Measurement Matters- What is Measurement Uncertainty?

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Measurement Matters- What is Measurement Uncertainty?

Why we know measurement

As a UKAS accredited calibration laboratory, precision measurement is a fundamental part of the calibration services that we offer. We specialise in temperature and humidity measurements using highly accurate test equipment as a reference point when calibrating your test items and ensuring that everything is functioning as per your requirements. We also sell a wide range of measurement equipment including data loggers and thermocouples and laboratory equipment such as calibration baths and climatic test chambers. With a huge aspect of our business involved in the field of measurement we wanted to discuss some of the important considerations about how and why we measure.

How do we define measurement?

So, what is measurement? Measurement provides a numerical value for a physical property. Measurements provide us with information about a wide range of physical properties such as weight or length. Or, in our case, we primarily specialise in the measurement of temperature (degrees) and humidity (relative humidity). Measurements are fundamental to the world that we live in with millions of measurements taken daily and especially important in technical fields such as engineering, medicine, and construction. Accurate measurements keep us safe and well, ensure the quality of products 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 highlighted below.

How do we measure and why is calibration important?

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 measurement 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 all the way back to a primary standard. This is known as traceability and a primary standard is a good as it gets in terms of accuracy. Primary standards normally consist of a known physical entity- for example in temperature we know the physical melting points of certain metals. We all know some of the basic 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 equipment and our customers equipment. We get lots of questions about determining the suitability and accuracy of a calibration and to answer these questions we often must discuss the topic of 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. So, what is an example of one of these factors? You might be surprised at just how many factors there are to consider, for example a clock might start to show the wrong time as its battery begins to die, a thermometer measurement might be influenced by radiation effects of direct sunlight, a datalogger accuracy can be limited by the resolution of the display. More static examples might include the environmental variables such as light, vibration, 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.

Why is measurement uncertainty important?

The reporting of measurement uncertainty is a requirement of ISO 17025:2017. This is the international standard used by all accredited laboratories and any calibrations that are conducted within the scope of accreditation will adhere to the requirements of this standard. You will find the measurement uncertainty stated on the calibration certificate supplied by your accredited lab. Measurement uncertainty can give you a real insight into the level of calibration that has been completed and of course it might be a factor to consider when determining which type of calibration is right for your needs. 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. Sometimes lower measurement uncertainties are not even possible as functionality of the unit under test are taken into consideration. If a calibration is completed on-site, instead of in a controlled environment such as a laboratory then this, as well as restrictions on the suitability of test equipment will usually mean a higher measurement uncertainty. Therefore, when considering measurement uncertainty, it is important to weigh up all considerations when deciding what is most suitable for your requirements.

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