Miller Magazine Issue: 132 December 2020

68 ARTICLE MILLER / december 2020 as the bonds between water molecules affect the sticki- ness and caking effect of the powder. PELLETISING Pelletising is the process of extruding the formu- lation into cylindric shapes that are more easily con- sumed by animals. The content of the mix is extre- mely variable between the various applications and recipes. However, even in this process and after all the other steps, the water content is still an impor- tant factor to measure the quality of pellets. Additi- onally, the pellets may need to be dried for storage. CONTROL AND SENSORS In summary, the moisture affects the costs and the qu- ality of the products. Knowing and subsequently cont- rolling the water content of the material in every step of the process is necessary to improve efficiency, to reduce carbon footprint, and to save money. To achieve these results, sampling the material is not enough because the samples may not be representative of the full batch and the speed of the feedback process is not adequate. It is possible to achieve real-time control in the process with inline sensors. As water content is an indirect measurement, this me- ans the value is attainable only by calculating it from ano- ther measured characteristic; therefore, it is necessary to keep as constant as possible all other variables such: • Material composition (mix recipe) • Particles size • Pressure on the sensor • Flow speed For this reason, it is key to calibrate the sensors for each recipe or formulation, only after the system instal- lation. The calibration must be realized by accurate lab tests, calibrating any sensor with another different sensor can cause a sum of errors resulting in incorrect calibrati- ons, defeating the initial objective completely. Independently from the method used in the process, whilst calibrating any sensor, during the lab test it is criti- cal to completely “cook” away the moisture of the samp- le to reach the dry weight, as this is what will be used to define the moisture reported by the sensor. There are many moisture sensors available on the mar- ket, and we can summarise the different technology used in five categories: Figure 1 Resistive Low cost, low repeatability, affected by impurities and temperature, limited range. Capacitive Inexpensive, marginally more reliable than resistive, measure water impurities NIR/Infrared Contactless, expensive, fragile, affected by dust, colour, temperature, high maintenance costs, measure only surface material. Nuclear Very expensive, very accurate, require safety maintenance, requires a highly trained operator, measure large volumes. Microwave • Analog: High sensitivity to water, affected by salts and impurities, nonlinear calibration. • Digital: Accurate, high sensitivity to water, not affected by impurities, cost-effective, simple to calibrate.