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Comment PDF Processing & Handling

Weighing Equipment Selection

By Chemical Engineering |

The number of options available for scales and weighing equipment can make it difficult to determine which instruments will offer the best value and which will meet the application requirements (Table 1). Focusing on the following ten areas can help reduce the time needed to research available models and help ensure a good value.

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1. Primary use. Identifying the instrument’s primary use is the first step in selection. Will the instrument be used for weighing solids or liquids? Will the device be used at laboratory scale, or larger? Is it needed to weigh large quantities of uniformly sized objects, such as capsules, tablets or small parts? Will the instrument be used for weighing moving items on a production line? Do you need to control conditions inside a weighing vessel, such as heating, cooling or mixing?

2. Capacity. What is the largest possible load that a scale would be required to handle? Do you need overload protection? What will the overall footprint of the scale be and how will the items being weighed fit within the weighing area? Would a below-balance setup, where weight is measured via tension instead of compression, work for your application? An unofficial guideline recommends use of a balance for samples from microgram levels to approximately 10 kg, and load cells for those samples from 10 kg to several metric tons. Try to have the weighed quantities lie mostly in the middle of the range of the unit’s specific capacity to minimize stress or damage to sensitive internal electronics, and also to ensure greater accuracy.

3. Accuracy.In the context of weighing, accuracy can be thought of as a combination of several different factors, including the quantifiable specifications of resolution (the smallest mass change that can be read on a scale), reproducibility (ability to weigh consistently over time and with different operators), linearity (the variance in accuracy over the weight values within the scale’s capacity) and uncertainty of measurement (difference between measured weight and true weight due to environmental variances).

4. Materials of construction. Basic materials include aluminum alloy, carbon steel, aluminum-coated steel and galvanized steel. For these, cleanliness and corrosion-resistance are not critical. When higher levels of cleanability and chemical and environmental protection are required, AISI-304 and 316 stainless steels are possibilities.

5. Environment. Environmental conditions can affect weighing. Large temperature fluctuations, vibration, humidity, magnetic fields, air currents, corrosive chemicals and electrical interference can all influence weight measurements, especially at higher resolutions. Consider whether a particular environment would require specialized padding, protective covers, or more frequent calibrations.

6. Industry regulations.Many industry-specific regulations exist and may be relevant for some sectors and not others. Table 2 contains a list of websites for several of the organizations that set standards and regulations.

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7. Features. Additional features can customize the scale for enhanced flexibility, ease of use, functionality, protection and others. Consider whether your scale would need explosion protection, internal calibration software, interfacing ability with a computer network, wireless connectivity, scale readouts that are separated from the weighing platform, multi-language displays, backlit display for dimly lit areas, or other needs.

8. Price. Choosing a scale should never be based solely on price, but the most expensive scale is not necessarily the best choice.

9. Installation. When installing, it is recommended to place scales in a permanent location and connect to peripheral equipment, including a remote display. The resolution and readability should be set, and an initial calibration should be performed. For multiple load cells on a large vessel, a corner load test should be performed to ensure even weight distribution. Scales should generally not be moved from their point of use after installation, if possible.

10. Calibration and service.Regularly scheduled calibration of weighing equipment is necessary, because with use, normal stress can cause the accuracy of a scale to drift slightly. A series of certified test weights are placed on the weighing platform and the results recorded. When displayed results do not correspond to the test weight, manual or automatic adjustments can be made to correct the drift.

 

Editor’s note: This column is adapted from the following article: Titmas, R. and Carey, S., Weighing Your Options: The 10 Most Important Scale Considerations. Chem. Eng., December 2007, pp. 61–65.

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