A chronometer is not to be confused with a chronograph – that’s any watch with a stopwatch function for measuring time. A chronometer watch is essentially a fine-tuned watch that keeps better time than most. This is confusing on both counts when you dissect the Greek root. Chronograph means “time writer” and chronometer means “time measurer”, which is what every watch does.

This is not the time for a lesson in ancient Greek, however. What makes a watch a chronometer and what practical value does it have in today’s world? First, you need to know that a chronometer is technically a particularly precise movement, not the entire watch. To be certified, it must pass stringent tests, which last 15 days, at one of the three laboratories of the Contrôle Officiel Suisse des Chronomètres (COSC), losing no more than four seconds a day or gaining no more than six.

A quartz watch gains or loses just 15 seconds a month, let alone a day, but the delicate mechanics of a traditional mechanical timepiece pose a whole different challenge to watchmakers. Creating a single watch with a very high degree of precision (and therefore a very low level of daily variation) is a painstaking technical challenge that requires hands-on expertise. Making thousands of watches a year, all of which meet the same standard of –4/+6 is an equally impressive achievement.

In 2019, 2.15 million mechanical watches graduated with distinction from COSC. This comprised 30 per cent of Switzerland’s mechanical exports. Admittedly, as much as half of COSC’s work is cut out by Rolex alone, followed by a quarter from Omega and slightly less by Breitling, then Tudor, Tissot, Chopard, Panerai, Zenith and Bremont, in no particular order, since 2016, when COSC stopped publishing brand-by-brand stats.

Today, the value of certifying chronometers can be debatable (see below), but there’s no doubt that having a COSC movement ticking on your wrist is a reassurance, in terms of both build quality and reliability.

Long before COSC was established in 1973, so-called observatory trials were held annually at star-gazing facilities. Brands were invited to submit one or two of their star pupils for intense scrutiny. Getting top A-rated marks would then be a potent marketing tool for their bread-and-butter output.

One of the most prestigious trials was at the Kew Observatory in London. For it was London’s watchmakers who first knuckled down in the 18th century, focusing more than ever on horological precision, once clockmaker Mr John Harrison had proved that navigating at sea was better using a robust, reliable “chronometer” than observing the heavens. He won the Longitude Prize in the process.

The precision of a mechanical watch movement is defined by its consistency of timekeeping under varying environmental conditions (accuracy concerns whether it’s telling the right time, which, as the saying goes, even a stopped clock does twice a day). To achieve any sort of precision, let alone chronometer precision, the movement must be regulated by a watchmaker. This is essentially a trial and error approach to tweaking the escapement, the ticking heart of every watch, whose oscillating balance wheel metes out the kinetic power that runs through the geartrain.

Back in the day, temperature was a big factor, which explains Mr Harrison’s early invention of the bimetallic strip – in essence, a spring made of two metals, thereby less subject to shrinking and expanding in the heat. But with modern alloys mitigating the effect of most real-world thermal variations, chronometry comes down to shock resistance and wear. And that largely depends on how fast and consistently the balance is oscillating.

“A higher frequency helps to achieve higher precision,” says Mr Pascal Ricci, head of production at Breitling, “but the power consumption increases as well and some parts could wear out faster than they would with a ‘normal’ frequency. The industry’s standard rate of 28,800 vibrations per hour [4 Hertz] achieves a good balance between precision, power reserve and the wear on the components.”

So, given a chronometer’s margin for error, or daily rate of –4/+6 seconds, is about three times better than a standard Swiss mechanical movement, there’s presumably a fair bit of extra work involved.

“It is significant, though we never disclose the exact figures,” says Mr Ricci. “There is not only the regulating job, but also specific components, such as the escapement, the balance wheel and so on, which have to be at the higher end to be able to achieve it.”

Watchmakers submit bare movements in robot-compatible plastic trays, fitted with plain white dials and basic black hands that are readable in an instant by digital cameras. After winding, the movements are stored at 23°C for 15 days in five constantly adjusted orientations, except for days 11 and 13 when they’re cooled to 8ºC and heated to 38ºC respectively. Testing in different positions is important because gravity differently affects friction at pivot points, as well as the oscillation of the balance.

Because smaller calibres have smaller mainsprings, they have less power. They must also make do with a smaller balance, and tolerances for all parts are generally more generous, relatively speaking. For this reason, daily rate requirements are slightly easier on movements that are less than 20mm in diameter. A COSC certification means daily variance of –5 to +9 seconds per day.

The daily rate of a COSC-certified chronometer movement will not always characterise the watch it finds a home in back at brand HQ, particularly when it’s actually worn, since moving about on the wrist in all orientations and trajectories cannot possibly be adjusted for.

For those who’d rather their –4/+6 seconds per day really was guaranteed on the wrist, there’s Qualité Fleurier, the first qualitative horological certification for complete, cased-up watches, whose Fleuritest involves a watch-wearing robot that mimics everything from cycling to brushing your teeth.

But for less than £2,500, something such as Breitling’s irresistibly accessible, COSC-certified Colt or a Bremont MBII buys you tremendous peace of mind, given the care and better-quality parts that have gone into the mechanics steadfastly ticking beneath your shirt cuff. It is also worth pointing out that many manufacturers regulate their watches to COSC standards, but do not submit them to COSC for testing and are therefore unable to describe their watches as chronometers.