When an instrument is calibrated, the measurements should be traceable back to a common standard such as the international system of units (SI). By definition, one cannot have a traceable measurement without uncertainty, which means we have to estimate it and communicate it in each step of the traceability chain. If traceability is important to you, then you must be able to understand and quantify uncertainty.
Estimating measurement uncertainty is one of the most challenging tasks that metrologists and calibration technicians face. The ISO Guide to the Expression of Uncertainty in Measurement (GUM) was produced in 1993 by the Comite international des Poids et Mesures (CIPM) and the International Bureau of Weights and Measures (BIPM) as ISO/IEC Guide 98.
The GUM was developed to
- Provide rules on the expression of measurement uncertainty
- Promote full information on how the uncertainty statements are arrived at
- Provide a basis for the international comparison of measurement results
All calibration laboratories around the world use the GUM method to estimate measurement uncertainty. You can download it here: https://www.bipm.org/en/publications/guides/
Here is an overview of the GUM 8-step process for calculating uncertainty:
- Describe the measured value in terms of your measurement process.
- List the input quantities.
- Determine the uncertainty for each input quantity.
- Evaluate any covariances/correlations in input quantities.
- Calculate the measured value to report.
- Correctly combine the uncertainty components.
- Multiply the combined uncertainty by a coverage factor.
- Report the result in the proper format.
If you would like more information on this topic, you may watch a recorded version of a webinar I presented:
What you will learn in this on-demand, recorded web seminar
Estimating measurement uncertainty is one of the most challenging tasks that metrologists and calibration technicians face. This web seminar introduces you to the ISO Guide to the Expression of Uncertainty in Measurement (GUM). The GUM document was produced in 1993 in order to have a standard way to compute and communicate measurement uncertainty. All calibration laboratories around the world use the GUM method to estimate measurement uncertainty. We’ll provide information on the history of the GUM’s development and the fundamental concepts of the GUM. This web seminar is intended for calibration technicians, new metrologists or metrology engineers. The information presented provides a foundation for other Fluke Calibration web seminars that present measurement uncertainty analyses for specific calibration applications.