The word isochronism is bandied around a lot when discussing the quality of high-end timepieces and deserves a solid definition so that it can be easily understood.
It comes from Greek and literally means ‘Same’ (Iso) ‘Time’ (Chronos). It relates to the consistency of a watch’s timekeeping in all tested positions of wear. This is largely controlled by the poise of the balance wheel and the flatness and centring of the hairspring.
There are several factors that can impair the isochronism of a watch. Eight such disturbances are:
Out of poise balance/out of poise hairspring
So how does the escapement – the distribution organ of a mechanical watch – affect isochronism? The Swiss Lever escapement has an inherent loss. Impulse before the centre line (0 degrees) causes a gain. Impulse after the line of centres causes a loss. A disturbance before the line of centres causes a loss. A disturbance after the line of centres causes a gain.
Since the unlocking (a disturbance) occurs before the centre line, causing a loss, and the majority of the impulse occurs after the line of centres, a further loss is caused. Therefore the effect of the escapement interference causes a loss. As the lift angle is 52 degrees (in a standard watch), and becomes proportionally more influential as the amplitude decreases, the loss too becomes greater.
The lift angle is the angle of travel through which there is contact between the impulse jewel and the notch. It is classified as a disturbance and thus as the amplitude drops and the lift angle remains the same, it becomes a relatively longer and more influential disturbance.
So how do we define the isochronous error of a watch? To check the isochronism of a watch, you must test the watch in all five positions when fully wound, and then again when half wound (often 24 hours later). The readings are then totalled (separately) and an average rate is found. The difference between those rates is then referred to as the isochronous error. Here’s a theoretical example:
Test 1 (fully wound):
Dial up: +25
Dial down: +5
3 o’clock up: -5
6 o’clock up: +0
9 o’clock up: +5
Test 2 (half wound):
Dial up: +20
Dial down: +10
3 o’clock up: -8
6 o’clock up: +0
9 o’clock up: +6
In this case, the Isochronous error is 0.4
The mean daily rate would be 6+5.6/2=5.8