Sensor, moisture content

Capacitive Sensors. This device usually consists of a capacitor which is formed either from two concentric cylinders or from a pair of parallel plates. The solid sample to be analyzed for moisture content is passed between these plates. Since w has a large dielectric constant, the w content of the sample causes a significant change in the dielectric constant of the solid, which is measured using bridge or frequency techniques. 

Fig. 1.98. Schema of the fiber-optic sensor to measure the residual moisture content (RM) through the bottom of the container with the product.

Belt with nucleonic sensor <25 (or <0.3 m3/s) 0.5 to 1 Preferred when material is difficult to handle, e.g. corrosive, hot, dusty, or abrasive. Accuracy greatly improved when particle size, bulk density, and moisture content are constant, when belt load is 70-100 per cent of maximum. 

Figure 9.11 Predicted and measured plot for granule moisture contents, calibrated on data from 5 months of production. Sensor B was used in this model (eight PLS components). Cray curve measured, black curve predicted.

J. Rantanen, S. Lehtola, P. Ramet, J. Mannermaa, J. Yliruusi, On-line monitoring of moisture content in an instrumented flnidized bed grannlator with a multi-channel NIR moistnre sensor. Powder TechnoL, 99(2), 163-170... 

Hinrichs and Thuen [28] used ultrasonic attenuation to determine the proper time for pressure application during an otherwise traditional pre-established cure cycle. Because dielectric is an electrical property, it is influenced by moisture content and temperature as well as viscosity, so it may vary quantitatively. Ultrasonic measurements are also affected by other parameters (i.e., void content), but they are a mechanical measurement rather than an electric one. The ultrasonic sensors used by Hinrichs unfortunately were less reliable than the dielectric sensors. 

Rantanen J, Lehtola S, Ramet P, Mannermaa JP, Ylirussi J. On-line monitoring of moisture content in a instrumented fluidized bed granulator with multichannel NIR moisture sensor. Powder Technol 1998 99 163-170. 

In one operational mode, the system pw is established by a large sample and is characterized by the sensor. The amount of moisture transfer from the sample does not significantly change its moisture content. The sensor is queried to identify its condition, and the result is converted into an estimation of pw. 

In the heat-transfer-type flowmeters, heat is added to the fluid stream with an electric heater, and the resulting temperature rise is detected (Figure 3.73). These types of flowmeters are best suited for the measurement of homogeneous gases (such as H2) and are not recommended for applications in which composition or moisture content is variable, because, for these sensors to work, both the thermal conductivity and the specific heat of the process fluid must be constant. 

Bielecka, M. and Majkowska, A. Effect of spray drying temperature of yoghurt on the survival of starter cultures, moisture content and sensoric properties of yoghurt powder, Nahrung, 44, 257, 2000. 

The sensor output signal should be insensitive to the moisture content... 

In our discussion of electromagnetic techniques, we omitted a few available technologies that provide some unique capabilities and, with further development, can attain practical application. One such technique involves the use of a microwave resonance sensor (Kobyashi and Miyahara, 1984) that uses a microwave cavity to measure solids concentration and velocity by monitoring the resonance frequency shift. However, this technique suffers from some shortcomings the frequency shift may be positive or negative, depending on the dielectric properties of the solids, and the cavity is extremely sensitive to changes in moisture content and temperature. 

Several applications can be found in literature regarding the use of NIR for the paediction of the main physical and rheological parameters of pasta and bread. De Temmerman et al. in 2007 proposed near-infrared (NIR) reflectance spectroscopy for in-line determination of moisture concentrations in semolina pasta immediately after the extrusion process. Several pasta samples with different moisture concentrations were extruded while the reflectance spectra between 308 and 1704 ran were measured. An adequate prediction model was developed based on the Partial Least Squares (PLS) method using leave-one-out cross-validation. Good results were obtained with R2 = 0,956 and very low level of RMSECV. This creates opportunities for measuring the moisture content with a low-cost sensor. 

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