Setting Minimal Sample Weight

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. 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.

If the weight is below the set minimum weight a warning appears on the balance display warning you of this; these values are also marked in the printout and in the statistics.

The required minimum weight 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, as it performs and subsequently logs the requisite series of measurements. Once the minimum weight has been determined, it can be input into the balance.

The balance's weighing parameters settings (refresh rate, environment etc.) are also fixed in such a way as to guarantee compliance with tolerances in future measurements and can only be changed by a user with Administrator or Quality Manager access rights.

Information about the used MSW settings could, along with the report on the statistical measurement series, be used as a QM certificate.

The minimum weight is dependent on the ambient conditions. Therefore, it must be determined on-site and must be reviewed periodically.

Tho make use of the “Minimal Sample Weight” you need to setup this in as below.

MSW Off / On

Minimum weight [g]

Sets the minimal weight as determined and calculate as described below.

Refresh rate

Fix this setting as used to determine the repeatability to calculate the minimal weight. For more information see the settings in “Weighing parameters”.

Environment

Fix this setting as used to determine the repeatability to calculate the minimal weight. For more information see the settings in “Weighing parameters”.

Filter profile

Fix this setting as used to determine the repeatability to calculate the minimal weight. For more information see the settings in “Weighing parameters”.

Value release

Fix this setting as used to determine the repeatability to calculate the minimal weight. For more information see the settings in “Weighing parameters”.

Notes

Space to enter notes.

Expiry date

It's often required for QM reasons to renew the minimum sample weight periodically. This can be accomplished by setting an expiry date.

Expiry warning

Time when a warning is displayed in advance to the expiry date.

Calculation of the minimum weight

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 Pharmacopeia), the workflow differs from the previously described 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 recommend 10 measurements at 50% of the capacity)

2. Calculate the smallest possible weighing 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

3. 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