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Proper Specification Leads to Better Weighing

Image courtesy of Mettler Toledo Joseph_Wilt_METTLER_TOLEDO.JPG
Joseph Wilt, OEM account manager, Mettler Toledo
Several factors should be considered to ensure weighing devices are fit-for-purpose

Proper specification of weighing components during the initial stages of a project is critical to quality, safety, uptime, and return on investment. Several factors should be considered to ensure that weighing devices are fit-for-purpose

Four simple steps, along with proper consultation from your weighing supplier, can ensure that your weighing systems produce accurate batches, safely measure bulk material, and fit within your project scope and budget.

Step 1: Define Functional Requirements (weighing tolerance)

To select the proper components for your weighing application, start by defining the intended scope of application requirements. Meaning: what are these measurements for and how critical are they to decision making or downstream processes?

This could broadly be categorized in terms like inventory management (low criticality), filling/dosing into a vessel (medium criticality), quality control checks (medium criticality), or formulation (high criticality).  Understanding the criticality or importance of the measurements will help you determine an acceptable weighing tolerance. This is different from production process tolerance which defines the boundaries of pass/fail according to quality (i.e. the soup has too much salt or not enough salt to be acceptable for the brand). Whereas weighing tolerance is the acceptable amount of uncertainty in the displayed or reported weight. Measurement uncertainty is omnipresent, yet it largely gets overlooked as a source of production error. But if you define a weighing tolerance according to the criticality of the application you can avoid under specifying as well as over specifying. A low criticality weighing tolerance could be 2-3% measurement uncertainty on the displayed (net) weight. Versus a high criticality measurement should likely have <1% measurement uncertainty. For example, most pharmaceutical measurements are required to have 0.10% measurement uncertainty or less.

Step 2: Functional Design

To ensure weighing system accuracy, find a scale or combination of weighing components that delivers lower measurement uncertainty than your weighing tolerance at smallest target weight value. This means you must know the intended smallest measurement to be made. For example, the smallest ingredient in a batch. First, look at the manufacturer's specifications, and then calculate the measurement uncertainty of the weighing system based on manufacturer specifications (if they provide typical values such as repeatability). You can use a root mean square of specifications such as repeatability, error of indication, and eccentric load deviation if the manufacturer lists those values.

Avoid using weights and measures tolerances such as NTEP or OIML. They include allowable error as determined by lawmakers not scientists. The purpose is fair trade, not accurate trade.

Step 3: Asset Specification

Once measurement uncertainty of a weighing system is calculated, compare the findings against your weighing tolerance at the smallest target to ensure the system is fit-for-use. While this process may seem straight forward, pay close attention to some key points:

* Even though the measurement uncertainty has been calculated for the device this assumes that it is calibrated. Scales need to be routinely tested and calibrated if necessary. Doubling that measurement uncertainty value you calculated first before making the comparison to the intended target will give you some buffer room for gross errors.

* Prove to your quality department and regulatory officials that your scale is fit for purpose through documentation that includes the statement of measurement uncertainty for the entire weighing system.

4: Asset in Operation

Routine testing and calibration are required to maintain the accuracy of the weighing system. Scales and load cells are precision measurement devices that need to be periodically compared to reference standards to determine they are working properly. A routine testing plan ensures the right tests are being performed with the right standards at a frequency, which minimizes business risk. 

Selecting the proper device for your application is critical to efficient weighing, and maintaining accuracy through routine testing ensures the device is operating as designed. These four simple steps are proven to create weighing systems that produce better batches, more consistent products, and greater ROI. Talk with your weighing provider about how their products fit your application requirements to ensure you are making the right choice when selecting new or replacement weighing components.

Joseph Wilt is an OEM account manager at Mettler Toledo, providing tailored weighing solutions for all industrial applications.

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