The big T in measuring refers to the concept of traceability. Traceability is the ability to chronologically interrelate uniquely identifiable entities in a way that matters. This allows tracing history, application, and location of an entity. For measurements this means establishing an unbroken chain of comparisons relating an instrument’s measurements to a known standard. Traceability ensures accuracy and consistency of measurements across time, space, and measuring instruments. It is a fundamental requirement in science, engineering, and commerce.
Why is traceability important?
Traceability is crucial for ensuring the reliability and comparability of measurements. Without it, there is no way to determine if a measurement is accurate or consistent with previous measurements. Traceability provides:
- Accuracy – Measurements can be traced back to known measurement standards to validate their accuracy.
- Consistency – Enables different instruments in different locations to produce comparable measurements.
- Reproducibility – Allows measurements to be reliably reproduced over time.
- Quality assurance – Provides confidence in the calibrations, measurements, and testing procedures used.
- Legal defensibility – Provides documented evidence that can be used to defend the validity of measurements.
Traceability is now a mandatory requirement for many regulated sectors like pharmaceuticals, medical labs, food manufacturing, and automotive manufacturing. It helps ensure product quality and safety for consumers and aids regulation compliance. For scientific research, traceability allows results to be verified and replicated in different labs. In general, traceability improves process control, risk management, due diligence, and decision making.
How is measurement traceability established?
There are four key requirements for establishing measurement traceability:
1. Transfers through an unbroken chain of calibrations
Each instrument must be calibrated by another instrument of higher accuracy, forming a documented chain of calibrations back to the primary standard. This may involve multiple intermediate instruments over several hierarchical levels, but gaps are not permitted.
2. Traceability to SI units through national metrology institutes
The calibration chain must eventually lead to primary standards maintained by national metrology institutes that realize the SI units – the globally recognized system of units like the meter, second, kilogram etc. This ties the measurement into the agreed international system of units.
3. Measurement uncertainty analysis
Each calibration must be accompanied by an analysis of the contributing uncertainties and how they propagate through the chain. This determines the final measurement uncertainty relative to the standard.
4. Established quality system
A documented quality system must ensure calibrations are done by trained personnel using approved procedures, with appropriately calibrated equipment under suitable environmental conditions. Records of all activities must be maintained.
Adhering to these four requirements provides objective evidence of traceability of measurements to required standards. Accreditation bodies like ISO 17025 reflect these requirements.
Calibration standards and traceability
Traceability chains are only as strong as their weakest link. High accuracy calibrations are vital. Key terms regarding calibration standards and traceability include:
Primary standards
These are standards established and maintained at the highest metrological level, usually by national metrology institutes. They have the highest metrological qualities and directly realize the SI units. Examples include atomic clocks for the second and kilogram prototypes for the kilogram. They are the origin of traceability chains.
Secondary standards
These are standards calibrated against primary standards, usually by a national metrology institute. They are then used to calibrate working measurement standards and instruments at lower levels. They disseminate traceability from primary standards.
Working standards
These are standards used routinely to calibrate or check instruments and measuring systems. They transfer calibration from secondary standards to field instruments. They are calibrated periodically by primary or secondary standards.
Reference standards
These have very high stability and accuracy. They are calibrated against primary or secondary standards and are used to maintain working standards. They preserve and transfer calibration between less frequent calibrations.
Transfer standards
These are stable portable devices used to compare reference standards at different locations. This enables local calibrations when shipping reference standards is impractical.
Proper selection, care, and calibration of these standards at each step is vital for maintaining traceability. National metrology institutes like NIST in the USA play a crucial role in providing the infrastructure and scientific expertise to realize and maintain primary standards and disseminate traceability.
Traceability in specific fields
While traceability principles are universal, implementing them requires interpreting them into the context of each application domain. Here are some examples:
Medical testing labs
lab instruments like blood analyzers must have calibrations traceable to standards maintained by national metrology institutes or standards developing organizations. Reference material providers play an important traceability role by providing materials for checking test accuracy.
Pharmaceutical manufacturing
Measuring instruments for processes like weighing active ingredients must have calibrations traceable to SI through an accredited lab. Traceability applies to the entire supply chain from raw materials to finished drugs.
Food manufacturing
Key instruments like thermometers, scales, and flow meters must have traceable calibrations. Traceability requirements extend to supply chain inputs like ingredients from suppliers. HACCP programs integrate traceability.
Automotive manufacturing
Measurement processes for fabrication and assembly require traceability. Gauge blocks, hardness testers, torque wrenches etc. must be calibrated with SI traceability. Traceability also applies to supplied components like engine parts.
Forensic investigations
Measurement results like DNA, toxicology, and ballistics tests must demonstrate traceability. Traceability of sampling, evidence handling, and calibration of lab equipment is also vital. This is required for legal admissibility.
Environmental monitoring
Traceability applies to pollutant measurements like stack emissions, water quality, and air quality. Networked monitoring stations must have consistent SI traceable calibrations. Traceability enables comparing local and global datasets.
While details vary by field, the necessity of measurement traceability remains constant. Adhering to traceability principles is key for quality, safety and meaningful comparisons across space and time.
Challenges in achieving traceability
While critical, establishing full end-to-end traceability can be challenging:
- Complex supply and distribution chains make tracing back to primary standards difficult.
- Many laboratory instruments don’t have clear calibration routes to SI units.
- New measurement technologies often lack recognized standards and traceability infrastructure.
- Traceability adds costs which companies may seek to avoid.
- Lack of awareness and training on traceability requirements.
- Coordinating traceability across multiple entities and processes.
Overcoming these challenges requires effort and commitment from both private and public institutions. Companies must invest resources in traceability and governments should expand metrology outreach and services. International collaboration is also essential. Growing recognition of the benefits should continue driving improved traceability across more fields.
Conclusion
Traceability is a vital concept that enables measurements to be connected through an unbroken chain to required standards like SI units. It establishes measurement accuracy and consistency. Traceability provides extensive benefits but requires rigorous calibration procedures and infrastructure. Continued advancement in national and international metrology capabilities combined with regulatory and commercial drivers should further expand measurement traceability and its crucial role in science, trade and safety.