Falling liquid film thickness measurement by an optical-electronic method

An optical-electronic method was developed for measurement of falling liqui d film thicknesses. The method is based on the postulate that a sheet light beam passing tangentially through a vertical liquid film on a cylinder wil l 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 op posite side of the cylinder from the light source will be reduced by an amo unt proportional to the film thickness. The shadow of the transient film sh ape will also appear on a screen behind the falling film. Therefore, the va riation 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 pictu res of the shadow using computer aided image analysis. The average film thi ckness 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 x 10(-2) mm. Compared with popu lar conductance parallel-wire probes, this method has the advantages of con venience, noninvasive, higher spatial and temporal resolution, smaller hyst eresis and it does not disturb the model's surface. (C) 2000 American Insti tute of Physics. [S0034-6748(00)00504-9].