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What is the difference between Calibration and Verification?

Posted by Phil Wiseman on Jan 16, 2017 9:43:40 AM
Phil Wiseman

 

 

 

 

A quick Google search will show about 500,000 results for verification vs. calibration. Obviously, there is a wide range of answers to what appears to be a simple question.

What does it mean to have something calibrated?

There seems to be a good consensus on this question. Calibration is the comparison of a UUT- Unit Under Test- to a known standard. As an ISO 17025 accredited laboratory we use standards that are traceable to the SI unit through NIST or the appropriate NMI-National Metrology Institute.

The International Vocabulary of Metrology-VIM- defines calibration as:

VIM 2.39

calibrationmicrometer calibration- alliance calibration.jpg

operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication

NOTE 1 A calibration may be expressed by a statement, calibration function, calibration diagram, calibration curve, or calibration table. In some cases, it may consist of an additive or multiplicative correction of the indication with associated measurement uncertainty.

NOTE 2 Calibration should not be confused with adjustment of a measuring system, often mistakenly called“self-calibration”, nor with verification of calibration.

For a practical explanation please read: The word Calibration: What does it really mean?

Download the VIM International Vocabulary of Metrology

 

What does it mean to have something verified?

An internet search will provide a broad range of answers that are confusing. You will see terms such as verification calibration, calibration verification and validation used interchangeably. Verification simply means an instrument meets the specified requirements. This is different than calibration.

ISO 10012:2003 section 7.1.4 Records of the metrological confirmation process offers this guidance: "In some instances, a verification result is included in the calibration certificate or report where it is stated whether the equipment complies with (or fails to comply) with specified requirements." This commonly referred to as Pass or Fail.

A more informative answer is provided in Annex A section A.4 Verification and metrological confirmation;" After the calibration, the MEMC(Measurement Equipment Metrological Charasteristics) are compared to the CMR(Customer Metrological Requirements) before confirming the equipment for its intended use. For example, the reported error of indication of the measuring equipment would be compared to the maximum permissible error specified as a CMR. If the error is smaller than the maximum permissible error, then the equipment complies with that requirement, and may be confirmed for use. If the error is greater, action should be taken to remove the nonconformity or the customer should be informed that the equipment cannot be confirmed.

Such direct comparison of MEMC and CMR is often termed verification (see ISO 9000). The metrological confirmation system is firmly based on such verifications, but should also include detailed consideration and review of the complete measurement process in order to give assurance of the quality of the measurements made with the equipment, in support of determining the compliance of a product with the customer requirements.

EXAMPLE Following the example in A.2, it is assumed that the error found by calibration is 3 kPa at 200 kPa, with a calibration measurement uncertainty of 0,3 kPa. Therefore, the instrument does not meet the requirement of maximum permissible error. After adjustment, the error found by calibration is 0,6 kPa and the uncertainty in the calibration process is 0,3 kPa. The instrument now complies with the maximum permissible error requirement and it may be confirmed for use, assuming that evidence demonstrating compliance with the drift requirement has been obtained. However, if the instrument was submitted for reconfirmation, the user of the instrument should be informed of the results of the first calibration since corrective actions may be required concerning product realization for a period before the instrument was taken out of use pending reconfirmation.

Whether performed by the user or by the metrological function, the results of the verification process may be compiled into a verification document, in addition to any calibration or test certificates or reports, as part of an audit trail within the metrological confirmation system. The final stage in the metrological confirmation system is the proper identification of the status of the measuring equipment, for example by labeling, marking, etc. After this, the measuring equipment may be used for the purpose it has been confirmed for."

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Topics: calibration, verification

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