From horological terminology to essential concepts, this concise glossary of watchmaking terms will provide you with a clear understanding of the language used in the world of watches. Expand your knowledge and navigate the fascinating world of timepieces with our watch industry glossary.
Rate accuracy refers to the instantaneous measurement of the deviation in the beating frequency of a watch’s balance wheel. It is typically expressed in seconds per day [s/d]. A watch’s rate accuracy can vary due to various factors, including internal imperfections in the gear train, aging of oils, gravity and vibrations, temperature fluctuations, magnetization of the hairspring, and more.
The accuracy of a watch is determined by its daily variation, which should fall within a specific range defined by the brand. Different brands may have different accuracy requirements. For instance:
- Superlative Chronometer certification (Rolex): -2…+2s/d
- Master Chronometer certification (Oméga, Tudor): 0…+5s/d
- COSC certified movement: -4…+6s/d
- Other ranges may include: -10…+10s/d, -5…+15s/d, etc.
It’s important to note that while the instantaneous rate accuracy may occasionally fall outside the specified range, the average daily rate of the watch can still remain within the brand’s specified range.
The amplitude of a balance wheel, measured in degrees [deg], represents the angle from its equilibrium position to the maximum extent of its rotation.
Typically, when the movement is fully wound, the amplitude values fall within the range of 260° to 310°. However, this range can vary due to factors such as gravity, frequency, aging of oils, and other influences.
It’s important to note that the calculation of amplitude is an estimation, and the results should be interpreted with caution. The amplitude is determined by measuring the time between the first and third pulses of the beat noise. During this interval, the balance wheel rotates a specific angle known as the lift angle, which is determined by the construction of the movement.
The lift angle refers to the angle in degrees [deg] covered by the balance wheel between the first and third peak of the escapement signal.
This geometric feature is determined by the construction of the movement and is provided by the manufacturer.
In the watchmaking industry, the lift angle is known to have some degree of inaccuracy. It’s not uncommon to observe a variation of +/- 3° in the lift angle between two movements of the same production. Since a 1° change in the lift angle corresponds to approximately a 7° change in the amplitude value, the acoustic measurement of the amplitude remains an estimation. Manufacturers often rely on more precise laser measurements.
For most standard watch movements, the lift angle typically falls around 51°.
The beat error refers to the time difference between the “ticks” and “tocks” of a watch’s balance wheel and is measured in milliseconds [ms]. It represents an asymmetry in the vibrations of the balance wheel.
Ideally, the beat error should remain within the range of 0.0 to 0.8ms. If it exceeds this range, it can have negative effects such as reducing the amplitude of the balance wheel, degrading the accuracy of the watch, and increasing the time required for the movement to start.
The frequency refers to the number of oscillations performed by the balance wheel within a given time period. In the ONEOF® Accuracy App, the frequency is expressed in Hertz [Hz], which represents the number of oscillations per second. Watchmakers also commonly use the term “beats” or “vibrations” per hour [b/h].
The ONEOF® Accuracy App has the capability to automatically detect the most common beat frequencies ranging from 2Hz (14,400 b/h) to 10Hz (72,000 b/h).
Due to phenomena related to acoustic physics, the rate accuracy of a watch needs to be averaged over a specific period known as the integration time, measured in seconds [s].
There are various integration time values commonly used, ranging from 2 seconds to several minutes.
The stability of the measurement decreases with a lower integration time, but it allows for more detailed fluctuations to be observed.
In general, a longer integration time (greater than 30 seconds) is preferred to achieve a stable measurement that is resistant to ambient noise. This is particularly useful when monitoring the watch or movement over its full power reserve.
On the other hand, a shorter integration time (less than 4 seconds) is typically employed for adjusting the timekeeping of the movement or analyzing phenomena occurring near the escapement wheel.