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Stopwatch and Timer Calibrations 5.C.4.c. Combination of Uncertainties In the following examples, the uncertainty budgets were developed for a calibration using traceable land lines (Table 8 and Table 9), and for a calibration using a cellular phone or satellite signal (Table 10) based upon the data previously provided. The human reaction time was based on the worst case data presented in Section 5.C.2. The uncertainties are rounded to the nearest millisecond. The uncertainty components are considered to be uncorrelated, so they are combined using the root sum of squares method. Table 8 - Uncertainty analysis for direct comparison method (digital DUT) using a land line. Source of uncertainty Human reaction time bias Human reaction time standard deviation Telephone delay deviation Magnitude, ms Method of evaluation 120 Type B 230 Type B 30 Type B Distribution Rectangular Standard uncertainty, ms 69 Normal (k = 1) 230 Rectangular 17 ½ DUT resolution 5 Type B Rectangular 3 Combined uncertainty 241 Expanded uncertainty (k = 2, representing approximately a 95 % level of confidence) 482 38
The Direct Comparison Method Table 9 - Uncertainty analysis for direct comparison method (digital DUT) using a cell phone. Source of uncertainty Human reaction time bias Human reaction time standard deviation Telephone delay deviation Magnitude, ms Method of evaluation Distribution Standard uncertainty, ms 120 Type B Rectangular 69 230 Type B Normal (k = 1) 230 150 Type B Rectangular 87 ½ DUT resolution 5 Type B Rectangular 3 Combined uncertainty 255 Expanded uncertainty (k = 2, representing approximately a 95 % level of confidence) 511 Table 10 - Uncertainty analysis for direct comparison method (analog DUT) using a land line. Source of uncertainty Human reaction time bias Human reaction time standard deviation Telephone delay deviation Magnitude, ms Method of evaluation Distribution Standard uncertainty, ms 120 Type B Rectangular 69 230 Type B Normal (k = 1) 230 30 Type B Rectangular 17 ½ DUT resolution 100 Type B Rectangular 58 Combined uncertainty 248 Expanded uncertainty (k = 2, representing approximately a 95 % level of confidence) 495 39
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