Regulating a Watch: Amplitude, Rate and Beat Error

A timegrapher gives you three numbers and they are not equally important. Read in the wrong order they will send you chasing a rate problem that is actually a power problem, or adjusting a regulator on a watch whose real fault is somewhere else entirely.

The order is: beat error, amplitude, rate. Always in that order.

Beat error first

Beat error is the asymmetry between the two halves of an oscillation — how far off centre the impulse happens. It is measured in milliseconds and it should be below about 0.5 ms. A watch with significant beat error is being impulsed unevenly, and no amount of rate adjustment will make it run well.

The cause is geometric: the position of the hairspring stud relative to the impulse jewel. The fix is to rotate the hairspring at the stud — or, on some layouts, to move the roller — until the two halves of the oscillation are equal. It is not adjusted at the regulator.

• Below 0.5 ms — good
• 0.5 to 1.0 ms — acceptable on older movements, worth improving
• Above 1.0 ms — fix before anything else
• Above 2.0 ms — the watch will not run consistently in any position

Amplitude second

Amplitude is how far the balance rotates from rest, measured in degrees from the point of reversal. A healthy modern lever movement in the flat runs roughly 270° to 310°. Below about 250° you have a power problem; above about 320° you risk knocking.

Amplitude is the diagnostic that tells you whether the rest of the work was done properly. Low amplitude after a service means friction — dried oil somewhere, excessive endshake, a bent pivot, a weak mainspring, or too much oil in the wrong place. Regulating around low amplitude just hides the fault.

• Below 200° — do not regulate; find the power loss first
• 200° to 250° — marginal; investigate before adjusting
• 270° to 310° — typical healthy range for a modern lever movement
• Above 330° — risk of knocking and rebanking

Measure in several positions. A large difference between dial-up and dial-down points specifically at the upper or lower balance pivot and its jewels. A large difference between vertical positions points at poise.

Rate third

Rate is the number everyone cares about and the one to touch last. It is expressed in seconds per day and it is changed by altering the effective length of the hairspring.

On an index-regulated movement the regulator index carries two curb pins that grip the outer coil. Moving the index changes where the spring is effectively held, which changes its active length. Shorten the active length and the balance runs faster; lengthen it and the watch runs slower.

On a free-sprung balance there is no index. Rate is set by changing the inertia of the wheel: timing screws turned outward increase inertia and slow the watch, inward reduces it and speeds it up. It is slower to adjust and far more stable afterwards, because there are no curb pins introducing positional error.

The procedure

Regulation is not how you make a watch run well. It is the last five percent, after the other ninety-five is right.