Accurate Double Transmission Measurement Concepts for the Permittivity Determination in Pneumatic Conveying Tubes with Microwaves Baer, C. #1 , Gerding, M. #1 , Pohl, N. *1 , Vogt, M. %1 , Musch, T. #1 # Institute for Electronic Circuits, Ruhr-Universitaet Bochum Universitaetsstr. 150 44801 Bochum Germany 1 christoph.baer@rub.de * Institute for Integrated Systems, Ruhr-Universitaet Bochum % High Frequency Engineering Research Group, Ruhr-Universitaet Bochum Abstract—In this contribution, two measurement concepts for the determination of volume fractions in multi phase flows are presented. Essential requirements for the proper functionality of double transmission measurement concepts, such as the feed side reflection suppression, are presented. The first measuring method uses a two state modulator which allows a high accurate feed side reflection suppression. The second measuring method makes use of a transpolarizing reflector which improves measuring speed and decreases setup complexity. In order to verify the introduced measuring methods, various electromagnetic field simulations and measurements on a borosilicate glass tube are presented and discussed in detail. Index Terms—Double transmission measurement, FMCW RADAR, permittivity composition. I. I NTRODUCTION Pneumatic conveying systems are used for the transportation of bulk materials for loading ships or silos and e.g. for loading combustion chambers of furnaces and coal-fired power plants. The transported bulk always consists of at least two materials. While one material is a transportation gas, the other material is the transported bulk. By dispersing the bulk inside the gas flow, a quasi-homogeneous medium that passes the conveyance tubes is created. In case of coal-fired power plants, the mass flow determination of the transported bulk inside the conveying tube is of interest as it allows operators to optimize the combustion process.In [1] it has been shown that the particle’s velocity v and the volume fraction ζ are two physical quantities to be investigated. Common approaches for the determination of the volume fraction make use of backscatter measurements [2], resonator tuning measurements [3] or cut-off frequency measurements [4]. In [5] additional methods like optical, acoustical and electro statical methods are described. Another way to determine the volume fraction is the measurement of the propagation speed of electromagnetic waves inside the conveying tube which is directly connected to the effective permittivity of the tube’s interior. While the effective permittivity depends, by means of an adequate mixture formula, on the volume fraction of the conveyed material composition, a transmission measurement through the cross section of the conveying tube is a precise way for determining the volume fraction. The proposed measuring concept in [1] uses a vector network analyzer which is cost intensive and not applicable for industrial purposes. In [6] a double transmission measurement concept was proposed that, by means of a two state modulator, already showed the functionality of a double transmission measurement. In this paper, an advanced double transmission two-state modulator concept and a novel double transmission cross polarization measurement method is presented. II. FUNDAMENTALS A. Materials In order to perform microwave measurements on coal dust- gas compositions, some relevant properties of the transported coal must be investigated. Before transported by a pneu- matic conveying system to a combustion chamber, the coal is pulverized to a particle size of 5..100 μm. Additionally a heating process reduces the coal moisture content to 10..14 %. Within these restrictions the relative permittivity of the coal is about ε r,coal =2.4 .. 2.8. With an electric conductivity of σ coal ≈ 1.5 · 10 −6S m coal can be treated as a dielectric medium [7]. Because of the small particle size compared to the wavelength of microwaves, the coal dust-gas composition can be described as a quasi-homogeneous dielectric medium with an effective permittivity. In literature, various mixture equations are known which all depend on the permittivities of the compound materials and their volume fraction. In this work the mixture equation by Maxwell-Garnett which describes the mixture of spherical matters with a background material is used [8]. Due to the assumption of the effective permittivity and the quasi homogeneous medium of the tubes interior, simulations and measurements can also be performed on solid materials. B. Double Transmission Measurement Concepts In [6] it was shown, that the determination of the volume fraction ζ can be performed by delay time measurements. Therefore the time of flight (TOF) of an electromagnetic wave propagating through the tube’s cross section is an indicator for the effective permittivity which depends on the volume fraction. When analyzing the signal that, due to a defined reflection on the tube’s opposing wall, passes the tube’s cross section twice, a measuring device with only one measuring port, e.g. a RADAR-system, can be used. The TOF T doub of the double transmitted signal is given by: