Important Precision Scale Terminology You Need to Know

Digital scales and analytical balances play a crucial role across a broad spectrum of industries to help maintain high GLP and GMP standards in pharmaceutical labs and manufacturing, or in any industry where the need for weighing accuracy is paramount.  Written operating procedures governing accurate weighing and record keeping go a long way in supporting any organization’s goal to achieve excellence.

What I’ve found is there are discrepancies in how certain digital scale manufacturers describe the performance of their products.  Incorrect scale specifications and term definitions can lead to problems when these scales are part of precision manufacturing processes.

A case in point:  one website I checked equates readout to accuracy.  Readout is what you see on the digital display.  A long row of digits may look impressive but if the scale is not calibrated properly it most likely is not accurate. “Accuracy” as defined by weighing-systems.com is the extent to which the weight readout of a scale (the numbers you see) approaches the true value of the quantities being weighed – or counted, or whatever the case may be.

The resolution of the readout defines the preciseness of the weight displayed.  Kern Models ABT 120-5DM and 220-5DM analytical balances, for example, provide resolution to 0.01mg for load ranges of 1mg to 32 and 82mg respectively.  These scales, by the way, provide automatic calibration under four separate scenarios. As above, the key word is “calibrated.”  This is not to be taken lightly as it is the method by which laboratory and production personnel take steps to assure the digital scale maintains its accuracy.

There are other terms that are important to understand when making purchasing decisions for digital scales and balances.

Calibration, for instance, is not the same as adjustment, although they would appear to be the same. Calibration is defined as determining the relationship between the displayed value and true mass by comparing that with a known mass.  During calibration no intervention occurs that would change the parameters of the weighing instrument. Calibration can occur automatically or by using calibration weights such as used to calibrate Kern moisture analyzers. Moisture analyzers also require periodic temperature calibration.

Should there be a deviation between what you see on the digital display and the known calibration weight, then an adjustment – or intervention – is necessary to bring the scale into line within maximum permissible errors. Activities that may call for a calibration and possible adjustment include moving balances to different locations, changes in room temperatures and being disconnected from a power source.  Some balances automatically calibrate after X hours of operation.

Getting it right also requires care on the part of the digital scale operator.  Not unknown is what is called an eccentric loading error or corner load error.  This is the change in readout when the same object is placed in various positions on the weighing pan.  Certain models of precision balances are designed to compensate for off-center loading, but GLP and GMP procedures should be such so as not to tempt errors.

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Can you recall of instances where scale specifications were incorrect?  What were the outcomes of incorrect readout vs. accuracy?  What are some of your more challenging weighing or counting operations and how often to you (1) calibrate and (2) adjust your balances?

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