KAN Policy on Traceability of Measurement

1. INTRODUCTION

Traceability of measurement results to the SI unit of measurements is needed by every economy to be active participated in the era of global trade. The quality of measurement in controlling the production processes is very important for the quality of product. The traceability of such measurement that can be achieved by calibration of measuring and testing equipment is necessitated by the growth of national and international demand that manufactured parts be interchangeable: the suppliers that make products and customer which install them to the other parts shall measure based on the “same measure”

Traceability is one of the criteria that shall be met by KAN accredited laboratories as laid down in chapter 5.6 of SNI 19-17025-2000 “General Requirements for the competence of testing and calibration laboratories”. For implementing the requirement, Policy on Traceability of Measurement is needed. This policy is intended to explain the KAN requirements pertaining to traceability of measurement.

2. ELEMENTS OF TRACEABILITY

The formal definition of traceability is given in the International Vocabulary of Basic and General Terms in Metrology (VIM 1993) as: “property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparison all having stated uncertainties.” Based on the definition, it can be characterized by six essential elements:

1. An unbroken chain of comparison: traceability begins with an unbroken chain of comparisons originating at national, international or intrinsic standards of measurement
2. Measurement uncertainty: the measurement uncertainty for each step in the traceability chain must be calculated according to defined methods and must be stated at each step of the chain so that an overall uncertainty for the whole chain can be calculated;
3. Documentation: each step in the chain must be performed according to documented and generally acknowledged procedures and the results must be documented, i.e., in a calibration or test report;
4. Competence: the laboratories or bodies performing one or more steps in the chain must supply evidence of technical competence, e.g., by demonstrating that they are accredited by a recognized accreditation body;
5. Reference to SI units: where possible, the primary national, international or intrinsic standards must be primary standards for realization of the SI units;
6. Recalibrations: Calibrations must be repeated at appropriate intervals in such a manner that traceability of the standard is preserved.

KAN requires that all calibrations and verifications of measuring and test equipment, reference standards, reference materials and ancillary equipments those have significant effect to the results of test and/or calibration shall be conducted by KAN’s accredited calibration laboratories or by a recognized national metrology institute or by calibration laboratories accredited by accreditation bodies that have signed Multilateral Recognition Agreements or Arrangements (MRAs) in the international organization, i.e. International Laboratory Accreditation Cooperation (ILAC) or in the regional organization, such as Asia Pacific Laboratory Accreditation Cooperation (APLAC) and European cooperation for Accreditation (EA). List of KAN’s MRA partners and their logos are given in Annex.

These calibrations or verifications must be documented in a calibration certificate or report endorsed by the accreditation body’s logo, or otherwise refers to accredited status.

All laboratories shall have policy for achieving measurement traceability and also for achieving traceability for reference materials if applicable. The policy must be in compliance with this policy document.

3. SCOPES OF ACCREDITATION

The scopes of accreditation are important to the customers before they place work with an accredited laboratory. The customers should request a copy of laboratory scopes to ensure that the laboratory can meet their requirements.

The scopes of accreditation document specifically the measurements a laboratory is accredited to make, include the ranges of the accredited measurand along with the associated best measurement capability (BMC) expressed as an uncertainty for each measurand and range.

4. BEST MEASUREMENT CAPABILITY

BMC is defined as “the smallest uncertainty of measurement that a laboratory can achieve within scope of accreditation, when performing more or less routine calibrations of nearly ideal measurement standards intended to define, realize, conserve of reproduce a unit of that quantity of one or more of its values, or when performing more or less routine calibration of nearly ideal measuring instruments designed for the measurement of that quantity”

In relation with the accreditation of calibration laboratories, BMC is stated as expanded uncertainty at 95% confidence level.

The important thing in the definition of BMC is that the BMC assigned to accredited calibration laboratory shall reflect the capability of the laboratory to carry out routine calibration of nearly ideal UUT, which can be calibrated by the respective laboratory.

Based on the definition, BMC consists of the components which depends on many factors required for demonstrating the competence of a calibration laboratory, such as

• Personnel education, training, technical knowledge
• Environmental conditions of calibration laboratory
  
• Maintenance of equipment, including calibration and verification intervals

BMC can be evaluated by assessing a budget contributing uncertainty components, and/or by means of measurement audit using the nearly ideal measuring instruments or nearly ideal measurement standards that can be calibrated by the respective laboratory.

Detail discussions and examples on the evaluation of BMC are given in the KAN Guide to the Evaluation and Expression of Uncertainty in Measurement.

5. CALIBRATION AND TEST REPORTS

For demonstrating measurement traceability, calibration certificates should, wherever applicable, indicate the traceability to national or international standards of measurement, should provide the measurement result, and associated uncertainty of measurement.

Wherever uncertainty calculations are applicable, and when suitable for customer requirements, a statement of compliance with an identified metrological specification can be accepted instead of measurement results and associated uncertainties.

6. EVALUATION AND EXPRESSION OF UNCERTAINTY

Where uncertainty calculations are applicable, testing and calibration laboratories accredited by KAN shall calculate the uncertainty of measurement based on the ISO “Guide to the Expression of Uncertainty in Measurement”.

Uncertainty reported shall be supported by uncertainty budgets those will be expressed as expanded uncertainty, typically using a coverage factor of k = 2 to approximate the 95% confidence level.

When reporting the calibration and/or test result and its uncertainty, the use of excessive numbers of digit should be avoided.

In most cases, the uncertainty should be expressed to no more than two significant figures.

During the stage of the estimation and combination of uncertainty components, at least one more figure should be used to minimize rounding errors.

If the rounding brings the numerical value of measurement uncertainty down by more than 5 %, the rounding up value should be used.

The numerical value of the measurement result in the final statement normally should be rounded to the least significant figure in the value of the expanded uncertainty assigned to the measurement result.

If a calibration certificate or report contains a statement of the measurement result and the associated uncertainty, then the uncertainty statement must be accompanied by an explanation of the meaning of the uncertainty statement which specify at least the coverage factor and the confidence level of the reported uncertainty. An example of such an explanation might be the statement:

“Reported uncertainties represent expanded uncertainties expressed at approximately the 95% confidence level using a coverage factor of k=2”.

Detail discussions and examples on the evaluation and expression of uncertainty in measurement are given in the KAN Guide on the Evaluation and Expression of Uncertainty in Measurement.

7. STATEMENTS OF COMPLIANCE WITH SPECIFICATION

Clause 5.10.3.1 of SNI 19-17025-2000 states: ”... where necessary for the interpretation of the test results, include the following: ... b) where relevant, a statement of compliance/non-compliance with requirements and/or specifications.”

Clause 5.10.4.2 of SNI-19-17025-2000 states: “... when statement of complance with specification are made, the uncertainty of measurement shall be taken into account.”

In harmony with those clauses, when a test and/or calibration is carried out to a stated specification require a statement of compliance; the report must contain a statement indicating whether the test and/or calibration results show compliance with the specification.

KAN requires that, if certificates or reports include the statement of compliance with specification, it shall calculated using expanded uncertainty of measurement. In addition, the coverage factor and confidence level must be stated.

Detail discussions and examples on statement of compliance with specification are given in the KAN Guide on the Evaluation and Expression of Uncertainty in Measurement

8. STATEMENTS OF TRACEABILITY IN THE REPORTS

In addition to the information required in section 6, calibration reports and certificates shall contain a traceability statements of the calibration results to give evidence that the results of calibration reported was carried out using standards whose values are traceable to SI unit through an appropriate national, international, intrinsic or mutual consent standards. The statements of traceability in the certificate may take one of the following forms or other words to that effect:

1. If the traceability chain for given laboratory originates at the recognized NMI, the statement may be, “The calibration result reported was traceable to SI unit of measurement through the NMI (state the NMI or NMI of other countries),”
   
2. If the traceability chain for given laboratory originates at the KAN’s accredited laboratories, the statement may be, “The calibration result reported was traceable to the SI unit of measurement through LK- (accreditation number)-IDN”
   
3. If the traceability chain for given laboratory originates at the highest own standards and calibrate by in house calibration, the statement may be, “ The calibration result reported was traceable to the SI unit of measurement through (the laboratory that conducted calibration for the highest standard of the laboratory)”
   
4. If the traceability chain for given laboratory originates from intrinsic or mutual consent standards (e.g. certain CRM), the statement might be, “The calibration result reported was traceable to (producer of the standard)”

9. IN-HOUSE CALIBRATION

The task for in-house calibration laboratories is to calibrate regularly the measuring and test equipment used in the calibration and testing carried out in the laboratories. The in-house calibration shall be carried out against its reference standards that are traceable to the SI unit through calibration by KAN’s accredited calibration laboratories or by a recognized national metrology institute or by calibration laboratories accredited by accreditation bodies that have signed Multilateral Recognition Agreements or Arrangements (MRAs) in the international organization, i.e. International Laboratory Accreditation Cooperation (ILAC) or in the regional organization, such as Asia Pacific Laboratory Accreditation Cooperation (APLAC) and European cooperation for Accreditation (EA)

Accreditation of in-house calibration laboratories is not always necessary to carry out separately from the accreditation of its parents organization. However, to ensure the traceability of measurement, all in-house calibrations shall be supported by the following minimal set of elements and shall be assessed by the assessors who have sufficient knowledge in the field of metrology and calibration:

1. The in-house laboratory shall maintain documented procedures for the in-house calibrations and the in-house calibrations shall be evidenced by a calibration report, certificate, or sticker, or other suitable method, and calibration records shall be retained for an appropriate, prescribed time;
   
2. The in-house laboratory shall maintain training records for calibration personnel and these records shall demonstrate the technical competence of the personnel performing the calibrations;
   
3. The in-house laboratory shall be able to demonstrate traceability to national or international standards of measurement by procuring calibration services from accredited calibration labs or a national metrology institute;
   
4. The in-house laboratory shall have and apply procedures for evaluating measurement uncertainty. Measurement uncertainty shall be taken into account when statements of compliance with specifications are made;
   
5. Reference standards shall be recalibrated at appropriate intervals to ensure that the reference value is reliable. Policy and procedures for establishing and changing calibration intervals shall be based on the historical behavior of the reference standard.

REFERENCES

1. ILAC G2: 1994 Traceability of Measurements
2. ILAC G8: 1996 Guidelines on Assessment and Reporting of Compliance with Specification (based on measurements and tests in a laboratory)
3. ILAC G16:2001 ILAC Policy on Traceability of Measurement Results
4. APLAC-EA Policy on Traceability of Measurements
5. APLAC TC 004: 09/2001 Method of Stating Test Results and Compliance with Specification
6. OIML Vocabulary of Legal Metrology (VML)
7. International vocabulary of basic and general terms in metrology (VIM), 1993
8. ISO Guide to The Expression of Uncertainty in Measurement (GUM), 1993