Beat error is the difference in time between the two alternating halves of a balance’s swing: the clockwise ‘tick tock’ and the counter-clockwise ‘tock tick’. In theory the ideal is zero, with both beat intervals lasting the same time. Watchmakers try to keep it as low as they can while keeping amplitude high.
How is beat error measured?
A timing machine picks up each beat of the watch by sound or vibration. The tick it hears is the escapement stopping a tooth of the wheel. Electronic timers time two consecutive beats and compare them. The difference between the two is the beat error. Timing machines usually give it in milliseconds. A Witschi machine, for example, shows beat error alongside rate and amplitude. One example watch read 9.3 milliseconds on such a display.
What causes beat error, and how is it corrected?
Beat error comes down to how the balance assembly is positioned relative to the escapement. When the two beat intervals differ, the movement is described as out of beat. When they match, it is in beat. To put a balance in beat, a watchmaker turns the hairspring collet until the fork sits centred. The collet is rotated in the opposite direction to the one the wheel needed to move. The aim is a low beat error with a high amplitude.
Not to be confused with
Beat error is not the same as rate. Rate is the time a movement gains or loses, usually given in seconds per day. Beat error measures something else: how evenly the balance divides its swing between the two directions. The two are separate measurements. A Witschi timing machine shows them side by side, with amplitude as well, so each can be read on its own.
Does beat error apply to clocks?
Beat error is not only a watch measurement. Clocks have it too. Their beats are slower, so the difference is often shown as a percentage instead of milliseconds. The principle stays the same. The two halves of the beat should take equal time. As with a watch, a clock whose two beat intervals differ is described as out of beat.
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