Introduction
Calibration is an act which establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties. Then refers to the relation from the indication before establishing a confirmed measurement result.
Calibration shall not be confused with adjustment of a measuring system, often mistakenly called self calibration nor with verification of calibration.
Also, property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty should be referenced before establish a finalized calibration statement.
Metrologicial Traceability
The diagram below shows traceability chart regarding calibration laboratory references (where SI unit will always be on the top of the metrological traceability chain)
International Laboratory Accreditation Cooperation (ILAC) considers the elements for confirming metrological traceability to be:
- An unbroken metrological traceability chain (see above) to an international measurement standard or a national measurement standard
- A documented measurement uncertainty
- A documented measurement procedure
- Accredited technical competence
- Metrological traceability to the SI
- Calibration intervals.
Lastly, in order to satisfy metrological traceability chain, there are 4 options to maintain the traceability chain.
- Option 1: Calibration service from National Measurement Institute (NMI) with CIPM Mutual Recognition Arrangement (MRA) scope, the calibration service shall include the calibration range and uncertainty.
- Option 2: Calibration service from certified calibration laboratory (with certification logo on the report). If the laboratory has the ILAC’s dimension measurement approval while providing measurement results with uncertainty can also be considered.
- Option 3:
- a. Calibration service from National Measurement Institute (NMI) but without CIPM MRA scope, the calibration service shall include the calibration range and uncertainty.
- b. Calibration service which complies with requirement, but does not include in ILAC agreement or ILAC MRA scope.
- Option 4:
- a. Use reference materials or certified reference materials for traceability references, although the laboratory shall be in compliance with reference material policy.
- b. Use official procedure and/or including a agreed standard specification with relative organizations. Laboratory shall provide relative document approval.
ISO/IEC 17025 Laboratory Requirements
In order to be considered as a calibration laboratory, the laboratory must be qualified for the ISO/IEC 17025 laboratory accreditation under the calibration scope, this chapter will list out the general clauses from respective chapters for readers who would like to have a better understanding regarding the calibration laboratory’s qualification.
Chapter 6.4 (For Laboratory Equipment Compliance) :
- Laboratories shall verify equipment is in compliance with requirement before equipment installation or resume for use. (6.4.4)
- For measuring equipment, the measurement accuracy and/or measurement uncertainty shall be sufficient for accurate & effective results. (6.4.5)
- When the following occurs, measuring equipment needs to be CALIBRATED (6.4.6):
- When measurement accuracy or measurement uncertainty effects report EFFECTIVENESS.
- This equipment’s calibration is required for report’s metrological traceability.
- Laboratory shall establish calibration plan, and also be reviewed and adjusted when necessary in order to maintain calibration’s confidence (6.4.7)
Chapter 6.5 (For Metrological Traceability) :
- Laboratory shall document unbroken metrological traceability chain, to establish and maintain the metrological traceability while ensuring chain’s detail are contributing towards measurement uncertainty (6.5.1)
- Laboratory shall ensure measurement result can be traced to SI units by: (6.5.2)
- Calibrated by capable laboratories (can be traced to ILAC associated national laboratories); or
- Capable production organization’s declaration can trace to reference material value (SI verified)
- SI unit’s direct comparison via direct or secondary comparison between International standard or national standard.
- If metrological traceability cannot be traced to SI units, laboratory shall verify the metrological traceability to appropriate reference standard (6.5.3) :
- Capable production organization’s declaration can trace to reference material value (SI verified)
- Refer to measurement procedure, required method or results from mutual standard. Then describe the measurement result is in compliance with expected usage. Then also compare accordingly to assure the measurement result.
Chapter 7.6 (For Measurement Uncertainty) :
- Laboratory shall identify measurement uncertainty’s source contribution. When evaluating measurement uncertainties, all obvious uncertainties (including sampling uncertainty) shall be considered via appropriate analysis. (7.6.1)
- Laboratory needs to evaluate all CALIBRATION’S MEASUREMENT UNCERTAINTY when execute calibration (including self-owned device) (7.6.2)
Calibration Terminology Glossary
- Can quantize determined phenomenon, object or material characteristics
- Normal quantity: length, time, resistance…etc.
- Specified quantity: length of rod, wire resistance…etc.
- Use appropriate numeric measurement unit to represent quantity.
- Perfect and idea value from actual situations.
- Value to replace true value. Usually applied to capability comparison on calibration (national lab comparison scale).
- Operation to determine measured value (measurand).
- Measured value
- A value which influence the measurand without measurement.
- Value obtained from measurement.
- Measurement result before systematic error adjustment.
- Adjusted measurement result after considering systematic error.
- Proximity between measurement result and true value (conventional true value)
- Differences when measurand subtract true value.
- Errors which occured within the measurement system either by single or multiple measurements.
- Differences when multiple measurement’s average subtract the true value when measurement is repeatable.
- Use adjustment for the non-adjusted measurement result to compensate system error. (same as systematic error, but opposite signs)
- Chances occur for event randomly from 0 to 1.
- The range for value to fall in under finite probability or confidence level.
- A level of confidence to expect when the value falls into the region. Generally use 95% for uncertainty evaluation.
- A factor used for combining standard uncertainty to obtain extended uncertainty.
- Parameters with measurement result relevance to represent measurand’s distribution level.
- Use standard differences to represent the measurand’s uncertainty.
- Combination of all standard uncertainty when measurement results were obtained from multiple sources.
- The expected distribution range for the measurand with high probability, this is under a defined measurement result interval.
- Use standard statistical analysis for single or multiple sets of measurement and respective error (used to be called random error).
- Evaluate systematic error and refer to the variance quantity (if it exists) to evaluate variance, standard deviation, average and degrees of freedom (DoF) for measurement uncertainty variance.
- Indicates the proximity for the same measurand’s measurement result comparison under SAME CONDITION. It can indicate quantitative representation of measurand’s distribution within the same condition.
- indicates the proximity for the same measurand’s measurement result comparison under DIFFERENT CONDITION. It can indicate quantitative representation of measurand’s distribution within different condition.