Selecting Calibration Weights

Selecting Scale Weights for Calibration Testing

Calibrating precision digital scales, analytical balances and moisture analyzers that rely on accurate weight measurement requires periodic tests using a scale weight closely matching the maximum capacity of the instrument being tested.

Why Testing with Scale Weights is Important

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

Daily we depend on weighing accuracy – such as 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.

But in research labs and in food, drug and other production facilities, weighing accuracy assumes a highly critical criterion.  This applies to weighing and measuring instruments where readout (the smallest weight difference displayed) and linearity (the ± deviation from the actual weight) are governed by GLP, ISO and other standards.

Adding to challenges, the accuracy of precision weighing and measuring instruments can be compromised by changes in location, fluctuations in ambient temperature, use and other factors. These points are covered in our post on weighing accuracy. While it focuses on analytical balances, the importance of calibrating digital scales and moisture analyzers is no less important

This is why testing instruments by using a properly matched scale weight is important. They are required when instruments with capacities from milligrams to tons are not internally calibrated. (The type of calibration, whether internal or external, is indicated on instruments’ spec sheets.)

Selecting Calibration Weights for Digital Scales

As noted above, when using an externally calibrated instrument 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. 

Once the test weight is specified, the scale weight itself can be selected based on seven 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.

Scale Weight Care and Calibration

Calibration test weights, as indicated earlier in this post, are manufactured to exacting conditions because, as noted above, the equipment will never be more accurate than the scale weight used to adjust it. Test scale weights can represent a substantial investment to be protected by proper handling, care and periodic recalibration. 

As with the equipment they test, test weights should also be recalibrated as part of ongoing GLP. The frequency of recalibrating test weights depends on the frequency of use, the conditions of use and the security needs of your organization in terms of standards compliance. 

Tovatech suggests that scale weights used intensively for testing should be recalibrated at least every six months, otherwise at least once a year. Recalibration is done under strict laboratory conditions. Weights that pass are issued a recalibration certificate detailing the exact conditions under which the recalibration was made.

Sometimes test weights that fail recalibration can be brought back into tolerance limits by repair or remanufacture. Otherwise, they must be replaced. 

Usage procedures for test weights should be clearly specified relating to handling and storage. 

For example, acids on your hand could cause etching that could invalidate the accuracy of a 1 mg test weight – especially one with a tolerance of ± 0.003 mg.

For this reason gloves should always be used when handling scale test weights. Gloves, along with tweezers and dust brushes are supplied by companies such as Kern as a further means to promote safe handling.  Store scale weights in fully lined wooden boxes and milligram weights in plastic boxes.

Precision scale weights for testing can be obtained individually or in multiple-unit sets. Tweezers, gloves, dusting brushes and individual or multiple-unit storage container and carrying cases are also available to maintain and protect these precision test weights. These help organizations meet internal or external guidance procedures supporting precision weighing and measuring requirements.

For additional information please check our post on how to calibrate a digital scale. It covers a lot of useful information including purchasing considerations, equipment protection, and proper use.

Need More Info on Scale Weights for Testing?

Contact the weighing professionals at Tovatech for help in selecting scale weights to calibrating scales, balances and moisture analyzers used in your organization.

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