The majority of watches competing in the Grand Prix d’Horlogerie de Genève (GPHG) feature a three-hand movement based on a traditional escapement, a choice which, in terms of reliability, has been proven for more than a century.
However, in the “Chronometry” category in which the FB 2RE.2 model of Chronométrie Ferdinand Berthoud is positioned, the dominant criterion is not only reliability, but above all precision – a quest that once animated the master- watchmaker Ferdinand Berthoud and who still guides the work of the house that bears his name today.
To achieve the highest level of chronometric performance, the Manufacture has opted for certain radical technical choices. In fact, the Chronomètre FB 2RE.2 is the result of an unprecedented combination of two regulation mechanisms: it is the only timepiece with a one-second winder, associated with the constant force of a rocket-chain device.
Driven by the same spirit of transparency characteristic of the Age of Enlightenment, the manufacture unveils here, exclusively, its results of independent tests.
The equality winder, at the heart of chronometry
The fusee-chain regulates the torque at the outlet of the barrel so that it is constant, whether the watch is fully wound or at the end of the power reserve. The equalizer winder follows suit: positioned at the end of the gear train, it smooths the torque delivered to the escapement. For this, it uses an equality spring, substantially similar to a hairspring, which contracts and disarms, always releasing the same amount of energy, or “torque”.
This equalizer works with the escape wheel. It turns in 5 times, following the alternations of the balance. During the first 4 alternations, the equality spring disarms releasing constant energy. At the 5th alternately, the three-arm stop wheel which until then had blocked re-winding is released. This position allows the movement to reset the equality spring, which will then again release constant energy during the next 4 alternations, and so on. The whole cycle lasts 1 second.
To complete the winding mechanism of equality and thus ensure the best chronometric performance, Caliber FB-RE.FC has been fitted with a new variable-inertia balance, with two arms, aerodynamically shaped, equipped with four screws. balancing. Its large diameter guarantees its inertia and stability, while its low frequency of 18,000 (2.5Hz) vibrations per hour preserves the power reserve.
Three successive devices dedicated to chronometry
The Chronometer FB 2RE thus comprises 3 segments which, over the kinematic chain, gradually eliminate all variations in torque likely to affect its precision.
First, there is the barrel spring, the primary source of energy. It is technically capable of delivering 80 hours of power reserve. In the case of Caliber FB-RE.FC, only 60% of this energy is conserved (50 hours), a period during which its torque is the most linear. This selection of the most constant torque is made by a Maltese cross visible at the top of the barrel drum. This one can make 8 turns on itself but the Maltese cross allows only 6, materialized by the 6 teeth on its profile. The other two less pronounced inserts of the Maltese cross prevent the mainspring from delivering to the gear train, on the one hand too much energy (at the start of the power reserve) and on the other hand too little energy (at the end of power reserve).
The fusee-chain device then provides a second level of equalization of the torque delivered to the escapement during these 50 hours of power reserve. It acts as an automatic gearbox. In fact, depending on the level of winding, the torque delivered by the barrel varies. When the movement is fully wound, the chain is completely wound around the small diameter of the spindle. The barrel spring then has its maximum power. This force decreases as time passes, the chain wrapping around the drum and moving from the small to the large diameter of the spindle.
The variation in the diameter of the spindle then compensates for the decrease in torque at the barrel spring. At this stage, the FB 2RE model already has a level of isochronism well beyond the standards in force, as shown by the tests carried out on the FB 1 collection.
The energy thus smoothed out finally engages in the third and final device, the winding mechanism of equality. It aims to reduce the tiny irregularities that occur during tooth pickings and to gain even more linearity. Finally, the energy transmitted to the exhaust is almost perfectly constant.
Some standard movements, without a constant force device, can lose up to 10 s / d after 36 hours of operation. At the end of the stroke, the loss of torque is such that a variation of state of nearly one minute is not uncommon.
Keen to demonstrate the chronometric performance of its collections, Chronométrie Ferdinand Berthoud undertook in 2019 to carry out a new independent test: the Fleuritest, within the Fleurier Quality Foundation. In addition to the measurements carried out in the laboratory by the Swiss Official Chronometer Testing Institute (COSC) for 15 days on each movement, necessary to obtain the “Chronometer” designation, the Fleuritest provides a fundamental and complementary notion: that of experimentation. in real conditions.
Among the most demanding in the watchmaking industry, this test faithfully reproduces everyday gestures, alternating active and resting phases, during which the rate variations are analyzed using digital cameras. Carried out on a nested movement and carried out during the entire power reserve, these measurements are therefore much more meaningful and close to the real conditions of a customer who wears his watch daily.
Pursuing the principle dear to the master watchmaker of sharing his knowledge and research results, Chronométrie Ferdinand Berthoud has therefore subjected the new Chronomètre FB 2RE.2 to this same series of tests. The fusee-chain winding equality movement has therefore been tested by two independent laboratories; first for 15 days at COSC then, once fitted, for 2 days (50 hours of power reserve) at Fleuritest.
The results are clear: Throughout the power reserve, the readings taken differ by less than 0.5 seconds on average, compared to the reference optical measurement of the GPS clock. The FB 2RE model therefore has very little difference in rate between its first minute of operation and its last., and this under real test conditions, simulating the movements of a wrist watch. This exceptional level of isochronism is correlated by the measurement of the amplitude of the FB 2RE: an average amplitude of 301 °, showing only a very small variation throughout the walk (standard deviation = 2 °).
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