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3 Big Risks You Are Taking In Calibration

Posted by Phil Wiseman on Aug 15, 2018 4:56:14 PM
Phil Wiseman

 

 

 

 


ISO 9001 is all about Risk and Risk Management. Your first inclination might be that as long as you use an ISO 17025 accredited calibration company you have eliminated your risk. This is simply not true. We see several risks that are either overlooked or not understood in calibration.

Assuming Accredited means Capable

If a calibration company is accredited to ISO 17025 the assumption is made that they are capable of performing every calibration. It is obvious that items not covered on their scope of accredited services do not qualify. However, just because a capability is listed on a scope of accredited services, this does not mean the laboratory is capable of performing the calibration to meet your needs. An outrageous example would be calibrating gage blocks with a yardstick. 

The National Physical Laboratory shares this advice: 

  • "Purchasers of calibration services should review their expectation of an instrument before requesting a calibration - to ensure that the calibration is likely to meet their needs."

Before you purchase calibration services whether internal or from an external laboratory you need to know the capability of the measurement & test equipment to be calibrated and compare that with the capabilities of the calibration service. Understand the difference between Accuracy and Resolution. 

You may want to read: How Do I know If My Lab Is Capable?

Not Using Data To Determine Calibration Intervals

How frequently should an instrument be calibrated?

"The frequency with which calibrations should be carried out is an important, if sometimes difficult, question; there are two main considerations. Firstly, all measuring devices - whether they are simple, 'fundamental', or sophisticated - change characteristics with time; the issue is how much do they change? New devices should be calibrated relatively frequently in order to establish their reproducibility - 3 big risks in calibration_alliance calibrationessentially their metrological stability or the change in their measuring ability between calibrations. Initial estimates of reproducibility are sometimes made using type-test data from earlier calibration results of similar instruments but the resultant uncertainty of measurement has to be cautiously higher, until real data is available.

Secondly, the required uncertainty of measurement should be assessed. If the instrument's reproducibility is shown, by successive calibrations, to be substantially better than the uncertainty required then the interval between calibrations can be extended - perhaps even up to five years or so (if requirements in Standards don't specify otherwise), but at the other extreme - where the instrument's reproducibility approaches the uncertainty needed - the calibration intervals should be much shorter, perhaps on a weekly or even daily basis."

Source: NPL

"So, how can you determine an appropriate calibration frequency?  One good place to start is to obtain test data from the instrument manufacturer regarding the stability of the 
instrument over time, that is, how long it takes, on average, for the device to drift out of specification.  Some manufacturers may make this kind of data readily available to their customers, while others may not have such data or might consider it proprietary information.

If the stability information is not forthcoming from the manufacturer, you must develop it for your own organization.  One way to do this is to perform periodic calibration verification tests (sometimes called “interim checks”) to determine whether the instrument is currently operating within specifications.

When these tests are performed in-house, certain costs are involved; both in hours spent and in equipment needed to provide known, accurate inputs to the device, sometimes called a “calibration reference.” In some cases, the instrument manufacturer may offer a calibration verification service that is less costly than the price of doing a full calibration.  Regardless of who runs the tests, keep a log of the test results over a period of months and years.  This log provides the information needed to develop a calibration cycle that is appropriate for the instrument."

Source: Design World

An excel spreadsheet is adequate. If you don't want to write all the formulas in excel then consider software.There are several commercially available software programs that allow you to easily perform analysis of verification and calibration data to inform your decision regarding calibration frequency. 

Not Accounting For Measurement Uncertainty

Wikipedia defines measurement uncertainty as follows:

"In metrology, measurement uncertainty is a non-negative parameter characterizing the dispersion of the values attributed to a measured quantity. All measurements are subject to uncertainty and a measurement result is complete only when it is accompanied by a statement of the associated uncertainty. By international agreement, this uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity value.[1]

The measurement uncertainty is often taken as the standard deviation of a state-of-knowledge probability distribution over the possible values that could be attributed to a measured quantity. Relative uncertainty is the measurement uncertainty relative to the magnitude of a particular single choice for the value for the measured quantity, when this choice is nonzero. This particular single choice is usually called the measured value, which may be optimal in some well-defined sense (e.g., a mean, median, or mode). Thus, the relative measurement uncertainty is the measurement uncertainty divided by the absolute value of the measured value, when the measured value is not zero."

If you don't have a background in Metrology that definition might not be real helpful.

This might be more helpful.

"The uncertainty of a measurement tells us something about its quality. Uncertainty of measurement is the doubt that exists about the result of any measurement. You might think that well-made rulers, clocks and thermometers should be trustworthy, and give the right answers. But for every measurement - even the most careful - there is always a margin of doubt. In everyday speech, this might be expressed as ‘give or take’ ... e.g. a stick might be two metres long ‘give or take a centimetre’."

Consider downloading A Beginner's Guide To Uncertainty of Measurement.

Simply put, measurement uncertainty is the +/- in a measurement.

No measurement is perfect.

When considering measurement uncertainty 3 big risks in calibration_measurement uncertainty_alliance calibrationin calibration, think back to using a yardstick to measure gage blocks.This method would not give you much confidence in the result and you certainly would not want to risk producing a product with a tolerance less than a yardstick.

Check out FAQ's: Measurement Uncertainty for more information.

Determine what measurement uncertainty for each type of calibration will satisfy your risk tolerance and review the scope of accreditation BEFORE selecting a calibration provider.

Summary

Go slow to go fast. Get past simply using ISO 17025 accreditation as a checkbox in your list. Have a meaningful conversation with your audit-checklist1-7calibration provider and determine if they have the capabilities to meet your requirements. Collect and use data to inform your calibration program. It is only a matter of time before your auditors will no longer accept " we use an accredited lab" as compliance to risk management.

Topics: calibration, risk management

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