Repeatability in Measurement

Repeatability is a term in metrology that is an important aspect to consider when you are going to use a certain measuring tool if you demand a high degree of precision. It also indicates the quality of measurement provided by your tool.

What is Repeatability?

Simply, repeatability is a condition of how consistent the measuring result performed in a short period of time interval under a replicated measurement.

If you measure an object using, for example, a caliper, you do measure it several times and in a short interval time, and then find the measurement result always exactly the same or similar, it means your caliper has a high degree of repeatability.

Vice versa, if you don’t get the same result, it means low or even poor repeatability. Another term is repeatability error. 

To gain a replicated measurement, you have to use the same measuring tool (the same caliper you are using), the same procedure (applied pressure, etc), the same place (laboratory, machine shop, outdoor, etc), the same environment conditions (temperature, air pressure, humidity, etc), and the same operator (you).

For Example

The easiest thing you can do to recognize this metrology term is by performing the zero reading of your linear precision measuring tools such as dial caliper, digital caliper, dial indicators, test indicators, micrometers, or etc). You can run the zero reading several times within 10 minutes. Getting the same zero reading after a number of tests is considered consistent. Herewith, the measuring tool is reliable to use.

The experiment can be so much fun especially if you use the digital caliper or digital micrometer with the USB data output. You perform the zero reading, and let the reading automatically transferred to the spreadsheet on your computer or tablet.

Is there perfect repeatability that has a value exactly “0”?

However, it’s not actually what your eyes see (the zero reading). If your test your measuring tool has a 0.001″ resolution for example, then it may deviate to within the range of +0.0001 to -0.0001 (expressed with the “±” sign). In this range, your eyes are not able to realize it. A consecutive test will generate a number of different readings under a higher resolution measuring tool.

an analogy of repeatability

The above picture can be an analogy. If the shootings happen under the same environmental conditions, then all the dots in the yellow area are considered high while the red and blue areas are considered low repeatability.

Summary

When you replicate the measurement, run it in a short time and find a similar result, you get great repeatability. A deviation value happens due to the replicated measurement and therefore expressed in the “±” sign. Repeatability and other terms such as accuracy, precision, resolution, traceability, reproducibility, etc are some basic important terms you have to know in regard to precision measuring tools.

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