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Eliminating Weighing UncertaintyAdds Confidence to Manufacturing Process

Contents

1 Measurement Uncertainty in Weighing

2 Factors Affecting Uncertainty

3 Impacts of Uncertainty

Determining which scale and maintenance schedule is right for an application, location and applicable regulations can be daunting, but a new methodology makes it easy to eliminate weighing uncertainty.

The industrial scale plays a critical role in many manu-facturing facilities. From measuring raw ingredients to verifying final product packaging, scales are used throughout the entire manufacturing process. If a scale that is not suited for a particular application is used, or if it is not maintained correctly, weighing can be inac-curate, significantly hurting a company’s profit.

2 METTLER TOLEDO - White Paper - GWP® Verification © 2013 Mettler-Toledo, LLC

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Any measuring device, whether it is a ruler, a speedom-eter or a scale, is associated with some measurement uncertainty. Uncertainty means that no measurement is perfect; it is always distorted by random errors and unknown systematic errors. It is often expressed as an error associated with the weight (10 lbs. ± 0.001 lbs.).

As with any measurement device, uncertainty directly affects the accuracy of the reported value in a weigh-ing system. The error is less significant when you are weighing something large relative to the capac-ity of the scale. However, as you attempt to weigh something small on the scale, the relative uncertainty becomes more significant. At some point, the relative uncertainty is so significant that you lose your confi-dence in the reported weight value.

For example, suppose you have a scale that is accu-rate to plus or minus 1 gram. At 10,000 grams (10 kg.), this uncertainty represents one hundredth of one percent (0.01%) of the weight. In many situ-ations, that uncertainty is small enough that it won’t affect quality. Now suppose you are weighing a 10 gram sample on this scale with an uncertainty ± 1 gram. Now the uncertainty represents a full 10% of the reported weight. Your actual sample may be 10% larger or 10% smaller than what this scale is reporting just due to the uncertainty!

In the same way that you can’t use a standard 12-inch ruler to measure the width of a human hair, you can-not use a truck scale to weigh a feather accurately.

U [kg] = U0 + Constant x Weight Absolute measurement uncertainty [kg]

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Relative measurement uncertainty [%]

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Subject to technical changes©01/2013 Mettler-Toledo AG

www.mt.com/GWP

3 Impacts of Uncertainty

The accuracy of your scale directly affects qual-ity. Using the correct scale for your application and maintaining it properly will ensure the quality of your product.

Good Weighing Practice™ (GWP®) methodology from METTLER TOLEDO provides the necessary information to make decisions about your weighing system. By testing the scale in place, weighing experts can cer-tify the weighing process, meet final product quality requirements and ensure the scales are proven to be fit for their intended use. GWP® Verification can also identify weighing installations that may not be suited for the particular application and could affect quality.

Finally, testing in place can determine a proper main-tenance schedule for the weighing application. The

measurement uncertainty of a scale will change over time, so proper maintenance is critical. By maintain-ing the scale on the proper schedule, you can ensure the accuracy of the scale throughout its lifetime. After evaluating a scale system, it is common to find scales being maintained more often than necessary. The result is extra downtime and cost that isn’t needed to ensure accuracy. In other situations, scales are being under-maintained resulting in questionable results over time.

The scale is a key component in many operations. Be sure that you have the correct weighing system for your application and the best maintenance schedule to ensure a long life of accurate results.

2 Factors Affecting Uncertainty

The measurement uncertainty of a weighing system is a combination of many factors. The readability, sensi-tivity, repeatability, non-linearity and eccentricity of the scale are all factors affecting measurement uncertainty and are associated with the scale design. The scale manufacturer can calculate the uncertainty of the weighing system associated with these factors.

However, the environment in which the scale is used also impacts the uncertainty of the weighing system.

The environment is unique to every scale installation and cannot be calculated at the time the scale is man-ufactured. Significant impacts from the environment on weighing uncertainty can be attributed to wind, dirt or dust; temperature fluctuations; vibrations; and operator errors among other factors. The only way to calculate the measurement uncertainty associated with the envi-ronment is to test the scale installed in the environ-ment using the appropriate tools and methods.

GWP® is the only science-based global standard for efficient lifecycle management of weighing systems, applicable to all kinds of weighing systems of any manufacturer. It helps you to

• choose the right balance or scale• calibrate and operate your weighing equipment with security• comply with current quality, compliance, laboratory and manufacturing standards