Traceability is the property of a measurement result whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons, each step in the chain having stated uncertainties. The purpose of requiring traceability is to ensure that measurements are accurate representations of the specific quantity subject to measurement, within the uncertainty of the measurement.
Traceability is characterized by six essential elements:
An unbroken chain of comparison: Traceability begins with an unbroken chain of comparisons originating at national, international or intrinsic standards of measurement and ending with the working reference standards of a given metrology laboratory;
Measurement uncertainty: The measurement uncertainty for each step in the traceability chain must be calculated according to defined methods and must be stated at each step of the chain so that an overall uncertainty for the whole chain can be calculated;
Documentation: Each step in the chain must be performed according to documented and generally acknowledged procedures and the results must be documented, i.e., in a calibration or test report;
Competence: The laboratories or bodies performing one or more steps in the chain must supply evidence of technical competence, e.g., by demonstrating that they are accredited by a recognized accreditation body;
Reference to SI units: Where possible, the primary national, international or intrinsic standards must be primary standards for realization of the SI units;
Recalibrations: Calibrations must be repeated at appropriate intervals in such a manner that traceability of the standard is preserved
Summary of Specific Requirements
All calibrations and verifications of measuring and test equipment, reference standards, and reference materials shall be conducted by accredited calibration laboratories (in some instances an accredited testing laboratory may also be able to satisfy traceability requirements) or by a recognized national metrology institute.
Calibrations or verifications must be documented in a calibration certificate or report endorsed by a recognized accreditation body’s symbol, or otherwise makes reference to accredited status.
Laboratories shall define their policy for achieving measurement traceability. This policy shall be in compliance with this policy document.
Where uncertainty calculations are applicable, testing and calibration laboratories shall calculate measurement uncertainty in accordance with the ISO “Guide to the Expression of Uncertainty in Measurement.” These uncertainties shall be supported by uncertainty budgets, and they shall be represented as expanded uncertainties typically using a coverage factor of k=2 to approximate the 95% confidence level.
If a calibration certificate or report contains a statement of the measurement result and the associated uncertainty, then the uncertainty statement shall be accompanied by an explanation of the meaning of the uncertainty statement. (For example, “This uncertainty represents an expanded uncertainty expressed at approximately the 95% confidence level using a coverage factor of k=2.)
Uncertainty ratios or TURs shall be calculated using the expanded uncertainty of the measurement, not the “collective uncertainty of the measurement standards”.
Implicit uncertainty statements (generally expressed in terms of a TUR) shall be accompanied by words to the effect that the uncertainty ratio was calculated using the expanded measurement uncertainty. In addition the coverage factor and confidence level shall be stated.
Calibration reports and certificates issued by JANAAC-accredited calibration laboratories shall contain a traceability statement.
All in-house calibrations shall be supported by the following minimal set of elements:
- The in-house laboratory shall maintain documented procedures for the in-house calibrations and the in-house calibrations shall be evidenced by a calibration report, certificate, or sticker, or other suitable method, and calibration records shall be retained for an appropriate, prescribed time;
- The in-house laboratory shall maintain training records for calibration personnel and these records shall demonstrate the technical competence of the personnel performing the calibrations;
- The in-house laboratory shall be able to demonstrate traceability to national or international standards of measurement by procuring calibration services from accredited calibration labs or a national metrology institute;
- The in-house laboratory shall have and apply procedures for evaluating measurement uncertainty. Measurement uncertainty shall be taken into account when statements of compliance with specifications are made;
Reference standards shall be recalibrated at appropriate intervals to ensure that the reference value is reliable. Policy and procedures for establishing and changing calibration intervals shall be based on the historical behavior of the reference standard.