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1.1 Metrology and Its Classification 1.1.1 Metrology Metrology is a fundamental and supportive discipline applied in various fields such as scientific research, industrial and agricultural production, and commercial trade. It is the science of measurement. 1.1.2 Metrological Classification Currently, in China, it can be classified by specialty as: length (geometric quantity) metrology, mechanical metrology, thermal metrology, electromagnetic metrology, radio metrology, time and frequency metrology, acoustic metrology, optical metrology, chemical metrology, and ionizing radiation metrology. That is the so-called top ten metrics. 1.2 Definition and Characteristics of Measurement 1.2.1 Definition of Measurement An activity aimed at achieving unit consistency and accurate, reliable measurement values. 1.2.2 Characteristics of measurement ① Accuracy: Accuracy is a fundamental characteristic of measurement. A result that has only a numerical value but no degree of accuracy is not a measurement result. In other words, a measurement result should not only provide the value of the quantity being measured but also its measurement accuracy (or measurement uncertainty); otherwise, the measurement result is meaningless. ② Consistency: As long as the measurement is taken at any time, in any place, using any method or instrument, and by any person, provided that the required metrological conditions are met, the measurement results should be consistent within a specified error range. This enables international mutual recognition, eliminating the need for numerous repeated measurements and thus saving a great deal of manpower, resources, and time. ③ Legal framework: The social nature of measurement (involving all industries and aspects) requires legal safeguards. Ensuring uniformity of measurement values relies not only on technical means but also on legal measures. Not only are laws and regulations needed, but there must also be corresponding institutions to ensure their enforcement. In our country, the **metrology administrative authorities are the quality and technology supervision bureaus at all levels** (officially known as the General Administration of Quality Supervision, Inspection and Quarantine). The legal testing laboratories are referred to as “legal metrology calibration institutions,” and they are established by the **metrology administrative authorities in accordance with the law or authorized to do so by law**. ④ Traceability: Definition of traceability: It is the characteristic that enables the value of a measurement result or a measurement standard to be linked to a specified reference standard, usually through an unbroken chain of comparisons with defined uncertainties, often in relation to **measurement standards or international measurement standards. To ensure the accuracy and reliability of measurement results, any quantity must be traced back to **a benchmark, an international benchmark, or a physical constant. Measurements with traceability are meaningful.