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Internal calibration: The laboratory must establish its own calibration procedures, but the calibration standards used must be verified or calibrated to meet the required standards (in accordance with the regulations for value transfer). Calibration refers to adjusting machines, instruments, etc. to make them more accurate. 01 Difference between internal calibration and self-calibration: “Self-calibration” generally involves using the calibration procedures or functions built into the measuring device itself (such as the auto-calibration procedure that runs when a smart instrument is turned on), or standard samples provided by the device manufacturer that do not come with traceability certificates. Usually, this type of calibration does not constitute a valid method for establishing value traceability, unless otherwise specified in specific fields. “\"Internal calibration\" refers to the calibration carried out within a laboratory or its organization, using its own facilities and measurement standards; the results of such calibration are intended for internal use only, and it is conducted in order to establish a traceability for the values of measurement equipment used in recognized testing activities. Therefore, compliant internal calibration is a traceability method recognized by CNAS, whereas self-calibration is not. 02 Scope of application for internal calibration: For testing laboratories that have obtained CNAS accreditation, or those preparing to apply for such accreditation, regarding the measuring equipment related to their accredited capabilities, if the traceability of measurement values is achieved through internal calibration, then they must comply with CNAS-CL31:2011 \"Requirements for Internal Calibration\". However, if the laboratory possesses the capability for internal calibration of the measuring equipment related to its accreditation or certification requirements, and has already been accredited as a CNAS calibration laboratory or has received authorization from a statutory metrological verification institution, it can be considered to meet the \"Internal Calibration Requirements\", and thus no further assessment in accordance with CNAS-CL31:2011 \"Internal Calibration Requirements\" is necessary. For instance, some laboratories have a large number of glass measuring instruments. If all of them were to be submitted for calibration, the costs would be substantial and the process would take a long time, thereby affecting their testing operations. If such a laboratory applies to the provincial metrology administrative authority to establish its own measurement standards and successfully passes the assessment, thereby obtaining a certificate of compliance with measurement standard requirements for glass measuring instruments, then it can be considered that the laboratory’s internal calibration of these instruments constitutes a traceability pathway that meets CNAS requirements. Relevant regulations stipulate that measuring equipment subject to mandatory calibration must be calibrated as required, and internal calibration is not permitted. 03 Requirements for personnel: Those responsible for internal calibration should possess the appropriate knowledge in physics and mathematics, as well as knowledge of metrology and the ability to assess measurement uncertainty. Personnel responsible for internal calibration must receive necessary training in relevant metrology knowledge and calibration skills, pass the relevant assessments, hold the required certificates, or be authorized to perform such tasks. 04 Requirements for environmental conditions and facilities: The calibration environment and facilities used for internal calibration in the laboratory must meet the requirements of the calibration methods. It must be ensured that its environmental conditions do not render the results invalid or adversely affect the required calibration quality. The technical requirements for the facilities and environmental conditions that affect the results of internal calibration should be documented. When internal calibration methods are required or have an impact on the quality of the results, the laboratory shall monitor, control, and record environmental conditions. Calibration should be stopped when environmental conditions threaten the results of internal calibration. 05 Requirements for equipment applicable to internal calibration and reference standards: Internal calibration shall be carried out by configuring and using reference standards and/or reference materials (metrological standards) as well as auxiliary equipment in accordance with the requirements of the calibration methods. The traceability of measurement values must meet the requirements of Article 5.6 \"Measurement Traceability\" in CNAS-CL01 \"Criteria for the Accreditation of Testing and Calibration Laboratories\" and the requirements of CNAS-CL06 \"Requirements for Traceability of Measurement Values\". To ensure the accuracy and reliability of calibration, specific procedures should be established for the establishment, verification, maintenance, and proper use of reference standards and/or reference materials (metrological standards). Reference standards and/or reference materials (metrological standards) shall be verified in accordance with the specified procedures and schedules to maintain the confidence in their calibrated status. Reference standards and reference materials (metrological standards) shall undergo verification of measurement uncertainty, assessment of repeatability, and assessment of stability. 06 Requirements for calibration methods: The internal calibration methods for measuring equipment should preferably be based on **calibration specifications or departmental calibration specifications; in the absence of such specifications, the corresponding **verification procedures or departmental verification procedures should be used instead. In the absence of appropriate calibration specifications, verification procedures, or other standard methods, laboratories may use non-standard methods such as internally developed methods or those recommended by the manufacturers of the measuring equipment. When using methods recommended by the manufacturers of measuring equipment, these methods must be documented in the laboratory’s own files. Non-standard methods should undergo method validation, and the records of this validation must be kept. 07 The quality control procedures and quality supervision plans of the quality control laboratory shall cover internal calibration activities. Reference standards and/or reference materials (metrological standards) should participate in inter-laboratory comparisons, measurement audits, or proficiency testing programs recognized by CNAS. 08 Uncertainty requirements: The laboratory shall evaluate the measurement uncertainty for all internal calibration tasks (parameters) it carries out. It shall report the measurement uncertainty in the calibration certificate or records, and/or provide a statement confirming compliance with its metrological specifications or relevant regulations. Under normal circumstances, the calibration results should include the numerical value y of the measurement result and its expanded uncertainty U. The coverage factor and coverage probability for the uncertainty should be specified in the calibration certificate; it can be described using the following wording: “The expanded uncertainty given in this report is the standard uncertainty multiplied by the coverage factor k for a coverage probability of approximately 95%.” ” The value of the expanded uncertainty should not have more than two significant figures, and the following requirement must be met: the last digit of the measured value reported in the final report should align with the last digit of the expanded uncertainty ； Numerical rounding shall be performed in accordance with the rules specified in GB/T 8170 “Rules for Rounding of Numerical Values and Expression and Judgment of Limiting Values”. When reporting the sources of measurement uncertainty in the calibration certificate, the short-term uncertainty components during calibration should be included. 09 Records and reports of internal calibration: The calibration certificate for internal calibration can be simplified or omitted, but the contents of the calibration records must meet the requirements of the calibration methods and accreditation criteria. Calibration records should be retained for a long period to monitor the stability of the calibrated measuring equipment. The calibration results produced by the calibrators must be verified by the reviewers. The calibration record should include the name of the measurement standard used and its unique identification number, to ensure the reproducibility of the calibration process. 10 Requirements in CNAS accreditation processes: During the initial review, re-evaluation, and expansion review, if internal calibration activities are carried out for the testing capabilities for which accreditation is sought, the laboratory must declare this in its application. CNAS will then assign reviewers specializing in calibration to conduct on-site evaluations. If the results of these evaluations show that the laboratory’s internal calibration capabilities do not meet the requirements, then the relevant testing items or parameters for which accreditation is applied will not be approved. If there are internal calibration activities in the laboratory but they are not reported at the time of application, and during the on-site audit the audit team lacks the expertise to assess such internal calibrations, then the relevant testing items or parameters for which approval is sought will not be approved. During the surveillance review, internal calibration activities should be covered; generally, the scope of coverage for internal calibration capabilities is consistent with the scope of surveillance for the recognized testing capabilities. The results of internal calibration can only be used within the laboratory; it is not permitted to claim that the internal calibration process has been recognized by CNAS or to use such recognition marks.