From purpose based interpretation to appropriate application of Calibration and Interlaboratory comparison results

Transcription

1 From purpose based interpretation to appropriate application of Calibration and Interlaboratory comparison results Mulugeta Derebew Test & Measurement 2017, Conference and Workshop, National Laboratory Association South Africa, 31st July - 1st August 2017

2 1. Calibration and Inter-laboratory comparison for traceability Traceability It is to establish globally comparable Measurement and laboratory results Calibration Calibrations and Measurements made by the laboratory shall be traceable to the International System of Units (ISO/IEC 17025, )

3 Calibration and Inter-laboratory comparison for traceability (cont.) Traceability in ISO/IEC Reference standards and reference materials Reference standards programme and procedure for calibration used for calibration only to avoid possible invalidation calibrated before and after any adjustment Reference materials be traceable to SI units of measurement, or to certified reference materials. Internal reference materials shall be checked as far as is technically and economically practicable Intermediate checks Checks needed to maintain confidence in the calibration status of reference, primary, transfer or working, standards and reference materials shall be carried out according to defined procedures and schedules.

4 Calibration and Inter-laboratory comparison for traceability (cont.) There are certain calibrations that currently cannot be strictly made in SI units. In these cases, competent traceability establishing options include (ISO/IEC 17025, ) the use of certified reference materials the use of specified methods and/or consensus standards participation in a suitable programme of interlaboratory comparisons

5 2. Calibration and Inter-laboratory comparison for compliance verification and as evidence of competency ISO/IEC 17043:2010 Conformity assessment General requirements for proficiency testing Used in confirming or recognizing the competence of a PT provider Proficiency testing Evaluation of participant performance against pre-established criteria by means of inter-laboratory comparisons, whereby Inter-laboratory comparison is organization, performance and evaluation of measurements or tests on the same or similar items by two or more laboratories in accordance with predetermined conditions

6 Calibration and Inter-laboratory comparison for compliance verification and as evidence of competency (cont.) ILAC-P9:06/2014 Mechanisms a laboratory can provide evidence of its competence to its stake holders (clients, interested parties and the accreditation body) participation in interlaboratory comparisons or proficiency testing programs (ISO/IEC 17025,ISO 15189, ISO/IEC 17011) regular use of reference materials (ISO/IEC 17025) replicate tests (ISO/IEC 17025) calibrations using the same or different methods (ISO/IEC 17025)

7 Calibration and Inter-laboratory comparison for compliance verification and as evidence of competency (cont.) Calibration certificates issued by competent service provider shall contain the measurement results, including the measurement uncertainty and/or a statement of compliance with an identified metrological specification (ISO/IEC 17025, ) The calibration certificate shall relate only to quantities and the results of functional tests. If a statement of compliance with a specification is made, this shall identify which clauses of the specification are met or not met. (ISO/IEC 17025, )

8 Calibration and Inter-laboratory comparison for compliance verification and as evidence of confidence (cont.) ISO 13528:2015 Statistical methods for use in proficiency testing by inter-laboratory requirements concentrates on methods for evaluation and assessment ISO describes several possibilities for the determination of the assigned value Known values from formulation Certified reference values Reference value from one lab Consensus values from expert laboratories Consensus values from participants

9 3. Evaluating The Fit For Purpose Of Proficiency Testing Schemes Both owned capabilities (competence of our experts, our laboratory facilities, our analytic instruments and in general performance of our method) and The requirement of our target customers are the bases to define the scope of laboratory activities. PT/ILC results are acceptable to demonstrate scope of tests or calibrations declared to accreditation bodies and customers. Therefore, PT schemes/ilcs shall be evaluated to fit for purpose during the planning process.

10 4. Cases for failures in Evaluating the fit for purpose of proficiency testing / Insufficient ILC schemes care for input quantities in the determination of En numbers and z- scores : insufficient data, major sources of uncertainties are not well addressed and estimated, standard deviation from limited number of data X10 X11

11 Cases for failures in Evaluating the fit for purpose of proficiency testing / ILC schemes (cont) Insufficient knowledge of input quantities in the determination of En numbers and z-scores insufficient data, major sources of uncertainties are not well addressed and estimated When the expanded uncertainties are calculated using a coverage factor of 2 - a critical value of 1 for an En number is equivalent to the critical value of 2 used with z-scores.

12 En, the error would have been investigated En while the error was not investigated Cases for failures in Evaluating the fit for purpose of proficiency testing / ILC schemes (cont) When expert laboratory is the source of traceability either by provision of the calibration service or provision of calibration item such as the CRM - Situation of undetectable bias transfer It can more serve as only staff competence evaluation than the competence of the laboratory Case plot of a 10 ppm undetectable bias transfer

13 En Values Cases for failures in Evaluating the fit for purpose of proficiency testing / ILC schemes (cont) The case when En numbers reveals the limitation of the laboratory when selecting ILC expert laboratory with an uncertainty higher than the required scope of the laboratory U of lab is better than the expert lab 2.50 U of Lab is equal to U of expert lab Comparison values

14 6. Why Laboratories worried about PT evaluation statistics while PT providers result are used directly? Why Laboratories worried about PT evaluation statistics while PT providers result are used directly? Alternative corrective action may be through selfretesting/re-evaluation Important to run own acceptable Inter-laboratory comparison Inter-laboratory Comparison can serve as intermediate checks

15 7. A Case where the laboratory and interested parties disagreement on importance of PT/ILC While a laboratory has proved competency at the level of RL (such as by accreditation, PT/ILC successful participation), assessors require PT/ILC participation of the Lab at the level of B L and M L Duplication activities with higher uncertainty than RL. B L R L M L Rather asking objective evidence of the dissemination of calibration values from RL to the lower class labs help.

16 8. Intermediate checks Importance: Clause of ISO/IEC 17025:2005 Checks needed to maintain confidence in the calibration status of our instruments. Know how to establish intermediate check schedules Using fixed characteristics (triple point of water, ice point) Using well established reference values (crms, calibrated items, measured values) Using performance characteristics (fall rates, zero indications, repeatability, reproducibility, stability) Checking Consistency with similar instruments at our facility (measurement of the same environmental conditions using different environmental loggers)

17 9. Intermediate checks (cont) Inter-laboratory Comparison can serve as intermediate checks

18 8. Calibration

19 9. Evaluating The Fit For Purpose Of Calibration Services External calibration source, Demonstrated competence, measurement capability, and traceability Certificates contain measurement results associated with measurement uncertainty best when accredited or when no option try to confirm by competent second party audit

20 Evaluating The Fit For Purpose Of Calibration Services (cont) Using internal calibration source, An unbroken chain linking to the SI units. Competent staff, equipment and environmental condition Produce Certificates contain the measurement results with the measurement uncertainty Confirmation by accreditation is best, when difficult try to realize using competent internal audit and inter laboratory comparison

21 Evaluating The Fit For Purpose Of Calibration Services (cont) When Calibration cannot be strictly made in SI Units (ISO/IEC 17025) Use of Certified reference Material Use of Specified methods and /or Conesus standards Participation in Interlaboratory comparison/pt

22 10. Evaluating The Fit For Purpose Of Calibration Services Evaluation of calibration results and application of correction factors when calibration and measurement points fit, correction is just the instrument reading + correction - U is the U given in the cal certificate When calibration is done at set points, actual values in the certificate are used with the U given in the certificate. Too big U- inappropriate instrument could be used for our diagnosis When regression is used, the U due to the curve fit shall be included in the original U and checked against the lab requirements. The residual standard deviation is used as an estimate of the uncertainty and shall be included to confirm the performance of the measuring instrument against specification.

23 11. Calibration: Correct use and interpretation of calibration certificate When correction is applied by fitting regression line- the uncertainty due to regression u reg shall sum on to the uncertainty reported on the calibration certificate U(k=1) if it has not been issued from the cal. Service. n u reg ( s( r)) i 1 y i n yˆ m i 23

24 Calibration: Correct use and interpretation of calibration certificate (cont) E U req U wih no correction

25 Calibration: Correct use and interpretation of calibration certificate (cont) Calibration certificate may not always be the source of correction factor, rather always some kind of information about our equipment or method. The measured Value is considered to be the Nominal Values (all possible Set Values) Measured Values U (k=2) actual value and is not permanent. So, if the measured Values does not fit the need; correction factor has nothing here; but instrumental adjustment or change the instrument will be appropriate solution

26 Thank You