Estimation of measurement error and depth of field in PDIA experiments: a comparative study using water droplets and a calibration target

Abstract

The particle/droplet image analysis (PDIA) technique is often used for droplet sizing in sprays. Successful use of PDIA for drop size measurement in sprays mandates careful calibration, and determination of the depth of field (DOF) for particles of different size classes. This is typically done with a calibration target that has circles etched on it. In the present study, a calibration experiment was performed with a calibration target as well as droplets, to evaluate the uncertainties in the measured sizes and also to compare the measured DOF values. A continuous-jet droplet generator was used to produce mono-disperse droplets of sizes ranging from 54–393 µm, and the calibration glass plate used contained circles of sizes ranging from 20–400 µm. Images of these were captured using a CCD camera equipped with a long-distance microscope, and were processed using a code written in MATLAB®. Differences in the measured sizes of droplets and circles, and their DOF values are reported. It was observed that the use of the calibration target led to an underestimation of the error in droplet size measurements, and an overestimation of the DOF. For the droplets, the measurement error was observed to be less than 5%, with the parameters chosen.