Physical measurements are always subject to certain influence factors that affect the measurement result. Such influences might include fluctuations in the ambient temperature or inherent errors of the reference devices. To make these factors more comprehensible, it is important to state not only the actual measurement result but also the measurement uncertainly of the test equipment.
The value resulting from the physical measurement always deviates from the "true value" of the physical quantity. This is why it is necessary to determine these deviations or measurement uncertainties. This is the only way to evaluate and compare the actual measurement results.
The measurement uncertainty (MU) of the test equipment is stated along with a measurement or calibration if the result of a measurement must also meet requirements of quality, reliability and comparability.
MU results from the measurement process. The dispersion of the measurement can be used to calculate the standard deviation, and from this the MU. That means the more measurements are made, the smaller the MU will be, because the mean value comes closer and closer to the "true value". The measurement uncertainty can be an absolute or relative value keeping within a certain range.
According to the precise definition: "The measuring uncertainty is a non-negative parameter associated with the result of a measurement characterizing the dispersion of the quantity values being attributed to a measured value, based on the information used." It localises a range of values, within which the "true value" of the measured value lies with a level of confidence to be indicated of 95.45 %.
The measurement uncertainty characterizes the quality of a measurement result.
Simply put, this value can be calculated by using a uniform procedure, thereby expressing the quality of a result, since the measured value lies within a certain value interval under specified conditions. The smaller the MU of a device or test setup is, the more it allows a clear statement how close a measurement result is fitting to a stated limit value. Thus, the MU also shows whether limits are being observed only barely or very clearly.
Measurement results can be affected, among other things, by inherent errors or by the long-term stability of the reference standard. In this case, it is called a systematic deviation. Additionally, the influences of external fields, contact resistances in the area of the meter/transformer under test, or the arrangement of the test setup can also affect the result of a measurement. These influences are referred as non-systematic deviations. >> For more information, please refer to our info paper.
Summarized the indication of measurement uncertainty has the following significance:
- Measurement results become objective
- Calculations are consistent and reproducible
- Measurement uncertainties are traceable, results can then be compared worldwide as long as the reference devices are traceable to national standards
For whom is a measurement uncertainty budget important?
International regulations (ISO 17025) require that calibration or testing laboratories indicate the measurement uncertainty along with the measurement result of a test procedure or calibration. However, the creation of a measurement uncertainty budget* is also important for test laboratories that attach value to high-quality measurement results. In addition to the measurement result of their meter/transformer under test, they also deliver a reliable and comparable value to their customer.
* A measurement uncertainty budget is a tabular overview of the influences affecting the measurand that are associated with measurement uncertainty.
If the result of a measurement is to meet requirements of quality, reliability and international comparability, the measurement uncertainty of the test equipment must be considered. ZERA GmbH will gladly advise you in case you have questions about creating a measurement uncertainty budget for your test equipment. We will provide training if required or create a customized measurement uncertainty budget for you. Feel free to contact us. We will be happy to advise you.