Accredited Calibration of Standards in Manufacturing Operations

By: Angel Pabellon, PVSR Corporation dba ProvenSmart Metrology Lab / Quality Manager

Introduction

In manufacturing, calibration standards ensure the traceability and reliability of measurements. When these standards are used to calibrate other instruments, the type of calibration—standard (non-accredited) vs accredited—affects measurement quality, legal compliance, and risk management. ISO 17025 and ANSI Z540.1 provide guidance on when accreditation is essential.

Definitions

In metrology, precise definitions are critical for clarity and compliance. Understanding the distinctions between standard and accredited calibration is foundational to proper measurement management:

·       Standard Calibration (ANSI/NCSL Z540.1): Refers to calibration activities that, while traceable to national standards (e.g., NIST), are not conducted under third party-accredited systems. These calibrations may lack full uncertainty analysis, or the rigorous quality oversight demanded by accreditation bodies.

·       Accredited Calibration (ISO/IEC 17025): Performed in laboratories accredited by recognized bodies (e.g., A2LA, ANAB, NVLAP) under ISO/IEC 17025 standards. These calibrations include documented uncertainty budgets, rigorous measurement procedures, traceability to SI units, and consistent quality assurance through periodic audits and proficiency testing.

·       Traceability: Is defined 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. For each metrology discipline, there is defined a relationship between traceability and the measurement uncertainty hierarchy; when closer to primary standard, tighter shall be the level of uncertainty.

·       TAR (Test Accuracy Ratio): The comparison between the accuracy of the Unit Under Test (UUT) and the accuracy of the standard used to calibrate it. The idea is assuring the accuracy of the standard used would be several times less than UUT accuracy (usually in absence of measurement uncertainty analysis).

·       TUR (Test Uncertainty Ratio): The comparison between the accuracy of the UUT and the estimated calibration (measurement) uncertainty of the standard used to calibrate it. The idea is to assure the uncertainty of the standard used would be several times less than UUT accuracy.

Regulatory & Quality Frameworks

ISO/IEC 17025 is the international standard for testing and calibration laboratories. Key requirements include:

·       Demonstration of technical competence.

·       Implementation of a quality management system aligned with ISO 9001 principles.

·       Traceability to national or international standards.

·       Reporting of measurement uncertainty according to ISO GUM (Guide to the Expression of Uncertainty in Measurement).

·       Participation in interlaboratory comparisons or proficiency testing.

·       Document control and data integrity measures.

·       This standard is globally recognized and often mandated in regulatory environments, especially in aerospace, defense, and medical sectors.

ANSI/NCSL Z540.1 is a U.S.-based standard, although withdrawn in 2007, is still prevalent in legacy defense and industrial contracts. Key points include:

·       Establishment of a calibration system that ensures accuracy ratio (TAR ≥ 4:1).

·       Traceability to national standards (e.g., NIST).

·       Quality assurance and documented calibration intervals.

·       Emphasis on controlling measurement errors and ensuring repeatability.

While ANSI Z540.1 does not require third-party accreditation, it does require a documented system and qualified personnel.

When Accredited Calibration Is Necessary

Determining the necessity for accredited calibration requires evaluating the risk and application criticality. Situations where accredited calibration is indispensable include:

1.     Calibration of Transfer Standards or Reference Instruments: Any instrument used to calibrate other equipment must have a well-characterized uncertainty and traceability. Using accredited calibration minimizes cumulative error across the measurement chain.

2.     Compliance with Regulatory or Customer Requirements: Contracts in aerospace, defense, pharmaceuticals, and automotive industries often mandate ISO 17025-accredited calibrations.

3.     When Measurement Uncertainty Affects Product Quality: If product conformance decisions depend on measurements with tight tolerances, accredited calibration ensures data reliability and supports defensible quality control.

4.     During Internal or External Audits: ISO 9001, IATF 16949, and AS9100 audits often scrutinize calibration records. Accredited certificates with uncertainty statements and traceability facilitate smoother audits.

5.     Legal and Liability Considerations: In disputes over product performance or safety, accredited calibration provides defensible evidence of measurement integrity.

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Comparative Table: Standard Calibration vs Accredited Calibration

Criterion	Standard Calibration	Accredited Calibration
Traceable to NIST	✔ Traceable	✔ Accredited traceable
Measurement uncertainty provided	✘ Often not	✔ Required per ISO GUM
Quality system audited	✘ Optional	✔ Regular third-party audits
Accreditation symbol displayed	✘	✔ ILAC-MRA logo
Acceptable for contracts	Limited	Required for military/regulated industries

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Implementation Guidelines

Implementing a robust calibration program that integrates accredited calibration involves:

·       Risk Assessment: Assess the impact of instrument uncertainty on measurement results. Use TUR (Test Uncertainty Ratio) or GUM-based evaluations.

·       Policy Development: Establish a metrology policy outlining when accredited calibrations are mandatory, e.g., for reference standards or customer-required instruments.

·       Supplier Qualification: Select only those calibration labs with accreditation scopes that cover the required measurement parameters.

·       Recordkeeping and Review: Maintain certificates of calibration, uncertainty data, and traceability records. Regularly review calibration intervals and requalification requirements.

·       Training and Awareness: Ensure that metrology personnel understand the implications of using non-accredited versus accredited services.

Illustrative Example

Case Study: Automotive Manufacturing Plant

Scenario: A plant uses a master micrometer standard (±0.002 mm uncertainty) to calibrate handheld micrometers used on critical brake rotor dimensions (±0.01 mm tolerance).

·       Using a standard-calibrated gauge without documented uncertainty risks TAR < 2:1, which could compromise product conformance decisions.

·       Switching to an ISO 17025-accredited gauge with ±0.0005 mm uncertainty improves the TUR to 10:1, enhancing confidence in downstream measurements.

Result: Improved audit outcomes, reduced measurement risk, and alignment with IATF 16949 calibration requirements.

Conclusion

Accredited calibration is a cornerstone of modern quality assurance in manufacturing. Its importance extends beyond compliance, serving as a proactive approach to minimizing measurement uncertainty, reducing risk, and ensuring consistency in product quality. As global supply chains become more integrated and regulatory scrutiny intensifies, the demand for traceable, reliable, and defensible measurements continues to grow.

ISO/IEC 17025 offers a comprehensive and internationally recognized framework for establishing metrological competence, while ANSI/NCSL Z540.1 remains a valuable reference in domestic and legacy systems, especially within U.S. government and defense sectors.

Organizations that adopt a strategic approach to accredited calibration—by integrating it into risk management, audit readiness, and quality control—gain a competitive edge. They can better assure customers and regulators of the integrity of their measurement systems and safeguard against costly non-conformities. Investing in accredited calibration is not merely a compliance exercise but a critical element in sustaining manufacturing excellence, innovation, and trust in technical outputs.