Saturday, September 6th, at the COSC space inside the Geneva Watch Days Pavilion. Between two appointments, I found time for a bit of horological action: attempting the precision timing of a Sellita SW 200-1 movement as part of the Calibership Challenge. The movement would then undergo the official COSC chronometer certification tests for mechanical movements.
Fate—and my eternally youthful 25 years—landed me with caliber number 25. A sign? Maybe. After going through the certification tests, the result was in: not a worrying 25-second daily deviation, but a precise 0.14142 seconds. A result well within the tolerance range (-4/+6 seconds) measured according to ISO 3159 protocol, earning the top distinction: "Certified Chronometer"—a hallmark of Swiss excellence.
To earn this label, a movement must pass seven criteria: average daily rate, greatest deviation, mean deviation, rate resumption, greatest variation, rate variation due to temperature, and positional variation (horizontal vs. vertical). The movement is tested over 15 days, in five positions and at three different temperatures (8°, 23°, and 38°C). Enough to seriously shake up the leaderboard as participants rise and fall throughout the process.
One of the most nerve-racking stages? The rate resumption test, which checks how the movement's precision holds up after 14 days of "stress tests" compared to its initial accuracy. “This is the moment when everything can change,” warned Sylvain Broillet from COSC, cautioning me not to get too confident about my lead before this decisive test. In the end, my caliber number 25 handled the pressure like a pro and held on to first place.
Behind the Feat
How to achieve—and repeat—such a result? That’s surely the question on every watchaker’s mind. My secret? Deep breaths in and out before every adjustment, and walking around the bench three times while invoking Chronos in Ancient Greek. My other secret weapon: the support of students from the watchmaking school, who pointed out which components to fine-tune for precision—saving me from dismantling the entire movement. Don’t worry, it wasn’t favoritism: all participants had access to the same guidance, strictly limited to instructions.
To keep track of how effective the adjustments were, accuracy was calculated almost instantly using acoustic measurement of the tick-tock. Based on experience, rushing is a mistake—it’s better to wait a few seconds for the instrument to display the effect of each adjustment.
The timing is done in two stages. First, a rough adjustment, which involves gently nudging the setting index with a stylus, one way or the other. This is when I realized, with some alarm, just how delicate you need to be—any heavy-handedness can lead to wild deviations in rate. Technically, the index acts as a lever to change the effective length of the balance spring, and therefore adjust the oscillation speed. Some watchmakers achieve great results at this stage alone and don’t need to go any further.
But to fine-tune it—if needed (as was my case)—you move on to the precision timing. Using a small screwdriver, you turn the setting screw to get closer to absolute zero (thankfully, not absolute zero in temperature—no frozen calibers here!).
In the end, I have to say the experience was truly enjoyable. On one hand, it required both physical and mental concentration, especially since I’d been bouncing from brand to brand in 30-minute bursts for days—clearly showing signs of hyperactivity, maybe even a few compulsions. On the other hand, the process was punctuated by genuinely fascinating conversations with COSC staff and the students.
The final result—for glory—was just the cherry on top (or maybe the ruby on the pivot) of an amazing experience. More importantly, it helped shine a spotlight on the skill of regulating assembled balances and the rigorous standards of COSC testing. And this protocol is set to evolve—remember, they hinted at a "Super-COSC"... To be continued!
