Abstract: This paper analyzes and expounds the specific content, fault phenomenon, influencing factors and repair methods of the balance measurement performance.
[Keywords] stability sensitivity correctness invariance The measurement performance of fault-maintaining balances mainly includes stability, sensitivity, correctness and invariance of the balance indication. These four characteristics are interrelated and inseparable. Stability refers to the ability of an already balanced balance to be automatically disturbed to its original equilibrium position after being disturbed by external forces and leaving the equilibrium position. The better the stability, the more stable the balance. The stability of the balance depends on the position of the center of gravity of the balance beam. When the center of gravity of the beam is at a moderate position below the fulcrum, the balance is more stable (of course, the position of the center of gravity of the beam is moderate below the fulcrum, not the lower the better, otherwise it will destroy the sensitivity of the balance). Conversely, if the position of the center of gravity of the beam coincides with the fulcrum or above the fulcrum, the more unstable the balance, the worse the stability. Since the stability of the balance is closely related to sensitivity and indication invariance, the measurement of the balance only requires verification of the three metrological properties of sensitivity, correctness and invariance. The sensitivity, correctness and immutability of the balance are three common measurement phenomena and the causes and adjustment methods are:
1. Sensitivity Sensitivity refers to the ability of a balance to reflect small changes in weight or mass. It also refers to the ability of a balance to detect the amount of change in the mass of an object placed on a weighing pan. It is usually expressed in terms of sensitivity, which is the specific magnitude of the sensitivity of the balance. After the balance is balanced, the distance the pointer moves on the label is called sensitivity when each 1 milligram weight is added. The sensitivity of the left and right discs of the balance at no load and full load refers to the ratio of the linear displacement or angular displacement of the tip of the balance pointer to the displacement mass produced. The sensitivity of the balance is independent of the mass of the substance referred to in the two plates. The balance indexing sensitivity is the number of divisions of the balance sign caused by the unit mass. The greater the distance the balance pointer moves (that is, the more the offset number is offset), the higher the sensitivity, the more sensitive the balance. The index value is the quality value that causes the unit index, so the smaller the mass value, the better the sensitivity of the balance.
The fault phenomenon, the influencing factors and the repairing methods of the balance's sensitivity failing to reach the technical indicators are:
(1) The center of gravity is too high or too low. It can be solved by adjusting the position of the center of gravity ball by lowering or raising it. (2) The knife and the knife are broken or worn seriously. The knife and knife can be replaced. (3) The ambient humidity is too large. The balance chamber should be dry, bright, and the temperature fluctuations are not large, to prevent water vapor and corrosive gases from entering the room, and the window is open to the north. In order to avoid uneven heat and cold between the two arms of the balance, the heat source should be separated by appropriate methods around the balance. It should be kept at an appropriate distance from heat sources such as heating, electric furnace, oven, lighting, etc., and electric fans with high power, such as exhaust fans and air conditioners. The single arm of the balance is affected by cold and hot air. (4) Technical problems in installation of various components. The correct installation and use of the balance has a direct relationship between the measurement performance of the balance and the service life. The measurement performance is reduced, and the weighing error will occur, which will affect the measurement result. Therefore, the balance should be placed on a stable, firm, and off-wall cement platform. If the surrounding environment is vibrating and cannot be eliminated, the weather should be placed on a table with anti-vibration protection function to reduce the balance and protect the blade. . (5) Inconsistent air and full load sensitivity Fault: 1 Offline: Full load sensitivity is lower than no load. The three-blade can be removed by raising the edge cutter. 2 eating line: full load sensitivity is higher than no load. The three-blade can be removed by flattening the edge cutter. (6) The sensitivity of the left and right discs is inconsistent. Fault: The friction between the two sides of the blade and the cutter bearing is inconsistent. The position of the middle knife and the knife holder can be adjusted. If the wear is serious, the middle knife should be replaced. 2 If the length, hardness and sharpness of the two sides of the knife are inconsistent, the two sides of the knife should also be replaced.
Second, the invariance indication invariance is to turn on the balance multiple times without changing the existing state of the balance, and the reproducibility of its equilibrium position. It also indicates the consistency or reliability of the weighing result obtained by weighing the same object continuously for the same object under the same conditions. Invariance of indication value For multiple measurements of the same object, the smaller the variability of the measured indication value, the better the invariance of the indication of the balance, and vice versa, the worse the invariance of the indication of the balance. The magnitude of the invariance of the indication is related to the position of the center of gravity of the balance, and also related to various factors such as the adjustment of the beam itself, especially the three-knife adjustment state, operation, temperature difference, airflow, vibration, and static electricity. Generally, the smaller the variability of the balance, the better.
The influencing factors of the invariance of the indication value and the technical indicators are not affected by the technical indicators:
(1) The texture of the knife and the knife itself is not good, and the expansion coefficient of the beam material at each point is inconsistent. It can be solved by replacing the knife, the knife holder and the beam. (2) Technical problems of assembly. When installing, the three cutting edges are not parallel, the precision of the knife bearing is not enough, the braking position is not accurate, the eating is too large, the force of each beam is uneven, and the tightening degree of each screw is inconsistent. The solution can be eliminated by reinstalling correctly. (3) No-load variability. Mainly caused by mechanical transmission failure, it can be eliminated from the mechanical transmission part. (4) Full load variability. Mainly because the three blades are not horizontal and non-parallel, the parallelism of the three blades can be adjusted so that the blade is perpendicular to the centerline of the beam and in a plane. (5) Impact of working environment, there are vibrations, heat sources, air currents, excessive humidity, static electricity, etc. in the indoor environment of the balance, which can be solved by improving the environment of the balance chamber. (6) The operator's operation method is improper. It can be ensured by strengthening training, improving the operation level of the post operators, and strictly abide by the operating procedures and other management methods.
3. Correctness Correctness is usually called the unequal arm or deviation of the balance. It is used to indicate whether the difference between the lengths of the two arms of the balance beam has a correct fixed proportional relationship. For an equal-arm balance, the difference in the length of the two arms of the beam (deviation) should meet certain requirements, and its correct proportional relationship is 1:1. If the two arms are not equal, the measurement result will be inaccurate, and as the load increases, the unequal arm error will also increase. For single-disc balances, because the beam design requirements are in the form of unequal arms, there is no unequal arm error.
For an equal-arm balance, the weights of the same mass are placed in the two weighing pans of the equal-arm balance. In theory, the moments on both sides of the balance should be equal and balanced. In fact, the lengths of the arms of the equal-arm balance are not absolutely equal. Although the weights of the two plates are exactly the same, the balance pointer will not completely return to the equilibrium position at the time of no-load, and will always deviate from the original equilibrium position. The gap, the magnitude of the deviation increases as the load increases.
There are many reasons for the unequal arm (deviation). (1) The technical level of assembly. (2) The two-arm expansion material of the beam is not equal, the density of the manufacturing material is not uniform, the length of the left and right arms of the beam is different, and the deformation of the beam is affected. (3) The influence of ambient temperature, the indoor temperature is not uniform, so that the balance is heated or cooled by one arm or part.
In short, in order to ensure that the measurement performance of the balance meets the technical requirements, the stability of the balance must first be guaranteed, and secondly, the measurement requirements of the other three performances can be guaranteed. The balance of the four metrology performances complement each other, affect each other, and restrict each other. Only by ensuring the good performance of the four majors can the balance weighing data be accurate and reliable.
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