Since I became involved in the calibration industry in 1972, the calibration interval has been a common topic of interest and confusion. I was asked to write an article for the newsletter & blog, and after some thought, I determined there was no need. Many people have published excellent content on the subject:
One of the best and most practical ways for a laboratory to justify calibration, intervals are to maintain Shewhart control charts (SPC – Statistical Process Control charts). This is achieved by monitoring and charting key parameters of the equipment in between currently established calibration intervals for the equipment. Several established control limit criteria are then used to determine the stability of the artifact. This satisfies many requirements for ISO/IEC 17025:2005 ( Dilip Shah’s “ kill 5 birds with one stone” quotation – the “stone” being the SPC tools and techniques and the “birds” being the ISO/IEC requirements ).
5.5.9 When, for whatever reason, equipment goes outside the direct control of the laboratory, the laboratory shall ensure that the function and calibration status of the equipment are checked and shown to be satisfied before the equipment is returned to service.
5.5.10 When intermediate checks are needed to maintain confidence in the calibration status of the equipment, these checks shall be carried out according to a defined procedure.
126.96.36.199 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.
5.9.1 The laboratory shall have quality control procedures for monitoring the validity of tests and calibrations undertaken. The resulting data shall be recorded in such a way that trends are detectable and, where practicable, statistical techniques shall be applied to the reviewing of the results. This monitoring shall be planned and reviewed and may include, but not be limited to, the following:
- a) regular use of certified reference materials and/or internal quality control using secondary referencematerials;
5.9.2 Quality control data shall be analyzed and, where they are found to be outside pre-defined criteria, planned action shall be taken to correct the problem and to prevent incorrect results from being reported.
Having the historical data in the SPC format helps to determine and provide justification if the calibration interval needs to be shortened or lengthened. It is also a preventive tool which will help identify issues before they actually become real problems ( ISO/IEC 17025:2005 Clause 4.12 ).
Listed below are links to several articles on calibration intervals for your reference:
ASTM E74 or E2428 from Morehouse
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