A Discussion On Bleeding Edge Accuracy In A Wristwatch
Posted by Mycroft on May 09, 1998 at 10:18:04:
A BACKGROUND TO MY MADNESS
I am obsessionally compulsive about accuracy in my watches. For me, a correctly adjusted mechanical wristwatch (worn 24 hours a day) is running well at 0 to +3 secs/day. It is tolerable at 0 to +6 secs/day, and anything that exceeds 10 secs/day disturbs me enormously.
In comparison, a “regular” quartz watch generally varies in rate by about 2 minutes (120 sec.) a year, i.e. 10 seconds a month or 0.333 sec/day. The most common factors contributing or affecting this are:
- temperature-related frequency drift
- age-related frequency drift – often attributed to the trimmer in the circuit and may vary with its quality
- variation in battery voltage with time and temperature
In a conventional quartz watch, the most common frequency of quartz oscillators is 32,768Hz. A traditional circuit then divides this frequency by two, 15 times in succession – down to 1 Hz. This means the stepping motor of the watch receives one impulse per second (as shown by the jump of the second hand in analogue displays).
Some of the methods used by manufacturers to improve on these accuracy parameters over the common quartz, to create the ultra-accurate wristwatch include:
- increasing the oscillator frequency, e.g. in the Omega 24 MHz Marine Chronometer,
- temperature compensation of the crystal frequency of oscillation, e.g. in the Longines Conquest VHP or Pulsar 10PSR and (I think) the late Kreiger Marine Chronometer
The current bleeding edge of accuracy in a wristwatch stands at 10 secs/year or 1 min in 5 years. This translates into less than 1 sec/month or 0.02 sec/day. When you compare this with a run-of-the mill quartz, there is at least an order of magnitude of difference in precision (with apologies if I get the application of precision vs. accuracy wrong).
The execution or implementation of thermo-regulated quartz varies with the movements mentioned above. This discussion will centre on this technology, and its implementation in the Longines Calibre 276 found in the Conquest VHP.
The 276 appeared in the late 1970s/early 80s with a then revolutionary concept of a thermo-compensated oscillator which in one fell sweep, put paid to the quartz’s greatest enemy making it virtually immune to temperature variations.
An extremely sensitive, high frequency quartz thermometer operating at 262,144 Hz, measures the surrounding temperature and corrects its effect by constantly adjusting the oscillator’s frequency to cope.
A conventional dividing circuit simply cannot achieve an accuracy of 0.02 sec/day. So the traditional trimmer was replaced by a system of digital frequency adjustments obtained through suppression of steps in the dividing sequence. This process is known as inhibition. Hence, there is no trimmer in the Conquest VHP – negating another source of inaccuracy through frequency drift and ageing.
The software programme for this adjustment is stored in the non-volatile memory of an integrated circuit. Thus even when there is no voltage, as during a battery change, the programme remains unaffected. When the movement leaves the factory, it has been adjusted for life.
When temperatures fluctuate in the course of the day or the month of the season, impulses to the stepping motor no longer arrive precisely on the dot at every second, but slightly earlier or later. The high frequency quartz thermometer senses these temperature variations and instructs (through the encoded programme) the frequency divider to omit 1 or more steps in the dividing sequence. This happens when the quartz oscillator slows down or accelerates in response to temperature variations and acts to compensate for these effects.
Essentially, 2 different types of corrections are made in this movement:
- Variable corrections are made at any time that the temperature variation exceeds a certain threshold value;
- Regular corrections are made once every eight minutes.
Timekeeping variations are thus kept within the very narrowest limits possible. This is the critical factor that contributes the characteristic of extreme accuracy in this movement.
The other factor that has a great potential to affect accuracy is unstable or varying power supply voltage over time or temperature. With the advent of the Lithium battery, the first iteration in the VHP allowed a continuously stable, reliable power source extending its useful life to 5 years (12 years in movements with only hours and minutes indication). This vastly supersedes the previous average of about 2 years with older battery technology. The latest iteration of the Lithium battery gives a battery life extending out to 10 years.
As an addendum, the Lithium battery is somewhat larger than a silver oxide battery and therefore, it was decided not to accommodate it within the movement itself. It was designed to be lodged in the back of the case, where it adds 1mm to the thickness of the watch. The extra 1mm was hidden by clever design of the case to successfully conceal it visually.
OK, I know its heresy, sooo, what watch do you think I most recently acquired? 🙂
Posted by Mycroft on May 09, 1998 at 16:17:38:
In Reply to: A Discussion on Bleeding Edge Accuracy in a Wristwatch posted by Mycroft on May 09, 1998 at 10:18:04:
It was 2 am when I completed this piece (it is now 6am). In my befuddled state, I omitted to say that magnetism is another important external influence in determining variance from accuracy. Something you wouldn’t even think about, yet is important when you open the fridge door with its magnetic seal, load the washing machine with its powerful spin motor, or go near your 10,000 watt hi-fi speakers with their Samarian Cobalt magnets.
Magnetism is often measured in A/m or Oersted units, the conversion factor is not clear to me (so if some TZer more qualified than I would like to comment and expand on this, I would be grateful, CCS perhaps?).
This is just an excuse to say that the watch I got hold of is a NOS Longines Conquest Titanium VHP 1000 Oersted with a soft iron shell, screw down crown and case back (this is different from the more common non-shielded VHP). This is an unusual piece as I have not seen it in the catalogues or Longines literature, nor in in any of the many shops I frequent. It is the only (to my knowledge) such ultra-accurate quartz to have a magnetic shield cladding to eliminate one of the last remaining common day problems encountered by watches, which may encourage deviant behaviour.
Can I get input from the rest of you guys regarding details of other forms of implementation of the ultra-accurate quartz technology in watches mentioned earlier?