====== Twin Quartz ======
**Twin Quartz** is the name for two similar [[thermocompensation]] technologies employed by [[Seiko]] in the late 1970s and early 1980s, resulting in astonishing accuracy as high as 5 seconds per year in certain [[high-accuracy quartz]] movements.

===== Overview =====
All [[quartz]] movements vary in rate according to temperature, and the challenge of thermocompensation has long presented challenges. Ever since the first [[Beta 21|Beta project]] [[chronometer]]s, various techniques have been employed.

In the 1970s, Seiko added a second quartz crystal to their leading movements, resulting in various Twin Quartz movement families. [[Suwa Seikosha]] was first, introducing their [[Seiko 99|Cal. 99 family]] in August [[1978]]. Their approach used a second quartz crystal not for timing but to detect changes in temperature and tune the timing of the primary crystal accordingly. This resulted in accuracy as good as 5 seconds per year in the [[Seiko Superior]] Cal. 9983. Other members of the Cal. 99 family achieved 10 or 20 seconds per year in [[Seiko Grand Quartz|Grand Quartz]] and [[Seiko King Quartz|King Quartz]] watches.

[[Daini Seikosha]] used a different approach, averaging the timing between two crystals with different temperature characteristics. This also delivered excellent accuracy, especially in later movements like their [[Seiko 96|Cal. 9682]] which was also good to 5 seconds per year.

===== List of Twin Quartz Movements =====
  * [[Seiko 99]] (1978-1985) - Suwa
    * [[Seiko 99|Seiko 9920/9921/9923]] (1978-1985) - 20 seconds per year, [[Seiko King Quartz]]
    * [[Seiko 99|Seiko 9940/9942/9943]] (1978-1985) - 10 seconds per year, [[Seiko Grand Quartz]]
    * [[Seiko 99|Seiko 9980/9983]] (1978-1985) - 5 seconds per year, [[Seiko Superior]]
  * [[Seiko 92]] (1978-1985) - Daini
    * [[Seiko 92|Seiko 9223]] (1981-1985) - 10 seconds per year, [[Seiko King Quartz]]
    * [[Seiko 92|Seiko 9256]] (1978-1980) - 10 seconds per year, [[Seiko Grand Quartz]]
  * [[Seiko 97]] (1979-1981) - Daini
    * [[Seiko 97|Seiko 9721/9722/9723/9726]] (1979-1981) - 20 seconds per year, [[Seiko King Quartz]]
  * [[Seiko 96]] (1979-1981) - Daini
    * [[Seiko 96|Seiko 9641/9642]] (1980-1983) - 20 seconds per year, [[Seiko King Quartz]], [[Seiko Dolce]]
    * [[Seiko 96|Seiko 9661]] (1980-1983) - 20 seconds per year, [[Credor]]
    * [[Seiko 96|Seiko 9681/9682]] (1981-1985) - 5 seconds per year, [[Seiko Superior]]
  * [[Seiko 94]] (1980-19??) - Suwa
    * [[Seiko 94|Seiko 9441/9442/9443]] (1980-1985) - 20 seconds per year, [[Seiko King Quartz]], [[Seiko Dolce]], [[Seiko Lassale]]
    * [[Seiko 94|Seiko 9461]] (1981) - 10 seconds per year, [[Seiko Grand Quartz]]
    * [[Seiko 94|Seiko 9481/9483]] (1980-1985) - 5 seconds per year, [[Seiko Superior]]


{{tag>Seiko}}