©Fluke Calibration 2011 Basics Of Measurement Uncertainty for DMM Calibration 23
Some sources of measurement
“doubt” when verifying a DMM
• The most obvious & significant sources of doubt:
− Inaccuracy of the calibrator’s output value
• 100.0000 mV might actually be 100.0000 mV .0030 mV
− Repeatability or randomness in measurement values from the DMM
• 100.0003 mV, 99.9995 mV, 100.0010 mV, etc.
− Resolution or sensitivity limits on the DMM
• It’s value is ½ the least significant digit,
• in this example it represents 0.05 V
• Many other factors that could also contribute to uncertainty:
− ambient temperature effects, thermal emfs, noise, loading, power line
conditions, etc.
• Consider all factors and include if they significantly contribute to
measurement uncertainty
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