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Stopwatch and Timer Calibrations Table 13 - Uncertainty analysis of using a calibrated stopwatch to calibrate another device. Source of uncertainty Unit under test resolution ½ digit Human reaction time bias Human reaction time standard deviation Magnitude, ms Method of evaluation Distribution Standard uncertainty, ms 500 Type B Rectangular 289 120 Type B Rectangular 69 230 Type B Normal (k = 1) 230 60 Stopwatch accuracy Stopwatch resolution ½ digit Stopwatch calibration uncertainty 208 Type B Rectangular 120 5 Type B Rectangular 3 8 Type B Normal (k = 2) Combined uncertainty 394 Expanded uncertainty (k = 2, representing approximately a 95 % level of confidence) 789 9.B. The Effects of Stability and Aging on Calibrations of 32 768 Hz Crystals Aging is the systematic change in frequency over time due to internal changes in an oscillator. All quartz oscillators are subject to aging, and the rate at which they age is often a key factor in determining their calibration interval. The aging rate of a quartz crystal oscillator often depends upon its surface area to volume ratio, and small, low frequency crystals generally have low aging rates [18]. Thus, the 32 768 Hz crystals found in stopwatches and timers usually tend to age slowly and have very good long term stability, and their frequency changes by a surprisingly small amount over time. 4
Other Topics Related to Measurement Uncertainty Figure 22. Graph of the frequency stability of two stopwatches. To illustrate this, Figure 22 is a graph of the frequency stability of two stopwatches that were continuously measured over a period of more than one month using a previously described (Section 7.B.1) time base measurement system. Both stopwatches were low cost devices; stopwatch A had a suggested list price of about $25 USD, and stopwatch B sells for about $55 USD. The laboratory temperature during the measurement was about 23 °C ± 1 °C. The graph shows the Allan deviation (ADEV) of each stopwatch for averaging times ranging from 0.5 days to 7 days. ADEV is a commonly used statistic for estimating frequency stability [19]. It differs from the conventional standard deviation because it does not use the average frequency as a point of reference. Instead, it compares the frequency offset of the DUT during each measurement period with its frequency offset during the previous measurement period. By doing so, it reveals how the frequency of an oscillator changes over time due to effects such as aging. Stopwatch A had an average frequency offset (accuracy) during the test of about 8 × 10 -6 . As indicated in Figure 22, the stability (ADEV) was slightly better than 1 × 10 -7 after one week, or about 80 times better than the accuracy. Stopwatch B had an average frequency offset (accuracy) during the test of about 5 × 10 -7 . 61
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Acknowledgements ACKNOWLEDGEMENTS
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Section 1 Introduction to Stopwatch
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Introduction and their traceabilit
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