Capacitance wire-mesh sensor

Conductivity wire-mesh sensors have been successfully employed in the investigation of two-phase flows in the past. Since the measuring principle requires at least one continuous conductive phase, wire-mesh sensors have almost exclusively been used for the investigation of air-water or steam-water systems. Nevertheless, non-conducting fluids such as oil or organic liquids often occur in industrial applications, for instance, in chemical and petrochemical industry. The experimental investigation of multiphase flows involving non-conducting fluids is therefore of large interest. For this reason we developed a novel wire-mesh sensor based on measurements of the electrical permittivity (capacitance) which is suitable for the investigation of non-conducting fluids.

The main idea of the conductivity wire-mesh sensor has been maintained. One plane of wires is used as transmitter. The other one is used as receiver. During the measuring cycle, the transmitter wires are activated in a successive order while all other wires are kept at ground potential. For each time slice a transmitter wire is activated, the receiver wires are sampled in parallel. However, while in the conductivity wire-mesh sensor a bipolar excitation voltage and a DC measuring scheme is employed, for the capacitance measurement we have moved to an AC excitation and measuring scheme. Therefore a sinusoidally alternating voltage is now employed for excitation and the receiver circuit must encompass a demodulator. For the capacitance measurement of the crossing points we have employed an AC based capacitance measuring method, which is typical for many types of capacitance measuring circuits and has also been successfully used in electrical capacitance tomography.

Selected results

Static measurements with selected organic liquids, air and water in a relative permittivity (ε_r) range from one (for air) to 80 (for water) have been carried out. Maximum deviation of 10 % was found when compared with reference values. A noise level of 0.27 % or 0.9 fF was obtained. Thus, the measured capacitances have shown good linearity, accuracy and reproducibility. This way, the system is able to well distinguish permittivity differences of up to one (Δε_r = 1), which corresponds, for instance, a two-phase flow of air and oil (ε_r = 2). Furthermore, the comparison with a conductivity wire-mesh sensor has revealed a very good agreement. The figure below shows an example of a silicone oil/air bubbly flow measured with the capacitance wire-mesh sensor. The system enhanced the range of substances which may be investigated with a wire-mesh sensors, thus, being a valuable new tool for the investigation of multiphase flow.

Images of a silicone oil-air bubbly flow measured with the capacitance wire-mesh sensor. Four selected frames are shown. The image on left is an axial slice image taken from the central electrode, i.e. along a central chord of the channel.

The feasibility in the use of the capacitance wire-mesh sensor for the imaging of a three-phase flow is being investigated in a current research work. The figure below depicts the result of three-phase air-oil-water flow measurement. Besides the correctly visualization of each one of the three phases involved, the emulsion formation, characterized by relative permittivity values between 2 (for oil) and 80 (for water) is also properly captured by the sensor.

Three-phase flow measurement. The upper image shows the cross section of a stratified structure air/oil/water. The lower image depicts the time evolution of the separation process, where the central chord of the channel is shown.

Publications

Da Silva, M. J., Schleicher, E.; Hampel, U.
Capacitance wire-mesh sensor for fast measurement of phase fraction distributions
Measurement Science and Technology 18(2007)7, 2245-2251
doi:10.1088/0957-0233/18/7/059⁽¹⁾

Da Silva, M. J.; Schleicher, E.; Hampel, U.
Novel wire-mesh sensor for the investigation of non-conducting fluids
Proceedings of 6th International Conference on Multiphase Flow, paper S7_Thu_B_51, 978-3-86010-913-7

Da Silva, M. J.; Schleicher, E.; Hampel, U.
Capacitance wire-mesh tomograph for multiphase flow applications
Proceedings of 5th World Congress on Industrial Process Tomography, pp. 624-629, 03.-06.09.2007, Bergen, Norway

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