MEASURING DROPLET SIZE OF AGRICULTURAL SPRAY NOZZLES-MEASUREMENT DISTANCE AND AIRSPEED EFFECTS

Abstract

With a number of new spray testing laboratories going into operation and each gearing up to measure spray atomization from agricultural spray nozzles using laser diffraction, establishing and following a set of scientific standard procedures is crucial to long-term data generation and standardization across the industry. It has long been recognized that while offering ease of use as compared to other methods, laser diffraction measurements do not account for measurement bias effects due to differential velocities between differing sized spray droplets, and in many cases significantly overestimate the fine droplet portion of the spray. Droplet sizes and velocities were measured for three agricultural flat fan nozzles (8002, 8008, and 6510) each at three spray pressures (138,276, and 414 kPa) at four downstream distances (15.2, 30.5, 45.7, and 76.2 cm) across a range of concurrent air velocities (0.7−80.5 m/s). At air velocities below 6.7 m/s, large gradients in droplet velocities resulted in over-estimation of both the 10% volume diameter (Dv0.1) by more than 10% and the percent volume of the spray less than 100 µm (V<100) was overestimated two- to three-fold. The optimal measurement distance to reduce droplet measurement bias to less than 5% was found to be 30.5 cm with a concurrent air velocity of 6.7 m/s for measuring droplet size from ground nozzles. For aerial spray nozzles, the optimal distance was 45.7 cm. Use of these methods provides for more accurate droplet size data for use in efficacy testing and drift assessments, and significantly increases inter-lab reproducibility.