A class belongs to the state mandatory verification of measuring equipment, namely, "the organization's highest measurement standard instruments and instruments used for trade settlement, medical and health care, safety and environmental monitoring included in the mandatory verification directory of the work of measuring instruments". This type of measuring equipment should be implemented in accordance with national laws and regulations for mandatory verification. These must be external calibration.
B class belongs to the enterprise for process monitoring, quality testing, internal accounting, business management and other aspects of measuring equipment. This type of measuring equipment can be used by the enterprise according to its use, frequency, use of environmental conditions, regulations, etc., to determine the calibration interval and implementation of calibration and confirmation. This type of internal calibration can be carried out, but need to organize the relevant calibration capabilities, such as personnel (after training on the job), equipment, environment, methods and so on. Do not have the ability can also be external calibration.
Category C belongs to the enterprise equipment installed on the monitoring instrumentation is not easy to dismantle (such as oil pressure gauge, pressure gauge), only play a role in indicating the instrumentation does not participate in the measurement (such as switchboard table), no measurement characteristics of the test equipment (such as platforms, leveling) and so on. This type of measuring equipment can be implemented by the enterprise one-time confirmation, without the need to implement the subsequent verification and calibration, that is, free of school.
Of course, there is also a class of measuring equipment, this type of measuring equipment should be industry-specific equipment or the state does not have calibration procedures for experimental/testing, measuring equipment, calibration methods can be developed by the enterprise or the use of industry standards for their own calibration, or to find a calibration laboratory or unit with calibration capabilities for calibration.