Accuracy in watchmaking is a term associated with the consistent rate and precision of timekeeping.
Mechanical timepieces are adjusted through various mechanisms built into the calibres and adjusted and measured by means which detect either sound or through visual measurements.
A watch with a single balance can be regulated with a simple timing machine which has a microphone/sensor which effectively listens to the sound of the watch beating. With multiple balances, the sound system with a microphone is ineffective due to two escapements working in unison, and is replaced by optical systems which measure the rate of the watch using lasers.
Early Timing machines from a Bergeon Catalogue published in 1958.
Optical sensor timing machine
The passing of the second’s hand is detected by an optical sensor. The sensor’s signal is analysed to give both instantaneous and absolute rate measurements.
Images courtesy of Icoflex Sàrl