Fluke 5730A Calibration Manual page 60

5730A
Calibration Manual
There are many ways to develop test limits from decision rules. Two of these
rules are shown as examples to develop test limits through guardbanding.
ILAC G8 (ISO 14253-1) Decision Rule
This guardband strategy addresses example 5 shown above. The test limit is
determined by the subtracting the uncertainty of measurement from the
specification.
Test Limit = Specification Limit – Expanded Uncertainty of Measurement
The specification limits can be obtained from the performance verification
procedure or can be computed using information from Determining Specification
Limits for other Calibration Intervals as guidance.
Once the calibration laboratory has determined the expanded uncertainty for a
measurement, the test limits for this method may be computed as follows:
Using the example test point in the previous section, for the 90 day 99 %
confidence specification for 2 mA at 1 kHz, the specification limits are ±274 nA.
If for example, the expanded uncertainty of measurement at this test point was
55 nA, the test limit would be:
Test Limit = 274 nA – 55 nA = 219 nA
This creates upper and lower test limits of 1.999781 mA to 2.000219 mA. If the
measured value obtained in the calibration were between these limits, by this
decision rule the Product would be in tolerance or indicated as Pass.
If the measured value obtained was between 219 nA (the test limit) and 274 nA
(the specification limit) this is known as an indeterminate measurement by ISO
14253-1. Some organizations elect to call this a conditional pass as it is more
likely that the measurement indicates an in tolerance condition than not.
If the measured value is >274 nA, but <329 nA (the sum of the specification limit
plus the uncertainty) ISO 14243-1 indicates that this is an indeterminate
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Figure 6. Test Limits Established by Guardbanding
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