Laser based measurement of water film thickness for the application in exhaust after-treatment processes

Abstract In this work, an absorption based laser sensor for the investigation of dynamically changing liquid film thicknesses is developed and validated. For the wavelength selection of the single-ended, fibre-coupled, diode-laser sensor, near-infrared spectra of liquid water are measured at various thicknesses and temperatures. To reduce the influence of the film temperature, the evaluation is supported by a calibration procedure. Unknown film thicknesses of up to 440 µm could then be measured. To adapt the sensor to particular boundary conditions of different systems, the single-ended setup can be changed to a transmissive configuration. The sensor is validated in both configurations against a commercially available chromatic confocal resonance (CHR) sensor. A comparison with respect to the CHR sensor leads to an accuracy of 2.8 µm and a precision of 3.9% of the new laser based sensor.