Isochronism

The word Isochronos 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:

Gravity
Friction
Regulating pins
Escapement
External Shock
Temperature
Out of poise balance/out of poise hairspring
Magnetism

G FREE TOM

A few of them, such as Temperature, Magnetism, and External Shocks are really difficult to observe with your eyes alone, and may not occur to their fullest extent until the watch is being worn. This is kind of true with gravity as well, although part of the watch’s susceptibility to gravitational changes can be accounted for by the poise of the balance wheel and balance spring.

Getting the poise of the wheel and spring right should be your first port of call when attempting to improve the isochronism of a watch. This is something you can see, easily test for and rectify manually.

Crucially, the spring and wheel will react differently when operating at different amplitudes.

Before I go any further, allow me to give you a textbook definition of amplitude:

The amplitude, expressed in degrees, is the maximum displacement of any point on the balance wheel from its initial starting point.

If a balance is well poised, the centre of gravity will be in the centre of the balance staff, and it will, when free of any external influence, operate in an isochronous fashion.

If your balance has a poising fault, it will behave differently at different amplitudes. You can use the awareness of this fact to diagnose the issue and, hopefully, correct it.

Let’s start with the magic number. For some providential reason, an amplitude of 220 degrees will CONCEAL poising faults. When your watch is running in this threshold, you will not be able to tell if there is anything wrong with it.

Additionally, poising faults do not manifest when the watch is in either horizontal position. Imagine a roundabout with one child riding it. You give the kid a push and the roundabout spins at a constant rate, eventually slowing down to a halt as the inertia diminishes.

Now strap the kid into his/her seat and turn the roundabout on its side so its spinning like the Wheel of Fortune.

Ignore the screaming.

When you set the kid spinning, he/she will career towards the ground at breakneck speed, but as soon as the kid passes the lowest point of the journey, the Wheel of Fortune will slow down immensely. You see, the wheel is using up all the force exerted upon it during its downward travel in order to drag the kid back to the high point. The second he/she passes the highest point, the wheel will speed up again.

As entertaining as this may be, the wheel is not moving at a constant rate and is therefore useless to us as watchmakers.

At 160 degrees however, changes can be observed. In our roundabout example, this is because there is not enough inertia in the roundabout to carry the child from the low point, back to the apex, so the roundabout would swing back and forth until coming to rest with the child – the heavy point – at the bottom. As always, the horizontal position has no effect on the balance. In the vertical positions, if the heavy point is below the centre of gravity it will cause a gain because it is shortening the distance travelled by the wheel, and if the heavy point is above the centre of gravity, it will cause a loss for the opposite reason.

To dynamically poise a watch you should let down the mainspring and place the watch on the timing machine. Wind the watch until you reach 180 degrees amplitude, rotate it through eight equal stages (on its edge), taking a reading at each stage. The position that shows the maximum gain is the position where the heavy point is (at the bottom (the lowest) when the balance is at zero). This is where to remove material. Material must be removed from the screws of under the balance in an aesthetically sympathetic fashion.

Dynamic poising should only be used to fine-tune a watch. It should not be relied upon to poise a balance that is wildly out of time. As always, the removal of material is a last resort.

So that’s a brief introduction to Gravity for you to digest. Now let’s turn our attention to temperature and the problems it can cause the isochronism of a watch.

Before you can really visualise the effects of temperature change on a watch and its components, it is important to grasp the concept of temperature coefficiency. Different materials react differently under the influence of temperature. This value is known as the temperature coefficient. Other coefficients, such as friction, also exist in watchmaking and these ratings are used to determine the best pairings of materials in order to guarantee optimal functionality.

The temperature coefficient is a measurement taken in +/- seconds per degree per day to rate the resistance of different materials to the disturbances caused to isochronism by temperatures variation. For example, Glucydur has a +/-0.6 temperature coefficient. It is calculated thusly:

Example: Rate = -4 @ 36 degrees and -1 @ 4 degrees. So…

T1 – T2
Temp1-Temp2

Therefore:

-4 – (-1)
36 – 4

Which equals 0.09 seconds.

The material has a temperature coefficient of 0.09 seconds per degree per day (it will gain 0.09 seconds for every 1 degree rise in temperature).

While on the subject of temperature, we should spend a moment getting to grips with Secondary Error.

We arrive at the value known as Secondary Error by testing a watch’s timekeeping capabilities in a variety of different temperatures so that the fluctuations in isochronism, due to the temperature coefficiency of the watch’s components, can be observed.

The first test is taken at a low temperature (8 degrees) and the timekeeping performance is noted.

Temperature is then taken at a high temperature (38 degrees) and recorded. Both temperatures are placed on a graph and a line is drawn between them to show the expected effects of temperature variation on timekeeping.

Then the watch can be tested at the mean temperature (23 degrees) and its timekeeping recorded and compared to the expected performance, as according to the graph. The difference between the real value and the expected value is the Secondary Error and is expressed as a gain or a loss.

Es = Secondary error
M1 = Rate @ 8 degrees
M2 = Rate @ 23 degrees
M3 = Rate @ 38 degrees

So

Es = M2 – ((M1 + M3)/3)

Ideally, the value would be 0.

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