- Practice · 8 marks
Traceability and hierarchy of standards
Explain traceability of measurement. Describe the hierarchy of standards from the international prototype to the workshop gauge, and classify standards of measurement.
Traceability is the property of a measurement result by which it can be related to national or international standards through an unbroken chain of comparisons, each with a stated uncertainty. It assures that a reading taken in a workshop agrees with readings taken elsewhere.
Classification of standards
| Standard | Use |
|---|---|
| Primary | Realises the unit with the highest accuracy; kept at the national laboratory, used only to calibrate secondary standards |
| Secondary | Calibrated against the primary standard; reference for the calibration laboratories |
| Tertiary | Reference standard of an industry or calibration lab, used to calibrate working standards |
| Working | Used daily in the workshop for inspection and for calibration of gauges and instruments |
Standards may also be classified as line standards (length between two lines, such as the old metre bar), end standards (distance between two flat faces, such as slip gauges) and wavelength standards (based on the wavelength of light).
Hierarchy
International standard (BIPM)
|
National standard (national lab)
|
Accredited calibration laboratory
(secondary / reference standard)
|
Company reference standard
(master slip gauge set)
|
Working standards (workshop gauges)
|
Inspection of the product
Accuracy decreases by a factor of about 3 to 10 at each step, and each step is documented by a calibration certificate. For length, the unit is now defined by the speed of light: the metre is the distance travelled by light in vacuum in s.
Importance
Traceability ensures interchangeability of parts made in different places, gives confidence to customers, and is required by quality systems such as ISO 9001 and ISO/IEC 17025.
✓ Answer checked by Claude Opus 5.5 (high effort)
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