Measurement uncertainty is of great economic importance. The measurement uncertainty is decisive for the level of testing costs. Even small measurement uncertainties can lead to high testing costs.

It is therefore all the more important to know how the measurement uncertainty is calculated and which factors influence it.

Here you can find out which methods you can use to determine the measurement uncertainty, how you can reduce the measurement uncertainty and how you can keep an eye on the causes of the measurement uncertainty.

How the measurement uncertainty is determined

The measurement uncertainty is determined by means of experimental procedures, also known as measurement series. Various other methods are also used.

In these procedures, the causes are identified and indicated in a numerical value.

These causes are then summarized in a so-called measurement uncertainty budget.

A model is created, so to speak, which determines the overall measurement uncertainty.

Comparable measurement results are essential in the field of calibration and measurement technology. For this reason, standardized procedures are necessary when determining the measurement uncertainty.

Interpret measurement uncertainty and reduce testing costs

Knowledge of measurement uncertainty forms the basis for comparing and accepting measurement results.

Knowledge is therefore extremely important. Sound testing and calibration decisions cannot be made without knowledge of the measurement uncertainty.

Please refer to the “Golden Rule of Metrology”.

This means that the ratio of measurement uncertainty to tolerance should not exceed 10%.

This will give you an initial indication. You can decide how suitable a particular measurement process is for a specific application.

The measurement uncertainty also determines how you design and monitor certain measurement processes.

Ensure a sensible ratio of tolerance, production spread and measurement uncertainty. This creates the conditions for economical production.

Unnecessarily precise measurements drive up your inspection costs, but a measurement process with high measurement deviations increases your error costs.

Decide on the right ratio to reduce your testing costs.

Causes of measurement uncertainty

There are basically five different causes. Each of these has a different influence on the measurement deviation.

A distinction is also made between random and systematic measurement deviation. The random measurement deviation cannot be influenced.

At best, this can be estimated using mathematical-statistical methods.

The systematic measurement deviation is made up of the known or unknown influencing factors. Known factors are added together. Unknown influencing factors are treated as random measurement deviations.

All components must be taken into account so that the measurement deviation can be calculated as accurately as possible.

The relationships between the various influencing factors must be understood as well as possible. Therefore, the determination of measurement uncertainty requires extensive knowledge and experience of random and systematic measurement deviations.

All causes of measurement uncertainty are also dependent on the methods and measuring devices used for calibration.

The five basic causes are explained in more detail below.

Surroundings

An important factor in the environment is the temperature. This can influence measurements.

Temperature deviations from the reference temperature or temperature fluctuations during the measurement ensure this.

Temperature changes are also noticeable, for example, in the linear expansion of measuring equipment. If the temperatures are measured continuously, you can take them into account in your measurement by making a corresponding correction in the measurement result.

Humidity can also influence the measurement result, especially with optical measurements.

Why?

Air has a refractive index. The humidity in the air influences this refractive index. If the humidity changes, this affects the refractive index, which in turn influences the measurement result of optical measurements.

Vibrations are partly generated by the environment, but also by the respective measuring device. Seismic vibrations can be reduced by foundations and other vibration-damping measures.

Soiling must also be taken into account. Contaminated measuring equipment usually causes deviations in shape and position.

You should therefore ensure that your measuring equipment is cleaned thoroughly and regularly.

Measuring device

The design of the measuring device to be used is particularly important.

This has a decisive influence on the deformation. Deflection due to dead weight or the weight of workpieces influences the measurement result.

The measuring standard used for the measurement must also be taken into account when determining the measurement uncertainty. These dissolve over time, causing deviations in the measurement.

It is important for the evaluation software to use certified sample data with input and result data. Otherwise, imperfect algorithms can have an impact on the measurement uncertainty.

Material

The material you use has many properties.

Density, elasticity, hardness and many other factors influence the measurement uncertainty.

Deformation of the material is caused by the dead weight of clamping forces and by one-sided heating.

Therefore, support your material with suitable workpiece holders and select their support points favorably in order to keep the deformations as small as possible.

Users

The calibration or measurement technician usually has the greatest influence on the measurement result. It is crucial that it interprets the measurement task correctly.

Measurement technicians must be well trained to reduce any influences.

The choice of clamping is the first decisive point. Workpieces with a large length/cross-section ratio, for example, should be supported in such a way that they bend as little as possible due to their own weight.

The measurement process also plays a major role. The measurement result is positively influenced by a clever choice of measurement procedure.

Of course, people sometimes make mistakes. Subjective influences are therefore also possible. This includes reading or transmission errors. However, careful, experienced and trained working methods reduce these errors.

Measurement strategy

Measuring equipment, auxiliary equipment and the type of measurement are defined in a measurement strategy.

Clamping devices, lighting and positioning devices can influence the measurement uncertainty. Both positive and negative.

Select your tools to support the measurement process and thereby reduce measurement uncertainty.

The measurement procedure defines the principle and method of measurement. Physical principles are defined here. It is also decided whether the measurement is relative or absolute.

The evaluation strategy determines the association procedure used. This is an essential criterion for determining functional measurement results. This therefore has a decisive influence on the measurement uncertainty. Here you can also specify whether any measuring points are filtered and which parameters are used for these filters.

Conclusion

The influences and causes of measurement uncertainty therefore consist of a large number of factors. To calculate the measurement uncertainty, all components must be taken into account.

Therefore, proceed step by step:

• List all relevant influencing variables
• Create a model with which you can determine the measurement uncertainty
• Determine the influence of the individual influencing variables
• Summarize the influences to a combined standard measurement uncertainty
• Specify the determined measurement uncertainty

If you have any further questions about the calculation, please contact us or write your question in the comments.

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Measurement technology is used in many areas. Whether in electrical engineering, mechanical engineering, medicine, chemistry or environmental technology, applied measurement technology plays a major role.

In this article, you will learn about the basic terminology and the tasks of measurement technology.

Tasks of measurement technology

In the field of industrial measurement technology, the achievement of new findings in the course of scientific developments is often overlooked. In addition to traditional metrological activities such as testing, calibrating, adjusting or verifying, science is using metrology to research the further technical development of measurement processes.

This enables improvements to be made in environmental protection or telecommunications, for example. The use of special and increasingly precise measuring methods makes it possible to detect and eliminate faults.

The task of measurement technology can generally be described by three characteristics:

• Measurement technology is not objective. Measuring must therefore be distinguished from estimating.
• Metrological activities are repeatable, controllable and can be reproduced.
• All measurement results are marked with a number.

Digitalization is also advancing in the field of measurement technology.

Electrical measuring methods are being used more and more. Nowadays, non-electronic variables are also determined by digital measuring systems.

Electrical measuring methods offer many advantages. The measured values can be recorded without a great deal of effort. In addition, the high resolution, the fast and simple processing of the measurement data and the good transmission capability are further important advantages in the digital age.

Typical measurement technology activities

The actual measurement is dedicated to the determination of a measured variable multiplied by a unit. However, there are other typical activities that must be distinguished from standard measuring.

Check

We speak of testing when we determine whether the measured object fulfills one or more conditions. In other words, the result is always reflected in a yes/no decision. Are certain requirements met? The activity of testing deals with this type of question. Examples here are the testing of resistance values of certain resistors within a tolerance limit.

Calibrate

By definition: Calibration is the determination of the measurement deviation, i.e. the correlation between the output value and the true value of a measured variable. This checks whether the measuring system complies with the promised accuracy. A table can be drawn up to correct systematic deviations. During calibration, the reference value is determined using a standard. The reference measuring device is called Normal.

Adjust

The measuring device is calibrated during adjustment. The aim of an adjustment is to reduce measurement deviations. The difference between calibration and adjustment is that during adjustment, for example, a precisely known voltage is applied and measured at the same time as the standard. The adjustment elements are changed until the display value matches the reference value as closely as possible.

Oaks

Calibration is often associated with official authorities. In simple terms, calibration is the testing of measuring devices in accordance with legal regulations and requirements. If a measuring device complies with all requirements, a calibration certificate is issued. Many measuring instruments are subject to calibration, especially in commercial transactions. Calibration is most frequently used for scales or gas flow meters.

Basic terminology

In the following, some basic terms of measurement technology, or metrology (the science of measurement) are explained.

Measured variable

The measurand is the physical quantity that is to be determined by the measurement. For example, the resistance value, energy, voltage or direct current can be determined.

Standards

Standards and guidelines are not necessarily popular. However, they have the important purpose of describing the state of the art and thus support the safety and quality of products and services. They are regarded as the basis for international trade.

Measuring device

A measuring device is the device required to measure a measurand. The measuring device contains a display that shows the measured value. The measured variable can also be transformed or processed by the measuring device in the same way as a current-voltage transformer does, for example.

Measuring device

A measuring system is a system consisting of one or more measuring devices together with the equipment required for measurement. These facilities include, for example, the energy supply. A digital voltmeter or a calibration device are examples of such measuring devices.

Transducer

The transducer is also commonly referred to as a sensor or measuring probe. It is part of the measuring device or the measuring system. The sensor responds directly to the physical variable. The output signal of the sensor is further processed and output. Examples of such sensors are current sensing resistors or Hall sensors for power measurement.

Measured value

The measured value is an explicitly measured value of a measured variable. It is given as a numerical value multiplied by a unit. For example, this could be 223.2 V as a measured voltage value.

The true value

The true value is the uniquely existing value of the measured variable. The target of the measurement, so to speak. As a rule, the true value cannot be determined. External circumstances, influences or repercussions do not permit recording. The measured value is virtually falsified by these influences on the measuring device itself.

Measurement deviation

The measurement deviation is the difference between the measured value and the true value. The aim of calibration is to record this measurement deviation.

Measurement result

The result of a measurement can be a single measured value or determined from several measured values. Special calculation rules apply here. If necessary, individual measured values are also corrected.

Measurement uncertainty

The measurement uncertainty forms an interval around the measured value in which the true value lies with a certain probability. For a certain measured value x with a measurement uncertainty u , the true value lies in the interval x±u.

Traceability

Traceability is a property of a measurement result which is characterized by being related to corresponding national standards through an uninterrupted chain of comparisons. Traceability in accordance with ISO 9000 is mandatory. Compliance with quality assurance is checked in specific audits by independent and accredited companies. If this test is passed, the respective quality assurance system receives certification. For ISO certification, measuring equipment must therefore be calibrated with standards that are traceable to international or national standards.

Conclusion

Measuring and testing have been part of everyday life since time immemorial. These are activities that are of great benefit to science, trade and industry. They are also indispensable for global cooperation.

We hope we have been able to give you a brief but instructive insight into the tasks and basics of measurement technology.

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