9 Moisture Monitoring

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1 9 Moisture Monitoring Microwave techniques have been considered for moisture sensing in many food processing and agriculture-related industries (Trabelsi, et al. 1998b). Chapter 7 highlighted the strong dependence of dielectric properties for organic materials on the moisture content of the material. Both bound water and free water have much higher dielectric properties than dry organic materials; therefore RF and microwave technologies have been used to automatically monitor moisture content. 9.1 Free-Space Moisture Detection The previous chapter introduced the idea of using a free-space transmission technique for detecting decay in wood. The same technique can be used for measuring moisture in various materials. As the microwaves propagate through a layer of organic material, they will be attenuated and experience a phase shift (Kim, et al. 2002, Kim, et al. 2006), as illustrated in Figure 8.5 in the previous chapter. The attenuation and phase shift can be compared to an empty sample holder between two antennae (Kim, et al. 2002, Kim, et al. 2006). The change in RF and microwave field attenuation is given by (Kim, et al. 2002, Kim, et al. 2006): D " A = p k d l k ' (9.1) o where DA is the difference between the attenuation, with and without samples, in db, l o is the free space wave length, k and k are the real and imaginary parts of the relative permittivity of the material, and d is the thickness of the material under test. The phase delay is given by (Trabelsi, et al. 1998a, Kim, et al. 2006): ( k ' 1) 360 DF = - d (9.2) l o Where DF is the difference between the phase, with and without samples, in degrees. As illustrated in equation (9.1), the change in attenuation depends on the dielectric properties of the samples under test when the frequency and sample thickness are constant. Because the dielectric properties are closely linked to moisture content, it is possible to measure the moisture content by detecting the peak voltage of the microwave signal from the receiving antenna (Kim, et al. 2002); however, with

2 Free-Space Moisture Detection 123 particulate materials, bulk density fluctuations cause significant errors in moisture content determination (Trabelsi, et al. 1998b). k" k ' Trabelsi et al. (1998b) demonstrated that plotting r against r for varying moisture content, where r is the bulk density of the material, resulted in a straight line of the form: k" æk' ö = a - b r ç çèr ø (9.3) Slope (a) and intercept (b) values for hard red winter wheat of different moisture contents are listed in Table 9.1. All the lines cross the axis at a common point, which corresponds to the coordinates, in the complex plane, of the dielectric properties of a totally dry kernel mixture (Trabelsi, et al. 1998b). The slope factor (a) depends solely on frequency (in GHz) such that (Trabelsi, et al. 1998a): a= f (9.4) Table 9.1: Slope and intercept values for hard red winter wheat of different moisture contents as a function of frequency (Source: Trabelsi, et al. 1998b). Frequency (GHz) a - measured a calculated from equation (9.3) b - measured r Provided a and b are known for any given material, the bulk density can be determined from the dielectric properties at any given moisture content and temperature (Trabelsi, et al. 1998a):

3 124 Moisture Monitoring r ak' -k" ab = (9.5) The slope and intercept values (a and b) must be determined experimentally for each material to be tested. A density-independent function can also be defined (Trabelsi, et al. 1998a): k" x = k' k' k" ( a - ) (9.6) The square root of x varies linearly with moisture and is ultimately independent of bulk density: x = am + b (9.7) where a and b are constants, and m is the moisture content of the sample Practical Applications Various free-space systems moisture detection systems have been developed for use in agriculture for monitoring the moisture of grains and hay going into storage (Figure 9.1). For example, a commercially available moisture detection system for quickly measuring the moisture content of hay bales has been developed (Figure 9.1). Analysis of this commercial microwave hay moisture meter showed a close relationship of the meter s readings compared to the lab results (R 2 = 0.867) (Keith, et al. 2013). Similar RF and microwave based free-space systems are available for measuring moisture of other agricultural products that are susceptible to moisture related degradation, including: grain, sugar, and cotton. In all cases, the dielectric properties of the dry material are very low compared with the dielectric properties of bound and free water; therefore moisture detection is moderately easy to achieve. Free-space moisture monitoring relies on transmitting a RF or microwave field through a material under test. Although this arrangement is ideal for monitoring many agricultural and forestry products, such as grains and sawn timber, it is not always possible to have access to both sides of a material. Under these circumstances, microwave emissions from the material or radar assessments are better.

Microwave Emissions as a Measure of Moisture 125 Figure 9.1: Microwave system for monitoring moisture in hay bales mounted on the outlet of a large square baler (Source: Anonymous 2013).

4 Microwave Emissions as a Measure of Moisture 125 Figure 9.1: Microwave system for monitoring moisture in hay bales mounted on the outlet of a large square baler (Source: Anonymous 2013). Other moisture monitoring techniques rely on close range radar analysis. These also respond to changes in the dielectric properties due to changes in the moisture content of the material being monitored. In particular these changes in the dielectric properties change the reflectivity of the material, which therefore enhances the reflections that the radar system detects. One commercial example of a radar-based moisture detection system was developed for the measurement of moisture and temperature in rotating pan, static and planetary mixers (Hydronix Limited 2004). The main sensor has a simple temperature sensor and a compact modular microwave radar system mounted on an arm. This can be inserted into a mixer system, such as an animal feed mixer, and monitors the moisture content of the material as it passes by the radar module. Changes in the moisture content correspond to changes in the output voltages from the radar module. 9.2 Microwave Emissions as a Measure of Moisture Heat is transferred through space by conduction, convection and radiation. Radiative heat transfer refers to the transfer of energy by broad spectrum electromagnetic radiation from some adjacent hot object (or from a hot environment) to the heated object. Any object that is above zero degrees Kelvin will radiate energy in the form of electromagnetic photons. The German physicist, Max Planck ( ), deduced that the ideal radiation spectral density (r ideal ) given off from a hot object depended on the wavelength of interest and the temperature of the object. This spectral density can be described by:

5 126 Moisture Monitoring r ideal 2phc = hc 5 æ ö kt l e l -1 ç çè ø (9.8) Where h is Planck s constant ( J s), c is the speed of light, l is the electromagnetic wavelength of interest, k is Boltzmann s constant ( J K -1 ), and T is the temperature in Kelvin. A typical set of spectral distributions is shown for different temperatures are shown in Figure 9.2. Although the peak emissions occur in the infra-red part of the spectrum for ambient temperatures, some energy is emitted in the RF and microwave bands. Real objects radiate slightly less energy in different parts of the spectrum. The ratio of the real emission at any given wavelength and the ideal emission is defined as the emissivity (e) of the surface: e r r ideal = (9.9) The spectral emissivity of a material is directly related to its reflection coefficient (G) at the wavelength of interest (Lakshmi 2013): real e = 1-G (9.10) This is the same as the transmission coefficient defined in equation (4.40) in Chapter 4. Combining and manipulating equations (4.33) and (4.40) yields: 2 k e = (9.11) 1 + k where k is the complex relative dielectric constant of the material. It has already been demonstrated in Chapter 7 that the relative dielectric constant of organic materials and soils in the RF and microwave bands is strongly linked to the moisture content of the material while the influence of moisture on the dielectric properties of these materials in the infra-red part of the spectrum is negligible; therefore monitoring natural electromagnetic emissions from organic materials and soils using duel band sensors (thermal infra-red and microwave) can remotely determine the moisture content of these materials (Lakshmi 2013), as illustrated in Figure 9.3.

6 Microwave Emissions as a Measure of Moisture 127 Figure 9.2: Ideal radiative spectral density at different temperatures as a function of temperature and wavelength. Figure 9.3: Estimated microwave emissions from bare clay soil as a function of frequency and soil moisture content at 25 C. Although the electromagnetic power emissions from soil at microwave are very small, radiometers that measure microwave emission brightness from the earth s surface have been deployed in satellites for some time and are routinely used to determine the moisture content of soils on a large scale (Lakshmi 2013). Close range systems, mounted on farm machinery, could also be developed; however the requirement for dual band monitoring makes these systems expensive.

7 128 Moisture Monitoring 9.3 Radar Moisture Measurement The reflection coefficient of surface is also dependent on the dielectric properties of the material: 1 G = 1 + k k (9.12) Therefore the radar cross section of an object, which was introduced in Chapter 8, also depends on the dielectric properties of the surface material of an object. If radar is directed at an organic material or the soil, changes in moisture content will affect the reflection coefficient of the surface (Figure 9.4). Figure 9.4: Reflection coefficient of bare clay soil as a function of frequency and soil moisture content at 25 C. Comparing figures (9.3) and (9.4) suggests that reflection provides better discrimination between soil moisture status than emissions, therefore research into remote moisture measurements using radar is being actively pursued (Zhan, et al. 2006). Mass produced radar modules at fixed frequencies have been available for some time. These can be deployed on agricultural machinery to measure the moisture content of soil and produce in real time. Aerial and satellite radar systems can also be employed to assess large areas of the landscape. These large scale radar systems will be explored in the next chapter.

8 References 129 References Anonymous Gazeeka Moisture Gauge. International Stock Foods Hydronix Limited Hydro-Probe Orbiter User Guide Keith, E. W., Mathew, D., Jeff, A. and Drew, S Production Scale Single-pass Corn Stover Large Square Baling Systems. Proc Kansas City, Missouri, July 21 - July 24, Kim, K., Kim, J., Lee, S. S. and Noh, S. H Measurement of Grain Moisture Content Using Microwave Attenuation at 10.5 GHz and Moisture Density. IEEE Transactions on Instrumentation and Measurement. 51(1): Kim, K. B., Kim, J. H., Lee, C. J., Noh, S. H. and Kim, M. S Simple instrument for moisture measurement in grain by free-space microwave transmission. Transactions of the American Society of Agricultural and Biological Engineers. 49(4): Lakshmi, V Remote Sensing of Soil Moisture. ISRN Soil Science. 2013: Trabelsi, S., Kraszewski, A. W. and Nelson, S. O. 1998a. A Microwave Method for On-line Determination of Bulk Density and Moisture Content of Particulate Materials. IEEE Transactions on Instrumentation and Measurement. 47(1): Trabelsi, S., Krazsewski, A. W. and Nelson, S. O. 1998b. New Density-Independent Calibration Function for Microwave Sensing of Moisture Content in Particulate Materials. IEEE Transactions on Instrumentation and Measurement. 47(3): Zhan, X., Houser, P. R., Walker, J. P. and Crow, W. T A Method for Retrieving High-Resolution Surface Soil Moisture From Hydros L-Band Radiometer and Radar Observations. IEEE Transactions on Geoscience and Remote Sensing. 44(6):

Microwave Moisture Measurement Technique In Industrial Application Dr. T. Jayanthi 1, B.Shiny Navya 2, K.Vanitha 3

Microwave Moisture Measurement Technique In Industrial Application Dr. T. Jayanthi 1, B.Shiny Navya 2, K.Vanitha 3 2,3 UG Students, Department of Electronics and Communication Engineering, Panimalar Institute