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发表于 2015-11-19 16:16:35
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非常感谢李老师的总结。 我正在查阅 USP 36 <1251 WEIGHING ON AN ANALYTICAL BALANCE> 的内容，其中提到“
QUALITY ASSURANCE PROCEDURE FOR MEASUREMENT OF BALANCE DRIFT
Over an extended period of time, balance drift and other day-to-day variations are monitored by weighing a fixed check-weight on a regular basis; this check should be performed after the balance has been calibrated at the ambient laboratory temperature. The check should be made before
the first weighing of the day or after any event that might disturb the balance’s calibration (power failure, moving the balance to a new location, etc.). The check-weight may be any object whose mass remains constant and does not exceed the load limit of the balance. A balance weight makes a reliable check-weight. Each balance should be provided with a check-weight, which should be stored in a protective container near the balance.
Perform the following procedures to reduce balance errors and the possibility of an incorrect reading because of drift:
1. Make certain that the electrical power to the balance is on and that the level bubble is in the center of the interaction
2. Calibrate the analytical balance or the microbalance.
3. The first person to use the balance each day should weigh the check-weight and record the weight in the log book for comparison with previous readings. If a deviation greater than those indicated below for Analytical Balances and Microbalances is observed, the balance should be reported for service.
。。。。
The check for drift at most sensitive position will show whether a problem exists; the variation in the observed weight does not exceed±0.2 mg. For example, with a 20-g weight, if the mean value of the readings were 19.9984, the tolerance would be from 19.9982 to 19.9986 g. Thus, several readings must be taken before one can establish a tolerance. [NOTE—The check-weight need not be of high accuracy, but it is essential that its mass remain constant. In addition, the tolerance does not correspond to the value of 0.1%, specified under Weights and Balances á41, for weighing material accurately. Rather, the tolerance is purposefully tight to reveal possible drift or calibration errors; this tolerance is readily achievable with modern electronic balances.] 以上看出USP对日常检查用的砝码本身的准确性并不高，只要保持恒定即可。（甚至都不用一定是砝码），但对分析天平的漂移限度似乎非常紧，只有0.2mg。另外，关于分日校和校准的顺序，我之前也觉得应该先做日校，但USP上我怎么看它的意思是先做校准，再check。 请问李老师这该如何理解。
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