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Does improving
measurements have any practical value?
What's the value of increasing
from 5 to ?????
V
eff
Degrees of
freedom
68.27
90
1
1.84
6.31
2
1.32
2.92
3
1.2
2.35
4
1.14
2.13
5
1.11
2.02
6
1.09
1.94
7
1.08
1.89
8
1.07
1.86
9
1.06
1.83
10
1.05
1.81
20
1.03
1.72
50
1.01
1.68
100
1.005
1.66
1
1.645
©Fluke Calibration 2011
U
beyond ±31 mA by taking more
m
Fraction p in percent
95
95.45
99
12.71
13.97
63.66
4.3
4.53
9.92
3.18
3.31
5.84
2.78
2.87
4.6
2.57
2.65
4.03
2.45
2.52
3.71
2.36
2.43
3.5
2.31
2.37
3.36
2.26
2.32
3.25
2.23
2.28
3.17
2.09
2.13
2.85
2.01
2.05
2.68
1.984
2.025
2.626
1.96
2
2.576
Basics Of Measurement Uncertainty for DMM Calibration
The test tolerance is ±250 mA
• 5 measurements,
− k
= 2.57,
99.73
235.8
19.21
I
10
.
9.22
6.62
5.51
4.9
4.53
• With a
4.28
4.09
3.96
the test ratio is already 8:1
3.42
(TUR =
3.16
3.077
3
0.25A ÷ 31mA = 8.06)
So to satisfy a minimum test ratio of 4:1,
5 measurements are more than adequate!
V
eff
U
=
31 mA
m
04
0.031
= 31mA,
U
m
Test Spec ÷ Total Uncertainty
= 5.2
Amps
59
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