A look at three measurement techniques for bubble size determination

Three methods for measuring the volume and the equivalent radius of air bubbles at detachment from the tip of different capillary tubes in quiescent water were tested. The aim was to provide an accurate cross-calibration of two of the methods against a standard laboratory method. The inverted funnel method is the laboratory standard and performed within a 0.5% repeatability error for 50 bubble sets. The passive acoustic method performed within an accuracy between 97% and 99% with respect to the inverted funnel method. The photographic method gave an accuracy between 88% and 96%. After improvement of the photographic method by an empirical cutting edge criterion, its accuracy was raised between 95% and 99%. The bubble shape at detachment was found to be fitted by a Cassini's oval or by a Bernoulli's lemniscate according to the formation time. Qualitative observations on the bubble formation stage indicated an early vertical nose growth for the larger diameter capillary tubes at short formation times. The major-axis vertical and horizontal lengths were measured as functions of time at the formation stage.