Minimal Sample Weight and Quality Management
Only very small quantities are used in many applications and, thus, only a small part of the balance's weighing capacity is used. However the lower the weight the greater the relative measuring uncertainty. The minimum weight which is required is determined on the basis of the QM criteria and of the statistical data from repeated weighing procedures. The minimum weight which is required is determined on the basis of the QM criteria and of the statistical data from repeated weighing procedures.
If the weight is below the minimum weight a warning is appears on the balance display warning you of this; these values are also marked in the printout.
The required minimum weights should be determined on the basis of the QM specifications by means of the statistical analysis of certain series of measurements.The balance's own Statistics function could be used for this purpose, for example. It is used to perform and subsequently log the requisite series of measurements. Once the minimum weight(s) has/have been determined, it/they can be input into the balance. Up to three tare ranges can be defined with the corresponding minimum weights.
The balance's weighing modes, as well as measuring time and stability, are also fixed in such a way as to guarantee compliance with tolerances in future measurements. Tare ranges, minimum weights and weighing modes cannot be changed by the user.
The values which are input can be logged by means of an configuration status print and could, along with the report on the statistical measurement series, be used as a QM certificate.
If work is performed on the basis of the minimum weight application, this serves to ensure that the weighing results conform to the certificate specifications and, thus, to your QM guidelines.
The minimum weight is dependent on the ambient conditions. Therefore, it must be determined on-site and must be reviewed periodically.The following parameters influence the minimum weight:
MSW
Can be activated by switching on/off
Minimum Weight
Sets the minimal sample weight 1/2/3 for tare range 1/2/3
Tare Range
Sets the tare range 1/2/3
It's often required for QM reasons to renew the minimum sample weight periodically. This can be accomplished by selecting a expiration date as depicted below
Expiration date
The following parameters influence the minimum weight
Repeatability
The repeatability is determined by the standard deviation of the balance on-site. It is determined by the ambient conditions, the nature of the goods being weighed and the balance settings.
Uncertainty (U)
The tolerable uncertainty is determined by the user or defined by standards.
Extension factor (k) (generally 2 or 3)
The extension factor determines the likelihood of occurrence. The factor is defined by the user or is defaulted.
Calculation
The minimum weight is calculated as follows:
Minimal Weight = k * S/U
USP Chapter 41
If work is conducted in accordance with the USP 41 (United States Pharmacopoeia), the workflow differs from the previously decribed procedure. USP requires that the repeatability is determined based on at least 10 comparable weighed values. Additionally the accuracy of the balance must be determined. To determine the standard deviation according USP 41 the following steps are required :
1. Determined the repeatability (Precisa recommeds 10 measurements at 50% of the capacity)
2. Calculate the smallest possible weighin sample :
2a. If the standard deviation < 0.41 digits :
The smallest sample weight is 820 digits
2b. If the standard deviation is >= 0.41 digits :
The smallest sample weight is the calculated as follows : 2 * repeatability*1000
4. Check the accuracy of the balance by using a weight (maximum permissible error) of 0.03%. The displayed value must not differ more than 0.1%
Remark : The value of the weight must be within 5% and 100% of the capacity
Notes
Carry out the tests locally and if possible under the conditions that normally prevail.
Ensure the best possible environmental conditions.
Make sure that these conditions will not deteriorate significantly during normal operation.
First configure the weighing parameters.
Define the due date for the next review according to your QA requirements.
Define the coverage factor and relative uncertainty according to your QA requirements.
First check the behavior of the scale in different tare ranges: Divide e.g. scale the scale into 10g sectors and then measure repeatability for each sector.