Dear michela,
Since the thread interested me, I hv done a little more searching. The links below may be of some value to people who are interested in this topic. I have put the tougher material first (section1) with the, perhaps, more immediately useful stuff following in section2.
Section1
This topic, when treated rigorously, overlaps into metrology and (IMO) requires quite heavy statistical ability to progress very far. If you wish to see what I mean, just google “estimation calibration intervals., eg here is one extract -
The process of determining calibration intervals is a complex mathematical and statistical process requiring accurate and sufficient data taken during the calibration process.
Calibration_0.ppt 189KB
743 downloadsThis link is another fairly basic (!!) introduction
http://www.qualitydi...ement-equipmentTo demonstrate a practical usage of the heavy stuff, this is a link to a comprehensive procedure for pipette calibration which includes an interestingly simple graph for predicting calibration intervals utilising the MBTF (mean time before failure) characteristic. This table may well be generic but my (lack of) statistical ability prevents a yes/no.
http://www.labautope...cy_for_pipettessection 2In a more friendly way, one of the other "techniques" I noticed includes a route nicely defined as “engineering intuition” (EI) which I suspect is more useful to readers of this forum.
Some justification for the EI idea also occurs in this quote from another forum -
the calibration frequency you set (barring industry/regulatory/customer requirements) is yours to choose. You will need to assess risk. Basically how risky is it should this item be out of tolerance? The lower the risk, the longer you can go between intervals.
Another description of the idea is introduced (see option No.1) in this extract –
calibration_1.png 471.64KB
51 downloadsMore officially, the principle is apparently also included within the BS EN 30012-1 standard as described in this calibration procedure for industrial weighing systems.
calibration_3.PDF 1.83MB
651 downloads(see pg13)
I do believe this semi-intuitive starting procedure will satisfy the typical (food) auditor expectations if backed up by an appropriate risk assessment (ie data
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A table of (I presume) EI derived “starting” intervals is in this quite readable (except the statistics part) short book from an instrument supplier (see Ch3) with examples of interval shortening / lengthening procedures. However many intervals seemed rather long for (audit) comfort to me and I noticed in another forum that most people recommended setting intervals for maximum 1 yr unless the item was very infrequently used. (Maybe johnmcmaster's earlier logic will pass in some circumstances if one is brave enough.
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calibration_2.PDF 3.29MB
519 downloadsRgds / Charles.C