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January 2020 / 41 manual or have poorly de ned protocols or method- ologies. Poor repeatability erodes the sensitivity of a method, but techniques are inherently different in this respect. Sensitivity is a function of the variable being measured, the instrumentation used, and, in some instances, sample type. For example, current shear cells are moderately sensitive for more cohesive materials because the absolute value of the measurements being made is relatively large but much less sensitive for more free- owing powders, which generate lower val- ues of shear stress. Judging inline solutions for bulk powder character- ization against the criteria of relevance and sensitivity is vital when it comes to robustly determining their potential to improve your process. Inline powder characterization by DFF measurement An inline drag force ow (DFF) sensor is a relatively new technology for real-time bulk powder character- ization that measures the local forces associated with the ow of powders, granules, or wet masses within a process. This technology uses ber-optical strain gauges that offer integrated temperature compensation and measurement and are well-suited to the process envi- ronment. While there are alternative inline technologies available, the DFF sensor is of particular interest because it has been shown to produce data that correlates closely with of ine dynamic data, which has widespread appli- cability to the bulk powder processing industries. As you'll read in this article, the DFF sensor has proven relevance in mixing and blending applications. The DFF sensor, or pin, as shown in Figure 1, has a ne, needle-like structure with a hollow core and an outer diameter of approximately 1 to 4 millimeters. The size of the sensor results in minimal ow disruption when the sensor is inserted into a process. Mounted opposite one another on the inner surface of the sen- sor are two ber-optical strain gauges or ber Bragg samples are extracted from the process and analyzed of ine to assess the degree of homogeneity. Of ine measurements provide limited opportunity to closely track the blending process toward comple- tion and/or rapidly detect any mechanical failure of the equipment's impeller. Operators will feel the urge to safeguard the homogeneity speci cations due to the time delays associated with of ine analysis. Operators may also experience a general lack of con dence in being able to exert close control over the process. As a result, executing the process using of ine measure- ments is highly likely to cause routine overmixing. This "safety margin" carries the unintended costs of wasted energy, compromised throughput, and an increased risk of particle damage, which could impact product quality. Inline technology that precisely and instantaneously detects the endpoint of a mixing pro- cess eliminates these issues, resulting in considerable economic return. Assessing requirements for inline powder characterization technology To successfully deliver the bene ts of real-time measure- ment, inline technology must answer to a demanding set of requirements. Process environment reliability and suitability are critical for any continuous monitor- ing technology, but for bulk powder characterization, relevance and sensitivity are of particular importance because of their uniqueness to the bulk powder process. The sampling technique must measure a property that correlates process or material performance with suf - cient repeatability and sensitivity to detect subtle but crucial differences. These issues similarly impact the value of of ine powder testing methods. Relevance. Powders are routinely tested in many ways across the various powder processing industries, a common goal being to rank or quantify material's owability. Traditional methods for assessing owabil- ity include measuring the material's angle of repose, a manual technique that involves measuring the angle at which a powder settles when poured onto a at surface. Though simple measurement methods can differentiate powders to some degree, the relevance of the resulting data to a speci c process may be unclear. For example, if Powders A and B each have an angle of repose of 34 and 41 degrees, respectively, what does that mean in terms of their relative blending perfor- mance? Powder testing methods vary signi cantly in terms of their ability to generate data that correlates with performance in a process, but without such cor- relations, the data is of limited value. Sensitivity. Many of ine powder testing methods suffer from poor repeatability often because they are FIGURE 1 A drag force ow sensor de ects in response to the ow of process materials. Flow direction De ection of the pin F Drag DFF sensor Process wall