Stopwatch and Timer Calibrations - National Institute of Standards ...

Stopwatch and Timer Calibrations
Its stability as estimated with ADEV was near 7 × 10 -8 after one week, or about
seven times better than its accuracy.
Because of their slow aging rate, stopwatches tend to produce very repeatable
results over long periods of time. When compared to the requirements for
stopwatch calibrations discussed in Section 3, the small changes in frequency
due to aging are usually insignificant. This makes it possible for a laboratory to
allow long intervals (perhaps exceeding one year) between calibrations.
9.C. Factors That Can Affect Stopwatch Performance
Many factors can cause a quartz crystal oscillator to change frequency. Some of
these are shown in Figure 23. Sharp temperature changes, vibration, and shock
are some of the more common phenomena that can affect the time base on a dayto-day
basis. Also, if power is removed from an oscillator and later restored, there
can be a shift in the frequency. This is true of quartz crystal stopwatches when
the battery is changed. This frequency change is usually small when compared
to the stopwatch’s time base specifications, usually less than 10 to 20 parts in
10 6 (~10 to 20 ms/day) and usually short-term, with the time base returning to
its original frequency after a period ranging from a few hours to a few days.
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Figure 23. Factors that can change the quartz time base frequency.