RATE

The rate of a watch refers to its chronometric precision, meaning the difference between the Time actual and the indicated time, expressed in seconds per Day (s/d) and not to be confused with the power reserve, which measures the Autonomy of the Movement. A distinction is made between the Daily rate — drift measured over twenty-four hours — and the instantaneous rate, recorded at a given moment. A Chronometer certified must display, according to the ISO 3159 standard, an average rate between –4 and +6 s/d, checked in five positions and at three Temperatures (8, 23, and 38 °C) for fifteen days. The rate varies according to the position of the wrist (Dial up, Crown to the left or up, etc.), the Temperature ambient temperature and the state of charge of the Barrel, whose drop in torque towards the end of letting down tends to slow the movement. In the workshop, the Horologist (watchmaker) measures it by means of a Timing machine, which acoustically picks up the beats of the Escapement in order to deduce the actual frequency of the balance. Corrections are made by acting on the Regulator assembly to modify the active length of the hairspring or by modifying the moment of inertia of the balance, in order to reduce the discrepancies between positions and optimize the isochronism of the Balance.