Multi-range sensors for the measurement of liquid film thickness distributions based on electrical conductance

• Multi-range approach presented for an electrical film thickness sensor for fast two-dimensional liquid film thickness measurements.
• Three interlaced electrode matrixes allow extending the measuring range by a factor of 4 compared to our earlier solutions.
• High spatial resolution in the small film thickness range is combined with low spatial resolution in the high film thickness range.
• Extension of the measuring range uses low impedance features of existing signal acquisition circuits for wire-mesh sensors.
• Functionality of the sensor has been successfully demonstrated in a wavy stratified air-water flow in a horizontal pipe.

The paper presents an approach toward an enhancement of the measuring range of high-speed sensors for the measurement of liquid film thickness distributions based on electrical conductance. This type of sensors consists of electrodes mounted flush to the wall. The sampling of the current generated between a pair of neighboring electrode is used as a measure of the film thickness. Such sensors have a limited measuring range, which is proportional to the lateral distance between the electrodes. The range is therefore coupled to the spatial resolution. The proposed new design allows an extension of the film thickness range by combining electrode matrices of different resolution in one and the same sensor. In this way, a high spatial resolution is reached with a small thickness range, whereas a film thickness that exceeds the range of the high resolution measurement can still be acquired even though on the costs of a lower spatial resolution. A simultaneous signal acquisition with a sampling frequency of 3.2 kHz combines three measuring ranges for the characterization of a two-dimensional film thickness distribution: (1) thickness range 0–600 µm, lateral resolution 2×2 mm2, (2) thickness range 400–1300 µm, lateral resolution 4×4 mm2, and (3) thickness range 1000–3500 µm, lateral resolution 12×12 mm2. The functionality of this concept sensor is demonstrated by tests in a horizontal wavy stratified air–water flow at ambient conditions. Using flexible printed circuit board technology to manufacture the sensor makes it possible to place the sensor at the inner surface of a circular pipe.