Determination of Dielectric Properties of Wheat in Various Moisture Contents with a Parallel-Plate and Cylindrical Coaxial Capacitor

Transcription

1 Determination of Dielectric Properties of Wheat in Various Moisture Contents with a Parallel-Plate and Cylindrical Coaxial Capacitor Parviz TOMARAEI *, Kamil SAÇILIK Ankara University, Faculty of Agriculture, Department of Agricultural Machinery, Ankara TURKEY prvztmr@yahoo.com Abstract: The moisture content of grain is an essential factor affecting the physical properties related to storage, processing and quality control. Dielectric properties of grain have been mainly used to measure the moisture content of grain because of their practicality for rapid moisture sensing method. In this study, a parallel-plate and a coaxial sample holder that grains were placed in them were designed. These sample holders were used as a capacitor in the radio frequency (RF) moisture measuring system. The dielectric properties of wheat were determined in the ranges of % wet basis (w.b.) moisture content, MHz for frequency of applied electric field using a parallel-plate and cylindrical coaxial capacitor sample holder. Effects of the parameters such as moisture content and frequency on the dielectric properties were investigated. The dielectric constant and loss factor were greatly affected by the moisture content and frequency. The moisture content was the most significant factor affecting the dielectric properties of wheat seeds. The dielectric constant and loss factor increased with increasing moisture content. Dielectric measurements provided new information concerning moisture content and frequency dependent. Behaviour of dielectric properties of wheat seeds that may be useful in sensing of the moisture content. Key words: Moisture content, dielectric constant, loss factor, parallel-plate coaxial capacitor, cylindrical coaxial capacitor, RF INTRODUCTION Use of electrical properties of grain for moisture measurement has been the most prominent agricultural application for dielectric properties data. Interest generally focused on the influence of a grain sample on the response of an electrical circuit, and the instrument readings were calibrated with values measured by drying-oven techniques or other standard procedures for moisture determination. The need for quantitative values of the dielectric properties arose from research on the application of radio-frequency dielectric heating to agricultural problems. The first quantitative data on the dielectric properties of grain were reported for barley along with a method for reliable measurement of those properties in the 1- to 50-MHz frequency range (Nelson et al, 1953). Relative permittivity or dielectric properties of cereal is proportional to the amount of water in grain. Since the dielectric constant of free water is much larger than that of the grain dry matter. It is relatively easy to sense the mass of water within a grain sample (Kurt, 1998). For example, if the water dielectric constant is 80, dielectric constant of dry wheat is about 3. Therefore, used of the dielectric properties is appropriate method to measuring moisture content in grain. Dielectric properties of agricultural products and materials, an electromagnetic field investigated behavior by various researchers. (Nelson, 1965, Corcoran et al., 1970, Jorgenson et al., 1970, Stetson and Nelson, 1970, Nelson and Stetson, 1976, Sokhansanj and Nelson, 1988, Kraszewski and Nelson 1991, Lawrence and Nelson, 1993, Berbert and Stenning, 1996a, Berbert and Stenning, 1996b Lawrence et al., 1998, Lawrence et al., 2001, Berbert et al., 2002, Kim et al., 2002, Kim et al., 2003, Boldor et al., 2004, Saçılık and Çolak, 2005). Electromagnetic signal is sent to an object is divided into three parts, a part of the energy is reflected, some is transmitted across the surface, the rest is absorbed by the object. This is separated into 552

2 three energy rates, referred to as dielectric properties (Venkatesh and Raghavan, 2004). The complex relative permittivity ε* of a material can be expressed in the following complex form: ε* = ε' - jε'' (1) ε*: Complex relative permittivity, ε' : Dielectric constant, ε'': Dielectric loss factor. The real part ε' is referred to as the dielectric constant and represents stored energy when the material is exposed to an electric field, while the dielectric loss factor ε'', which is the imaginary part, influences energy absorption and attenuation, and Loss tangent δ is also often used as the power dissipation in a dielectric and can be expressed as follows: " (2) tan δ ' Frequency and moisture content are the major factors affecting dielectric properties of agricultural materials and food products. Wheat is one of the most important cereal grains in the world. Reliable dielectric data for wheat in the radio frequency range will be very useful in developing a calibration method for nondestructive sensing of the moisture content. Therefore, the aim of this study was to present dielectric properties of static samples of wheat seeds at excitation frequencies in a range of MHz for moisture content in a range of % w.b. MATERIALS and METHOD Wheat seed used in this study was obtained from Ankara University, Faculty of Agriculture. The seeds were cleaned manually to remove foreign matter, immature and broken seeds. Initial moisture content of clean seeds at reception was found to be 9.04% w.b. The moisture content of the seeds was determined by placing three samples, each weighing about 10 g, in a air-oven at 130±1 ºC for 19 hours (ASAE, 2002). All moisture contents were calculated on a wet basis. In order to preserve its original quality, the sample was stored at an ambient temperature of 22 3 C in sealed plastic bags prior to any conditioning. Seed samples were conditioned to obtain different moisture contents in the range of % w.b. Samples of desired moisture level within the above range were prepared by adding calculated amounts of distilled water, then mixed and placed in sealed plastic containers. The samples were kept in a refrigerator at 5 ºC for at least one week and stirred frequently by rotation of the containers to ensure uniform moisture distribution throughout the sample. Then, each moistened test sample was divided into two sub-lots which were used independently to perform test runs at two different bulk densities in loose and dense fills. Dielectric property measurements of the seed were conducted at the moisture content levels of 9.04, 10.27, 11.26, 12.41, 13.37, and 15.53% w.b. Impedance analyzer used for dielectric measurements has a measuring range of MHz, 16453A dielectric test setup, E4991A-010 connection kit, 16192A and other parts of the parallel test system contains (Figure 1). Data obtained from the impedance analyzer to aid in advanced software was recorded in computer. The values obtained by electrical impedance analyzer, used to develop a RFbased moisture measurement system. Figure 1. Agilent 4991A impedance analyzer A coaxial cylinder, diameter 89 mm made of stainless steel, the inner electrode diameter of 16 mm. Dimensions of mm two Teflon rods rectangular prism-shaped from top and bottom placed in the cylindrical capacitor. Cylindrical coaxial measurement box, with aid a cable 50-Ω and SMAtype connector connected to 16453A test head. Total volume of 1512 cm 3 Figures 2 and 3. A parallel-plate capacitor box, the height of 200 mm, width 150 mm and depth is 46 mm. External electrode 2 mm thick made of aluminum, as suggested by (Lawrence and Nelson, 1993) is designed. Under the parallel-plate capacitor is made 553

3 of Teflon sliding sled used for the unloading of wheat manufacturer. Figure 2. Sectional views of the cylindrical coaxial capacitor for dielectric measurements of wheat (measurements in mm) Figure 4. Sectional views of the parallel-plate coaxial capacitor for dielectric measurements of wheat (measurements in mm) Figure 5. The parallel-plate coaxial capacitor sample holder Figure 3. Cylindrical coaxial capacitor sample holder after the measurement. Parallel-plate capacitor measurement box, with aid a cable 50-Ω and SMAtype connector connected to 16453A test head. Total volume of 936 cm 3 parallel-plate capacitor which also serves as a sample container Figures 4 and 5. In order to determined dielectric properties of wheat, sample holder include a cylindrical coaxial or a parallel-plate capacitor were connected to the impedance analyzer through a 50 Ω coaxial cable and SMA type connector. Tests for each combination of moisture content were carried out at intervals of 1 MHz from 1 to 100 MHz. Impedance analyser transfer data into the computer. Before the experiments, the impedance analyzer was calibrated according to recommend by the For this study, the most appropriate frequency range from MHz has been decided. Seven different moisture content of the experiment materials introduced, dielectric constant ( ε ) and dielectric loss factor ( ε ) values calculated by the following equation: ' C C 0 C: Capacitance filled with a dielectric material (pf), C 0 : Capacitance is empty ( pf). (3) Loss factor is calculated with following equation: (Saçılık and Çolak, 2008): G G " 2 fc 0 0 ε : Dielectric loss factor, (4) 554

4 G: Conductance of the cylinder coaxial filled with a dielectric material (Siemens), G 0 : Conductance of the cylinder coaxial is empty (Siemens), f: Frequency (MHz). RESULTS and DISCUSSION Capacitance values of the empty coaxial cylindrical holder sample in the range of MHz changed between pf (Figure 6). The graph showed until 40 MHz increase conductance values monotonous, above 40 MHz stability is corrupted. Therefore, coaxial cylindrical sample holder frequency range 1-40 MHz is considered. (Figures 8 and 9) It was observed that the values of dielectric constant increased more rapidly at lower frequencies than at higher frequencies. The dielectric constant decreased monotonically with increase in frequency at all moisture contents. At higher moisture contents, the effect of frequency on the permittivity is much more pronounced than at lower moisture contents. These results reaffirm by (Stetson and Nelson, 1970; Sokhansanj and Nelson, 1988; Berbert et al., 2002). Figure 8. Relationship between dielectric constant and frequency at various moisture contents in the cylindrical coaxial capacitor Figure 6. Capacitance values of the empty cylindrical coaxial sample holder Capacitance values of the empty parallel-plate holder sample in the range of changed between pf (Figure 7) The graph showed until 30 MHz increase conductance values monotonous, above 30 MHz stability is corrupted. Therefore, parallel-plate holder sample frequency range 1-30 MHz is considered. Figure 7. Capacitance values of the empty parallelplate sample holder In the studied frequency range MHz dependence of the dielectric constant of wheat sample at various moisture content are shown in Figure 9. Relationship between dielectric constant and frequency at various moisture contents in the parallel-plate capacitor In the studied frequency range MHz dependence of the loss factor of wheat sample at various moisture contents are shown in (Figures 10 and 11). The changes in the values of loss factor were greater at higher moisture content than at lower moisture content. It was observed that the values of loss factor increased more rapidly at lower frequencies than at higher frequencies. The results of the frequency dependence of loss factor for wheat sample were similar to that of wheat flour in the same frequency range (Nelson and Trabelsi, 2006). 555

5 Figure 10. Relationship between loss factor and frequency at various moisture contents in the cylindrical coaxial capacitor Figure 13. Moisture contents dependence of the dielectric constant of the wheat at various frequencies for parallel-plate capacitor Moisture contents dependence of the loss factor of the wheat at 1, 5, 10, 15 and 20 MHz frequencies for cylindrical and parallel-plate coaxial capacitors were given in Figures 14 and 15, respectively. At low frequency, loss factor increased with increasing the frequency in all moisture levels. The results of the frequency dependence of dielectric properties for wheat sample were similar to that of wheat (Stetson and Nelson, 1970; Berbert et al., 2002). Figure 11. Relationship between loss factor and frequency at various moisture contents in the parallel-plate capacitor Moisture contents dependence of the dielectric constant of the wheat at 1, 5, 10, 15 and 20 MHz frequencies for cylindrical and parallel-plate coaxial capacitors were shown in Figures 12 and 13, respectively. Greater increase was observed in dielectric properties when moisture content was above 12%. It shows that dielectric constant increased with decreasing the frequency in all moisture levels. Figure 14. Moisture contents dependence of the loss factor of the wheat at various frequencies for cylindrical coaxial capacitor Figure 12. Moisture contents dependence of the dielectric constant of the wheat at various frequencies for cylindrical coaxial capacitor Figure 15. Moisture contents dependence of the loss factor of the wheat at various frequencies for parallel-plate capacitor CONCLUSIONS 556

6 In this study the following conclusions were reached: The dielectric constant and loss factor were shown to be dependent on the moisture content and frequency of the applied electric field. The moisture content had a dominating influence on these dielectric properties because of related effect of moisture changes. Capacitance values of the parallel-plate and the cylindrical coaxial capacitors between MHz, changed between and pf and and pf, respectively. Conductance values of the parallel-plate and the cylindrical coaxial capacitors increased until 30 and 40 MHz, respectively. From this points, these values suddenly changed. In both of system, the measurement frequency range 1-20 MHz is considered. The dielectric constant and loss factor decreased monotonically with increase in frequency at all moisture content levels. At higher moisture content, the effect of frequency on the permittivity is much more pronounced than at lower moisture content. ACKNOWLEDGEMENTS The data used in in this paper were partially taken from the MSc Thesis titled Development of RF Moisture Sensor for Measuring Moisture Content in Grain completed at the Ankara University, The Graduate School of Natural and Applied Sciences. REFERENCES ASAE Standards S352.2, Moisture measurement - unground grain and seeds. ASAE, St. Joseph, MI. Berbert, P.A. and Stenning, B.C, 1996a. Analysis of the density-independent equations for determination of moisture content of wheat in the radiofrequency range. Journal of Agricultural Engineering Research, 65, Berbert, P.A. and Stenning, B.C, 1996b. On-line moisture measurement of wheat. Journal of Agricultural Engineering Research. 65(4), Berbert, P.A., Queiroz, D.M., Sousa, E.F., Molina, M.B., Melo, E.C. and Faroni, L.R.D, Dielectric properties of parchment coffee. Journal of Agricultural Engineering Research, 81(1), Berbert, P.A., Queiroz, D.M. and Melo, E.C., Dielectric properties of common bean. Biosystems Engineering, 83(4), Boldor, D., Sanders, T.H. and Simunovic, J., Dielectric Properties of in-shell and Shelled peanuts at Microwave Frequencies. Transactions of the ASAE, 47(4), Corcoran, P.T., Nelson, S.O., Stetson, L.E. and Schlaphoff, C.W., Determining Dielectric Properties of Grain and Seed in the Audio- frequency Range. Transactions of the ASAE, 13(3), Jorgenson, J.L., Edison, A.R., Nelson, S.O. and Stetson, L.E., A Bridge Method for Dielectric Measurements of Grain and Seed in the 50-to 250-MHz Range. Transactions of the ASAE, 31(6), Kim, K.B., Lee, J., Lee, S.S., Noh, S.H. and Kim, M.S., On-line Measurement of Grain Moisture Content Using RF Impedance. Transactions of the ASAE, 46(3), Kim, K.B., Kim, J.H., 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), Kraszewski, A.W. and Nelson, S.O., Densityindependent Moisture Determination in Wheat by Microwave Measurements. Transactions of the ASAE, 34(4), Lawrence, K.C., Funk, D.B. and Windham, W.R., Dielectric Moisture Sensor For Cereal Grains And Soybeans. Transactions of the ASAE, 44(6), Lawrence, K.C. and Nelson, S.O., 1993a. Radio-frequency Density-independent Moisture Determination in Wheat. Transactions of the ASAE, 36(2), Nelson, S.O., Dielectric Properties of Grain and Seed in the 1 to 50-mc Range. Transactions of the ASAE, 8(1), Nelson, S. O., L. H. Soderholm and F. D. Yung., Determining the dielectric properties of grain. Agricultural Engineering 34(9): Nelson, S.O. and Stetson, L.E., Frequency and Moisture Dependence of the Dielectric Properties of Hard Red Winter Wheat. Journal of Agricultural Engineering Research, 21(2), Nelson, S.O. and Trabelsi, S., Dielectric Spectroscopy of Hard Red Winter Wheat. An ASABE Meeting Presentation Paper Number: Saçılık, K. and Çolak, A., Dielectric Properties of Opium Poppy Seed. Journal of Agricultural Sciences. 11(1), , Ankara, Turkey. Saçılık, K. ve Çolak, A., Dielektriksel Yöntemle Bazı Tahılların Nem İçeriğini Sürekli Olarak Ölçebilen Bir Düzenek Geliştirilmesi. TÜBİTAK-TOVAG 104O263 nolu proje kesin sonuç raporu (basılmamış), 96s., Ankara. Sokhansanj, S. and Nelson, S.O., Dependence of Dielectric Properties of Whole-grain Wheat on Bulk Density. Journal of Agricultural Engineering Research, 39(3), Stetson, L.E. and Nelson, S.O., A Method for Determining Dielectric Properties of Grain and Seed in the 200- to 500-MHz Range. Transactions of the ASAE, 13(4), Venkatesh, M.S. and Raghavan, G.S.V., An Overview of Microwave Processing and dielectric properties of Agri-food Materials. Biosystems Engineering, 88(1),

7 Kurt C. Lawrence, Stuart O. Nelson, Bartley, Jr., and Philip G., Flow-Through Coaxial Sample Holder Design for Dielectric Properties Me asurements from 1 to 350 MHz. Transactions of the IEEE S (98) Tomaraei, P., Development of RF Moisture Sensor for Measuring Moisture Content in Grain. Masters Thesis, Ankara University, The Graduate School of Natural and Applied Sciences (Unpublished). 558