Rock Products

JUL 2019

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Page 26 of 79 ROCKproducts • July 2019 • 25 A second batch set point can be used to stop the feeding. Load out applications are much like batching applications and can use the accumulated weight signal to stop the filling once a set point has been reached. Variables in the process may affect desired performance. Air flow, excessively varying flow rates, environment, scale placement may be a factor. Material temperature, moisture, particulate size and flowability are considerations. Material abrasiveness and hours of operation affect maintenance. The means of feeding to and discharging from the scale is part of the process. Space for a Scale Space for a scale may be too confin- ing for some choices. Once a scale is placed, the scale will be calibrated under actual production conditions. The actual production conditions with all process variables takes into con- sideration during calibration all of the factors affecting accuracy. Material is run through the flow scale and the accu- mulated weight is compared to a known weight fed to the scale or collected after the scale. With this data, the rate span factor will be adjusted to a factor corrected to the material flow rate. The rate signal is integrated mathematically, which pro- duces the accumulated weight. Rate signals and accumulated weight signal outputs are available for pro- cess control use. Scale repeatability is most important. It is recommended that the first three comparisons are docu- mented with no changes to the span factor. Much like sighting in a rifle, a tight shot group is important. The reference scale net weight is divided by the accumulated scale total- izer weight and that ratio is multiplied by the existing span factor to give us the new desired span factor. If the first test results were 3.38% from the known weight, we would be happy if the fol- lowing two were 2.79% and 3.08%. The average repeatability error is 3.08%. Consequently, the error shot group would be a respectable +.3%. This would be acceptable to most applica- tions. This tells us, at one flow rate (300 lb./min.), we can be very accurate. Keep in mind we may be calibrating a scale to reference scale with its own built-in error. We mention this because the calibration is dependent on the per- formance of the reference scale. Now that a tight shot group has been established, we can adjust wind age and elevation to the bull's eye. In test four, perform the material calibration in the mid production rate range. As illustrated, the error is reduced to nothing. Flow rates in a process can change within a certain range. Adjust the rate up (400 lb./min) and down (200 lb./min) to the minimum and max- imum expected flow rates. Perform the fifth and sixth material calibration at the lower and upper pro- duction rate range. Document those tests and analyze the errors for linearity acceptability. As illustrated, the total accuracy is +.5%. Normally this is acceptable. If unacceptable, review this installation and process. If the accuracy is not acceptably linear throughout the range and the scale remains repeatable, the rate signal can be electronically profiled to make the scale performance linear. NTEP installations may require the linear profiling and/or controlling the feed rate to a specific flow. The calibration spreadsheet illustrated is a great tool and is provided free of charge by some scale providers for doc- umenting the above material calibration process. The spreadsheet organizes one test after another and does the error math and the corresponding new span number math. This tool comes in handy for trou- bleshooting problems as well. Scale providers often supply application data sheets to collect information potential users don't think about. There are nearly a dozen ways to continuously weigh. S Clarence Richard is a continuous weigh control applications engineer for Clar- ence Richard Co., Minnetonka, Minn. For more information, call 952-939-6000 or visit

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