The Measurements of Transmission of Long-wave Radiation through Materials Used for Crop Protection in Agriculture

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  • 農業用被覆資材の長波長透過率の測定
  • ノウギョウヨウ ヒフク シザイ ノ チョウハチョウ トウカリツ ノ ソクテイ

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Abstract

With recent commercial development of materials, some plastic films and cheese clothes are broadly used in agriculture. These materials are used for the protection of plants against unfavourable climatic conditions, so that it is important to consider the characteristics of heat loss by long-wave radiation. This paper deals with the total transmission of long-wave radiation for some commercial materials; plastic films and cheese clothes.<br>The radiative exchange of the ambient field between two radiative heating plates, A and B, can be written by eq. (1) (see Fig. 1-a). If the materials (named AM and BM) are inserted into this radiation field, the radiative exchange of the field between AM and BM can be expressed by eq. (2) (see Fig. 1-b). In eq. (2), the terms higher than second order of α and γ are neglected since their effects are small, and if the radiation flux from AM (RAM) is equal to from BM (RBM), the equation can be simplified to eq. (4). From eq. (1) and eq. (4), the transmissivity can be calculated by eq. (5).<br>The experimental arrangement is shown in Fig. 2. As preliminary experiment, the instrument was tested that the output of the net radiometer was zero when the surface temperature of the plates A and B were the same, and was the radiation flux calculated from the surface temperature of these two plates (see Fig. 3). Moreover, it is necessary to satisfy that the radiation flux from AM is equal to from BM, so that the material was rotated continuously around a fixed net radiometer by an electric motor. To determine the rotation speed of this material, tests were performed by using clear p.v.c., and silver p.v.c., and 20 r.p.m. was chosen as the standard rotation speed (see Fig. 4).<br>The measured transmission of some materials are shown in table 1. The transmittances for p.v.c., e.v.a. and p.e. 0.1mm thick are 21-25%, 49% and 71-74% respectively. But these values are increased by reducing their thicknesses. The transmittance values of cheese clothes for crop protection are 38-71%. These values are affected with the amount of thread per unit area and the thickness of thread in the cheese clothes.<br>(p.v.c.=polyvinyl chloride)<br>(e.v.a.=ethylenevinyl acetate copolymer)<br>(p.e.=polyethylene)

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