Sneak Peek: Withnell’s Instrument Calibration Services
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What is measurement uncertainty?
Why do we know measurement?
As a UKAS-accredited calibration laboratory, precision in measurement is a fundamental part of what we offer. As a calibration service provider we recognise that there will always be some degree of uncertainty in a calibration result.
When comparing the readings of your valuable temperature and humidity equipment to a known reference standard, we deliver the very best measurement uncertainties on the market. With the lowest uncertainties comes the highest confidence!
We calibrate equipment for many market-leading brands as well as perform 10,000+ individual calibrations annually. All this has led to us enjoying over a decade of accreditation. Since early 2011, we have been proud of our UKAS temperature calibrations.
Measurement plays a huge role in our business and is the foundation for many of our temperature and humidity services.
How do we define measurement uncertainty?
So, what is measurement uncertainty?
When calibrating an instrument, we use a reference standard to compare measurements against those from the unit under test. With every measurement comes a degree of uncertainty or doubt which is driven from characteristics of the process that cannot be evaluated. By outlining these characteristics, we can calculate a level of quantifiable doubt, leaving a true range that the measurement value sits within and an associated level of confidence.
Several external parameters can affect the accuracy and reliability of the measurement, which can lead to small variations in the final result, which is why the degree of 'uncertainty' must be accounted for and proved acceptable.
Things like operator influence, age of equipment, resolution of the instruments, environmental conditions, specification of test equipment and much more can all impact the measurement reading in one way or another. The aim is to calibrate your equipment to ensure that your instruments are operating within a small value 'uncertainty' range, to ensure the repeatability of your results.
Withnell Sensors' calibration laboratory offers temperature measurement uncertainties as low as ±0.005°C. These low uncertainties deliver ultimate confidence in the accuracy of the calibration.
Let’s try and illustrate measurement uncertainty in real terms, we will use the calibration of a data logger as this is common to most of our customers!
Let’s say we’re calibrating a temperature data logger against the triple point of water, which has a known, fixed temperature of 0.01 °C.
Now, imagine the data logger reads 0.015 °C during the calibration process. That’s slightly higher than the known value of the reference — but does it mean the logger is inaccurate when we report an error of 0.005°C
Here’s where measurement uncertainty comes in. If the calibration process has a measurement uncertainty of ±0.005 °C, this means that the true reading of the datalogger could reasonably fall anywhere between 0.010 °C and 0.020 °C.
Because the actual reference value (0.01 °C) sits within this uncertainty range, we can’t say the logger is wrong — it’s still measuring accurately within the expected limits. This is why understanding measurement uncertainty is so important: it helps us make informed decisions about whether equipment is performing as it should. It also illustrates why it is important to achieve uncertainties that are as low as possible.
Our measurement uncertainty
Our uncertainties are internationally recognised through our ISO 17025 accreditation, verified by UKAS and traceable to their national standards.
With expanded uncertainties as low as ±0.002 °C, this means our calibration services surpass ISO 17025:2017 minimum requirements. We're pleased to have set a new standard for accuracy across the technical industry.
Why is measurement uncertainty important?
Measurements are fundamental to the world that we live in, with millions of measurements taken daily. This data is especially important in technical fields such as engineering, medicine, and construction. Accurate measurements keep us safe and well and help us comply with a range of regulatory demands. Measurements also allow us to compare similar product properties, which is critical during calibration, as mentioned.
How we measure:
To conduct a measurement, you must compare the quantity or physical property in question to a known quantity/property of the same kind. A tape measure, a watch, scales, and thermometers are all measuring instruments with ‘known’ properties, whether it is length, weight, time, or temperature; these are everyday examples of measuring equipment.
How do we determine the known values of our measurement equipment? The answer to this is calibration!
All measurement equipment will need to be calibrated at some point. Calibration is the comparison of measurement values, checking the unit under test against a calibration standard of known accuracy. Think of a sequence of careful comparisons; you should be able to trace your equipment through an unbroken chain of comparisons back to a primary standard.
This is known as traceability, and is vitally important in many sectors since it is a fundamental of proving compliant practices. A primary standard is a good as it gets in terms of accuracy and normally consists of a known physical entity- for example, in temperature, we know the physical melting points of certain metals. We all also know some of the basics about temperatures that exist, such as the temperature at which water freezes.
Calibrations can vary hugely; over the years, we have seen all kinds of calibrations performed on our own equipment and our customers' equipment. To determine the suitability and accuracy of a calibration, we have to consider measurement uncertainty. Some calibration providers will not state a measurement uncertainty, this is usually because they aren’t accredited and so do not have the skills or understanding to define the measurement uncertainty. However it is a requirement of any ISO 17025 accredited calibration laboratory to define and declare the measurement uncertainty.
What is measurement uncertainty?
Every time we make comparisons between a measurement instrument and a calibration standard, there are several factors influencing the comparison.
Measurement uncertainty is a way of assessing all the variables that exist during the measurement period, allowing us to have a better understanding of the accuracy of the measurement. There are several ways of assessing measurement uncertainty to help us quantify these factors.
In the same way a clock might start to show the wrong time as its battery begins to die, a thermometer measurement can be influenced by radiation effects of direct sunlight, or a data logger's accuracy can be limited by the resolution of the display.
More static examples might include the environmental variables such as light, vibration, and ambient temperature, and the repeatability of a measurement is important if several different technicians will take measurements. These might be significant factors, but even with precision measurements using the most advanced equipment, there will always be a margin of deviation.
Have confidence with the lowest measurement uncertainties with Withnell Sensors' UKAS calibration services
ISO 17025:2017 and measurement uncertainty:
This international standard is used by all accredited laboratories, and any calibrations that are conducted within the scope of accreditation must adhere to the requirements of this standard, which includes measurement uncertainty and calibrations.
You will find the measurement uncertainty stated on calibration certificates supplied by accredited labs.
As we've already mentioned, the measurement uncertainty is what provides a real insight into the level of accuracy of the calibration, with the smallest range of uncertainty being the best. It may also be a factor to consider when determining which type of calibration is right for your needs.
Choosing the right calibration services:
You might assume that the lowest measurement uncertainty is always the best choice, but this might not always be the case. A lower measurement uncertainty can be cost and time-prohibitive for some customers due to the nature of the calibration process and the test equipment that is used.
It might also be unnecessary for your application, for example, sometimes lower measurement uncertainties are not even possible as the functionality of the unit under test is taken into consideration. If a calibration is completed onsite instead of in a controlled environment such as a laboratory, restrictions on the suitability of test equipment will usually mean a higher measurement uncertainty.
Here's some further factors to consider before choosing your calibration:
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Application specifics
Many applications have wide tolerances, particularly outside of the pharmaceutical/GMP scope, with certain equipment used for general storage having a wider boundary of uncertainty. For example, freezer and fridge calibrations in the food sector can start at ±1 °C of accuracy. Often, generalised industrial equipment doesn't need to operate to a high level of precision, and so the tolerance range can be broad.
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Cost and turnaround time
High precision calibrations vs standard calibrations are vastly different. UKAS calibration services often need controlled environments, trained technicians and stable reference standards (PRT calibration), and so with this comes increased costs. If your equipment doesn't stringently benefit from this degree of accuracy, then there's no need to invest in additional costs.
- Resolution and deviations
Depending on the piece of equipment being calibrated, the deviations can't always be solved. For example, some data loggers will not facilitate the resolution of the uncertainty range - they simply cannot record or display those kinds of fine values. In this case, the calibration uncertainty is more precise than your device's capabilities.
Some scenarios where the lowest measurement uncertainties might be required are: when you're calibrating a reference standard; when you're operating within an accredited laboratory; in applications where precision is critical (like vaccine storage, aerospace component testing); when traceability must be proven.
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Measurement uncertainty needs to be considered when assessing whether your test item has complied with a defined specification, for example, a pass/fail scenario. Now this does become a conversation topic all on its own, perhaps we will cover that in a later blog post. This is because there are different approaches as to whether to include the measurement uncertainty partially or wholly, and the impact on the initial specification. Your calibration laboratory should be available to discuss this with you. However, what is quite simple and clear is that you should always look at the measurement uncertainty and ensure that it is at least relative to your specification. Technically, it would be questionable to have a specification of 0.25°C if your measurement uncertainty were, for example, 1°C.
For further reading on measurement uncertainty, have a look at this guide from The National Physical Laboratory
