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If you don’t know the measurement uncertainty, why measure it !
Being aware of the concept of uncertainty in measurements is fundamental as it provides the assurance you need about the accuracy of the measures you rely on. It is essential that you know that the equipment you use is providing accurate measurements for what they are designed for.
Periodic verification and calibration of your equipment is conducted for a reason; to establish their accuracy and reliability. The same is true for the equipment and instruments we use to perform those verification and calibration. This is why measurement uncertainty is something we pay close attention to when providing those services to your organization.
Simply put, you can think of measurement uncertainty as the “uneasiness” you may have about the true accuracy of the measures taken, given the potential limitations and imperfections related to the instruments used, the expertise of the technician, the procedure followed, the conditions in which it was made, and much more.
The "uncertainty" can come from several sources, here are the most common ones.
One of the oldest tricks to ensure a measure is accurate is to repeat it multiple times (repeatability). Doing so will most likely unravel small differences in the measurements between repetitions. However, if hundreds of those measurements were to be performed, a pattern like the one presented below would emerge. This is called a standard deviation:
The concept of standard deviation had been proven reliable and evidenced by statistical methods, which we will not expose here. Let’s just say that if more than one measure is necessary to obtain statistical guarantee about the measurement uncertainty, when you calibrate similar kinds of equipment repeatedly, it is often enough to repeat the measurement only a few times and use the typical experimental standard deviation to calculate the measurement uncertainty within a statistically agreeable degree of certitude. This is enough to guarantee that the “real” measure will fall into the specified range n% of the time (usually 95%, also known as k2).
The verification report and calibration certificate we provide you present those calculations. They are used to assess the results of the measures we took against the standard applicable in your industry to determine compliance.
You should be aware that the reference equipment and instruments that are used for the verification and calibration of your equipment are also, by themselves, a source of uncertainty. This is where you should select your service provider carefully.
An Accredited ISO/IEC 17025 Provider is required to evidence that its equipment and instruments used as reference, to verify yours, are also calibrated periodically in order to maintain its acccreditation. Remember, your service provider’ measurement uncertainty must be factored in the calculation of your own uncertainty.
As discussed earlier, there are other sources of uncertainty that must be taken into account when calculating measurement uncertainty, they include, but are not limited to:
An Accredited ISO/IEC 17025 Provider gives you the guarantee that these other sources of uncertainty are correctly identified and taken into consideration in the results they provide you for better reliance on the validity of the measures.
Measurement uncertainty calculations are somewhat complex, but statistically robust and reliable. As you realize now, the ISO/IEC 17025 standard requires us to understand and make those calculations to determine the accuracy of the measures we provide and assess if they are within the range you require them to be in order to meet your compliance requirements.. In other words, replacing the nagging “uneasiness” with “trust”.
Measurement uncertainty is also important because it helps you to assess the best Service Provider to perform your verification and calibration, within your expected range for compliance. Accredited Providers are required to present their measurement uncertainty on their Accreditation Scope, so it is very relevant to check it as it impact your own measurement uncertainty calculations. They are inspected periodically to ensure their calculations are accurate and traceable.
Measurement uncertainty also keeps us honest. Your Service Provider should be in a position to demonstrate traceability to SI (the International System of Units) through NIST (the Institute of Standards and Technology). The larger the uncertainty becomes, the further away from the "true" measure you will be. We illustrate this in our topic on traceability.
Remember, understanding measurement uncertainty helps you make informed decisions about the conformity of your production output and evidence your compliance with the standards you are required to adhere to.
In calibration, when we compare your equipment against our reference equipment, the error is the difference between those two readings.
If an error is detected during a verification, we will perform a calibration to ensure that your equipment matches our reference standard, The equipment will then be verified again to ensure that all measurements, taking in account the measurement uncertainty, are in compliance with the standards you are required to comply with.
Accuracy indicates how close results are to the true or known value.
Precision indicates how close results are to one another.
Make sure the measurements obtained by your Service Provider are both!.
Use Service Provider with equipement and instruments measurement uncertainty that are within acceptable tolerance for your compliance.
Make sure your Service Provider can trace their measures back to the International standards and that the uncertainty of the calibration is known and suitable for your applications.
Take into account the environmental conditions as they may have a significant effect on the uncertainty of your measurements.
Be aware of the readability and display resolution of any indicating devices. Resolution can quickly add to measurement uncertainty.
Favor a Service Provider that is familiar with your equipment and understands the specificities of what they are measuring to avoid surprises down the line.
If you have questions about the reliability or traceability of the calibration we perform and how it impacts your business, don’t hesitate to contact us. We will be happy to answer any questions you may have.
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