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

The Direct Comparison Method
When a traceable time display was used, the uncertainty due to human reaction
time was found to be approximately the same as the human reaction time for an
audible tone. Keep in mind that these results are presented to illustrate the nature
of uncertainty due to human reaction time, and to provide a very rough estimate
of its magnitude. We strongly encourage each person calibrating stopwatches
and timers to perform repeatability and reproducibility experiments to help better
determine the uncertainty of human reaction time.
5.C.3. Device Under Test (DUT) Resolution Uncertainty
Since the direct comparison method requires observing data from the DUT
display, the resolution of the DUT must also be considered. For digital indicating
devices, resolution uncertainty is understood to be half of the least significant
digit, with an assumed rectangular probability distribution. For an analog watch,
the same method of determining resolution uncertainty may be used because the
watch moves in discrete steps from one fraction of a second to the next.
5.C.4. Uncertainty Analysis
This section provides an example of how data collected using the direct comparison
method can be used to perform an uncertainty analysis. For this estimate of
uncertainty we will include the mean bias as an estimate of uncertainty, rather
than correcting for it, because the mean bias can be either negative or positive,
and may vary from time to time for the same user [17]. In this calibration
process, the mean bias can be considered a measurement of reproducibility, and
the standard deviation a measure of repeatability.
5.C.4.a. Uncertainty Distributions
Because of the lack of knowledge regarding distributions of the mean bias and
delay deviation between telephone calls, both components of uncertainty are
treated as rectangular distributions. Since the resolutions of digital and analog
stopwatches have known, discrete quantities, their distribution is also rectangular
[12]. All other data are considered to be normally distributed.
5.C.4.b. Method of Evaluation
Even though the data provided in previous sections were treated statistically, they
were collected from previous measurements, and not during the actual stopwatch
calibration. Because the metrologist does not have statistical data based on a
series of observations to support these uncertainties, they are identified as Type
B.
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