Daniel and I get particular enjoyment from the old texts of watch-making and timekeeping, if you have not noticed.
And Why?
Because the old texts take us through some of the amazing advancements in watchmaking, the advancements are told by those who experienced it, as it happened. But all of these miss one of the most recent developments in timekeeping. This advancement brought incredible timekeeping accuracy to the world, common recognition of thousandths of a second, and heralded in a massive reduction in the cost of timekeeping instruments.
This Development? The Quartz Movement.
In 1927 Warren Marrison and J.W. Horton, who were based at the Bell Telephone Laboratories, built the first quartz clock. Yet it was not until 41 years later when Seiko, the now-famous Japanese watch company, popularised quartz timepieces with the introduction of the Astron movement in December 1969. With the advances in digital electronics from the 1980s, quartz movements shrank in size, manufactured in larger and larger quantities, and became less expensive.
What Made Quartz So Popular?
The accuracy and flexibility of quartz movements reinforced their position, over time, as the most popular and flexible timekeeping movement. In addition, the ability to pair the movement with a digital display and measure time periods to the millisecond or an analog display, and maintain accuracy over long periods of time made them the choice for utility.
Did you know Quartz movements are the world’s most widely-used timekeeping technology? Used in the vast majority of clocks and watches and computers and any other instrument that measures or relies on accurate time measurement. These movements are an order of magnitude more accurate than the best mechanical timepieces. In addition, they have no moving parts, which makes them more robust against environmental challenges. There is also no need for periodic maintenance – change a battery now and again.
What is Quartz?
These benefits come from a massively abundant compound found on the earth’s surface – quartz. However, chemically it is silicon dioxide, and for it to be used successfully in a watch movement, it must be a single crystal, and by definition, this is 100% pure.
Silicon dioxide is a very abundant substance on the earth. It is the primary constituent of sand, is used in glass manufacture, and is the largest component in clay for making plates, cups, and saucers. When it is purified and produced as a single crystal, it has piezoelectric properties. A piezoelectric material accumulates an electric charge when subject to mechanical forces. If an electric charge is placed across the same planes of the crystal, then the material will change shape – this is known as electrostriction.
How To Harness Quartz’s Unique Properties
An oscillator can be created by harnessing these physical properties. For example, when an electric charge is placed across the crystal, it will change shape, and when the charge is removed, the crystal will return to its resting position. By capturing the charge from the crystal as it physically returns to its original shape and amplifying it.
A quartz crystal can go through this cycle very fast. The maximum physical (and therefore electrical) changes will occur at the crystal’s resonant frequency. Resonance is determined by the crystal’s physical size, shape, and orientation. At resonance, the energy losses in the crystal circuit will be negligible, and the crystal can sustain the oscillations with only tiny amounts of additional energy. Similarly, indexing the oscillations of the pallet fork in a mechanical movement is the key to time measurement in a mechanical movement. By counting the number of oscillations of the quartz crystal, that time is measured in a quartz movement.
Interestingly, it is not the quality of the quartz crystal that determines the accuracy of a quartz movement. Still, it is the quality of the digital circuitry that counts the oscillations that is most important.
The Benefits Don’t Stop There.
Quartz movements have several additional advantages. In particular, temperature fluctuations do not materially impact the size of the crystal. This means that the ambient temperature will not materially alter the resonant frequency of the crystal. If the oscillating frequency does not fluctuate materially with temperature, then a quartz movement will remain accurate even as the temperature changes. But when you measure time to thousandths of a second, it will still impact the accuracy of the quartz movement.
The crystal used for a consumer-grade quartz oscillator is designed to be as insensitive to temperature fluctuations as possible. Still, in general, they are most accurate when held at a steady temperature of 28 °C.
How Is This Achieved?
To achieve the highest accuracy for quartz watches, they are designed to be worn regularly. The heat interaction between the human body (at 37oC generally) and the watch case is considered so that the crystal is maintained at as stable a temperature as possible to achieve the highest accuracy.
Oscillating – How Fast?
For the vast majority of quartz watches, the quartz crystal is configured to oscillate at a frequency of 32,768Hz. Alternatively, you can think about the crystal bending and relaxing 32,768 times in one second. This frequency is selected to be just high enough to exceed the human hearing range, yet low enough to permit inexpensive digital logic counters to derive a 1-second pulse accurately. As you can deduce from this, it is possible to measure a second into 32,768 equal periods – so milliseconds are not an issue.
What Determines Accuracy?
What does limit the accuracy for quartz movements is not the quartz crystal itself. It is actually the quality of the electronic counters that counts the vibration cycles of the quartz crystal and renders that into an accurate measurement of time. Generally, the more expensive the movement, the more accurate the counter, but even the cheapest quartz movement is more accurate than any mechanical movement. So it is easy to see why when you consider the oscillations of a quartz movement is over 8,000 times more than a mechanical movement, this additional resolution ensures the additional accuracy.
That is a Strange Number?
You may be scratching your head at this precise frequency that has been chosen for the oscillation, but there is a reason for this. The number 32,768 can be represented in binary form as 215, so a 15-bit binary digital counter driven by the crystal oscillating at its resonant frequency will accurately index a second once it has been through a complete cycle of counting. Thus, the counter will produce a digital pulse once per second and can then use this output to maintain time.
Each quartz movement relies on a single crystal that is manufactured physically but within tolerances which can still impact the resonant frequency. Therefore, instead of making each crystal identical, it is much easier to alter the timekeeping of each crystal through a process called inhibition compensation.
The crystal is deliberately configured in the oscillation circuit to resonate faster than the intended operating frequency. Once each circuit has been manufactured, it is compared to a reference frequency and adjusted to keep accurate time by programming the digital logic to skip a small number of crystal cycles at regular intervals, thus making it accurate. This process is analogous to regulating mechanical movements. The advantage is that adjusting the memory of the logic chip is less expensive than the historical method of physically trimming the quartz crystal to adjust the resonant frequency. This circuit in the digital controller is referred to as inhibition compensation logic. In more expensive quartz movements, the logic circuit is used to improve precision. In inexpensive quartz watch movements, this functionality is not available…
Accuracy is All Relative
More expensive quartz movements have additional processes that ensure accuracy by automatically adjusting to external conditions. This process is called self-regulation or self-rating. One such process is that instead of counting the absolute number of oscillations, the digital logic produces signals at predetermined intervals and then counts the oscillations between these signals. It is the ratio calculated between these counts and an epoch set at the factory that is used to determine the current time. In addition, these movements typically have special instructions for changing the battery as the counter is not permitted to stop.
Another complication is to include a logic circuit within the movement to measure the temperature of the crystal and adjust the logic for temperature variations. Both analog and digital temperature compensation methods have been implemented in high-end quartz movements. Still, in the most accurate movements, thermal compensation is implemented by varying the number of cycles in the digital logic depending on the output from a temperature sensor.
Part of Timekeeping History
The development of the quartz movement transformed timekeeping in the 20th century. It not only made accurate timekeeping available to the masses but was the catalyst for massive changes in the world of mechanical watches. From the 1970s onwards, makers of mechanical watches couldn’t compete with the accuracy or cost of quartz movements.
Our own Daniel Blunschi lived the aftermath of this phase in the history of the Swiss watch industry. The havoc this piece of technology wrought on the Swiss industry is the stuff of legends. You can read his experience through his blogs.
