Fluke 5720A service manual 130 + 1000000*0.000000040/0.002=130 ppm + 20 ppm = 150 ppm

Calibration and Verification 3

Determining Test Limits for Other Calibration Intervals

Example 1 shows you how to calculate a specification limit for a specific test level from a specification listed as a percentage (or parts per million) of reading plus a floor error in microvolts, millivolts, miroamps, or nanoamps.

The 99% confidence specification for a 5720A at 2 mA, 1 kHz is 130 ppm + 40 nA. We can convert the floor uncertainty of 40 nA to ppm of output by dividing 40 nA by 2 mA and multiplying by a million:

(130 + 1000000*0.000000040/0.002)=130 ppm + 20 ppm = 150 ppm

This test limit is shown in Table 3-28.

If we wanted to test to the 1 year specification of 140 ppm + 40 nA, the test limit would be the specification of 140 ppm + 40 nA = 140 ppm + 40 ppm = 180 ppm.

Example 2 shows you how to calculate a specification limit and a tighter test limit. For instrumentation that have the precision of the 5720A/5700A Series II calibrators, it is difficult obtain standards which are many times more accurate than the calibrator specifications. Therefore, for cases when the test uncertainty ratio (TUR) of the standards is less than 4:1, guardbanding is used to set test limits which are tighter than the specification limits. These limits provide the same risk of accepting out of tolerance instruments as a TUR of 4:1. The statistical analysis of the guardbanding techniques used are described in a series of technical papers referenced at the end of this chapter.

Guardbanding is used to set the test limits different than the specification limits for a 5720A at 2V, 1 kHz. The 90 day specification is 47 ppm + 10 uV. When you calculate the specification in ppm at the 2V level in the same way as in Example 1, the specification limit is:

(47 + 1000000*0.000010/2)=47 ppm + 5 ppm = 52 ppm.

99% Confidence Guardband Factors

Guardband Factor (K)

1.000

0.950

0.900

0.850

0.800

0.750

1.00 1.50 2.00 2.50 3.00 3.50 4.00

Test Uncertainty Ratio

CHART.EPS

Figure 3-20. Determining Other Test Limits

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