Selecting Calibration Weights for Digital Scales

Test weights can be purchased individually or in sets.
Test weights can be purchased individually or in sets.

Calibration weights for digital scales are key elements to ensure, in the simplest sense, that you get what you pay for.  For many of us this is exemplified by visits to the deli counter in the supermarket.  We want assurance that the weight shown on the scale is accurate.  The U.S. Government’s Weights and Measures Division sets the guidelines.  Here we describe the role of calibration weights for digital scales in keeping scales accurate.  

But first….

Why Digital Scale Calibration is Important

Granted, we all want assurances that the weight displayed by a scale is accurate.

But it does not take much to throw off the accuracy of a digital scale.  We cover this in our post on weighing accuracy.  While that post focuses on analytical balances, the importance of calibrating digital scales is no less important.

It is absolutely essential that all steps needed are taken to ensure precision digital scales consistently deliver accurate readouts (the smallest weight difference that can be displayed for a particular model).   

For example, the Kern PCB 2500-2 digital platform scale with a maximum capacity of 2500 g has a readability of 0.01 g.  The industrial digital platform scale DS 10KO.1S has a maximum capacity of 10 kg and a 0.1 g readout.

While certain analytical balances referenced in the above link offer internal calibration, digital scales require the use of external calibration weights to check their accuracy.  These weights are purchased separately.

External calibration of digital scales, by the way, is not that complex.  Operators do not have to perform adjustments during the exercise. 

Procedures are spelled out in operation manuals and usually consist of selecting the “calibration” mode, placing the correct calibration test weight (more below) on the weighing plate and activating the procedure.  The digital scale will adjust itself.

Selecting Calibration Weights for Digital Scales

When using an externally calibrated digital scale the calibration weight should be as close as possible to the maximum weight capacity of the scale.  Recommended test weights are provided in user manuals. 

But there is more.

Once the test weight is specified, the weight itself can be selected based on 7 International Organization of Legal Metrology (OIML) accuracy classes ranging from E1 to M3, where E1 is the most accurate and M3 the least. 

OIML recommendation R111-2004 covers multiple weight ranges from 1 mg to 5,000 kg showing maximum permissible errors (MPE) in terms of ± mg.  This increases to ± g as sample weights increase and accuracy decreases. 

As an illustration the E2 80 g MPE for a 100 g test weight is ± 0.16 mg.  The E1 MPE for this test weight is 0.05 mg; the M3 MPE is ± 50 mg.

Calibration weights for digital scales are also manufactured to precise standards and must be handled carefully.  Importantly, these test weights must also be recalibrated every six months if used intensively; otherwise annually.  You can learn more about this in our post that concludes with a description on the care and calibration of test weights.

And to help you pick out the scale best suited for your requirements, check our post on selection tips for precision digital scales. Please call the Tovatech scientists at 908 402-7243 for help in making your decision and as a source for calibration weights for digital scales with internationally valid traceable calibration certificates from Kern’s accredited calibration laboratory.

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