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Estimation of Measurement Uncertainty in Labs: a requirement for ISO 17025 Accreditation

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Knowledge of the uncertainty of measurement of testing and calibration results is fundamentally important for laboratories, their clients and all institutions using these results for comparative purposes. Uncertainty of measurement is a very important metric of the quality of a result or a testing method.

Main points covered:
• To introduce the basic concepts related to measurement results and measurement uncertainty
• Explain the relevance of these concepts to chemical analysis data
• Introduce mathematical concepts, uncertainty sources and important approaches for estimation of measurement uncertainty

Presenter:
This webinar was presented by Dotun Bolade, who is an Analytical Chemist/Environmental Scientist by training and practice with years of experience in laboratory instrumentation and automation. For him, ISO management systems have become second nature having worked in environments where ISO 9001, 14001, 18001 and 17025 have been fully implemented. He is a Certified PECB ISO/IEC 17025 Lead Assessor.

Link of the recorded session published on YouTube: https://youtu.be/AOpFou7_FVI

Published in: Education
  • Very Informative Presentation. ISO 17025:2017 documentation is coming up soon. Get more information here : https://goo.gl/wt4rsK
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Estimation of Measurement Uncertainty in Labs: a requirement for ISO 17025 Accreditation

  1. 1. ESTIMATION OF MEASUREMENT UNCERTAINTY: A REQUIREMENT FOR ISO 17025 ACCREDITATION
  2. 2. Dotun Bolade Managing Consultant Dotun Bolade is an Analytical Chemist/Environmental Scientist by training and practice has spent the past five years working in Nigeria’s Oil and Gas industry. For him, ISO management systems have become second nature having worked in environments where ISO 9001, 14001, 18001 and 17025 have been fully implemented/accredited. He is a Certified PECB ISO/IEC 17025 Lead Assessor, ISO 14001, 9001 & 29001 Lead Implementer and Certified Trainer. Dotun is a PECB Partner and Managing Consultant, EN2SOL PNP LIMITED. Contact Information 2348066884793 Dotun.bolade@energysolutiosng.com www.domain.com linkedin.com/Dotun Bolade twitter.com/dotunBolade
  3. 3. Objectives:  To introduce the basic concepts related to measurement results and measurement uncertainty & their relevance to chemical data  Introduce mathematical concepts, uncertainty sources and important approaches for estimation of measurement uncertainty  Understand availability and relevance of data in estimating measurement uncertainty  Understand selection of suitable approach for uncertainty measurement based on available data
  4. 4. Content:  Introduction  Accuracy, Precision & Trueness  Sources of Uncertainty in Measurement  Concepts in Uncertainty estimation  Elements of an Uncertainty Budget  Uncertainty Calculation in Pipetting; an example  References
  5. 5. Introduction: Key Terms:  Measurement/Chemical Analysis  Uncertainty/Uncertainty Range  Measurand  True Value  Measured Value  Error (systematic & random)
  6. 6. Accuracy, Precision & Trueness:
  7. 7. Common Sources of Uncertainty in Measurement:  Equipment  Unit Under Test  Operator  Method  Calibration  Environment Uncertainty in Volumetric Measurements: o Repeatability Uncertainty o Calibration Uncertainty o Temperature Uncertainty o Application-Specific Uncertainty
  8. 8. Concepts in Uncertainty Estimation:  Normal Distribution
  9. 9. Concepts in Uncertainty Estimation:  T-Distribution
  10. 10. Elements of an Uncertainty Budget:  Repeatability  Reproducibility  Stability  Bias  Drift  Resolution  Reference Standard  Reference Standard Stability To calculate Repeatability:  Repeat a measurement ‘n’ number of times  Record the results of each measurement.  Calculate the standard deviation.
  11. 11. Reproducibility: Common comparisons for Reproducibility Testing: Operator vs Operator Reproducibility Equipment vs Equipment Reproducibility Method vs Method Reproducibility Day vs Day Reproducibility Environment vs Environment Reproducibility To calculate Reproducibility:  Perform a Repeatability Test  Calculate the mean of average  Change a variable and repeat the Repeatability Test  Calculate the mean or average.  Calculate the standard deviation of the test averages
  12. 12. Stability: Property of a measuring instrument, whereby its metrological properties remain constant in time To calculate Stability:  Review your last 3 calibration reports  Record the results from each calibration report  Calculate the standard deviation of the calibration results.
  13. 13. Bias:  Estimate of systematic measurement error  Average of replicate indication minus a reference quantity value To calculate Bias:  Review your latest calibration report  Find the As Left value or measurement result  Find the Nominal value or standard value  Calculate the difference.
  14. 14. Drift:  Continuous or incremental change over time in indication, due to changes in metrological properties of a measuring instrument To calculate Drift:  Review your last 3 calibration reports  Record the results from each calibration report  Record the date each calibration was performed  Calculate the average daily drift rate  Multiply the average daily drift rate by your calibration interval (in days).
  15. 15. Resolution:  Smallest change in a quantity being measured that causes a perceptible change in the corresponding indication To Find Resolution:  Look at your measurement system or equipment  Find the least significant digit  Observe the smallest incremental change
  16. 16. Reference Standard Uncertainty:  Uncertainty of a measurement standard designated for the calibration of other measurement standards for quantities of a given kind in a given organization or at a given location  Reference standard uncertainty should be included in the every uncertainty budget. To Calculate Reference Standard Uncertainty:  Review your latest calibration report  Find the reported estimate of measurement uncertainty
  17. 17. Reference Standard Stability:  stability of a measurement standard designated for the calibration of other measurement standards for quantities of a given kind in a given organization or at a given location  Reference standard uncertainty should be included in the every uncertainty budget. To Calculate Reference Standard Stability:  Review your last 3 calibration reports  Record the uncertainty estimate from each calibration report  Calculate the standard deviation.
  18. 18. Uncertainty Calculation in Pipetting:
  19. 19. Uncertainty Calculation in Pipetting: In calculating the volume and Uncertainty of liquid delivered from a self-calibrated volumetric pipette, there are 3 main uncertainty components; Uncertainty due to repeatability, u(V,rep) Uncertainty due to pipette calibration, u(V,cal) Uncertainty due to the temperature difference from 20 °C, u(V,temp).
  20. 20. Uncertainty Calculation in Pipetting: In calculating the volume and Uncertainty of liquid delivered from a self-calibrated volumetric pipette, there are 3 main uncertainty components; Uncertainty due to repeatability, u(V,rep) Uncertainty due to pipette calibration, u(V,cal) Uncertainty due to the temperature difference from 20 °C, u(V,temp).
  21. 21. Uncertainty Calculation in Pipetting: Uncertainty due to repeatability of pipetting u (V, REP) is equal to this standard deviation 0.0057 ml
  22. 22. Uncertainty Calculation in Pipetting: Uncertainty of the calibration is expressed (when calibration is done) as the standard deviation of the mean:
  23. 23. Uncertainty Calculation in Pipetting: When there is a possibility that pipetting is performed at a different temperature from the calibration (and this possibility exists almost always), then an additional uncertainty source due to temperature change is introduced and it has to be taken into account.
  24. 24. Uncertainty Calculation in Pipetting: Combined Standard Uncertainty: When estimating the standard uncertainty of an output quantity then the standard uncertainties of all input quantities are taken into account The standard uncertainty of the output quantity obtained in this way is called combined standard uncertainty
  25. 25. Uncertainty Calculation in Pipetting: Expanded Uncertainty: • Expanded uncertainty is calculated from the standard uncertainty by multiplying it with a coverage factor, k. • In the case of the pipetting example the k = 2 expanded uncertainty is found as follows: U(V ) = uc(V ) · k = 0.0077 · 2 = 0.0154 ml
  26. 26. Uncertainty Calculation in Pipetting: Summary: The volume of the pipetted liquid is: V = (9.992 ± 0.015) ml, k = 2, norm The parentheses (brackets) mean that the unit “ml” is valid both for the value and the uncertainty. “norm.” means that the output quantity is expected to be approximately normally distributed. This, together with coverage factor value 2, means that the presented uncertainty is expected to corresponds to approximately 95% coverage probability
  27. 27. References: • A2LA. (2015). R205 – Specific Requirements: Calibration Laboratory Accreditation Program. Frederick: A2LA. • JCGM. (2012). International Vocabulary of Metrology: Basic and General Concepts and Associated Terms. Sèvres: BIPM. • Estimation of Measurement Uncertainty in Chemical Analysis. Available at https://sisu.ut.ee/measurement/10- approach-based-validation-and-quality-control-data-top- down-approach
  28. 28. ISO 17025 Training Courses  ISO/IEC 17025 Introduction 1 Day Course  ISO/IEC 17025 Foundation 2 Days Course  ISO/IEC 17025 Lead Implementer 5 Days Course  ISO/IEC 17025 Lead Assessor 5 Days Course Exam and certification fees are included in the training price. https://pecb.com/iso-iec-17025-lead-assessor | www.pecb.com/events
  29. 29. THANK YOU ? 2348066884793 dotun.bolade@energysolutionsng.com linkedin.com/Dotun Bolade twitter.com/dotunBolade oladotunbolade@gmail.com

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