When a calibration report on an instrument is sent back, it has a ton of information.  Measurement uncertainty is a result of the report that is an essential tool.  MU is one of the more difficult tasks that calibration technicians have to assess. While it is difficult, the measurement of uncertainty is defining for so many variables, including pass or fail.  Here is a look at what makes this valuable for calibration.

Measurement Uncertainty Defined

Within every measurement taken, there is some “doubt” of the measurement.  This in turn tells us how good the measurement is.  Every measurement has some doubt, it is about knowing how much doubt there is.

All calibration labs specifically 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 thing.  However, in 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 Calibration Measurement

In order to calculate the measurement uncertainty, GUM has an 8 step process.

1.  Measure – Define the measured value in terms of 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 to 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 true 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 significantly helps compare two measurement results that show which one is the measurement to be used.

An important 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 in relation to the uncertainty.

The measurement uncertainty reported in a calibration certificate can particularly define the tolerance limit.  The MU is the combination of all the valid sources of errors, so it can be used as the tolerance limit.

An Essential Reported Detail

Measurement uncertainty is specifically one of the most essential elements in calibration.  Without it, a lab cannot state that it has executed a complete calibration.  So, while it may not seem like a critical detail, it in fact has valuable use within the report.  If you ever have any questions about a calibration report, the Technicians at SRP control systems can help.  Contact them today for all of your calibration needs.