Measurement Traceability, often referred to as Metrological Traceability, is typically defined as “the property of a measurement result whereby the result can be related to a national or international reference standard through a documented, unbroken chain of calibrations, each contributing to the measurement uncertainty of the measurement”.
Measurement Traceability simply means that the result of any measurement can be traced back to being made by a calibrated measurement standard with an assigned measurement uncertainty.
The ‘unbroken chain’ in the definition means that the measurement standard must also have been calibrated by a higher accuracy measurement standard, and so on until the measurement or the value of a standard can be traced back to national or international reference standards. This ‘Calibration Hierarchy’ between each measurement standard must be documented, typically through a Calibration Certificate.
The easiest way to understand the concept of calibration hierarchy is by use of a traceability pyramid. At the top of the pyramid are the highest accuracy national or international reference standards. At the base level are the reference standards used within an organization that need to be sent out to an external calibration laboratory for calibration.
The levels in between contain the external calibration laboratories or National Metrology Institute’s that compare their standards directly with the international reference standards assuring that their calibrations are of the highest accuracy.
The top of the pyramid has the highest accuracy and the smallest uncertainty of measurement. Each level descending from the top of the pyramid loses a certain degree of accuracy as the measurement uncertainties get combined.
Misconceptions of Measurement Traceability
Measurement Traceability is only applied to the measurement result or the assigned reference value of a standard. One common misconception is that the measurement standard is itself traceable. The measurement standard cannot be traceable, only the result measured by the standard. Likewise, Measurement Traceability cannot be attributed to any specific calibration report, national or international standards laboratory or by following a particular procedure.
Another misconception is that a traceable measurement is suitable to be used for any application. Measurement Traceability only ensures that a measurement is an accurate representation of the value being measured. The individual specifications and uncertainty values for each measurement must be reviewed to ensure they are adequate for a specific purpose.
Meeting Traceability Requirements
Measurement Traceability is an important requirement for any organization. Measurement results may be used to make critical decisions that could significantly impact the health and safety of people’s lives, such as in the aviation or medical fields. Errors or omissions in traceability documentation are one of the top 10 cited deficiencies in regulatory audits. An organization needs to have an internal measurement assurance program in place to ensure that the requirements for Measurement Traceability are established and maintained. For a measurement in your organization to be considered traceable, the organization must:
Schedule Regular Calibrations – The calibration of each measurement standard in an organization must be calibrated at regular intervals. If the calibration expires for any standard in the traceability chain, the traceability is broken.
Use Competent Laboratories – The calibration of measurement standards needs to be conducted by an accredited calibration laboratory or by a National Metrology Institute. This ensures that the laboratory is competent to perform the calibrations and properly calculate the measurement uncertainty values.
It is critical that the calibration certificates for equipment that are sent to an external calibration laboratory are reviewed to ensure that the proper information is included on the certificate. The following items must be included on the calibration certificate to state that the measurement value is traceable:
Reference Standards Used – The calibration certificate must list the reference standards that the external calibration laboratory used in the calibration. Those standards provide the link to the chain of comparisons that establishes a connection to the national or international reference standards.
Documented Measurement Uncertainty – Every quantitative measurement needs to include the measurement uncertainty value. If the uncertainty information is missing from the calibration certificate, you cannot claim that the measurement is traceable.
Documented Measurement Procedure – The calibration certificate must list the procedure that was used for the calibration. The calibration must be completed according to a written procedure that is a part of the external calibration laboratory’s quality system.