Thermocompensated quartz

A quartz crystal's frequency is not fixed. NIST's account of frequency measurement states that, of the influences on a crystal oscillator, "temperature changes are the largest problem, but other parameters such as humidity, pressure, and vibration can also change the frequency". An ordinary watch crystal is trimmed to run correctly near one temperature, so a wrist warmer or colder than that shifts the rate.

Compensation does not prevent the shift; it measures it and subtracts it. NIST describes the standard method: "an alternative solution to the temperature problem is the temperature-compensated crystal oscillator (TCXO). These devices include a temperature sensor that generates a correction voltage that is applied to a voltage-variable reactance, or varactor."

The published watch figures are the brands' own. Grand Seiko states of its 9F quartz that "the temperature inside the watch is also measured 540 times a day", giving a stated "±10 seconds per year", and that its oscillators are "'aged' for three months" before selection. Citizen states: "we have achieved the highest levels of annual accuracy: ±5 seconds and ±1 second", with the internal temperature monitored "once per minute for a total of 1,440 measurements per day". These are company statements about company products.

On independent rating: COSC does test quartz (its FAQ states that its certification "attests to the high precision of the movement, whether this is mechanical or quartz") but the same page publishes numerical limits only for mechanical watches, saying of quartz merely that "quartz movements follow internal requirements inspired by this standard but adapted to their specific features". None of the figures above corresponds to a limit certified by a neutral body.