U2 S: water resistance to 2,000 meters, a rotatable bezel, a case made of submarine steel that’s been hardened and then coated with a layer of hard material, stay-dry technology with a filling of protective gas and stay-dry capsules, plus guaranteed functionality throughout a wide range of temperatures. Each of these features offers the owner a valuable benefit and increases the watch’s margin of safety. Some other brands ask significantly more for watches that contain the same movement in the same quality level but lack these technologies.

The Sinn U2 S faithfully accompanied me into the cave. I could rely on it throughout our expedition. I expected this extreme trip to leave some traces on the watch, but the minor wounds are just that: minor. The bottom line: if you’re looking for a faithful companion to join you when you indulge in extreme hobbies or work in a rigorous profession, or if you just want to own a watch with a wide margin of safety, then the U2 S is for you.

Sinn U2-S expedition group photo
From left: cave guide Udo Happ, diver Jörg Stolzenberg, diving instructor Jens Köppe, journalist Jens Koch, and photographer Nik Schölzel


Manufacturer: Sinn Spezialuhren, Im Füldchen 5–7, D-60489 Frankfurt, Germany
Reference number: 1020.020
Functions: Hours, minutes, seconds, date display, second time zone
Movement: ETA 2893-2, automatic, 28,800 vph, 21 jewels, stop-seconds function,  rapid reset for the date display, Incabloc shock absorption, Glucydur balance, fine adjustment via index adjuster and eccentric screw, 42-hour power reserve, diameter = 25.6 mm, height = 4.1 mm
Case: Submarine steel, Tegimented and coated with hard material; filled with  protective gas; three dehumidifying copper sulfate capsules; special oil for temperatures ranging from  -45° to +80° C; fully   threaded back; water resistant to 2,000 m; impossible-to-lose, unidirectional rotating bezel; domed sapphire crystal with nonreflective treatment on both sides
Strap and clasp: Silicone strap with secure folding clasp made of stainless steel, Tegimented and coated with hard material, with built-in divers’ extension
Rate results (Deviations in seconds per 24 hours):
Dial up         +9
Dial down         +5
Crown up        +2
Crown down        +7
Crown left         +6
Crown right         +5
Greatest deviation of rate    7
Average deviation    +5.7
Average amplitude:
Flat positions        301°
Hanging positions        260°
Dimensions: Diameter = 44 mm, height = 15.5 mm, weight = 176 grams
Variations: Without a coating of hard material ($2,950)
Price: $3,450


Strap and clasp (max. 10 points): The supple silicone strap and the robust safety clasp with divers’ extension are high-quality and work very well, but the strap attracts dust.             8
Operation (5): The crown, which uncouples from the winding mechanism when it’s screwed shut, is easy to turn and pull out; setting is convenient and speedy thanks to a stop-seconds function, quick setting in hourly increments for the second time zone, and a rapid-reset  function for the date.         5
Case (10): Well-crafted case with a high degree of water resistance, sophisticated stay-dry technology and scratch-resistant surfaces        9
Design (15): The handsome “tool watch” design looks even better in the black version.     13
Legibility (5): The legibility couldn’t be better day and night thanks to readily visible hands, high contrast and plenty of luminous material.        5
Wearing comfort (10): Despite its heavy weight, this watch fits very comfortably; the crown does not press against the back of the wrist.     9
Movement (20): The unaltered ETA movement with Glucydur balance is robust, reliable and time tested.       12
Rate results (10): Good rate results with a slight daily gain and no large differences among the various positions     7
Overall value (15): The price is low considering the watch’s many technological features, which make it extremely durable.     13

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  1. Gary Whitehead

    As an ex caver not something that I would do to any expensive watch, no matter how tough. The author has it right, caves are perhaps the toughest environment for any mechanical device (rocks, water, mud, grit, impacts etc). Think watch covered in layer of wet abrasive grit whilst squeezing through tight spaces.
    A cheap waterproof Casio would be the choice of most cavers (leaving the expensive watch for the bar afterwards)….
    Fair test though, and I am not surprised that signs suffering where seen on the finishes afterwards.

  2. A. G. (Alex) Tsakumis

    What a disappointing post! You cannot properly see the watch on the explorer’s wrist. I hardly care how good the follow up picture of the watch’s back looks if I can’t very well see the front!!! Not one of your better posts.

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