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# The definition of electromagnetic flowmeter error, the propagation of individual uncertainty

by：Sure     2021-08-04
10.1 Overview 10.1.1 Definition of error The measurement error of a quantity is the difference between the measured value and the true value of the quantity. There is no measurement of a physical quantity without uncertainty, which may come from systematic errors or random dispersion of measurement results. The system error cannot be reduced by repeated measurement, because it comes from the characteristics of the measuring instrument, installation and flow characteristics. Random error 10.1 Overview 10.1.1 Definition of error The measurement error of a 'quantity' is the difference between the measured value and the true value of the quantity. There is no measurement of a physical quantity without uncertainty, which may come from systematic errors or random dispersion of measurement results. The system error cannot be reduced by repeated measurement, because it comes from the characteristics of the measuring instrument, installation and flow characteristics. The random error can be reduced by repeated measurements, because the random error of the average value of n individual measurements is smaller than the random error of a single measurement. 10.1.2 Definition of standard deviation 10.1.2.1 If a variable X is measured multiple times, and each measurement has nothing to do with other measurements, then the standard deviation Sx of the n measurements of Xi distribution is in the formula-the arithmetic of the n measurements of the variable X Average; Xi——value obtained from the first measurement of variable X; n——total number of measurements of variable X. In short, SX is usually called the standard deviation of X. 10.1.2.2 If the variable X cannot be measured repeatedly, or the number of repeated measurements is too small, so that it may be unreliable to directly calculate the standard deviation on a statistical basis, and if the maximum range of the measured value of X can be estimated, the standard deviation can be taken as this One-quarter of the maximum range (that is, one-half of the estimated uncertainty above or below the adopted X value). In the same way, the system component of the assumed error can be described by a standard deviation, which is equal to half of the maximum expected value range of the component's positive or negative value. 10.1.3 Definition of uncertainty 10.1.3.1 For the purposes of this standard, in the measurement of a variable, the uncertainty can be defined as twice the standard deviation of the variable. When the measurement is required to comply with this standard, the uncertainty should be calculated and quoted under this name. 10.1.3.2 When the errors of each part of the combined uncertainty are independent, small and numerous, and have a Gaussian distribution, the probability that the true error is less than the uncertainty is 0.95. 10.1.3.3 Such as flow measurement qv The estimated standard deviation Sqv, the uncertainty eqv is given by the following formula: the relative uncertainty E is defined as: E u003d the result of flow measurement should be given in one of the following forms: a) flow u003d qv ± e (confidence The degree level is 95%); b) Flow rate u003d qv(1+ E) (confidence level is 95%); c) Flow rate u003d qv is within ±100% E (confidence level is 95%) 10.2 Flow measurement Uncertainty calculation 10.2.1 Error sources In the case of using electromagnetic flowmeters for flow measurement, the possible sources of error are basically: a) The system error caused by the equipment used in the output signal measurement; b) The output signal measurement The system error caused by the equipment used; c) the error caused by the flow condition is usually different from the conditions prevailing when the flow meter is calibrated, including the system error and the random error; d) due to the difference between the flow rate qv and the output signal X The error caused by the uncertainty of the piping qv(X). This error contains systematic and random components, it depends on the conditions of the flowmeter calibration, and can be changed at each test point of the calibration curve. 10.2.2 The uncertainty in the propagation flow measurement of the individual uncertainty is estimated by the combination of the individual uncertainties caused by the various sources listed in 10.2.1. Although systematic errors have been distinguished from random errors, the probability distribution of the possible values u200bu200bof each systematic error component is basically a Gaussian distribution. Therefore, the combination of random and systematic errors can all be handled as true random errors, and in accordance with ISO5168, the relative standard deviation of flow measurement can be taken as the square root of the sum of squares of relative standard deviations generated by various sources. Therefore, the result of the flow measurement is: its confidence level is 95%. Where S——standard deviation related to system error in output signal measurement; S——standard deviation of random error in output signal measurement; Sf——standard deviation produced by flow conditions; Sc——standard deviation in calibration relationship standard deviation. In the case of the simple form of qvu003dKiX proposed in the calibration relation, the above formula is written as: qvu003d1±2
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