Measurement uncertainty

Measurement terminology

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The measurement uncertainty is equal to measurement result Associated parameter , which is used to characterize the dispersion of the measured value. It can be used in the "uncertainty" mode, or it can be a standard deviation (or its given multiple ）Or give the half width of the confidence interval. This parameter is often composed of many components, and its expression (GUM) defines different methods for obtaining uncertainty.

Chinese name: Measurement uncertainty
Foreign name: uncertainty of measurement
Interpretation: Reasonably give Measured Of Dispersity

Contact parameters: measurement result
Source: 10 aspects
Related books: measure Uncertainty express guide
Chinese specifications: Issued in 1999

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seven Uncertainty

definition

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measure Uncertainty It represents that the measured value is reasonably assigned Dispersity , and measurement result Associated parameters ".

The "reasonable" in this definition means that the correction should take into account the influence of various factors on the measurement, especially the measurement should be in the state of statistical control, that is stochastic control In process. That is to say, the measurement is carried out under the repeatability condition (see Article 5.14 of JJF1001-2011 General Metrology Terms and Definitions, and Article X herein refers to the clause number of the specification) or the reproducibility condition (see Article 5.15). At this time, multiple measurements are made on the same measured object, and the result is measurement result The dispersity of can be Bessel It is calculated by formula and expressed by repeatability standard deviation sr or reproducibility standard deviation sR.

"Connected" in the definition means measurement Uncertainty Is a measurement result "Together" parameters shall include measurement uncertainty in the complete representation of measurement results.

Generally, the quality of the measurement results is measured by the measurement error, but the measurement error can only represent the short-term quality of the measurement. Whether the measurement process is continuously controlled, whether the measurement results are stable and consistent, and whether the measurement capability meets the requirements of production profitability, we need to use measurement uncertainty to measure. The greater the measurement uncertainty, the worse the measurement capability; Conversely, it means that the measurement ability is stronger. However, no matter how small the measurement uncertainty is, the measurement uncertainty range must include the true value (generally replaced by the agreed true value), otherwise it means that the measurement process has failed.

principle

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measure Uncertainty Comprehending from the word meaning means measurement result The degree of doubt or uncertainty about credibility and effectiveness is a quantitative explanation measurement result A parameter of the quality of. In fact, due to incomplete measurement and people's insufficient understanding, the measured values obtained are decentralized, that is, the results measured each time are not the same value, but many values scattered in a certain area with a certain probability. Although objectively existing systematic error It is a constant value, but since we cannot fully understand or grasp it, we can only think of it as probability distribution It exists in a certain area, and the probability distribution itself is also decentralized. measure Uncertainty It refers to the parameter indicating the dispersion of the measured value. It does not indicate measurement result Is it close to True value 。

To characterize this dispersion, measure Uncertainty Expressed as standard deviation. In actual use, they often want to know measurement result Of confidence interval Therefore, it is specified in Note 1 of this definition: measurement Uncertainty It can also be expressed as the multiple of standard deviation or the half width of the interval indicating the confidence level. In order to distinguish these two different representations, they are called standards Uncertainty And expanded uncertainty.^[1]

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In practice, measure Uncertainty It may come from the following 10 aspects:

(1) The definition of the measured is incomplete or incomplete;

⑵ The method to realize the measured definition is not ideal;

⑶ The representativeness of sampling is not enough, that is, the measured sample cannot represent the defined measured sample;

⑷ Incomplete understanding of the environmental impact of the measurement process, or incomplete measurement and control of environmental conditions;

⑸ There is artificial deviation for the reading of analog instrument;

(6) Limitations of measurement performance of measuring instruments. Inaccurate measuring instruments or insufficient resolution and discrimination of measuring instruments;

(7) Assignment Measurement standard Value and reference material（ Reference materials ）The value of is incorrect;

(8) The constants and other parameters quoted in data calculation are inaccurate;

(9) Approximation and assumption of measurement methods and procedures;

(10) The change of the measured repeated observation value under the seemingly identical conditions.

It can be seen that the measurement Uncertainty Generally from randomness and Fuzziness The former is due to insufficient conditions, while the latter is due to the unclear concept of things themselves. This makes the measurement Uncertainty It is generally composed of many components, some of which can be estimated by the statistical distribution of the measurement results (observations), and characterized by the experimental standard deviation (see 5.17); Other components can be estimated by other methods (based on the assumed probability distribution of experience or other information), and are also represented by standard deviation. All these components should be understood as contributing to dispersion. If it is necessary to indicate that a component is caused by a certain reason, random effects can be used Uncertainty and Systematic effect The resulting uncertainty, rather than the outdated or obsolete terms "random uncertainty" and "system uncertainty". For example: by Correction value and Measurement standard Brought Uncertainty Component, which can be called Systematic effect The resulting uncertainty.

result

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Uncertainty When by variance When obtained, take its positive square root. When the size of dispersion is expressed by the half width of the interval that indicates the confidence level, it is obviously meaningless to take a negative value as the half width of the interval. When Uncertainty divide measurement result Is called Relative uncertainty , this is a Dimensionless quantity , usually with percentage Or 10.^[2]

Cognitive process

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Measuring Uncertainty In the development process of, people traditionally understand it as "an estimate (or parameter) that represents (or describes) the range of the measured true value"; There was also a period of time when it was understood that " measurement result A measure of the possible error of the given measured estimate ". These used definitions are conceptually a process of development and evolution. They involve the two idealized or theoretical concepts of measured true value and measurement error (in fact, they are difficult to operate unknowns). What can be specifically operated is in the current definition measurement result The change of, that is, the dispersion of the measured value. As early as the early 1970s, there were more and more metering Scholars recognize the use of“ Uncertainty ”Substitution“ error ”More scientifically, since then, the term uncertainty has gradually been widely used in the field of measurement. In 1978, the International Bureau of Metrology proposed the experiment Uncertainty Indicates Proposal INC-1. Measurement Uncertainty The Guide has been approved by seven international organizations, namely BIPM, OIML, ISO, IEC, IUPAC, IUPAP and IFCC. It is published by ISO and is an important document of international organizations. China has also issued compatible measurements in 1999 Uncertainty Technical specifications for evaluation and indication measurement. So far, measure Uncertainty Evaluation has become one of the essential tasks of testing and calibration laboratories.

significance

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Measurement uncertainty is a new term, which fundamentally changes the traditional classification method that divides measurement errors into random errors and systematic errors. After the systematic error can be corrected, it divides all remaining errors into two types of errors that can be calculated by statistical methods (Class A component) and estimated by other methods). Type A components are characterized by the standard deviation σ calculated by statistical methods through repeated measurements, while type B components are characterized by the approximate "standard deviation" u estimated by other methods, and can be treated like the standard deviation. If the above components are independent of each other, the characterization value of the combined uncertainty can usually be obtained by the method of variance synthesis. Since uncertainty is the characteristic description of undetermined error, it cannot be used to correct the measurement results.^[3]

Uncertainty

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Uncertainty refers to the uncertainty of the measured value due to the existence of measurement error. Conversely, it also indicates the credibility of the results. It is a measure of the quality of the results. The smaller the uncertainty, the closer the said result is to the measured true value, the higher the quality and level, and the higher its use value; The greater the uncertainty, the lower the quality and level of the measurement results, and the lower their use value. When reporting the results of physical quantity measurement, the corresponding uncertainty must be given. On the one hand, it is convenient for users to evaluate its reliability, and on the other hand, it also enhances the comparability between measurement results.

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