Drop volume measurements by vision

A specific configuration for liquid flow metrology consists of a flow of falling drops coupled with a preferred measuring method that derives the flow directly from the drop count. Given the inaccuracy of this counting method, alternative methods have been proposed that measure the volume of each falling drop. The principle consists in deriving the volume from geometric measurements obtained by vision and the basic problem can be described as the estimation of the volume of a drop from its projection. This paper reviews methods previously used and provides an analysis of qualitative and quantitative aspects of drop volume estimation for flow metrology. Three drop shape models and the related volume estimation methods are defined in a first part. A second part is devoted to an experimental analysis of drop shape variations. In a final experimental part, the presented methods are compared and the good performance of a volume measurement method is experimentally demonstrated. It shows a rms-error of 1% in normal measurement conditions. These figures speak for the interest of the measurement by vision and represent a good base for predicting the suitability of the method in various applications.