In the last section we discussed both imperial and SI unit systems. By understanding the principles of measurement traceability or metrological traceability, we can know how a measuring instrument is traceable to SI units, and if it complies with ISO/IEC17025.
When you are using a ruler to measure length, do you ever think about whether the ruler is accurate or not? And is the equipment resolution high enough for your purposes? If we do not know how accurate an instrument is, we cannot be sure its measured values are true.
Instruments must therefore be calibrated regularly to make sure that they are accurate. In the product testing industry, most standards will specify the accuracy and resolution requirements for the instruments we use. For example: BS EN71-1: 2011, Clause 8.25.1.1, states that the apparatus shall be capable of measuring thickness to an accuracy of 1 µm according to ISO4593. See how this requirement is set out:
Even if you have considered these factors, you must also consider the other instrument that has been used to compare with the ruler, i.e. in this activity called 'calibration'. In other words, a chain of reliable measurement must be established to ensure the measurements we make are accurate. This is why we require calibration and consider metrological traceability in product testing.
The Joint Committee for Guides in Metrology (JCGM) defines metrological traceability as the 'property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty' (JCGM 2008, 29).
In the ISO/IEC17025: 2005 edition, Clause 5.6 Measurement Traceability (Figure 1.4) requires that all equipment used for tests and/or calibration shall be calibrated before being put into service. You will learn more about metrological traceability and the metrological traceability chain in the following sections.
Now complete the following reading.
Reading
Please read pages 4–5 from the American Association for Laboratory Accreditation (A2LA) (2011) P102: A2LA Policy on Measurement Traceability.
You should also have a look at the measurement traceability services provided by the Hong Kong SAR Government in the next reading.
Reading
Standard and Calibration Laboratory of the Innovation and Technology Commission, Hong Kong SAR Government.
You should now have a better idea of the requirements and definition of measurement traceability.
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