Please find a list of technical terms used both in the ONEOF Accuracy App and in the watch industry.
The amplitude of a balance wheel, expressed in degrees [deg], is the angle formed from its equilibrium state up to the maximum rotation.
When the movement is fully winded, the amplitude values are generally located between 260° and 310°, depending on the gravity, the frequency, the aging of the oils, etc.
The computation of the amplitude always remains an estimation and the result must be used carefully. Indeed, in order to calculate the amplitude, the time between the first and the third pulse of the beat noise is measured. Between these two pulses, the balance wheel rotates a certain angle: the lift angle which is determined by the construction of the movement.
The beat error is the time difference between the “ticks” and the “tocks” and is expressed in milliseconds [ms]. It indicates an asymmetry in the balance wheel’s vibrations. The beat error should remain between 0.0 and 0.8ms. Above, it can reduce the amplitude, degrade the accuracy and increase the time needed for a watch movement to start.
The frequency is the number of oscillations the balance wheel does over time. The ONEOF App expresses the frequency in Hertz [Hz] which is the number of oscillations per second. Watchmakers also commonly use the number of beats, or vibrations, per hour [b/h].
The ONEOF App automatically detects the most common beat frequencies from 2Hz (14’400b/h) to 10Hz (72’000b/h).
Due to phenomenons related to the acoustic physics, the rate accuracy must be averaged over a period called the integration time, expressed in seconds [s].
The different integration time values are: 2s, 10s, 20s, 30s or 60s.
The lower the time integration, the less stable the measurement. However, a low integration time allows the measurement to show more detailed fluctuations.
Basically, you would use a long integration time (30 or 60s) for inaccurate vintage watches or if you use the ONEOF Accuracy2 in a noisy environnement.
If the watch is stable, accurate, and you are doing the measurement in a calm environnement, you can use a lower integration time.
The integration time can be changed in the App settings in expert mode only, before or during a measurement.
The lift angle is the angle in degrees [deg] covered by the balance wheel between the first and the third peak of the escapement signal.
It is a geometric feature, determined by construction and given by the manufacturer.
In the watchmaking industry, the lift angle is known to be inaccurate: between 2 movements of the same production, a variation of +/- 3° of the lift angle is not rare. As a 1° change equates to about 7° change of the amplitude value, that is the reason why the acoustic measurement of the amplitude remains an estimation (manufacturers always use more precise laser measurement).
For the most of the standard watch movements the lift angle is about 51°.
The rate accuracy is an instantaneous indication of the deviation of the balance wheel beating frequency and is expressed in seconds per day [s/d].
A watch can run faster or lower and its accuracy changes over time as a result of a wide variety of perturbations: internal imperfections in the gear train, aging of the oils, gravity and vibrations, temperature variations, magnetization of the hairspring, etc.
A watch is “accurate” when its daily variation is within the range determined by the brand. For example:
- Rolex: -2…+2 s/d
- Omega, Master Chronometer certification: 0…+5 s/d
- COSC certified movement: -4…+6 s/d\
- Or any other ranges: -10…+10 s/d, -15…15 s/d, etc.
Please note that as the watch accuracy is constantly changing over time, the instantaneous rate accuracy can be out of the specs while the average daily rate is still within the brand’s range.