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