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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Other Topics Related to Measurement Uncertainty <br />
Figure 22. Graph <strong>of</strong> the frequency stability <strong>of</strong> two stopwatches.<br />
To illustrate this, Figure 22 is a graph <strong>of</strong> the frequency stability <strong>of</strong> two stopwatches<br />
that were continuously measured over a period <strong>of</strong> more than one month using<br />
a previously described (Section 7.B.1) time base measurement system. Both<br />
stopwatches were low cost devices; stopwatch A had a suggested list price <strong>of</strong><br />
about $25 USD, <strong>and</strong> stopwatch B sells for about $55 USD. The laboratory<br />
temperature during the measurement was about 23 °C ± 1 °C. The graph shows<br />
the Allan deviation (ADEV) <strong>of</strong> each stopwatch for averaging times ranging from<br />
0.5 days to 7 days. ADEV is a commonly used statistic for estimating frequency<br />
stability [19]. It differs from the conventional st<strong>and</strong>ard deviation because it does<br />
not use the average frequency as a point <strong>of</strong> reference. Instead, it compares the<br />
frequency <strong>of</strong>fset <strong>of</strong> the DUT during each measurement period with its frequency<br />
<strong>of</strong>fset during the previous measurement period. By doing so, it reveals how the<br />
frequency <strong>of</strong> an oscillator changes over time due to effects such as aging.<br />
<strong>Stopwatch</strong> A had an average frequency <strong>of</strong>fset (accuracy) during the test <strong>of</strong> about<br />
8 × 10 -6 . As indicated in Figure 22, the stability (ADEV) was slightly better than<br />
1 × 10 -7 after one week, or about 80 times better than the accuracy. <strong>Stopwatch</strong><br />
B had an average frequency <strong>of</strong>fset (accuracy) during the test <strong>of</strong> about 5 × 10 -7 .<br />
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