Fluke A40B, Instruction Manual

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Precision AC Current Shunt Set
Operating Instructions
17
Example: Estimating the Uncertainty of a 5790A Measurement
The following example shows how to estimate the measurement uncertainty of a
calibration measurement (ambient temperature 23
°
C). It illustrates one of the
performance verification tests for the Fluke 5720A calibrator. Specifically it tests
200 mA at 1 kHz, as documented in the 5720A's full verification procedure.
Source: Fluke 5720A – 200 mA output at 1 kHz
Measurement Equipment: Fluke Current Shunt, 200 mA (calibrated at 23
°
C) and a
Fluke 5790A
Make the measurement and estimate the uncertainty using the information given in Table
6. A sample calculation of Measured Current using arbitrary values is shown in Table 7.
As described in the 5720A full verification procedure, evaluating the measured current
uses limits based on the specification at 200 mA, 1 kHz of +/-145 ppm; calculated by the
5720A's 1 year, 99 % confidence specification of +/-(130 ppm of 3
μ
A). As
shown, the expanded measurement uncertainty is very adequate to reliably confirm this
level of performance.
Table 6. Estimating the Uncertainty of a 5790A Measurement
Error Component
Value
(ppm)
Distribution Divisor
Value at
k=1
Squared
Value
Standard deviation of measurement
noise
[1]
5 normal 1 5 25
Shunt current measurement
specification (95 %)
26 normal 2 13 169
Shunt temperature coefficient 0 rectangular 1.73 0 0
Shunt power coefficient
[2]
0 rectangular 1.73 0 0
5790A loading effect uncertainty
[3]
1 normal 2 0.5
5790A specification (2.2 V range) 22 normal 2.58 8.53 72.71
Sum of squared value
266.96
Std uncertainty = square root of the sum
16.34
Expanded, rounded uncertainty in ppm (k = 2)
33
Expanded Uncertainty
±
0.0000066 A
Notes:
[1] Value arbitrarily chosen for this example
[2] Power Coefficient = 0 as shunt is used at nominal current value
[3] Typical uncertainty at 1 kHz
Table 7. Sample Calculation of Measured Current (Arbitrary Values)
Givens/Calculations Values Units
5790A reading 0.799977 V
Calibrated resistance value 3.999770
Ω
Uncorrected calculated 5720A current output (V/R) 0.2000058 A
Add A40B AC-DC difference error (ppm) +1 ppm
Subtract 5790A loading error (ppm) 0 ppm
Total measurement correction +1 ppm
Corrected measurement result 0.2000060 A
Measurement uncertainty in Amperes
±
0.0000066
A