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42 / January 2020 powderbulk.com DFF sensors offer multiple advantages for bulk pow- der characterization when judged against the criteria discussed earlier. These include: • a sensing mechanism well-suited to the process environment that is unaffected by electromagnetic interference and doesn't present an ignition hazard, • high-frequency measurement (up to 500 hertz) at high resolution (less than 10 micronewtons), mak- ing it possible to precisely and sensitively track even rapidly changing processes, • an enclosed, stainless steel construction that offers excellent resistance to a wide range of mate- rials, boosting reliability, • and turnkey operation when integrated with an optical interrogator and associated software. Crucially, the data generated by DFF sensors has also been found to correlate closely with dynamic powder ow properties. 1 Dynamic testing is a high-sensitivity, of ine method with proven relevance for a wide range of industrial processes from uidiza- tion to granulation. 2 Correlations between DFF and dynamic data highlight the relevance of DFF technol- ogy and the potential to transfer valuable speci cations already in place directly into the processing environ- ment for real-time monitoring. Dynamic testing is an established method for assessing the mixing and blending of powders. The following case studies illus- trate the application of inline DFF sensor technology in comparable applications. Case Study 1: Monitoring mixing behavior and the in uence of particle properties Batch mixing studies were carried out using four grain samples: couscous (CC), green lentils (GL), pearl bar- ley (PB), and long-grain rice (LG). Baseline data was gathered for 20 seconds with the mixer off, and then samples were mixed for 120 seconds. The mixing, gratings (FBGs). An FBG, shown in Figure 2, is a short segment or structure of varying refractive index within the core of an optical ber; cladding surrounds the FBG to give it mechanical integrity. A critical characteristic of an FBG is that the application of ow force induces a shift in the wavelength of interrogation light that it re ects. Materials owing past the sensor cause a de ection of the pin (sensor), which exes from its anchored base as demonstrated previously in Figure 1. The magnitude of this de ection correlates with the local ow force, referred to as F Drag in Figure 1, which is associated with powder movement within the pro- cess. The FBG on one interior wall is subject to tensile forces while the FBG diametrically opposite undergoes compression, giving rise to the relative spectral shifts, labeled as ∆λ in Figure 3. The wavelength of light re ected by an FBG can also be shifted by temperature changes, but both FBGs would be equally affected where this occurs. Spectral shifts associated with force and temperature are, therefore, easily deconvoluted, allowing the sensor to self-calibrate for temperature. The force associated with the movement of particulate ow is measured precisely in real time by applying a light source (interrogation light) and tracking shifts in the re ected light or resonant wavelength. FIGURE 3 In a drag force ow sensor, ow force induces opposing shifts in the wavelength of light re ected by the two ber Bragg gratings (left graph) while a change in temperature causes an identical spectral shift for both ber Bragg gratings (middle and right graphs). P P P λ λ 1 λ 2 Spectral shift associated with exing of the sensor Spectral shift associated with a change in temperature FIGURE 2 A ber Bragg grating is a short segment or structure of varying refractive index within an optical ber. Optical ber Fiber Bragg grating Interrogation light Re ected light Cladding Core