Accuracy: What Is It, And Does Anyone Actually Care?

Accuracy: its pursuit is the reason watchmaking exists at all, and yet, it's easier to get an argument about date windows started than a discussion about accuracy. Take away watches as everything else that they are – expressions of personal taste or style; collectible rarities; objects of decorative ornamentation; amusing pieces of mechanical ingenuity – and what you have is a machine designed to keep time accurately. What accuracy actually is, however, is not as straightforward as you might think, and with atomic-clock accurate time available just about everywhere, it's useful – if mechanical horology is something that interests you – to look at this single most essential, yet surprisingly little-discussed, feature of watchmaking.

Accuracy for most of us means one thing: whether or not a watch keeps the same time as whatever we use as a time standard. Generally, nowadays, that means the time provided by a phone; it may also mean the time provided by a wearable connected watch. If the time kept by a watch always matches the time standard (the time on your phone is served to it over the Internet, and is provided ultimately by an atomic clock) then we say the watch is accurate. Of course, no mechanical watch is a perfect timekeeper, and so we generally determine if our watch is accurate enough for our purposes rather unscientifically: by deciding how unhappy we are with how inaccurate it is. (One of the most accurate watches I have that I wear regularly is a rather unassuming little IWC Portofino; if I wear it on my left wrist and leave it dial up on the nightstand, it gains one second every couple of days.)

The watch I'm using for illustration purposes here is a Lange & Söhne pocket watch, and I've chosen it because the way it's made illustrates a key point: the goal of precision watchmaking is not, surprisingly enough, the achievement of accuracy. Yes, accuracy is important, but it's the result of something more fundamental: rate stability. Rate stability means, simply put, that a watch always beats the same number of times over a given time interval, without ever changing. A watch with a stable rate is a precision watch.  The difference between accuracy and rate stability can be illustrated by thinking about a watch that does not have a stable rate: one day it's +6, the next, -10, the next, -2, the next, +7, and the next, -1.  At the end of five days, you are one second off a time standard, and you may feel you have an accurate watch.  However, you actually have is a one that is not very precise, and you've basically just gotten lucky, which if you want a precision watch is somewhat unsatisfying.

Any timekeeper depends on an oscillator – it could be a pendulum, it could be a balance and spring, it could be a quartz tuning fork shaped crystal drive by a trickle of current – but the more stable the rate of the oscillator, the higher precision timekeeper it is. The classic example is the marine chronometer. Marine chronometers were very carefully timed before being brought on board ship, and they weren't so much expected to be accurate as they were expected to be unvarying in their rate. If you knew your chronometer was always going to be five seconds fast per day, without changing, you could easily calculate the time at Greenwich for the purpose of getting a position fix based on astronomical observations (generally, to avoid upsetting the rate of the chronometer, you would use a deck watch set to the time the chronometer showed, and bring that on deck when you did your observations).

What you have above is a look at the heart of a high-grade Lange pocket watch. True, this would have been a very accurate watch for its time, but most of what you see is, to be precise, there to ensure stability of rate. The clear ruby endstone is there because the balance pivots are steel, and steel and ruby are almost frictionless bearings; this keeps the rate from changing when mainspring power changes (more friction would make the balance amplitude more sensitive to variations in power). The balance spring likewise makes the balance less susceptible to variations in power; the overcoil is there to make the balance amplitude – and thus, the rate – less variable with changes in position. The balance itself, you can see, is a circular sandwich of steel and brass; it changes its diameter as temperature changes, to compensate for the effect of temperature on the balance spring, which again, improves stability of rate. The single element in the image having the most to do with accuracy, rather than rate stability per se, is actually the beautiful swan's neck regulator – it's used to change the position of the regulator pins, in between which the outer coil of the balance spring passes. The position of the pins determines the effective length of the balance spring; to make the watch keep time in synchrony with an external reference (an accurate pendulum clock, for instance, in the 19th century, or an atomic clock time signal today) you would adjust the index, and that would probably have been the last step in bringing the watch to time.

One other point about the balance: its size and mass. Rate stability for an oscillator is a function of two things: the mass, and the frequency. Increase one or the other, or both, and typically, you have a more stable rate. Above is a closeup of the escapement of a pendulum clock; in pendulum clocks generally the approach was to use a massive oscillator swinging at a low period. In watchmaking, for many centuries you used the biggest balance you could for a given size movement, but in the 20th century and up to the present, there's been a move to using higher frequency balances (28,800 vph has become more or less standard, up from the 18,000 vph typical of most 19th-century pocket watches). Attempts are being made nowadays to push things even further, though.

Above is the movement of the Senfine watch, with Genequand escapement, that we covered when Parmigiani Fleurier introduced it at the SIHH. The oscillator is very unusual in materials and design and it vibrates at 115,200 vph (16 hertz, which also enables it to have a theoretical 70-hour power reserve).  

As frequency increases, of course, mass generally has to be reduced. Quartz watches do have mechanical oscillators (we often forget this) but the mass is pretty small and the frequency can be correspondingly higher; typical frequency for a quartz watch is 32,768 hertz, and that high frequency is exactly the reason quartz has mechanical beaten in terms of accuracy before the race even starts. Atomic clocks use the resonant frequency of, typically, a cesium atom, which is 9,192,631,770 hertz (more specifically, that's the frequency of the radiation emitted as the atom transitions between two energy states).

Does anyone still care about accuracy? Sure, we do. But I often think that when it comes to accuracy, how you get it is as interesting as that you get it. The Apple Watch, for instance, uses a temperature compensated quartz oscillator, which is a nice touch (TCXOs are usually found only in high-grade quartz movements, like Breitling's SuperQuartz or Seiko's 9F series of Grand Seiko quartz movements) but it also uses NTP (Network Time Protocol) to update its internal clock, so in practice, you're never really likely to notice it's using a better grade oscillator. More relevantly, there's no human behind its accuracy, at least not directly – the watch is watching a phone, that's watching an Internet protocol, that's watching a hierarchy of time servers, that are watching GPS satellites fitted with internal atomic clocks, that are watching a master atomic clock. The Lange shown in this story could have, with care in adjustment and regulation and actual use, probably kept time to around a second a day or less in variation in rate, but it would have done so thanks to the eye and hand of a master watchmaker.  

Precision mechanical watches nowadays don't require exactly the same skill set (nobody is out there hand-tuning temperature-compensated balances on a regular basis anymore) but many of the skills that would have been necessary to make the Lange in this story are still with us – one more dimension to mechanical clocks and watches that gives them the fascination they continue to have.

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