Mike Howell
Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO
This paper, presented by Mike Howell at the EASA Convention 2015, explores strategies for minimizing calibration costs for measuring and test equipment (M&TE) in service centers. Howell emphasizes the importance of inspection and testing activities, as well as controlling special processes like vacuum pressure impregnation and oven cycles. The goal is to minimize measurement error and ensure that measurement values are close to actual values, which necessitates the calibration of M&TE.
Howell begins by differentiating between accuracy and precision, using an archery example to illustrate these concepts. Precision refers to the repeatability and reproducibility of measurements, while accuracy is the degree to which the measured value agrees with the true value. Calibration, defined as a comparison between M&TE readings and standard measurement units, is crucial for assessing accuracy.
The paper explains the formal definition of calibration provided by the Joint Committee for Guides in Metrology (JCGM) and highlights the importance of not confusing calibration with adjustment or verification. Howell discusses the concepts of Test Accuracy Ratio (TAR) and Test Uncertainty Ratio (TUR), explaining their relevance in calibration processes. TAR is the ratio of the accuracy tolerance of the M&TE to the measurement ensemble, while TUR is the ratio of the accuracy tolerance to the measurement ensemble uncertainty.
Traceability in metrology is another key topic, requiring a calibration hierarchy where measurement uncertainty increases along the sequence of calibrations. Howell provides an example of this chain, starting from a national standard to the final measurement at the service center.
The paper outlines the steps for developing a calibration program, emphasizing the need to control and calibrate all M&TE used for activities affecting quality. Howell suggests compiling information about M&TE in an indexed manner, which can be done through a database or a paper-based system. This information helps service centers decide which M&TE to calibrate internally and which to subcontract.
Subcontracting calibration to accredited laboratories is recommended for certain types of M&TE, as these labs provide tools for managing calibration programs efficiently. In-house calibration is feasible for some M&TE, depending on available standards and personnel skills. Howell suggests starting with micrometers and digital multi-meters, leveraging existing skills and shadowing subcontract calibration suppliers.
The development of a calibration program involves evaluating equipment capability, identifying calibration requirements, selecting standards, establishing a recall system, and implementing documentation and reporting systems. Calibration procedures should be detailed and approved according to service center document control procedures.
Howell also discusses calibration checks, which provide added assurance that M&TE is acceptable for use. Impact analysis for out-of-tolerance conditions is crucial, requiring evaluation of previous measurements and actions based on the significance of the condition.
Finally, Howell emphasizes the importance of minimizing calibration costs through proper planning and sound business decisions. He provides a rough estimate of relative costs for subcontracted calibration of typical service center M&TE, highlighting the potential cost savings from a well-planned calibration program.
Key Points Covered:
- Importance of inspection and testing activities
- Differentiation between accuracy and precision
- Formal definition and process of calibration
- Concepts of Test Accuracy Ratio (TAR) and Test Uncertainty Ratio (TUR)
- Traceability in metrology and calibration hierarchy
- Steps for developing a calibration program
- Subcontracting vs. in-house calibration
- Calibration checks and impact analysis for out-of-tolerance conditions
- Strategies for minimizing calibration costs
Key Takeaways:
- Calibration is essential for minimizing measurement error and ensuring accuracy
- Understanding TAR and TUR is important for effective calibration
- Traceability requires a calibration hierarchy with increasing measurement uncertainty
- Proper planning and documentation are crucial for developing a calibration program
- Subcontracting calibration to accredited labs can simplify program management
- In-house calibration is feasible for certain M&TE with the right standards and skills
- Calibration checks provide added assurance of M&TE accuracy
- Impact analysis for out-of-tolerance conditions is critical for maintaining measurement validity
- Minimizing calibration costs requires sound business decisions and efficient program planning
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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