ON-LINE MOISTURE AND DENSITY MEASUREMENT OF FOODS WITH MICROWAVE SENSORS
R. J. King · Applied Engineering in Agriculture · 1997
New developments and applications are presented for continuous, on-line determination of moisture contentand dry density of food products with microwave sensors. The focus of this work is on applications to harvested foods(various grains and nuts), dried fruits and extruded foods. Two unique types of sensors and their ancillary electronics for process control and/or product quality measurement areintroduced. For both, measurement principles are based on the interaction of electromagnetic fields with the dielectricand power dissipative properties of matter, particularly of water. These properties are diagnostics of the partial water anddry densities. The first type of sensor is a contacting open reflection resonator which can be interfaced in various ways to thematerial being measured, e.g., sensors can be flat or curved for flush mounting in a shaker, hopper, glide sled, chute orpipe. The second type is noncontacting, wherein a microwave beam is transmitted through the test material. Throughmeasurement of the transmitted signal attenuation (absorption) and phase delay, the moisture and dry basis weights(mass/area) are determined by empirical algorithms. Either sensor type can be used for continuous, on-line measurementsor as stand-alone benchtop instruments. Choice of sensor type depends on the test material form (liquid, particulate orsolid), the ranges of material moisture content and thickness, and logistical constraints associated with installation.Generally, accuracies of better than 0.5% moisture content and 2% absolute density are achieved up to 20% moisture.Resolution of small changes in moisture is about 0.1%, depending on the application and the degree of on-line dataaveraging.