Two color coding schemes exist to guide applicators when selecting nozzles for pesticide applications. The color schemes are designed to help applicators achieve accurate and effective crop protection.
It may be confusing, but the color schemes represent flow rate and droplet size and are independent of one another.
The first color scheme, the color of the spray nozzle tip, describes the capacity (flow rate) of the nozzle orifice at 40 PSI (pounds per square inch) and is based on Standard 10625 of the International Standards Organization (ISO). Nozzle flow rates are mainly a function of orifice size and pressure.
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).
Droplet sizes within any spray are never completely uniform, so VMD is used as an indicator of the average droplet diameters within a spray. VMD is the droplet size at which half of the total spray volume is contained in droplets larger than the VMD and half of the total spray volume are contained in droplets smaller than the VMD.
Pesticide manufacturers use the droplet size categories on pesticide labels to specify the optimum droplet size for a particular product.
A nozzle produces different droplet sizes depending on the operating pressure, nozzle design, spray angle, and the components of the spray mixture. Flow rate increases with the square root of pressure, therefore doubling the flow rate will result in a four-fold increase in pressure. Increasing the operating pressure or spray angle of the nozzle decreases droplet size.
Nozzle design features, including pre-orifices, mixing chambers, and Venturi inlets, can have dramatic effects on the VMD and the range of droplet sizes produced by a nozzle. Generally speaking, combinations of wide fan angles, smaller nozzle orifice sizes (lower flow rates), and high pressures result in smaller droplets.
Choosing a nozzle: The sprayer calibration should include determining the proper droplet size for a pesticide (included on the label). The applicator can then refer to nozzle manufacturer catalogs and nozzle tables to select the proper nozzle tip color to obtain the droplet size based on the desired operating pressure, sprayer travel speed, and desired carrier volume (also from the pesticide label).
To view the related tables for this UA bulletin, click on this link: http://ag.arizona.edu/crops/cotton/files/ColorsofNozzles_Droplets.pdf.
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