Electronic scales have the advantages of fast weighing, convenient reading, and easy connection to computer networking for automatic control, and are widely used. At present, as various user units have increasingly higher requirements for the accuracy of electronic scales, corresponding weighing instrument manufacturers are constantly improving the resolution of electronic scales, and the number of graduations of electronic scales is also increasing. It is generally believed that the smaller the graduation value of an electronic scale, the more accurate it is.
The higher it is, actually this is a misunderstanding. The graduation value of the electronic scale should be determined as required. Modifying or reducing the graduation value at will will not only worsen the stability, repeatability and reliability of the electronic scale, but also affect the accuracy of the measurement data.
1 Standards and regulations
During the use of electronic scales, the setting of the graduation value is very strict. It is set in strict accordance with the calibration procedures and combined with the corresponding configuration of the electronic scale, such as weighing instruments, sensors, maximum weighing capacity, etc. At present, both international recommendations and product standards and calibration regulations have strict and comprehensive regulations on weighing instrument products. The calibration graduation value e is a value expressed in mass units used when grading and calibrating weighing instruments. It represents the absolute accuracy of the scale. Therefore, the smaller the calibration graduation value, the higher the absolute accuracy of the scale. The number of calibration graduations n is the ratio of the maximum weighing capacity to the calibration graduation value, that is, n = Max /e. The calibration scale represents
The relative accuracy of the scale, so the more calibration divisions there are, the higher the relative accuracy of the scale. It can be seen that the accuracy level of the scale is determined based on the calibration graduation value e and the calibration graduation number n. The accuracy level of the scale is divided into medium accuracy and normal accuracy.
Pass accuracy, can be called Level 3 and Level 4, represented by symbols III and IIII.
The "International Recommendation for Non-Automatic Weighing Instruments" No. R76 drafted by the International Organization for Legal Metrology (OIML) integrates the technical conditions and inspection methods of non-automatic weighing instrument products. Our country's metrological verification regulations JJG555-96 "General Verification Regulations for Non-automatic Scales" 》and GB/T7721-2005 "National Standard for Electronic Bench Scales" partially adopt international recommendations. Mainly in terms of technical content, it is equivalent to the international recommendation No. R76 "Non-automatic Weighing Instruments" of the International Organization for Legal Metrology (OIML). In the process of implementation and application of JJG555-96 for more than ten years, there are many clauses that have not attracted the attention of our quality and technical supervision department and the scale.
For example, (OIML) R76 and JJG555-96 "General Verification Regulations for Non-Automatic Scales" in Article 4.8.6: It is clearly stipulated in the influencing quantities and constraints of the weighing instrument that it is installed outdoors. And for electronic scales that do not take appropriate protective measures to prevent the influence of the atmospheric environment, if the value of the calibration division number n is relatively large, it may usually not be able to meet the measurement requirements of the electronic scale and the technical requirements of the display. Under general conditions, the maximum calibration division number n value It cannot be greater than 3000, otherwise the zero point and indication value of the electronic scale will be difficult to stabilize, and reliable data cannot be obtained. But there are quite a few electronic scales in our country
This provision has been ignored in the measurement verification and product supply and demand technical requirements of manufacturers and users. There are many electronic scales in use with a maximum verification scale n value greater than 3000. Scale manufacturers blindly cater to the needs of users, regardless of the on-site The use environment and conditions of the electronic scales are competing with each other for the high accuracy of their products, so that the calibration graduation number of electronic scales is n = 4000, 5000, 6000, 8000, or even 10000 or more, so that the metrology and calibration institutions cannot strictly implement the calibration regulations and have to If left unchecked, manufacturers of scales that strictly comply with measurement regulations and standards will have no market, while electronic scales that do not comply with regulations will have a huge market. So why do (OIML) R76 and JJG555-96 "General Calibration Regulations for Non-Automatic Scales" stipulate that the maximum calibration division n value of electronic scales cannot exceed 3000?
(1) Let’s start with the definition of the weighing instrument itself: A measuring instrument that determines the mass of an object through the gravity acting on the object is called a weighing instrument. Gravity is the earth's gravitational force on objects. The same object experiences different gravitational forces at different locations on the earth. The further away from the center of the earth, the smaller the gravitational force it experiences, and the smaller the gravity. The object also obtains a certain acceleration g under the action of gravity, so
The weight of the same object measured at different locations on Earth must also be corrected for acceleration.
(2) The load cell is the core component of the electronic scale. For each load cell, its maximum division number nLC should not be less than the calibration division number n of the electronic scale, that is, nLC ≥ n. This essentially limits the performance of the electronic scale. Graduation number. At present, the sensors of domestic electronic scales are generally C3 level sensors. According to the requirements of (OIML) R60 international recommendations, the maximum calibration graduation number nmax of C3 level sensors is 3000. According to (OIML) R76 international recommendations 4.4. 4 Error distribution principle, C3 level sensors are used to manufacture level III scales with calibration divisions not greater than 3000. The electronic scale uses a C3 level load cell, and the scale can only have a maximum calibration scale of 3000. Therefore, the measuring range is fixed, and if the scale value is reduced, the scale number will become larger. If it exceeds the range of 3000, the performance of the load cell will be exceeded.
(3) Influence of temperature, humidity and atmospheric pressure. Under normal use conditions, the applicable temperature range of electronic scales is - 10℃ ~ + 40℃. The actual situation is that under open-air conditions under the scorching sun in summer, especially the surface temperature of the upper cover or rail of an electronic truck scale or rail scale without sunshade measures is generally nearly 20 to 30°C higher than the ground air temperature, while the temperature of the electronic scale is The device is very sensitive to temperature, and small reactions in temperature will have a great impact on weighing accuracy. The influence of humidity and atmospheric pressure will also affect the weighing accuracy. Especially during the rainy season in our southern region, the atmospheric humidity of electronic scales used in the open air is much greater than 85%. In some areas, the actual water vapor even reaches saturation during the rainy season. status, the relative humidity is close to 100%.
In addition, the influence of wind, rain and snow, as well as the influence of uneven force on the scale body, interference and distortion in signal transmission, voltage fluctuations, the effect of wind and dust, industrial vibration, electromagnetic interference and many other objective factors will bring harm to the electronic scale. There is a certain weighing error. Therefore, it is objective to set the maximum calibration division number of the electronic scale as less than or equal to 3000.
To sum up, it is necessary to clearly realize that any idea of improving the accuracy of an electronic scale by adjusting the scale value is not advisable. Blindly adjusting the graduation values of some electronic scales to smaller values and pursuing high accuracy of electronic scales will be counterproductive and affect the stability and accuracy of electronic scales. Only by strictly implementing the calibration regulations and operating according to the calibration regulations can electronic scales be made better and more efficient.
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