Specification and calibration requirements for industrial scales in pharmaceutical applications

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Courtesy of Prime Technologies, Inc.

Performance and accuracy of industrial scales in pharmaceutical applications are covered by current Good Manufacturing Practices (cGMPs). Verification of proper operation of process scales is an important factor in finished product Quality Assurance (QA) programs. Incorrect weighing, additions of materials and components in validated processes of formulation, dispensing, and mixing are most likely not recoverable and costly to businesses. Mistakenly released products within an established QA program could be detrimental to patients’ health and manufacturers’ reputation including legal implications.

This article presents standardized classification of accuracies for weighing systems and describes specifications, methodologies, calibration procedures, routines, and related metrology theories in reference to the current standard. Calibration checks and certification methods are reviewed for illustrations of importance in preservation of weighing integrity.

The Product Master Formula and Batch Records contain information concerning weighing specifications for additions of chemical components. Weighing specifications are scientifically developed to control critical parameters related to scale functional activities. Production recipes include sequential order of chemical component additions, maximum and minimum amounts for each chemical component, mixing time, and feeding rates.

Weighing additions of chemical components will naturally fluctuate from batch to batch. Therefore, cGMPs require that Standard Operating Procedures (SOP) for weighing processes will cover maximum allowed deviations for weights in the process formulas. Products made outside of defined weighing specifications will oblige sanctions of product quarantine for investigation. The almost certain outcome from investigations will lead to destruction or rework of manufactured material.

Permitted variations in component weights need to agree with the scale capabilities. Qualification tests and procedures for scales in validated processes will provide the necessary assurance of accurate weighing execution. Verification of the scale’s compliance to the process requirements is an important phase for the system qualification and validation. Selections of calibration procedures, calibration frequencies, and certification methods for weighing systems depend on application, accuracies, and possibilities for in-time accuracy changes.

The manufacturing processes in weighing applications are limited by the scale’s calibration accuracy. Scale accuracies are established by scale classes. The product quality compliance requires certification of scales traceable to National Institute of Standards and Technology (NIST) Weighing Standards. Calibration procedures deal with formats of recorded data to establish documentation layout and flow designed to assure traceability of collected data.

Performance verification of weighing measuring devices consists of two parts. One is a calibration certification and the other is a calibration check. The calibration certification summarizes a methodical process defined by a written and approved procedure developed for a range of measurements. A calibration check is a simplified confirmation of the scale performance. Usually calibration checks are represented by one or two test measurements.

Properly established scale tolerances, calibration procedures, and scale functional tests are very important issues for QA programs and production costs. CGMP and metrology requirements issues related to scale capabilities, weighing tolerances, and calibration methodology are addressed in this paper.

The governing document for technical requirements of weighing and measuring devices is Handbook 44. Handbook 44 is the current standard published by NIST for all industrial scales and utilized in engineering practices for determination of weighing tolerances and calibration limits. Handbook 44 was adapted by the 84th National Conference on Weights and Measurements in 1999.

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