A calibration report for an instrument has a ton of information. Some of this information includes calibration methods, results, conditions, receipts, dates, and, more specifically, measurement uncertainty. Measurement uncertainty results from the report, which is an essential tool. MU is one of the more difficult tasks that calibration technicians must assess. While complex, uncertainty is defined for many variables, including pass or fail. Here is a look at what makes this valuable for calibration.
Measurement Uncertainty Defined
In every calibration done, an uncertainty ratio is completed. When this ratio is found, the measurement traceability can be calculated. Within every measurement taken, there is some “doubt.” This, in turn, tells us how accurate the measurement is and the amount of doubt that needs to be then calculated.
All calibration labs use the ISO Guide to the Expression of Uncertainty in Measurement (GUM) to estimate measurement uncertainty.
Error and Uncertainty are Not the Same
A common misinterpretation is that error and uncertainty are the same things. However, calibration error is when we compare the device to the reference standard. The error is the difference between the two. The error does not have significance unless the uncertainty is known.
Guide to Uncertainty in Measurement
To calculate the measurement uncertainty, GUM has an 8-step process.
1. Measurand – Define the measured value in your measurement process.
2. Model Equation – Calculate the output quantity from the input quantity listed.
3. Uncertainty Resources – Establish the uncertainty for each input quantity.
4. Evaluation – Assess any correlations or covariances in the quantities.
5. Calculation – Determine the measured value of the report.
6. Combined Standard Uncertainty – Merge the uncertainty components.
7. Obtained Uncertainty – Multiply the combined uncertainty by a coverage factor.
8. Reporting – The results must be reported within ISO standards.
What the measurement Uncertainty Establishes
Measurement uncertainty is used to establish the actual value of measurement results. By knowing the true value, you can define the implementation in conformity assessment, which decides the pass or fail outcome.
Measurement uncertainty provides a significant comparison of measurement results. It helps compare two measurement outcomes that show which one of the results is to be used.
An essential use of measurement uncertainty is to calculate Test Uncertainty Ratio (TUR). Before any calibration, it is a prerequisite to use correct reference standards. TUR is the ratio of tolerance of the test measurement concerning the uncertainty.
The measurement uncertainty reported in a calibration certificate can define the tolerance limit. The MU combines all the good sources of errors to be used as the tolerance limit.
An Essential Reported Detail
Measurement uncertainty is one of the essential elements in calibration. A lab must state that it has executed a complete calibration with it. So, while it may seem like a minor detail, it has practical use within the report. If you have questions about a calibration report, the Technicians at SRP control systems can help. Contact us today for all of your calibration needs!
