Each swing of the balance wheel, one way or the other, is a beat: the tick or the tock you hear. The beat rate counts them per hour, written vph (vibrations per hour) or bph (beats per hour); both mean the same thing. Two beats make one full oscillation, so the frequency in hertz is the beat rate divided by 7,200.
| Beat rate | Beats per second | Frequency | Examples |
|---|---|---|---|
| 18,000 vph | 5 | 2.5 Hz | Many vintage and some hand-wound movements |
| 19,800 vph | 5.5 | 2.75 Hz | Older calibres such as the Rolex 1570 |
| 21,600 vph | 6 | 3 Hz | Seiko 4R36, NH35 and 6R35, Omega 1861 and 3861, Powermatic 80 |
| 25,200 vph | 7 | 3.5 Hz | Omega Co-Axial 8500, 8800 and 8900 |
| 28,800 vph | 8 | 4 Hz | ETA 2824-2, Sellita SW200-1, Rolex 3235, Tudor MT5602, Grand Seiko 9S65 |
| 36,000 vph | 10 | 5 Hz | Zenith El Primero, Grand Seiko 9S85 and 9SA5 |
Is a higher beat rate better?
Not by itself. A faster balance is less disturbed by shocks and by the movement of your wrist, and the seconds hand sweeps in finer steps: eight a second at 28,800 vph, against six at 21,600. But it also uses energy faster and works its escapement harder, so a high-beat movement needs careful design and lubrication. Many excellent watches run at 21,600 or 25,200 vph.
Why a timegrapher needs to know it
A timegrapher compares each beat with where it should fall at the movement's nominal rate, so it has to know that rate first. Horology's Timegrapher detects it automatically among the six standard rates above. When the sound could fit two of them (a 36,000 vph watch can also look like 18,000), it prefers your calibre's rate.
Quartz and Spring Drive
A quartz watch is regulated by a crystal vibrating 32,768 times a second, and Grand Seiko's Spring Drive regulates its gliding seconds hand without a ticking escapement. Neither has beats an acoustic timegrapher can follow, so measure them over days with Time Check instead.
Beat rate also sets how fast a movement uses its stored energy: see power reserve.