Moisture measurement in harsh environments
Moisture in any bulk material influences many parameters whether sand for concrete production, wood chips for pellets production, process optimising in mining and chemical industry. Because most materials have inherent moisture contents, the final product recipe or quality is subject to error without an accurate analysis of the individual components. Since these inherent errors are dynamic in nature (changing with hydroscopic tendencies) the material mix demands ongoing total system moisture controls. Many problems may occur, starting with the ability to store the product, the material flow, filling procedures and different processing during production. The quality of the final products similarly demands proper moisture levels to ensure proper shelf-life, consistency, aroma and weight.
ACO Automation Components developed a sensor system (Fig. 1) which offers a maximum of operation comfort and excellent performance at a reasonable price. The ACO moisture sensor is designed to withstand the rigors of the harshest of industrial environments. The circuitry is completely encapsulated within an outer housing of stainless steel. The sensing face is a special ceramic disc, extra thick for mixer application, hard rubber or Teflon is available for special applications.
The ACO moisture measuring system is designed to be an in-line process control (Fig. 2) and offers a real time control of the process. Also non-contact measurement through glass or plastic is possible, for example at hygienic demands. The sensor is detecting approx. 6 inches (150 mm) into the product (depending on the material) and it detects average product moisture. This includes surface moisture as well as core-moisture of the material. Due to this measuring depth small dirt particles or a dirt film on the sensor head are not of importance for the measurement. The analogue output signal of 0-10 VDC, 0/4-20 mA from the sensor can be transmitted either directly or via ACO evaluation unit DIGISYS to a process control circuit, PLC, PC, or to the processing devices recommended by ACO.
The high frequency dielectric shift moisture measuring system for determination and control of the moisture is already in use for products such as sand, minerals, grain, granulates, quartz sand, ore, sludge, ceramic, clay, kaolin and its raw products, FGD gypsum and its products (boards etc.), chalk, concrete, wood, pellets, saw chips etc. Typical locations for the sensors are on a material slide on conveyor belts mounted on a sled, also from under the conveyor (Fig. 3), in feeder screws or in a screw conveyor, in a silo, under the silo discharge (Fig. 4), also inside a mixer, in extruding presses and many more. The application for all bulk products is straight forward and easily achieved.
• Reduction of costs in spare parts storage, as only modules must be changed
• High availability of production facilities due to no new calibration at sensor exchange. Function of measuring is still provided if one or several sensors must be replaced
• Full supply and evaluation of up to 16 pcs. moisture measuring sensors DMMS via one evaluation unit DIGISYS (cost saving at investment)
• Output of measured values: Profibus DP, RS 232
• Option: 4–20 mA rsp. 0 –10 VDC
• Digital outputs, digital inputs, external material switch
• Permanent (in-line or on-line) moisture measuring creates improvements in quality
• Production processes are continuously monitored and laboratory test times will be reduced and a “slipping” through of bad batches can be avoided
• Processes can be controlled and commanded by moisture e.g. in the field of granulation with spray dryers you can reproduce the end points of each batch and also you can drive a certain programmed set-point curve for the moisture
• Recipes for any fields (concrete, food, chemicals) and mixtures can be reproduced in equal quality and depending on the moisture content of the ingredients the necessary quantity can be corrected with the batch control circuitry
• Savings in the drying process and water consumption are affected by moisture control of the product
• The material moisture can be recorded (ISO 9000) and taken for process validation
ACO Automation Components, 79793 Wutöschingen (D), Tel.: +49 7746 91316, www.acoweb.de
