How to Choose Test Weights for Analytical Balance Recalibration

Analytical balances are precision instruments providing readouts* to an accuracy of 0.1 milligram and with a linearity** of ± 0.2 milligram. Maintaining this precision is essential to GLP and to be in compliance with regulations guiding the particular industry in which the instruments are used.

For that reason, an analytical balance must be recalibrated at regular intervals and records maintained in further conformance with GLP and ISO standards.  Some analytical balances such as the KERN ABT, ABJ and AEJ models available from Tovatech provide automatic internal calibration.  Others such as the ALS-A model are externally calibrated at intervals that should be defined in the company’s operations manual.

Precision scales and other weighing instruments likewise must be recalibrated on a regular basis.  If internal calibration is not provided, external calibration is performed using test weights.

Selecting Test Weights for Recalibration

As noted by KERN a balance can never be more accurate than the weight being used to adjust it.  Test weights, like balances and scales, are available in various classes of accuracy and priced accordingly.  For that reason the accuracy of a test weight should correspond to, rather than exceed, the readout of the balance.

KERN recommends that the size of the test weight should correspond to the heaviest weight accommodated by the equipment being calibrated.  This is shown on the display when the unit is in the “CAL” mode.  Once the size and accuracy or tolerance in ± milligrams or grams is specified, the weight can be selected based on International Organization of Legal Metrology accuracy classes E1 to M3.

As an example, for a balance with a maximum capacity of 2000 grams (2 kg) and readout of 0.01 gram (10 mg) the accuracy of the required test weight should be ±10 mg.  This balance will display in the “CAL” mode a choice of 1000 or 2000 grams.  The test weight should weigh 2 kg.

The importance of accuracy in analytical balances and precision scales cannot be overemphasized.  That’s why it is important to be certain that your recalibration procedures are in line with GLP and ISO recommendations.  Call on the experts at Tovatech as a source for test weights with internationally valid traceable DKD Calibration Certificates from Kern’s accredited calibration laboratory.   Note that test weights which are used intensively should be recalibrated every 6 months; weights used infrequently should be recalibrated annually.

What are your company’s procedures for recalibrating analytical balances and precision scales?

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*The smallest weight difference that can be displayed.

** The positive or negative deviation of the readout from the actual load.

About Rachel Kohn

So how did an MIT Ph.D. end up selling refrigerators? When I figured out that a lot more scientists buy lab refrigerators than innovative leading-edge instruments. I hope that my many years of lab experience will help you find the right equipment for your work. Before co-founding Tovatech I worked in business development and project management at Smiths Detection, Photon-X, Cardinal Health, and Hoechst Celanese. And before that I spent 12 years as an R&D chemist at Hoechst Celanese and Aventis working on advanced drug delivery systems, polymer films and membranes, optical disks, and polysaccharides. Some day, eventually, I’ll make enough money to develop an innovative technology that will change the world. Read More