Improving Test Ratios Using Reference Multimeters

Calibrator TUR Improvement

• Accepted levels of test uncertainty ratios (TURs) and test specification ratios (TSRs) have been set to various levels by different quality and calibration standards, such as 10:1 or 4:1 or 3:1.

• These levels are set as acceptable limits for calibration pass or fail decisions. – This minimizes the risk of incorrect decisions

on marginal units under test (UUTs)• However, there will always be instrument test requirements

whose TURs are less than the required levels.– Calibration instruments will always have test

requirements where the test requirement is for better uncertainty than the calibrator capabilities alone.

– In these cases, special metrology engineering work is done to design an acceptable test technique with appropriate levels of uncertainty.

Example: Improving calibrator voltage and resistance uncertainties to support a precision DMM• Verifying a high performance DMM of the 6.5 digit class is an example of a new

generation of T&M instruments with superior specifications (the Fluke 8846A)

• Using a multi-product calibrator (the Fluke 5520A), the most frequently used medium to high accuracy calibration source in modern DC/LF AC laboratories

• In this case, of the more than 130 test points are required to verify the DMM, many have Test Specification Ratios of less than 4:1, applying….

– 90 day specs for the DMM– 1 year specs for the calibrator– For volts (dc & ac) and ohms, 27 points of insufficient TSRs were studied for possible

improvement via characterization

Table Of Test Points Having Low Test Specification Ratios

Nominal Frequency

5520A 1-year

specification

8846A 90-day

specification8846/5520A

Ratio Comments

4WR Ohms 10 0.0014 0.0038 2.71Assume zero 5520A at beginning of each day.

4WR Ohms 100 0.0042 0.012 2.86

2WR Ohms 1.0E+6 34.00 90.00 2.655520A 4-W connection at 8846A terminals

2WR Ohms 10.0E+6 1350.00 2100.00 1.56 2WR Ohms 1.0E+9 15.5E+6 1.6E+6 0.10

DC V 0.1 0.000003 0.000006 2.00DC V -0.1 -0.000003 -0.000006 2.00DC V 1 0.000013 0.000025 1.92DC V -1 -0.000013 -0.000025 1.92DC V 5 0.00008 0.000115 1.44DC V -5 -0.00008 -0.000115 1.44DC V 10 0.00014 0.00023 1.64DC V -10 -0.00014 -0.00023 1.64DC V 100 0.00195 0.0033 1.69DC V -100 -0.00195 -0.0033 1.69DC V 1000 0.0195 0.041 2.10DC V -1000 -0.0195 -0.041 2.10

AC V 0.1 10 0.000038 0.00009 2.37AC V 1 10 0.00035 0.0008 2.29AC V 10 10 0.00365 0.008 2.19AC V 100 100000 0.25 0.68 2.72AC V 750 45 0.235 0.6 2.55AC V 750 1000 0.1975 0.6 3.04AC V 750 1200 0.1975 0.6 3.04AC V 750 10000 0.235 0.6 2.55AC V 1000 45 0.31 0.8 2.58AC V 1000 10000 0.31 0.8 2.58

Function

Lower thandesiredTSRs

Possible Alternative Solutions

• Acquire a more accurate calibrator – a major investment

• Perform limited calibrations – not usually acceptable

• Use advanced metrology techniques (guard banding for example) – a viable alternative on a case by case, test by test basis

• Improve calibrator’s working specs via characterization – also a viable alternative on a broader basis

Method 1: Real time characterization

• The higher accuracy 8.5 digit reference DMM is the actual traceable standard

• The calibrator serves as a stable source to the DMM as the UUT during verification

• The uncertainty for the standard is the combination of the reference DMM’s measurement specification plus any stability considerations for the calibrator

6 ½ DMM

6½ to 7½ digitcalibrator

8 ½ digit multimeter

Nominal Frequency

8508A 1-year

specification

8846A 90-day

specification8846/8508

Ratio Comment

4WR Ohms 10 0.000109 0.0038 34.86 8508A 4W Normal I 4WR Ohms 100 0.00085 0.012 14.12 8508A 4W Normal I

2WR Ohms 1.0E+6 10.00 90.00 9.00 2WR Ohms 10.0E+6 300.00 2100.00 7.00 2WR Ohms 1.0E+9 2.51E+6 1.6E+6 0.64 Use Standard Resistor

DC V 0.1 0.0000006 0.000006 10.00DC V -0.1 -0.0000006 -0.000006 10.00DC V 1 0.0000039 0.000025 6.41DC V -1 -0.0000039 -0.000025 6.41DC V 5 0.0000215 0.000115 5.35DC V -5 -0.0000215 -0.000115 5.35DC V 10 0.0000397 0.00023 5.79DC V -10 -0.000039 -0.00023 5.90DC V 100 0.00059 0.0033 5.59DC V -100 -0.00059 -0.0033 5.59DC V 1000 0.006025 0.041 6.80DC V -1000 -0.006025 -0.041 6.80

AC V 0.1 10 0.000018 0.00009 5.00AC V 1 10 0.000135 0.0008 5.93AC V 10 10 0.00135 0.008 5.93AC V 100 100000 0.077 0.68 8.83AC V 750 45 0.10725 0.6 5.59AC V 750 1000 0.10725 0.6 5.59AC V 750 1200 0.10725 0.6 5.59AC V 750 10000 0.10725 0.6 5.59AC V 1000 45 0.136 0.8 5.88AC V 1000 10000 0.136 0.8 5.88

Assume +/- 5C max temperature variation

Function

Test points with test specification ratios based on the 8½ digit reference DMM as the standard

ImprovedTSRs

Method 2: Create a “Golden Calibrator” with improved working specs

• A “golden calibrator” that has had its actual performance extensively measured so its short term and long term stability is known and this replaces the “generic specifications”

• Modern calibrators have generic specifications providing a greater than 99% confidence that all the population of calibrators will be within spec.

• Individual calibrators often perform well within 30% to 50% of the allowable performance errors

• Characterization of an individual calibrator usually permits using a calculated uncertainty of 2 to 4 times better than spec, or more.

Conclusion: calibrator characterization by a reference DMM

• Daily testing by a reference DMM confirms the DMM is a suitable tool to analyze and characterize a calibrator

• Long term testing by primary standards confirm the suitability of calibrators to be characterized to a better uncertainty.

• When characterizing with a DMM, the overall calibrator uncertainty is a combination of the measured stability of the calibrator and the uncertainty of the reference multimeter

Summary of both Accuracy Enhancement Methods

Table 6: Example Improvements In Calibrator Specifications Using Characterization.

Calibration Test Value

UUT Test Specification

non characterized calibrator spec Calibrator’s TSR

Example spec with long term characterization with

DMM

Example spec using real time characterization by

DMMBest TSR

10 volts ±23 ppm ±14 ppm 1.6:1 ±4.9 ppm ±3.9 ppm > 6:1

100 ohms ±120 ppm ±42 ppm 2.86:1 ±12.3 ppm ±8.5 ppm > 11:1

10 volts10 Hz ±800 ppm ±365 ppm 2.2:1 ±195 ppm ±135 ppm > 6:1

In either case, the final test ratios are very acceptablefor our test & quality requirements!