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Stopwatch and Timer Calibrations Figure 11. Reaction time measurements (four operators, 10 runs each) for the direct comparison method. Figure 12. Averaging measurement results for four different operators. 36
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. 37
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Page 7: Acknowledgements ACKNOWLEDGEMENTS
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