Why Measurement Uncertainty is Better Than a Simple "Reading"

Measurement Uncertainty – This term refers to a measurement plus or minus a particular range that best represents the true value. This allows for the fact that a true, actual reading may not be entirely accurate, and so the range accounts for that possibility.

A measurement without a degree of uncertainty is simply a "reading": even national standards laboratories such as the NPL, NRCC and NIST make use of measurement uncertainty over standard readings as these values will include all variables. Being aware of these variables and the range of numbers that the measurement encompasses provides certain benefits.

Benefits of Measurement Uncertainty
A reading will give the false impression that a measurement is accurate. Because no other variables are accounted for, critical applications that rely upon that measurement may have errors. Those experienced with working with measuring equipment are typically aware of this fact and often provide a guestimate of a reading to the best of their knowledge. But skill and experience often plays a role in how accurate those guestimates might be. However, as the benefits of measurement uncertainty are becoming more widely known, the practice is being put to use with greater credibility in many critical fields.

Because a realistic uncertainty measurement can account for actual variables that may be encountered, possible solutions can be provided and procedures can be enacted to account for those variables. A realistic uncertainty reading also lends greater credibility to the measurement information being produced. International standards and procedures have been created that outline how to take a realistic measurement uncertainty reading utilizing a tool known as the "Uncertainty Budget."

The Uncertainty Budget
The uncertainty budget is not an actual physical tool, but rather a procedure in which all factors that might affect a measurement are considered and applied. While knowledge of metrology is a critical factor in determining a proper estimate, the mathematics can be handled by a pocket calculator.

Using a form is the best practice in preparing the uncertainty budget, so that a method is applied to the goal of reaching a final value. The use of a form also enables you to more efficiently review and assess all the elements of your work.

To create an uncertainty budget, you are required to consider all the elements, and then process each one in order to arrive at a common dimensional effect in the form of a standard deviation. Running a statistical study for repeatability will help you to attain the standard deviation for each particular element. It should be noted that sometimes, a common element may in fact possess a different plus or minus distribution range due to specific conditions.

The best budget has proven values instead of estimates but this typically requires more time and capability resources. There are many resources that have published values of standard equipment in the calibration field.

Under ideal circumstances the measurement uncertainty would represent approximately 10 percent of the tolerance being measured. For practical applications a measurement of 25 percent or less is considered acceptable. Any ratio greater than 25 percent should be communicated to the customer by the lab and agreed upon.

In North America the industry standard is to accept and pass gages that have measurement readings within the prescribed gage limits discounting the measurement uncertainty. In Europe the gage is passed if the measurement plus the measurement uncertainty combined is within the prescribed gages limits

Reference Sources and publication can be found on the following websites and documents below

American Association For Laboratory Accreditation (A2LA)
www.a2la.org

National Conference of Standards Laboratories International
www.ncsli.org

ISO GUM
Guide to the Expression of Uncertainty in Measurement
ISO/IEC 17025
General requirements for the competence of testing and calibration laboratories
ANSI/ASME B89.1.2M
Calibration of Gage Blocks by Contact Comparison Method
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