Falling liquid film thickness measurement by an optical-electronic method

An optical-electronic method was developed for measurement of falling liquid film thicknesses. The method is based on the postulate that a sheet light beam passing tangentially through a vertical liquid film on a cylinder will be blocked by the falling liquid film. Hence, when the beam is much wider than the film, the output of a photodiode probe which is located on the opposite side of the cylinder from the light source will be reduced by an amount proportional to the film thickness. The shadow of the transient film shape will also appear on a screen behind the falling film. Therefore, the variation of the amplitude of the film waves can be measured from the output of the photodiode and the average film thickness can be measured from pictures of the shadow using computer aided image analysis. The average film thickness measured agrees well with commonly accepted empirical equations. The maximum relative deviation between the experimental and calculated results is 18.5% with a standard deviation of 4.34×10^-2 mm. Compared with popular conductance parallel-wire probes, this method has the advantages of convenience, noninvasive, higher spatial and temporal resolution, smaller hysteresis and it does not disturb the model’s surface. © 2000 American Institute of Physics.