Pesticide Application and Equipment in Greenhouses
Droplet Size Classification
Unfortunately, no practical spray nozzles are currently available to produce droplets that are all the same size. All commercial nozzles generate a range of droplet sizes. It is therefore difficult to exactly describe the output from a spray nozzle in terms of droplet size produced. The American Society of Agricultural and Biological Engineers (ASABE) developed a second color scheme used in nozzle literature and on pesticide labels that describes spray droplet sizes. This scheme defines droplet size ranges or categories using the Volume Mean Diameter (VMD).
Volume Mean Diameter
Droplet sizes within any spray are never completely uniform, so VMD is used as an indicator of the average droplet diameters within a spray. The VMD divides the droplet spectrum into two equal parts. One half of the total spray volume is made up of droplets larger than the VMD and the other half made up of droplets smaller than the VMD. A diagrammatic representation of VMD is shown in Figure 30.6. If droplets from a spray nozzle could be lined up in order of size, the VMD indicates the droplet size that would divide the sample in half by volume.
BCPC and ASABE S572.1 Classifications
The BCPC and ASABE S572.1 classifications describe spray quality (the range of droplet sizes produced by a nozzle). This spray quality is determined by comparing a nozzle’s output of different sized droplets (droplet spectrum) at a given pressure against the outputs of a set of standard reference nozzles. This is done using a laser-based instrument and, due to the comparative nature of the standard, nozzles will achieve the same classification regardless of testing technique.
Using Reference Curves to Understand Nozzle Outputs
By using the reference curves in conjunction with the spray quality classifications, nozzle outputs can be better understood. For example, a nozzle that has been assigned a FINE spray quality will produce droplet sizes within a particular range. Figure 30.8 shows that a fine nozzle will have a D[v,0.1] of 60 to 100 µm. This means that 10 percent of the spray volume is in smaller droplets than these sizes. A fine nozzle will also have a D[v,0.5] or VMD of 131 to 239 µm.
Using BCPC Spray Quality Classifications to Select Nozzles
When selecting a nozzle for a particular purpose, an understanding of what the classifications mean in terms of the types of droplets each classification includes is required. The behavior of various droplet sizes can then be used to select the appropriate nozzle for particular situations.
Droplet Sizes for Different Targets
Regardless of the target, the objective should be to obtain the best coverage possible while minimizing the off target losses, such as drift or run-off, as much as practicably possible. The understanding of the droplet sizes required for different targets is gradually improving.
Nozzle Selection
Nozzle selection is an important factor when considering spray drift. It is well known that spray drift can be minimized—and spray efficiency maximized—by selecting an appropriate nozzle for a spray job.
Spray Pressure
Spray pressure should be as low as possible, consistent with nozzle specifications and coverage requirements. Check the manufacturer’s nozzle catalogs for recommended pressure of operation.
Spray Volume
A larger nozzle orifice increases the droplet size when operated at the same pressure as a smaller nozzle orifice. It will also increase the rate of flow and thus the amount of spray used per unit of time.
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