Special report: why the F. Berthoud FB3 deserves to win the GPHG Chronometry prize

In the recent GPHG Awards 2023, the F. Berthoud FB 3SPC was awarded the Chronometry Prize. Here is a technical discussion on why this is well deserved.

Special report: why the F. Berthoud FB3 deserves to win the GPHG Chronometry prize

The Ferdinand Berthoud FB 3SPC won the Chronometry Prize in the 2023 Edition of the GPHG. This has been one of our favourite watches since we first saw the launch watches pre-embargo lifting. We subsequently took a closer look with a deep dive, comprehensive review of the FB 3SPC. We also came away super impressed.

Of course, we did not have the ability to perform chronometry testing on the FB 3, but the fact remains that this is the first watch with a cylindrical hairspring to have achieved COSC certification. While we sometimes throw water at COSC certification, it took a while for the FB 3’s achievement to sink in. Here is what we found. And stand in awe.

Why a cylindrical hairspring

Conventional hairsprings are wound concentrically from the center in ever expanding diameters to form a spiral. The entire spring can be laid on a table flat, safe for a when a terminal curve, which can be applied by hand by a master craftsman. This hairspring should develop concentrically when in use, maintaining the same center of gravity throughout the oscillation. If it develops irregularly, there is inconsistent spacing between the coils resulting in uneven expansion and contraction patterns which will, in turn lead to shifting of the center of gravity. A terminal curve is intended to ameliorate this effect. Furthermore, if left in one position, the weight of the spring will cause it to sag more in one side of the pivot than the other, causing a shift in center of gravity. Thus the rationale for a tourbillon in a pocket watch.

However, in a cylindrical hairspring, each consecutive revolution is made in the same diameter but one on top of the other. This construction form eliminates eccentricity of the effect of a spiral expanding and contracting, a motion poetically described by watchmakers as “breathing”. The spring is no longer eccentric, but the breathes from the center of gravity which remain stable throughout the oscillation.

Thus cylindrical hairsprings are typical in marine chronometers, which were the principle navigation in the days before GPS. Ferdinand and his nephew Louis Berthoud made marine chronometers back in the day. As an example, Marine Chronometer No.54, now in the Chopard collection, was equipped a cylindrical hairspring. Karl-Friedrich Sheufele wanted to produce a modern Berthoud wrist watch with the cylindrical hairspring which retained the superb chronometric performance of No.54. And that was the basis for the FB 3.

However, it has been long established that the cylindrical hairspring has excellent chronometry performance when kept in a horizontal position with the hairspring perpendicular to the ground, but is compromised otherwise. In the vertical position, the increased mass of the cylindrical hairspring would amplify the chronometric error.

Thus, in marine chronometers, the clockwork was kept in a gimbal system allowing it to be always in a horizontal position – the most favourable position for a cylindrical hairspring, regardless of the level of the ship’s deck. The treatise by J. Haag in Théorie du spiral et ses applications à la Chronométrie even adds “the cylindrical hairspring should not be used in a vertical position”.

This is constant position is not possible in a wrist watch, which encounters almost all positions, including the vertical as the wearer moves during the day. A tourbillon helps to even out these positions. See the modern examples below for executions of the cylindrical hairspring by Jaeger-LeCoultre, Montblanc, H. Moser and MB&F for details of how the hairspring is combined with a tourbillon. As noted below, the Alchemist as well as the Bvlgari were equipped with cylindrical hairsprings as well, but sans tourbillon.

“Initially, we were told that it would be impossible to achieve chronometer certification with a cylindrical hairspring. That was all the impetus we needed to rise to the challenge.”

Karl-Friedrich Scheufele – President and Founder Chronométrie Ferdinand Berthoud

Thus began the team’s challenging three year journey.

The Berthoud cylindrical hairspring

As they set out to work, the Berthoud team’s challenge was even larger. They wanted to use the cylindrical hairspring for its raison d’être of chronometric excellence as demonstrated by COSC certification. Yet wanted to do this without a tourbillon. A la Berthoud. They took inspiration also from a later work by Louis Berthoud. Enter the Decimal Watch No.26. We refer to the excellent scholarship of D. Reggiani of Savona and also from Jean-Clude Sabrier’s “La Longitude en Mer à l? Heure de Louis Berthoud et Henri Motel” Antiquorum Editions, Geneva, 1993. This watch was considered by Louis Berthoud to be the most important of the chronometers he made. No.26 showed decimal time, a system where the year was divided into 12 equal months having 30 days each and an additional 5-day (6-day in a leap year) holiday month; each month was divided into three decades, each having 10 days. The 24-hour day was divided into ten hours of 100 minutes each, and each minute was sub­divided into 100 seconds. Thus, for example, 12:30 PM was 5:20:83.3 in decimal time. The law introducing the changes was passed on 24 November 1793. Another example of this Decimal watch can be observed in Chopard’s LUCeum museum in Fleurier.

What is of interest to us in this discussion, is that No.26 had a cylindrical hairspring, which meant that its cylindrical hairspring would have been kept in a vertical position most of the time it was inside the pocket of the owner. Or it perhaps it was used as a desk clock in its deck box as shown, which allowed it to be horizontal at all times.

Even with this spectacular specimen of the cylindrical hairspring watch in the Chopard collection, the team had little to work on as there were hardly any published literature on the techniques on the cylindrical hairspring. The Berthoud team had to go back to first principles and use the time and tested Scientific Method to find the best settings. The theory was to optimise the characteristics of the hairspring while taking into account the movement height and construction. Thus, the critical considerations were the inertia of the system, the hairspring shape (height and diameter), the collet shape and the terminal curve. Berthoud buys the cylindrical hairspring without terminal curves from Precision Engineering, who manufactures them in their facility in Schaffhausen.

The shape of the terminal curve was first worked out by French engineer and mathematician Edouard Phillips in 1860. He established that there was a relationship between the period of oscillation, the balance’s moment of inertia, the length of the hairspring and the elastic moment. And by applying a terminal curve to the ends of the hairspring, whether flat or cylindrical, is to enable the spring to breathe concentrically.

The first task for the team was to set up with the calculations and bend the terminal curves. Initial testing showed that the cylindrical hairspring had a tendency to breathe asymmetrically when assembled into the balance and fixed to the collet and pin. This was despite that the calculations and tests of the unmounted spring was performing within the calculated expectations. But when assembled, with the mass of the collet and pin considered, and the center of gravity shifted out of the central pin position, causing the imbalance. This kind of asymmetry will cause a variation to the rate.

To address this problem, the obvious solution was to reduce the mass of the collet and re-center the center of gravity. But in a carefully balanced formula, changing one variable will inevitably involve having to adjust others as well. Thus, this one change leads to another, and as a consequence, the terminal curve needs to be recalculated and redesigned. This took three years. Several designs were considered. And each tested using the scientific method: hypothesis, test, and iterative refinement. It was decided to select the one with the optimum number of manipulations applied to the spring. This number turned out to be 3 folds. However, this design results in a compromise in the radius of the curve attachment point. The relationship between the collet and stud must be equal to 90° to guarantee isochronous oscillations. Which then led to a redesign of a new collet.

Finally with all these pieces together, the technical requirements of theory and practice come together. This requires precision adjustments being made in exactly the same way to each hairspring made. As the manipulations are done by hand, the work is done under 20X magnification. Each adjustment had to be made by a master watchmaker. The skilled master’s touch, perspective and experience is essential to achieve the desired level of perfection in mechanical construction, in energy equilibrium and in chronometric performance.

To optimise the fine adjustments, the balance wheel is variable inertia, equipped with 4 fine adjustment screws and 8 load screws. With these screws on the periphery of the wheel, the precise balance of the inertia of the wheel can be fine tuned.

The final desired outcome was that out of 40 movements were submitted in 2023 and passed the COSC so far, the average daily rate was + 2.08 seconds per day, with 8 out 10 achieving a daily rate between -1 +3 seconds a day. To provide some context, the COSC standard is -4 to +6 seconds per day, while the Rolex’s Chromometer Specifications is +/- 2 second a day.

The cylindrical hairspring: other examples

The cylindrical hairspring is not new in wrist watches. We have seen it in a multi-axis tourbillon as long ago as SIHH 2012 with Jaeger-LeCoultre’s Duometré Spherotourbillon. JLC followed up with the Master Grande Tradition Tourbillon Cylindrique and the Master Grande Tradition Tourbillon Cylindrique à Quantième Perpétuel Jubilee in 2016. While we understand the hairsprings were made by Lange in their facility in Glashütte, we are not sure if it is delivered in cylindrical form or a rolls, though, the terminal curves were probably manipulated in Le Sentier.

Bvlgari also used a cylindrical hairspring in their L’Ammiraglio del Tempo Minute Repeater in 2015, with the already impressive carillon minute repeater equipped movement with cathedral gongs being also equipped with a detent escapement driven by a constant force remontoire, no less.

Also in 2016, Montblanc Collection Villeret Tourbillon Cylindrique Geosphères Vasco da Gama was released, also with a cylindrical hairspring. This was not long after Jérôme Lambert who had been CEO of JLC till 2013, and moved to be CEO of Montblanc. In 2019 , the Alchemist Cu29 was introduced, also with a cylindrical hairspring, likely to have been made by Precision Engineering, a company owned by MELB Holdings who also own H. Moser and Hautlence. Moser themselves have used the cylindrical hairspring in the H. Moser x MB&F Endeavour Cylindrical Tourbillon as well as by MB&F with the LM Thunderdome.

Concluding thoughts

Of these earlier examples, all except for the Alchemist and the Bvlgari feature a tourbillon. As earlier pointed out, the constant movement of the tourbillon helps negate the effects of the cylindrical hairspring when not in a horizontal position. And all of these did not have COSC certification, and none have even published chronometry specifications, nor claimed to be chronometers. Which make the achievement by F. Berthoud all the more impressive.

So it is of great importance that the FB 3SPC takes the prize for Chronometry at the 2023 GPHG, and a deserving win.