Wire-Mesh Sensors
Wire-mesh sensors are measuring devices capable in acquiring the distribution of the electrical conductivity of a fluid within a matrix of electrodes. The conductivity signals are used to derive local instantaneous tracer concentrations in case of mixing experiments or local instantaneous gas fractions in two-phase flows. For the first purpose, tracer salts are used that increase the electrical conductivity of the fluid. In a two-phase flow, the inherent difference in the conductivity of both phases provides the primary signal. E.g. in a steam-water flow, the vapor is practically non-conducting, whereas the conductivity of pure water is sufficient to be detected.
The sensors were developed by Prasser and his team at the Research Center Rossendorf (Prasser et al. 1998) and are now widely used in numerous laboratories around the world. Two-dimensional distributions of the conductivity can be recorded with a high sampling frequency. Today, the limit is given by a dimension of the measuring matrix of 128x128 points, which are sampled with a maximum frequency of 1280 Hz. Smaller sensors can be read out faster, e.g. a matrix of 16x128 can be sampled at 10 kHz. The size of the matrix and the spatial arrangement of the electrodes defines the resolution of the sensor.
Electrodes usually consist of thin metallic wires that span over the measuring cross-section. The sensor consists of two grids of parallel wires crossing under a certain angle, preferably 90◦. During the signal acquisition, one plane of electrode wires is used as transmitter, the other as receiver plane. The transmitter electrodes are activated by supplying them with voltage pulses in a successive order. The current at a receiver wire resulting from the activation of a given transmitter wire is a measure of the conductivity of the fluid in the corresponding control volume close to the crossing point of the two wires. The currents from all receiver wires are sampled simultaneously. This procedure is repeated for all transmitter electrodes. After activating the last transmitter wire, a complete matrix of measured values is stored, which represents the complete two-dimensional conductivity distribution in the sensor cross-section.
A movie with animations of experiments with wire-mesh sensors can be downloaded from Downloadhere (WMV, 17.5 MB)vertical_align_bottom, for better resolution click Downloadhere (WMV, 51.9 MB)vertical_align_bottom.