When thinking about measurement devices and systems, the subject of accuracy comes up. Accuracy has been discussed in past columns of “Measurement Matters.” In the context of measurement system specifications, there’s more concern about the uncertainty of measurement, not its accuracy. So, what is measurement uncertainty? Is it measurement error?

Defining uncertainty Merriam-Webster defines uncertainty as “lack of sureness about something.” This isn’t the kind of thing you want to consider when performing critical measurements. You want to be sure of your measurement, especially if the measurement is critical to the success, or failure of a part or the system it goes into.

Accuracy is defined by the same source as “degree of conformity of a measure to a standard or a true value.” This definition is based on the use of standards against which other things are compared. The accuracy of a measuring device is the degree to which it can match the true value of the standard.

Length is an easy parameter to consider when thinking about uncertainty of measurement of a known standard. If a steel gauge block standard supposedly is 50.000 mm long and the device used to measure it reports 50.002 mm, is the measuring device inaccurate or is the part actually 2 µm longer than claimed? There’s some uncertainty in the measurement due to this error.

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