Transcript Dynamics to determine calibration intervals

Dynamics to determine calibration
intervals
SAAMF Roadshow Durban
CSIR NML
Eddie Tarnow
Metrologist: Torque & Automotive
14 June 2006
Calibration Myths
• A calibration certificate implies that an instrument meets its
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accuracy specification,
Calibration means that the instrument was adjusted to ensure it
meets its accuracy specification,
It is sometimes more expensive to have an instrument
calibrated than to replace it every year with a new one,
Only instruments which can be adjusted require calibration;
therefore e.g. LIG thermometers don’t need periodic calibration,
Re-calibration of an instrument only applies to instruments
where the manufacturer has specified a calibration interval,
Calibration interval is dictated by the calibration service provider
and the user is bound by this,
Safety regulations stipulate 1 year cal intervals
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What is the aim of calibration?
• To ensure that a piece of test or measuring equipment can
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perform the test or measurement within the required level of
accuracy
To maintain a balance between cost and the risk of a
measurement being performed outside of the required accuracy
• Excessive cost – zero risk
• Zero cost – excessive risk
• Reasonable cost – acceptable risk
Risk of a “safety critical” component erroneously being found to
comply with the technical specification requirements may be
very different to the risk associated with say the thickness of a
carpet erroneously being found to comply with the specifications
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What is a calibration interval?
• It is the interval between calibrations, chosen to ensure the test
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or measuring instrument continues to perform at the required
level of accuracy
At the end of the interval an accuracy must be predictable with a
certain level of confidence
It is NOT an interval dictated by the calibration service provider
It is NOT an interval determined by the freq of quality audits
It is NOT an interval determined by budgetary considerations
It is NOT an interval determined by the availability of the
instrument (taken out of service for cal)
It should form part of an integrated calibration plan (not be
arbitrarily chosen)
If managed correctly it can reduce the costs of calibration
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What factors influence a calibration interval?
• The type (quality/stability) of
instrument
• Selection of appropriate
instrument when purchasing
very important
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What factors influence a calibration interval?
(2)
• The accuracy required
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Higher accuracy usually implies
higher risk
High accuracy instruments
being used for low accuracy
applications
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What factors influence a calibration interval?
(3)
• The calibration service provider
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Do they always use the same calibration procedure?
Do they adjust the instrument?
Do they ensure compliance as an output of calibration?
Is the same calibration service provider used for regular calibrations
(important factor to consider when building up a history)
• Historical performance of the instrument
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• Can include factors such as environment, operator use, etc
The use/abuse of the instrument
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Who is responsible for determining the
calibration interval?
• The “user”, in conjunction with the calibration service provider
• This responsibility CANNOT be delegated to the calibration
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service provider without careful consideration and contract
review
This CANNOT be made the responsibility of the quality
department
This, by necessity, implies that instrument “users” must be more
intimately involved with their instruments
Financial constraints should not influence the determination of
calibration intervals
Calibration intervals should however be technically optimised to
reduce costs to the minimum required to maintain the desired
level of accuracy
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Suggested ways of determining a calibration
interval
• Industry – instruments on the factory floor
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Each calibration, check for conformance to the manufacturer’s accuracy
specifications
Ensure calibration service providers adjust instruments, if necessary, to
ensure they conform
Calibration service providers must record “before” and “after” adjustment
values in order for the user to take remedial action if required
This method assumes the accuracy of the instrument, as purchased, is fit
for the intended purpose
Track when adjustment is required – if no adjustment required lengthen
interval, if adjustment required shorten the calibration interval
This method should however not exceed a predetermined appropriate
maximum interval even if no adjustment is required as this increases risk
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Ways of determining a calibration interval (2)
• Test Labs – instruments used within a laboratory
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Calibration intervals can be determined based on actual drift
CRN-0038 10 V tap
Stability
-5,000
-5,200
Deviation (ppm)
-5,400
-5,600
-5,800
-6,000
-6,200
-6,400
-6,600
-6,800
-7,000
01-Jan-00
01-Jan-01
01-Jan-02
01-Jan-03
01-Jan-04
01-Jan-05
01-Jan-06
Period of Calibration
Slide 10
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01-Jan-07
Use of confidence checks to reduce risk
• Even a calibration interval selected on one of the two methods
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discussed is meaningless when:• The instrument fails but the fault is not instantly noticeable,
• The instrument is abused unknowingly,
• The instrument is adjusted without prior knowledge
“In-between” checks can reduce the risk associated with these
scenarios
Frequency and thoroughness of check depends on the risks
associated and to some degree on available resources
Confidence checks are NOT a substitute for calibration or
“cheap do-it-yourself” calibration
Confidence checks do NOT lengthen calibration intervals
Confidence checks REDUCE risk!
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Questions??