Revisiting the Appeal of the Ultra-High-Precision Breguet / Classique Chronométrie 7727

See for yourself the ultra-high precision that surpasses tourbillons

　The Classique Chronométrie 7727 is an ultra-high precision Breguet watch. Its high precision is due to its ultra-high frequency of 10Hz and magnetic pivot. These two elements give this classically-looking watch high isochronism and extremely small positional error. I witnessed this high precision firsthand in Switzerland, but what about the actual product? I went to Ginza, where the Swatch Group's maintenance department is located, and had a 7727 measured before delivery.

Extraordinarily small attitude difference error

The author and a watchmaker check the test results. The smallness of the positional error is noteworthy, but the watchmaker says, "Any 7727 will produce results of this magnitude."

　The magnetic pivot, a distinctive feature of the 7727, is an attempt to fix the balance shaft with a magnet. At first glance, it may seem tricky, but it ensures that the center of gravity of the balance wheel and hairspring always remains centered. Also, because a strong gravitational force is always applied to the balance shaft, the friction on the axis is almost the same in both the flat and upright positions, improving the "flat and upright position difference."

　Furthermore, the high frequency of 10Hz (=72,000 vibrations) increases the Q value, which indicates the stability of vibration, making the watch more resistant to external disturbances. In addition, a "stiff" balance spring that can withstand ultra-high vibrations should further reduce the shift in the center of gravity.

　What results will emerge from these data? The oscillation angle of the 7727 should be stable regardless of the orientation of the watch, and its accuracy should also be roughly consistent. What were the results?

　In this test, in addition to the usual five positions, measurements were also taken at 12 o'clock, which is not normally measured. Many high-precision watches sacrifice accuracy at the 12 o'clock position to improve accuracy in the five positions. As a result, when put through a tester, the accuracy at the 12 o'clock position is often unstable. However, with the 7727 we tested, the accuracy at the 12 o'clock position was almost the same as in the other positions. This means that fixing the balance with a magnet is effective.

　The smallness of the swing-down indicates the smallness of the positional error. The difference in swing angle when looking at all positions was only 11 degrees. The accuracy only differed by a maximum of 2 seconds. However, when I asked the watchmaker if this was a special case, he replied, "This is the case that will be delivered as is, and all models have the same accuracy."

　Without a doubt, the Classic Chronometrie 7727 is more accurate than a tourbillon. In the following pages, we will explore the reasons for its high accuracy.

Some of the test results. For a normal watch, the amplitude (Ampl) varies by about 20 to 30 degrees between the dial (DU) and 12 o'clock (CR). However, for the 7727, which uses a magnet to fix the balance, the difference is only about 10 degrees (though there are some differences depending on the individual watch). Note the "X" value, which indicates the average accuracy across the six positions. A value of 1 second is almost impossible for existing mechanical watches.