©Fluke Calibration 2011 Basics Of Measurement Uncertainty for DMM Calibration 59
Does improving beyond ±31 mA by taking more
measurements have any practical value?
What’s the value of increasing
V
eff
from 5 to ?????
The test tolerance is ±250 mA
• 5 measurements, V
eff
= 5.2
− k = 2.57, = 31 mA
• With a = 31mA,
the test ratio is already 8:1
(TUR = Test Spec ÷ Total Uncertainty
0.25A ÷ 31mA = 8.06)
68.27 90 95 95.45 99 99.73
1 1.84 6.31 12.71 13.97 63.66 235.8
2 1.32 2.92 4.3 4.53 9.92 19.21
3 1.2 2.35 3.18 3.31 5.84 9.22
4 1.14 2.13 2.78 2.87 4.6 6.62
5 1.11 2.02 2.57 2.65 4.03 5.51
6 1.09 1.94 2.45 2.52 3.71 4.9
7 1.08 1.89 2.36 2.43 3.5 4.53
8 1.07 1.86 2.31 2.37 3.36 4.28
9 1.06 1.83 2.26 2.32 3.25 4.09
10 1.05 1.81 2.23 2.28 3.17 3.96
20 1.03 1.72 2.09 2.13 2.85 3.42
50 1.01 1.68 2.01 2.05 2.68 3.16
100 1.005 1.66 1.984 2.025 2.626 3.077
1 1.645 1.96 2 2.576 3
0.031
So to satisfy a minimum test ratio of 4:1,
5 measurements are more than adequate!
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