Measurement Uncertainty

How does DMM accuracy affect your next measurement?

A 20 gallon automobile gas tank gauge has a measurement uncertainty of +/- 1 %, equating to an uncertainty of 4/10ths gallon. In terms of distance, at 20 Mpg fuel economy the uncertainty is 8 miles. When the gauge is close to empty, do you have enough to travel 1 mile or 8 miles?

Measurement uncertainty is an estimate of the possible error in a measurement. It's also an estimate of the range of values which contain the true value of the measured quantity. It's also the probability that the true value lies within a stated range of values.

The most significant factors contributing to digital meter measurement errors are:

• Drift, offset, and noise found in meter input signal conditioning circuits.
• Errors associated with analog to digital conversion processes, like range linearization and noise.

Digital multimeter uncertainty is:

• Expressed as +/- (percentage of measurement + percentage of range) and is dependant on temperature condition and time since last calibration.
• Typically specified at room temperature 23 +/- 5 °C. A temperature coefficient is provided to calculate uncertainty for measurements performed outside this range.

Manufacturers typically warrant the measurement uncertainty specification for up to 1 year after last calibration.

Impact of measurement uncertainty:

• If a meter's uncertainty is greater than the tolerance of the test, the test results are unreliable.
• Measurement uncertainty must be considered in order to determine the level of confidence in a measured value relative to the test requirements.
• To achieve high degree of confidence, best practices suggest the ratio of measurement uncertainty to test requirement uncertainty should exceed 4:1 (TUR).
• For example, testing a 1.25 Volt reference circuit (Burr Brown REF3312) that has an accuracy of 0.15 % or 1.25 V +/- 1.875 mV.