All types of instruments are devices used to directly or indirectly measure the quantity of the measured object. According to the definition of measuring instruments, all types of meters fall into this category.
During operation, measuring instruments may experience changes in their metrological performance due to various factors. Therefore, it is necessary to conduct regular verification or calibration on them.
Verification refers to all work performed to assess the metrological performance (accuracy, stability, sensitivity, etc.) of a measuring instrument and determine its eligibility.
Verification can be classified into the following types by nature:
- Factory Verification: After manufacturing measuring instruments, the manufacturer shall confirm their metrological performance. Only qualified measuring instruments are allowed to leave the factory.
- Sampling Verification: It involves selecting a certain proportion of measuring instruments from mass-produced batches for metrological performance confirmation. If the qualification rate fails to meet the specified proportion, double sampling verification shall be conducted. If the qualification rate still does not meet the requirement, the entire batch of measuring instruments shall be deemed unqualified. Sampling verification is generally only applicable to large-batch and relatively simple measuring instruments, such as glass measuring vessels and simple glass liquid thermometers.
- Initial Verification: The first verification conducted on a newly purchased measuring instrument after it is put into use is called initial verification. It also serves as the first verification in periodic verification.
- Periodic Verification: The interval between two verification works formulated based on the structure, performance, usage frequency, etc., of the measuring instrument is called the verification cycle. Verification carried out in accordance with the verification cycle is periodic verification. Periodic verification is a crucial link in metrological management. Only by formulating a reasonable verification cycle and strictly conducting verification accordingly can the performance of measuring instruments meet the specified requirements.
- Ad-hoc Verification: It refers to the verification conducted by government metrological administrative departments or enterprise competent departments to confirm the metrological performance of randomly selected measuring instruments during the supervision and inspection of enterprise metrological work.
- Arbitration Verification: It refers to verification conducted for arbitration purposes when metrological disputes arise.
Verification can be classified into the following types by management form:
- Mandatory Verification: For the highest-level measuring standard instruments used by departments, enterprises, and institutions as stipulated by the Metrology Law, as well as working measuring instruments included in the mandatory verification catalog for trade settlement, safety protection, medical and health care, environmental monitoring, etc., fixed-point and periodic verification shall be implemented, which is called mandatory verification.
- Non-mandatory Verification: It refers to the regular verification of used measuring instruments independently conducted by the user unit in accordance with the law.
Accuracy (or precision) mentioned in the definition of verification is the combination of systematic error and random error in measurement results, indicating the degree of consistency between measurement results and the true value. Stability refers to the ability of certain performance of a measuring instrument to remain unchanged over time under specified working conditions. Sensitivity refers to the ability of a measuring instrument to respond to changes in the measured quantity.
In accordance with the provisions of metrological management requirements, metrological verification must implement metrological verification regulations.
Verification regulations are legally binding technical documents formulated as the basis for verifying the metrological performance of measuring instruments. These regulations specify the scope of application, metrological performance of measuring instruments, verification items, verification conditions, verification methods, verification cycles, and handling of verification results.
National metrological verification regulations are formulated by the metrological administrative department of the State Council. In the absence of national metrological verification regulations, departmental metrological verification regulations and local metrological verification regulations shall be formulated by the relevant competent departments of the State Council and the metrological administrative departments of people's governments of provinces, autonomous regions, and municipalities directly under the Central Government, respectively.
Although the verification requirements for various measuring instruments are not entirely consistent, carrying out metrological verification work must at least meet the following basic conditions:
- An environmental condition (the impact of temperature, humidity, vibration, magnetic field, etc., on measuring instruments) that meets the requirements of verification regulations and allows the conduct of metrological verification work shall be available to maximize the metrological performance of the measuring instruments.
- Measuring standard instruments that meet the accuracy requirements shall be available. According to general regulations, the error limit of the standard instrument shall be at least 1/3 to 1/10 of that of the verified measuring instrument, and these standard instruments shall be traceable in accordance with metrological management requirements.
- Qualified verification personnel shall be available. Personnel engaged in metrological verification work must hold a "Verification Certificate". Only certified personnel are eligible to issue metrological verification certificates and verification result data. The "Verification Certificate" is issued by the government metrological administrative department or the competent department of the enterprise, with a validity period of generally 3 to 5 years.
These three are the most basic requirements for conducting metrological verification. After verifying a measuring instrument, detailed records shall be filled out, a verification seal shall be affixed, and the names of the verifying, reviewing, and supervising personnel shall be signed. A "Verification Certificate" shall be issued for qualified measuring instruments, and a "Verification Result Notice" shall be filled out for unqualified ones.
In the classic instrument management, the term "calibration" was used. Now, in metrological management, it is referred to as "calibration".
Calibration refers to all work performed to determine the indication error of a measuring instrument (and other metrological performance if necessary).
Similarities and Differences Between Calibration and Verification:
Calibration and verification are two different concepts but are closely related.
Calibration generally involves comparing a measuring instrument with higher accuracy (called a standard instrument) with the calibrated measuring instrument to determine the indication error of the latter. Sometimes, it also includes some metrological performance, but often only the indication error of the measuring instrument needs to be determined during calibration. If calibration is part of the indication error verification in the verification work, then calibration can be said to be a part of the verification work. However, calibration cannot be regarded as verification. Moreover, the requirements for calibration conditions are not as strict as those for verification. Calibration work can be carried out on-site, while verification must be conducted in a verification laboratory.
Some people understand calibration as the process of adjusting a measuring instrument to a specified error range, which is not entirely accurate. Although adjustments can be made during calibration, adjustment is not equivalent to calibration.
Basic Requirements for Calibration:
- Environmental Conditions: If calibration is conducted in a verification (calibration) laboratory, the environmental conditions shall meet the specified temperature, humidity, and other requirements of the laboratory. If calibration is conducted on-site, the environmental conditions shall be based on those suitable for the on-site use of the instrument.
- Instruments: The standard instrument used for calibration shall have an error limit of 1/3 to 1/10 of that of the calibrated instrument.
- Personnel: Although calibration is different from verification, personnel conducting calibration shall also pass effective assessments and obtain corresponding qualification certificates. Only certified personnel are allowed to issue calibration certificates and calibration reports, and only such certificates and reports are deemed valid.
