Techniques for measurement of droplet size and velocity distributions in agricultural sprays

Abstract

Two techniques for measuring the size and velocity of droplets in agricultural sprays produced by hydraulic flat fan nozzles have been evaluated for their ability to produce consistent results. These were a one-dimensional phase Doppler particle analyser (PDA) and a two-dimensional imaging probe of a Particle Measuring Systems (PMS) instrument, both measuring velocities in a single plane and operated in conjunction with a computer-controlled nozzle transporter to enable the whole spray to be sampled. The different operating principles of the instruments resulted in different droplet size and velocity distributions in relatively dense, polydispersed sprays. PDA gave consistently lower values for the volume median diameter and higher droplet velocities than PMS. PMS gave lower percentages of spray volume in droplets less than 100 μm in diameter, while the PDA indicated some large droplets that were not detected by PMS. The required sample size was smaller for PMS (3000 droplets) than for PDA (13,000 droplets). Both instruments are useful for measuring the characteristics of agricultural sprays, providing that their limitations are recognized. Appropriately configured, such systems enable droplet size and velocity distributions, entrained air velocities, droplet trajectories and the spatial structure of sprays to be determined.