In our daily metrology and testing work, one of the problems we often encounter is how to determine the calibration cycle of measuring instruments. Moreover, customers often question the calibration period or calibration validity period we give and ask for adjustment. How to explain this? In order to scientifically and impartially determine the metrology verification cycle and provide customers with a reasonable calibration date, we might as well standardize and discuss the verification (calibration) cycle of measuring instruments from the following aspects.

The JJF1139-2005 technical specification is formulated in reference to the international document OIML D10:1984 "Principles for Determining Recalibration Intervals for Measuring Equipment Used in Testing Laboratories" published by the International Metrology Organization of France and NCSL RP-1: 1999 "Confirmation and Adjustment of Calibration Intervals" published by the National Conference of Standards Laboratories of the United States. This specification stipulates the basic principles and methods for determining the verification cycle of measuring instruments.

The specification proposes three basic principles: First, when formulating or revising the verification procedures for measuring instruments, the verification cycle should be determined based on the characteristics of the applicable measuring instruments, the performance requirements of the measuring instruments and the usage of the measuring instruments; Second, when determining the verification cycle of measuring instruments, the metrological reliability target R of the applicable measuring instruments should be clarified first (the metrological reliability target R of general measuring instruments is 90%); Third, in order to determine the verification cycle of the measuring instrument, one or several appropriate methods, reaction method or maximum likelihood estimation method, should be selected for analysis and calculation. From these three principles, we can clearly see that the determination of the verification cycle of measuring instruments is determined by the performance, characteristics and usage conditions of the measuring instruments under the premise of ensuring a certain metrological reliability target; the verification cycle proposed in the national verification procedures for metrology is obtained under the premise of these three principles; (3) The calculation of this cycle is scientific.

The JJF1139-2005 specification proposes two verification cycles for measuring instruments: the reaction method and the maximum likelihood estimation method. There are three response methods: "fixed step adjustment method", "incremental response adjustment method" and "interval test method". There are three specific calculation methods for maximum likelihood estimation: classical method, binomial method and update time method.

The Notice on Strengthening the Management of the Verification Cycle of Measuring Instruments in Compulsory Verification Work was issued in October 2000. It is to strengthen the management of the verification cycle of measuring instruments in the compulsory verification work of legal, fixed (including authorized) metrological verification institutions, standardize the behavior of adjusting the compulsory verification cycle, and ensure that the compulsory verification work is scientific, fair and effective. This notice makes four provisions on the verification cycle of measuring instruments:

1. The verification cycle stipulated in the national metrological verification regulations or departmental and local metrological verification regulations (hereinafter referred to as "regulations") is the longest verification cycle under normal conditions and is generally applicable to the compulsory verification of working measuring instruments. Legal metrological verification institutions (including authorized ones) should strictly implement it and generally do not need to adjust it.

2. If the qualified rate of two consecutive calibration cycles is lower than 95% (the main metrological performance index of the measuring instrument) or the main metrological performance index of a certain measuring instrument fails to meet the requirements for two consecutive calibration cycles, the legal metrological verification agency (including the authorized agency) may shorten the verification cycle appropriately according to relevant regulations and in combination with the actual use situation, but the shortened verification cycle shall not be less than 50% of the prescribed verification cycle; for working measuring instruments with shortened verification cycles, if the qualified rate of two consecutive verification cycles is above 97% (including 97%) or three times qualified, the verification cycle specified in the implementation rules shall be restored.

3. Before adjusting the mandatory verification cycle, fixed (including authorized) metrological verification

4. The Provincial Quality and Technical Supervision Bureau shall strengthen the supervision of the mandatory verification work of legal (including authorized) metrological verification agencies, strictly review and adjust the mandatory verification cycle application procedures, and hire technical experts when necessary. Any arbitrary or unapproved filing adjustment of the mandatory verification period shall be corrected in a timely manner. If the circumstances are serious, the mandatory verification authorization of the project shall be revoked.

The above four provisions clearly define the principles and methods for how the metrology verification institutions determine the verification time intervals in their metrology verification work.