Development of a phase-doppler technique for mass balance and spray characterisation of orchard air-blast sprayers within New Zealand horticultural cropping systems

Abstract

The knowledge of the sprays emitted from orchard air-blast sprayers has historically been assessed using an array of samplers to capture airborne particles such as various strings/ribbons, paper material, and/or patternation structures. While these methods provide flux data, no other information is obtained which is pertinent to understand the potential movement of droplets. Qualitative droplet information can be acquired in situ which can be related to the deposition, coverage, and off-target losses. However, the quantitative analysis for agricultural sprays has predominately been conducted in a laboratory setting with the use of laser devices which are comprised of multiple pieces; ergo the necessity for controlled environments for the alignment of these pieces is essential. In this research, a new self-contained phase Doppler (pD) was tested to assess the droplet size spectrum, velocity, and flux in uncontrolled outdoor field conditions with the overall hypothesis that the pD will be a superior means of data collection in that the data will be more robust with fewer sources of error, highly repeatable, fast, and inexpensive. To test this hypothesis, a step-wise research plan was developed to determine 1) if pD could accurately measure flux by traversing through a similar spray plume to an orchard sprayer while still in the controlled setting of a laboratory; 2) compare and validate pD derived flux data to that of passive strings collectors in a wind tunnel in areas of heightened flux and droplet/air velocities; 3) compare these samplers in outdoor environments with no crop presence; and 4) determine if pD could be used in place of other collectors in a horticultural setting. Results demonstrated an average error of the computed flux versus measured flow rate was -3.3% using a disc core (D1/DC33) hollow cone nozzle at spray pressures of 3.1, 4.1, and 5.2 bar pressure (45, 60, and 75 psi) and at five heights (10, 20, 30, 40, and 50 cm). In the wind tunnel with varying wind speeds (1.4, 4.2, 8.3, 12.5, and 16.7 m/s) and spray exposures times (5, 10, 15, 30, and 60 s), the pD accurately measured the spray flux while the string samplers overload with saturation. From here, the pD was taken outdoors and displayed that the sampling volume of the pD was too small to acquire enough samples for sufficient flux data; therefore this research ceased. However, in all of these studies, regardless of the sampling frequency and inadequate flux output, important data was still acquired related to the droplet size distribution and droplet velocity. This is thought to be a major point of difference whereas pD may not yet be able to be the sole tool, but an important support tool for other instruments that can only measure flux. Lastly, the ability to quantitatively understand the droplet size differentiations at various heights and distances in relation to a crop can provide profound feedback to the application of plant protection chemistries, their fate, and their efficacy.