Applying Guardbands To Specification Limits - Fluke 5730A Calibration Manual

necessary to convert one quantity so that they are both in the same units of
measurement.
The 90 day 99 % confidence specification for the Product at 2 mA, 1 kHz is
117 ppm + 40 nA. Convert the range specification to the same units as the floor
specification by multiplying the range specification by the output value:
(2 mA X 117 ppm = [(2 x 10-3) A X (117 x 10-6)] = 2.34 x 10-7 A = 234 nA
Making the combined absolute specification 234 nA + 40 nA = 274 nA
The specification limits at 2 mA would be 1.999726 to 2.000274 mA
To test to the 1 year specification of 120 ppm + 40 nA, the specification limit
would be 240 nA + 40 nA = 280 nA.

Applying Guardbands to Specification Limits

The expanded uncertainty of measurement must be determined by each
laboratory that calibrates the Product. Even if the procedures for performance
verification in this manual are followed completely, there are different sources of
uncertainty due to traceability, environment, electrical cabling, electro-magnetic
interference, uncertainty of the reference standards used, and operator
influences that are unique to each calibration laboratory. These must be
accounted for their individual situations, so it is not possible for Fluke Calibration
to estimate uncertainty for all user calibrations.
Those doing the calibration of the Product can verify to 99 % confidence limits,
95 % confidence limits, 24 hour, 90 day or 1 year specifications. While the
calibration procedure included in this manual is applicable to testing every test
point, the uncertainty from the calibration process as compared to the
specification limit varies and can require further consideration of measurement
decision risk.
Some quality systems require adherence to particular rules for measurement
decision risk when making claims of compliance with a specification. Examples of
decision rules are:
1. The ratio of the specification tested to the specification of the accuracy of the
standard used (often referred to as a Test Accuracy Ratio (TAR) or Test
Specification Ratio (TSR)) must be greater than 4:1.
2. The ratio of the specification tested to the expanded uncertainty of
measurement (often referred to as the Test Uncertainty Ratio) must be
greater than 4:1.
3. The TUR or TAR must be greater than 3:1.
4. The probability of a false accept risk must be less than 2 %.
5. The Product cannot be determined as meeting specifications unless the
measurement at the test point is less than the value of the specification minus
the expanded uncertainty.
To comply with these decision rules, it can be necessary to establish a
guardband for each test point. A guardband creates a zone that is less than the
specification limits, and if the measured value obtained from the calibration is not
in the guardband area, the level of measurement decision risk is sufficient. The
inner edge of the guardband is the test limit for the calibration shown in Figure 6.
Multifunction Calibrator
Full Verification
49