GB/T1.1-2020, "Standardization Work Guidelines Part 1: Structure and Drafting of Standards" [1] stipulates the general principles, structure, drafting of elements, and expression of elements for product standard drafting, and also specifies other rules and formatting for ensuring standard drafting. Currently, product standards in our industry are developed according to the procedures outlined in GB/T1.1 national standards. The standard procedure is as follows: preliminary research stage, project initiation stage, drafting stage, consultation stage, review stage, approval stage, publication stage, re-examination stage, and repeal stage. All nine stages are indispensable. A tall building starts from the ground up, and the importance of the foundation cannot be ignored. What we want to emphasize today is that we must not underestimate the preliminary research stage, because it is the foundation of the entire standard drafting process, especially the following two conditions in the preliminary research stage: First, there must be a certain level of research and complete design and production capabilities for the product technology to which the standard is being drafted. Second, there must be the ability to conduct relevant tests on the product to which the standard is being drafted. This article focuses on the topic of "testing," specifically the metrological performance tests conducted on weighing instruments and the electronic and mechanical performance tests conducted on the components that make up the products. The importance of product testing is illustrated below through several case studies in the formulation and revision of weighing instrument product standards.
1. The Significance of Testing For weighing instrument products, the content of standards contains three important elements: metrological requirements, technical requirements, and test methods. Test methods are used to verify whether the indicators proposed in the metrological and technical requirements are necessary, accurate, rigorous, complete, and feasible. Test methods must correspond one-to-one with the content of the metrological and technical requirements.
Through the process of formulating and revising national, industry, and group standards for weighing instruments in our industry in recent years, we have identified an issue that warrants discussion: Standard drafters should provide the National Weighing Instrument Standardization Technical Committee with documents such as the draft product standard for comments, standard drafting instructions, and product test reports. Several product standards drafting units failed to provide product test reports, and even those that did provide reports often lacked comprehensiveness, particularly in that the test items did not correspond one-to-one with the requirements.
In reality, the test reports required by product standards are documents proving the feasibility of the metrological and technical requirements outlined in the standard. There has long been a misconception that test reports are merely a statistical summary of the metrological performance tests conducted on weighing instruments, neglecting the verification of test data for the technical requirements in the standard.
2. How to Conduct Tests
2.1 Tests for Metrological Requirements
When testing the metrological performance of weighing instruments, attention must be paid to:
1. Environmental conditions, standard instruments, test methods, and test procedures. Failure to correctly understand these test elements in the referenced standard may affect the quality of the standard's development and revision.
2.1.1 How to deal with the test methods in the referenced standards R76 International Recommendation on Non-Automatic Weighing Instruments [2] proposes an "off-center load test" in its metrological requirements. Although this test covers four types of off-center load phenomena that may occur in weighing instruments: "for weighing instruments with n > 4 support points", "for weighing instruments with n ≤ 4 support points", "weighing instruments subjected to very small off-center loads", and "weighing instruments used for weighing rolling loads", it also adds a note: "If the weighing instrument is designed to be loaded in different ways, it is appropriate to perform the following tests". The above note on the off-center load test requires that for weighing instruments of different structural types, the results of the tests listed must be considered for different loading methods. These requirements are referenced in the relevant standards we drafted. If the product is designed to be loaded in different ways, tests should be conducted on different loading methods, and test data should be provided to further prove this. (1) Relationship between the number of support points and the load value According to the international recommendation R76 "Non-automatic Weighing Instruments", for weighing instruments with "n ≤ 4 support points" and "n > 4 support points", different load values of 1/3 of the maximum weighing capacity and 1/(n-1) of the maximum weighing capacity are specified. Since the two test methods were not carefully compared, products with the same maximum weighing capacity but different specifications may have different test conclusions due to the different load values used for testing [3]. If it is uniformly stipulated that 1/3 of the maximum weighing capacity is applied to each support point, instead of according to the number of support points, then comparability can be achieved. (2) Multiple tests should be conducted for different loading methods. From a materials mechanics perspective, the loading of "weighing instruments with n ≤ 4 support points" and "weighing instruments with n > 4 support points" is a "locally distributed" load form, while the loading of "weighing instruments used to weigh rolling loads" is a "locally concentrated" load form. "Locally distributed" loads are evenly distributed over a local area of the weighing instrument's support structure, requiring lower stiffness from the support structure. "Locally concentrated" loads are concentrated at several points on the support structure, requiring higher stiffness from the support structure. For the same weighing instrument, the deformation caused by a locally concentrated load is greater than that caused by a locally distributed load, which may lead to tilting of the load cell, causing component forces in the transmitted force and thus affecting weighing performance. 2.1.2 How to deal with the test steps in the referenced standards When testing large weighing instruments, the number of standard instruments often affects the test, and the substitution method must be used. When using the substitution method, it is not enough to just meet the minimum number of weights. One basic condition to consider is "time". Not all weighing instruments can be tested by substitution as long as they meet the requirement that "the repeatability is not greater than 0.2e and the standard weight can reduce the maximum weighing capacity by 1/5". According to the standard weight of 1/5Max, the weight needs to be loaded and unloaded 5 times, and the substitute needs to be loaded 4 times. When the outdoor lifting equipment is not very convenient, the loading and unloading of these large items will take a long time, which will bring some nonlinear errors into the weighing error, such as "hysteresis error" and "creep error". Therefore, in the R60 "Weighing Sensors" [4] international recommendation, the loading, unloading and stabilization time of weighing sensor testing is clearly specified. 2.2 Test Methods for Technical Requirements
2.2.1 Requirements for the Load-Bearing Frame of Fixed Electronic Weighing Instruments
R76 "Non-Automatic Weighing Instruments" is an international recommendation that focuses on metrological requirements. Therefore, its technical requirements only vaguely state that "the structure of the weighing instrument should be robust and precise." Such requirements cannot guide manufacturers to produce qualified products. Therefore, when revising the national standard GB/T7723-2008 "Fixed Electronic Weighing Instruments" [5], we proposed detailed technical indicators and specified testing methods.
To ensure the scientific rigor and soundness of the testing methods, we specifically invested manpower and resources to conduct tests on various specifications of products under different loads within the load-bearing frame area.
2.2.2 Determination of Metrological Indicators for Electronic Body Scales
During the drafting and approval process of "Electronic Body Scales," the experts and drafters of the Weighing Instrument Standardization Committee had differing opinions on whether the metrological technical indicators of the R76 international recommendation should be adopted. Over 70% of the world's body scales are manufactured in China, making China the global manufacturing base for household body scales. According to marketing principles, China should be qualified to set product technical specifications. Furthermore, household body scales are not precision measuring instruments and do not require the same accuracy and stability as trade settlement scales. Therefore, it is entirely possible to set some measurement indicators that meet practical needs. However, these set measurement indicators must be verifiable through the test methods specified in this standard, and a detailed test report must be provided.
This test report should ideally include comparisons of different test methods and different data.
2.2.3 Issues Regarding the Selection of Load Cells for Digital Indicating Rail Scales
According to the international recommendation R76 "Non-Automatic Weighing Instruments," the verification division number (n) of the electronic weighing instrument must not exceed the maximum verification division number (nLC) of the load cell, i.e., nLC ≥ n. Digital indicating rail scales, since their first product was launched in 1991, have a maximum weighing capacity of 100t and a scale division of n=5000e. However, the load cells used were of class 0.02%FS (equivalent to the current C3 class), which does not comply with international recommendations for non-automatic weighing instruments. Therefore, this issue was avoided when formulating and revising national standards, metrological verification procedures, and type evaluation outlines for digital indicating rail scales. Theoretically, selecting such load cells should have met the performance specifications of digital indicating rail scales. Furthermore, in 1991, two major domestic manufacturers initially implemented this standard, and subsequently, many other manufacturers this standard in type evaluation testing. In over 30 years of actual verification and use, it has consistently met user requirements.
2.2.4 Issues in Selecting Weighing Indicators for Medium Accuracy Weighing Instruments In weighing instrument product standards, when mentioning weighing indicator modules, it is generally required that they comply with the metrological and technical requirements of GB/T7724. The international recommendation R76 also requires that the maximum verification division of the weighing indicator not be less than the verification division of the weighing instrument. For analog weighing indicators, the minimum input signal voltage (Umin) and the maximum input signal voltage (Umax) are specified by the weighing indicator manufacturer based on the design parameters of the weighing indicator's A/D conversion circuit (such as the input polarity of the ADC, signal amplification factor, reference voltage, ADC common-mode voltage variation range, etc.) and the software program processing requirements. When these modules are assembled into a weighing instrument product, many parameters, after being calculated and approved, must be tested and verified on the product system [7]. We have encountered similar situations where, although the system could display normally after assembly, the metrological performance could not be guaranteed, so the design data must be verified through testing. 3. Conclusion
3.1 Through the analysis of the above standard case studies, it can be seen that if drafters lack experience in standard drafting and lack knowledge of standardization and related laws, they should participate in training organized by professional institutions.
This also shows that drafters who only focus on metrological and technical requirements while neglecting testing work cannot draft a good product standard.
3.2 Because product standard testing requires significant financial, material, and time investment, drafting units should actively provide human and material support to ensure the quality of product standard drafting.
3.3 Drafting a product standard is actually a double-edged sword.
On the one hand, drafting units can use experimental verification and reverse engineering methods to improve the content of the standard; on the other hand, it also allows them to promote their own product technical requirements and testing methods within the industry.
