Hanoi University of Science & Technology Department of Precision Mechanical & Optical Engineering Measurements and Uncertainty of Measurements D.Eng. Vu Thanh Tung tung.vuthanh@hust.edu.vn 1 Metrology & Key Organizations Metrology: “the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology” Metrology is defined by the International Bureau of Weight and Measures (BIPM) [1]. Key Organizations ✓ The International System of Units (SI) ✓ General Conference on Weights and Measures (CGPM) ✓ International Committee for Weights and Measures (CIPM) Figure 1: NMIs of different countries. [1] URL: http://www.bipm.org/en/convention/wmd/2004/ 2 True value and Accuracy The true value is the actual value of the property being measured. Since the true value cannot be absolutely determined, in practice an accepted reference value is used. The accepted reference value is usually established by repeatedly measuring some NIST or ISO traceable reference standard. Accuracy is the closeness of agreement between a measured value and the true value. Figure 2: True value and measured value. 3 Error Error is the difference between a measurement and the true value of the measurand (the quantity being measured). ➢ Systematic error: tends to shift all measurements in a systematic way so that in the course of a number of measurements the mean value is constantly displaced or varies in a predictable way. The causes may be known or unknown but should always be corrected for when present. Systematic error can be corrected for only when the "true value" (such as the value assigned to a calibration or reference specimen) is known. ➢ Random error is a component of the total error which, in the course of a number of measurements, varies in an unpredictable way. It is not possible to correct for random error. Random errors can occur for a variety of reasons such as: lack of equipment sensitivity, no