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Measurement Uncertainty, Decision Rules and Impartiality: 3 Need-to-Know Calibration Terms

27 September 2021 /

Learn about measurement uncertainty, decision rules, impartiality, and why each calibration term is important to your advanced manufacturing operations.

Calibration is a complex process, but getting it right is essential to deliver dependable results and maintain the compliance of your advanced materials processing equipment.

In this article, we explain three often-misunderstood terms to help your site improve its calibration procedures and keep your operations running.

For more information about our UKAS accredited calibration services*, click here.

*UKAS Accredited Calibration Laboratory No.4338

Measurement uncertainty explained

Internationally recognised conformity authority UKAS defines measurement uncertainty in the following way:

​“Measurement Uncertainty (MU) can be used to estimate a range for values that could reasonably, with some defined probability, be attributed to a measured quantity. It is also critical to take account of measurement uncertainty whenever a conformity decision (e.g. pass/fail) is made.”

But what does this mean and why does it matter when measuring the conformity of your advanced materials processing equipment?

Simply put, it’s not possible to make a true, perfect measurement. A measurement is skewed by many factors. Identifying these factors is critical to understand how they affect the measurement as well as to normalise and quantify the significance of their effect.

The smaller the resulting uncertainty (or the better your ‘Calibration Measurement Capability’), the closer your measurement is likely to be to the ‘true’ measurement.

Why is measurement uncertainty important?

Working out the measurement uncertainty is important because a measurement that has been taken without regard for uncertainties is likely to be far from the ‘true’ measurement. 

  • As a result, it has an unquantifiable risk of being falsely accepted as complying with tolerance requirements.

  • Equally, a measurement may drive the conclusion that the equipment is not calibrated within acceptance limits when actually it is. 

Acknowledging and estimating the uncertainties at play offers operators the opportunity to improve compliance and reduce risk. Of course, identifying these contributing factors and actually calculating the measurement uncertainty is easier said than done.

Learn more about measurement uncertainty and how to calculate it in this article.

What is a decision rule in calibration?

It follows from the uncertainty that if you cannot say that the result is a ‘true’ measurement, then how can you say that it is ‘definitely’ within a specified tolerance?

It’s likely that in some circumstances, the result is within tolerance but adding the measurement uncertainty places it outside. When this happens, the benefit of taking steps to reduce uncertainties becomes clear. More of the tolerance band is available for the measurement and not taken up by the uncertainty.

When a business is taking a measurement because it requires a compliance statement, a ‘decision rule’ must be agreed in advance. The decision rule balances the risks of false accept/reject described above occurring. With an informed, impartial decision rule in place, a measurement can be said to pass or fail compliance based on the result and the amount of uncertainty at play.

What is impartiality in calibration?

It’s essential that the decision rule is impartial. As UKAS explains it in its policy statement, “Impartiality is one of the cornerstones of accreditation, and is of utmost importance in maintaining the trust that stakeholders should expect from accreditation.”

If you’re preparing to carry out calibrations on your advanced materials processing equipment, we encourage your laboratory technical management to get in touch to discuss this with us further. 

Our engineers have extensive experience performing a wide variety of instrument calibrations at our premises or on your site, including devices measuring temperature, humidity, pressure, vacuum, electrical properties and time. As a result, we’re expertly positioned to help you assess the performance and compliance of all your heat treatment and wider advanced processing equipment and make re-adjustments as necessary.

VFE calibration engineers will never (and must never be asked to) produce results that are favourable to a particular aim such as product acceptance. Any business that puts pressure on an engineer to make favourable results faces significant reputational and commercial damage from acquiescence.

UKAS accredited calibration services you can trust 

Calibrate your equipment to globally recognised standards and instil buyer confidence in the quality, safety and reliability of your products and services with VFE’s UKAS accredited calibrations (UKAS Accredited Calibration Laboratory No.4338).

VFE engineers can carry out UKAS accredited calibrations on a wide range of heat treatment and advanced materials processing equipment, either at our laboratory or on-site at your convenience.

For more information about the different kinds of equipment and instruments that we can calibrate, see here. 

UKAS accredited calibrations provide results that you can rely on. Our experienced team can also be your impartial guide to decisions about risk and conformity, helping you to comply with industry-specific standards like AMS2750.

If you haven’t worked with us before, this accreditation should give you the confidence that we have the competence required to deliver your calibrations.To find out more about our calibration services and how your site could benefit, click the image below and download our calibration services sheet.

UKAS Calibration