【 Xiangyi 】 Several problems of electronic balance measurement verification...

2013-01-18 12:00

The balance made by using electromagnetic force to balance the principle of gravity is called electronic balance. As the laboratory application becomes more and more common, the accuracy and reliability of its measurement become more and more important. As early as 1991, the "JJG98-90 Non-automatic balance measurement Verification Regulations" (trial) promulgated by the state, the description of electronic balance measurement verification is relatively general, and the operability is not good, so that the personnel engaged in electronic balance measurement verification work, the understanding of the regulations is not very thorough. Especially in the evaluation of an electronic balance "qualified or not" the scale is not the same, so that the accuracy of the instrument measurement, the reliability of the parameters can not be guaranteed. In order to effectively control the quality of electronic balance in practical application, this paper discusses the metrological verification of electronic balance and the problems that should be paid attention to. 1 How to grade the electronic balance and determine its tolerance At present, the production of electronic balance manufacturers are more, there are individual manufacturers in the product description failed to identify its performance indicators in detail, or the identification is not standardized, not uniform, some give the level, some do not give the level. For example, some electronic balances clarify the actual scale index value d, but do not indicate its verification index value e, which for users will mistakenly think that d = e, that the electronic balance can distinguish the smallest value is the accurate value that it can weigh itself. For metrological verification personnel, it is very important to determine the calibration scale value e of electronic balance, because e is used to evaluate its accuracy level and the maximum allowable error basis: In metrological verification, if the maximum error of each parameter index is not greater than 1d, we determine e = d; If the maximum error of each parameter index is less than 10d, we determine that e = l0d. Sometimes it needs to be determined according to the specific situation, such as when d: 0.2 mg, e = 5d; When d: 0.5 mg, e = 2d. In summary, the division of the scale value of the verification scale should be in the following form: l×l0k or 2×10k or 5×10kk is a positive integer, a negative integer or a zero). All cases except e = d are classified as ≠d, in which e = l0d is the most common. The following is a summary of the classification of the electronic balance and its maximum allowable error according to the verification scale value e. In the daily periodic verification and routine product quality inspection, it is generally necessary to verify the following contents: the verification of balance sensitivity and discrimination; Verification of the maximum allowable error (weighing linear error) of each load point of the balance; Verification of balance repeatability; Verification of balance partial load or four Angle error; Verification of balance balancing function. Table 1 Balance classification and maximum allowable error The author believes that the standard electronic balance used as a quantity value transfer is concerned. The error standard should be determined according to the error standard of "first verification". 2.1 Verification of electronic balance sensitivity and identification The sensitivity of electronic balance generally refers to the scale sensitivity, which should be numerically equal to the verification scale value of the corresponding load of the balance. For electronic balances with digital indications and automatic or semi-automatic calibration devices, the sensitivity of the balance can be exempted. When the electronic balance tests the index value e≥lmg, its discrimination can be determined by the following method: In the no-load or loaded electronic balance in the balance state, an external load (1.4d) equivalent to 1.4 times the digital scale index value, gently placed on the balance plate (or removed from it), the original balance value must be changed. 2.2 Verification of the maximum allowable error of each load point of the electronic balance first start up and preheat, and then calibrate the electronic balance according to the operating procedure of the manual (this step is very critical). After calibration, the zero position is displayed. Starting from zero load, gradually monotone loading up until the maximum weighing value of the balance is added, and then gradually monotone unloading until zero load. In the verification process, the verification operator selects the load point according to the specific situation of the balance (which is not clearly given in the verification regulations), but the following load points should be verified: (1) no load. (2) Full load. (3) Minimum weighing Min (Class I balance: 100e; Class II balance: 50e; Ⅲ level balance: 20e; Level IV balance: 20e). (4) The load corresponding to the "inflection point" that affects the error value of the balance, such as the class I balance: 50000e, 200000e; Class II balance: 5000e, 20000e; Ⅲ level balance: 500e, 2000e, etc. Error calculation is divided into two cases: (1) If e≠d, the calculation formula for indicating value error should be E = I-L (E: indicating value of the balance, I: indicating value of the balance, L load on the balance scale); (2) When e = d, the formula for calculating the indication error should be E = I-L+ (12) d-△L (d: the actual scale index value of the electronic balance, AL: the load added on the scale of the balance to round up the indication value). In either case, the error of each load point obtained is required to be less than the allowable error specified in the procedure. 2.3 Electronic balance repeatability verification Electronic balance repeatability verification should be carried out in no-load and loaded state, there are two kinds of load: one is full load, one is half load. It is required to read and record the balance position of loading and no-load respectively in the verification, and note that each load should be returned to zero once. It is required that the difference between the results of multiple measurements of the same load should not exceed the absolute value of the maximum allowable error of the balance at the time of the load. 2.4 Partial load verification of electronic balance (four Angle error verification) For standard balance, the test load is equal to the maximum weighing of the balance, and the four Angle error is equal to the maximum indication value minus the minimum indication value. For the working balance, the test load is equal to one-third of the maximum load of the balance, and the four Angle error is equal to the largest difference between the indicating value of each point and the indicating value of the center point. 2.5 Check the balancing function of electronic balance The newly purchased electronic balance should be checked its balancing function, generally select two load points, namely: (13) Max, (23) Max. The difference between the two results obtained under the same load shall not exceed the absolute value of the maximum allowable error under the load. 3.1 How to select Standard weights The standard weights selected can be grade weights or equal weights, but its error (for grade weights is the mass tolerance, for equal weights is the verification accuracy) shall not be greater than 13 of the maximum allowable error of the balance under the load. Secondly, the range of the standard weight can cover the maximum weighing range of the electronic balance. 3.2 Exemption problem of electronic balance (1) When d≤1mg electronic balance, the discrimination of the balance can be exempted; (2) When e≠d electronic balance, the discrimination of the balance can be exempted; (3) For balances with digital indications and automatic or semi-automatic calibration devices, the sensitivity of the balance can be exempted. 3.3 Verification Certificate should be standardized electronic balance verification after the completion of verification, verification certificate or verification result notification should be issued according to the actual verification results. It is considered that the issuance of certificates is a very serious and responsible matter, first of all, the format must be standardized, and the content must cover all the information of "4.1" elements in GJB2725-96  The results of calibration (verification) or testing should be accurate and objective.

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