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

Other Topics Related to Measurement Uncertainty
Section 9
Other Topics Related to Measurement Uncertainty
This section discusses other important topics related to measurement uncertainty.
The first involves estimating the uncertainty of a field calibration, where a
reference standard stopwatch is brought into the field and used to calibrate another
device. Next, the stability and aging of 32 768 Hz quartz crystal oscillators used
in stopwatches and timers is discussed, followed by discussions of how the
performance of a stopwatch can change when a new battery is installed, and how
extreme temperature conditions can affect stopwatch performance.
9.A. Uncertainty Analysis of Using a Calibrated Stopwatch to
Calibrate another Device
A calibrated stopwatch is often used to perform field calibrations of other timing
devices, such as an industrial timer or parking meter. In this example (Table
13), we are calibrating an industrial timer with 1 s resolution. The reference
for the calibration is a stopwatch that has a specified accuracy of 5 s per day
(about 208 ms per hour), 0.01 s resolution, and a calibration uncertainty of 0.2
s per day (roughly 8 ms per hour). As previously discussed in Section 5.C.2,
the human reaction time bias is 120 ms, and the human reaction time standard
deviation is 230 ms. The measurement time for the calibration is 1 hour (3600
s). Using these values and the analysis provided in Table 13, the uncertainty of
the industrial timer when measuring a 1 hour interval is 789 ms, or 0.02 %. All
values in Table 13 are rounded to the nearest millisecond.
When performing this analysis, it is important to include the specified accuracy
of the reference stopwatch, which takes into account long-term sources of error
during the calibration interval that may not have been noticeable during the time
of calibration. It is also important to consider the uncertainty of the stopwatch
calibration as part of the budget when using a stopwatch as a reference, because
it may be relatively large when compared to the other sources of uncertainty in
the measurement process.
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