Q.What kind of mechanisms do balances with built-in calibration weights incorporate to perform calibration? Also, when a balance with built-in calibration weights is used, aren't external weights needed when performing ISO 900, or GLP and GMP inspections?
A : Balances with built-in calibration weights have weights for calibration (or "plumbs" as defined in metrology), and are designed with the capability to place and lower these weights internally by a motor-driven mechanism. These "plumbs" themselves are originally not assigned mass values. The load value when a "plumb" is placed is compared with the load value when a standard external weight is placed, and the resulting value indicating how equivalent the "plumb" is to the mass of the external weight is stored to memory. Thus, the balance's internal "plumbs" can be substituted for standard external weights for calibrating the sensitivity of the balance.
Once it has been verified that the value obtained by calibration using a standard external weight and the calibration value obtained by using built-in weights are the same, then it is considered that deviation during regular use will hardly occur. So, daily inspection can be conducted by calibration using built-in weights (plumbs) based on this assumption.
Nevertheless, as one might expect, there is the possibility that deviation due to various factors might occur over prolonged use. So, it must be periodically verified that calibration by built-in weights (plumbs) is being performed correctly by using external weights. Periodic inspection also includes overall inspections that cannot be supported by daily inspection alone in addition to this inspection.