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Timing and Positional Rates of Watches

Watches run at a rate of course, but there is a difference between a watches accuracy over time, and the rate of the watch at any given moment.  In modern times, most people would think of "accuracy" as a reference to the rate, that is as how closely the passage of seconds on a watch matches the passage of seconds on a reference source.

However, this is not how watchmakers of old would think of accuracy in a timepiece.  In the days before quartz movements, which have an extremely consistent rate, accuracy in a watch was about how much it would drift away from a reference over a long period of time.  A "long period" being the intervening time between opportunities to set the watch to a reference source.  This could be a day, a few weeks, or even longer.  Accuracy would describe how far off a watch reads after a day, or a week, or a month.  It was important to build watches with this in mind because access to a good reference time source, a century ago, could be quite infrequent.

The rate at any given moment was less important, as a practical matter.  Railroad standards for example, refer to accuracy over time periods, but are silent on the question of rate.  And in fact during the course of a given day, the rate of a vintage watch varies by a surprising amount, even as over time, the accuracy may be quite good.

The rate at which a vintage watch runs varies for several reasons.  The most significant is that something is wrong, such as worn balance pivots, broken teeth, cracked jewels, unbalanced balance, distorted hairsprings, and so on...  Such problems in a watch typically cause large accuracy issues; errors observable within only one hour's time.  In my experience, once basic faults are corrected though, most mid-grade vintage watches are capable of accuracy of +/- one minute or less per 24 hours with little timing effort.

This discussion is about the discrepancies that remain after basic watch service.

Temperature

Temperature causes metal to expend or contract.  We see this impact watch timing because the balance wheel will literally get smaller of larger, thus moving its weight inward or outward and changing its period.  Compensating bi-metallic and split balance wheels were invented to help compensate for temperature changes.

Power

The rate of a mechanical watch varies over the course of running down from a full wind, to fully stopped.  The mainsprings of watches apply slightly more power when first wound, over when almost unwound.  Motor barrels and alloy springs were invented that reduce this effect, but it is not eliminated.

Gravity

All vintage watches will have a discrepancy in the rate in positions, especially between horizontal and vertical orientation in particular, due to gravity.

The root reasons for rate shift due to power and to gravity are changes in the center of gravity of the hairspring during its motion (here's the word of the day; isochronism, which is basically what the rest of this discussion is about).

Some watches have a flat hairspring.  But in some watches the outside end of the hairspring is bent up and across the spring in what's called an overcoil.  The overcoil helps make the motion of the spring more consistent, so watches with a flat hairspring will have greater rate errors, in general.  But the effects we're speaking of here are not eliminated by the overcoil, just reduced.  And the overcoil tends to make the watch more susceptible to rate error due to the watch moving around.

So, back to a watch that is otherwise functioning correctly, and focusing on positional rate error, it must be noted there is no simple way to adjust the rate for vertical and horizontal orientations independently.  There are things that have been done to deliberately change the vertical rate, but these are considered bad practices, for example deliberately inducing extra friction on the balance pivots in the horizontal by flattening the pivot ends to slow the watch when dial up or dial down.

Adjustment to Position

A watch adjusted to positions means that the watch has been run in various orientations for 24 hour periods (or more) and that the start and end times are recorded.  The errors after the test period in each position can thus be noted.  Watches may be tested in some number of positions.  These would be dial up, dial down, pendant up, pendant down, pendant left, pendant right and more.  Higher grade watches are typically tested in more positions.  More basic grades, just two or three, or perhaps none.

When a person wears a watch, it will spend some certain time in different positions. So, once serious problems are corrected, "adjustment to positions" means setting the rate over all slightly faster or slightly slower, so that on average, at the end of a period of time, the watch will read fairly accurately, even though during the course of that period of time, it has run at slightly different rates moment to moment.

The challenge is that the amount of time a watch spends in different orientations can vary from person to person.

A couple of times lately I have fielded questions from people noticing that the rate of a vintage watch seemed off under certain conditions.  But even noticing this is a result of modern exposure to quartz watches that have a rock-solid, constant rate.  The best vintage watches have rates that vary.  Adjusting was always a process of setting it so the errors cancel out more or less over a period of time.

As complex as modern electronics are, a quartz movement, supplied with the correct power, is essentially on or off.  In some ways, the mechanical designs are more complicated.  There are many, many factors that affect the rate of a watch.  I have left a lot out.  Whole books have been written about hairsprings alone.  Not to mention the special issues of vintage pieces that have been altered by watchmakers over and over.  It's a never ending learning process...


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