RATE

The rate of a watch refers to its chronometric accuracy, i.e. the difference between the time real 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 of 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 depending on wrist position (Dial up, crown on the left or at the top, etc.), the ambient temperature and the state of charge of the barrel, the drop in torque at the end of unwinding of which tends to slow down 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 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 changing the moment of inertia of the balance, in order to reduce the differences between the positions and optimize the isochronism of the balance.