On-Line Moisture Sensors
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Many methods are available for measuring moisture content on line, in real time: NIR, RF Capacitance, NMR (Nuclear Magnetic Resonance), and Microwave, among others. Finna Group is unique in that it has experience with most of these methods and is therefore able to recommend the best possible solution for any application.
However, without doubt, NIR (near infrared) is best suited for a great majority of all applications. These would include foods, chemicals, pharmaceuticals, paper and any other powder, granular, web, or coating application. This is primarily because of its ease of installation and the fact that it does not need to contact the product to assure accurate moisture measurements. A sensor about the size of a large flashlight is mounted over a conveyor (belt, screw, vibratory, etc.) and simply shines a light about 1 in. (3 cm) in diameter onto the surface of the product. An analog or digital output signal that’s proportional to the actual moisture content is then available to be used for closed loop moisture control if desired.
For thicker products such as gypsum board, OSB (Oriented Strand Board), plywood, particleboard, lumber products, etc., a full, non-destructive penetration of the product is necessary in order to assure total moisture content measurement.
RF (Radio Frequency) Capacitance sensors offer this penetration depth without harm to the measured product. The sensor is simply mounted below the bottom of the board and generates an RF field that penetrates through the thickness of the board. There is no direct contact with the board. RF sensors are manufactured in various lengths and widths to fit most any conveying system, but are typically mounted between two rollers either within a drying kiln or at the exit conveyor.
In gypsum board applications, a high temperature sensor is typically mounted within one of the last two zones of the drying kiln in order to allow temperature control to achieve optimum final product moisture results. Out kiln sensors are then used to confirm final product moisture content. Analog and digital closed loop control output signals are available representing actual moisture content.