Accuracy

ACCURACY

When it comes to accuracy there is one very important fact you need to know in advance. A $42 MWC Vietnam watch will keep time just as well as, and possibly better than, a top of the range MWC, CWC or Marathon mechanical or possibly even a $20,000 solid gold mechanical Omega, Rolex, or other high end watch.

If that last statement surprised you, read the rest of this section carefully.

All watches tend to gain or lose a few seconds over a period of time. These are small mechanical or electro-mechanical devices that are counting out 86,400 seconds per day. Even if a watch is 99.9% accurate, it will still be off by a minute and a half in only 24 hours! So even a mediocre wristwatch has to be well over 99.9% accurate to even begin to be useful on an ongoing basis.

So, what is a reasonable expectation of accuracy from a wristwatch? The table below is a pretty accurate guide as to what can be expected.



Seconds gain/loss per day



Worst

Typical

Best

Vintage mechanical watch
in good repair

+/-60

+/-15

+/-5

99.9826%

Modern mechanical watch
non-certified

+/-10

+/-5

+/-2

99.9942%

Modern mechanical watch
chronometer certified

+6/-4

+/-3

+/-1

99.9977%

Modern quartz watch
non-certified (normal)

+/-2

+/-1

+/-0.1

99.9998%

Modern quartz watch
chronometer certified (rare)

+/-0.02

+/-0.02

+/-0.0

99.9999%


While some people desire wristwatches with extremely high accuracy over long periods of time, it is seldom for any reason besides personal satisfaction. The few professions that depend on precision time synchronization (such as astronomy, global navigation, train scheduling, and broadcasting) base their operations on high precision time sources, not consumer wristwatches.

So why would anyone want a less accurate watch?

The short answer is that pretty much any modern wristwatch from a reputable brand is more than accurate enough for normal use. So some people (myself included) prefer older mechanical watch technologies over the small accuracy advantages of quartz watches. In the 1970s everything was heading towards quartz watches but by the 1990s handwound and automatic mechanicals were once again firmly establishing themselves in the mid ranges and high end market.

Are Quartz Watches alway more accurate than mechanical models?
Typically they are, but not always. Accuracy and precision are not exactly the same thing.

It is important to remember that even when a mechanical watch is allowed to vary +6/-4 seconds per day, that does not mean it will consistently vary by that high an amount each day. Mechanical movements--except the very rare 'turbillon' movements that correct for it--are noticably affected by the gravitational pull of the Earth. It only takes a performance distortion of 1/1000th of a percent for a watch movement to be one second less accurate in a day. This causes the performance of mechanical movements to be somewhat different from day to day when not stored in a fixed position. The good news is that the actual variations of a mechanical watch will often cancel each other out. This means a mechanical watch will tend to be more accurate over a longer period than the single-day COSC measurement may imply.

The day-to-day performance of quartz is much more consistent than mechanical under identical conditions. Quartz performance is affected mainly by temperature changes and weakened batteries. So a quartz watch that you measured to gains 0.5 second yesterday will be consistently increasingly off correct time by about that amount. You can be pretty certain that in 60 days, it will be about 30 seconds off. At the end of a year, it would be likely be over 180 seconds off.

Compare that to a mechanical watch that you measured to gain 2 seconds yesterday. It would seem that our example quartz watch is 4 times more accurate than this. But while the daily measured daily variations seem much higher, they are not likely to be as consistent, so will have a dampening effect. You cannot accurately predict that this mechanical would therefore be off by 120 seconds at the end of the same 60 days. It might be right on time, or it may be 200 seconds off. That broader range of variations allows most mechanical watches to stay closer to correct time than the daily variation rate implies. Over a year, some mechanicals can on average stay closer to correct time without having to be reset than a quartz watch might where others always tend to gain roughly the same amount each day.