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September 2020 / 31 range of weight. Therefore, you should choose the smallest one that covers your full range. Because the load cells will support and weigh the entire assembly, not just the material contents, the load cell or cells need to be rated for the combined weight of the hopper as well as everything inside and everything firmly attached to the hopper. For example, if your total assembly weight will range from 0 to 1,200 pounds, then a good choice would be to support the assembly with three load cells rated at 500 pounds each. In this case, you will be using 1,200 of the rated 1,500 pounds. However, if you had chosen three 1,000-pound load cells for the same application, then your accuracy and scale resolution — the smallest increment in applied weight that the scale can detect or display — would be worse since you would only be using a small part of the range (1,200 pounds of the rated 3,000 pounds). In addition, each of the load cells needs to support an equal portion of the weight. That's why we used three load cells in the previous example because if you have three support points on which to balance an object, then each of them will always be supporting their portion of the load. In contrast, if you have four or consistent. Production plants use volumetric feeders when the material is fairly consistent, a high level of accuracy isn't required, and the material has a rela- tively low value so that an accurate accounting record isn't required. An example of this might be feeding regrind or reprocessed material into an extruder when knowing the exact rate isn't critical. Be aware that material and environmental con- ditions can affect the accuracy and consistency of a volumetric feeder. Most importantly, if the material's bulk density changes from what you typically process, then even if the volumetric displacement remains con- stant, the weighed amount will increase or decrease in direct proportion to the bulk density variations. Den- sity change can occur without notice. For example, if the grinder becomes dull, this can change the particle shape and bulk density of the regrind material. Other factors can also cause material changes that affect accuracy. For example, powders might flow consistently into the feeder on a dry day but less consistently when the weather is humid. In addition, aeration and fluidization can affect the bulk density of many powders — air is lighter, so mixing solids with air will lower their bulk density. Another factor is consolidation weight from the material above. Material in the top of the hopper pushes down, compressing the material at the hopper bottom, increasing its bulk density. Furthermore, this compression or consolida- tion factor will continue to increase the material's bulk density over time. The material's bulk density will also increase or decrease as the hopper's material level goes up and down. In summary, volumetric feeders are simple and low cost. But their accuracy may be inconsistent, and they don't provide record keeping of the material usage. The next option in accuracy and price requires an actual weight measurement of the material being dispensed. Gravimetric feeding When a simple volumetric device isn't adequate for the process, then load cells are usually added to the assembly to measure the material's weight. A load cell is a type of transducer that converts force (weight push- ing or pulling on it) into an electrical signal that can be measured or sent to a controller. As the force increases, the electrical signal changes proportionally. In a simple, common configuration, several load cells are used to attach the hopper to a support frame, as shown in Figure 3. Proper selection of load cells is critical for a safe and accurate operation. Load cells are rated for the amount of weight they can handle. They have an electrical signal output that covers their entire