Accurate, precise and responsive gravimetric feed rate control is not possible without accurate, precise and responsive weight measurement. To highlight the scale and scope of the measurement challenge faced, consider the following typical application.
As part of a large, multi-ingredient continuous blending operation, a loss-in-weight feeder must discharge an additive to the extruder throat at a rate of 60 lb/hr, and do so to an accuracy of +/-0.5%. Additionally, end-product quality specifications require this accuracy to be based on feeder performance measured over a 10-second interval, reflecting the timescale of mixing within the high-speed extruder.
To meet these requirements, the feeder must continuously control the weight of ‘ten-seconds worth’ of discharged material to within a range of from 0.1658 to 0.1675 lb, or a little more than ¾ of a gram in metric terms!
Clearly, this is an extraordinary challenge all on its own. But add to this the fact that the feeder must weigh in a real-world process environment where measurement conditions are always less than ideal. And the feeder must perform just as accurately over a broad range of rates to formulate an entire family of compounds, blends or recipes.
In short, to meet today’s rigorous application requirements, a gravimetric feeder’s weigh system must possess the following attributes: