Dynamics to determine calibration intervals

SAAMF Roadshow Durban

CSIR NML

Eddie Tarnow

Metrologist: Torque & Automotive

14 June 2006

Slide 2 © CSIR 2006 www.csir.co.za

Calibration Myths

• A calibration certificate implies that an instrument meets its 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

Slide 3 © CSIR 2006 www.csir.co.za

What is the aim of calibration?

• To ensure that a piece of test or measuring equipment can 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

Slide 4 © CSIR 2006 www.csir.co.za

What is a calibration interval?

• It is the interval between calibrations, chosen to ensure the test 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

Slide 5 © CSIR 2006 www.csir.co.za

What factors influence a calibration interval?

• The type (quality/stability) of instrument• Selection of appropriate

instrument when purchasing very important

Slide 6 © CSIR 2006 www.csir.co.za

What factors influence a calibration interval? (2)

• The accuracy required• Higher accuracy usually implies

higher risk• High accuracy instruments

being used for low accuracy applications

Slide 7 © CSIR 2006 www.csir.co.za

What factors influence a calibration interval? (3)

• The calibration service provider• 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• Can include factors such as environment, operator use, etc

• The use/abuse of the instrument

Slide 8 © CSIR 2006 www.csir.co.za

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 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

Slide 9 © CSIR 2006 www.csir.co.za

Suggested ways of determining a calibration interval

• Industry – instruments on the factory floor• 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

Slide 10 © CSIR 2006 www.csir.co.za

Ways of determining a calibration interval (2)

• Test Labs – instruments used within a laboratory• Calibration intervals can be determined based on actual drift

CRN-0038 10 V tapStability

-7,000

-6,800

-6,600

-6,400

-6,200

-6,000

-5,800

-5,600

-5,400

-5,200

-5,000

01-Jan-00 01-Jan-01 01-Jan-02 01-Jan-03 01-Jan-04 01-Jan-05 01-Jan-06 01-Jan-07

Period of Calibration

Dev

iati

on

(p

pm

)

Slide 11 © CSIR 2006 www.csir.co.za

Use of confidence checks to reduce risk

• Even a calibration interval selected on one of the two methods 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!

Questions??