Droplet size and nozzle selection

Droplet size is the part of drift control most directly in the operator's hands. The nozzle type, orifice size and pressure together determine the spray quality — the range of droplet sizes produced — and that decides both how well the product covers the target and how much of it is at risk of drifting.

ASABE droplet size classes

The American Society of Agricultural and Biological Engineers standard ASABE S572 (revised several times; the S572.1 edition is still widely cited) classifies spray quality by comparing a nozzle's output with reference nozzles. Nozzle manufacturers publish the class for each nozzle at each pressure. From finest to coarsest, the classes are:

• Extremely Fine (XF)
• Very Fine (VF)
• Fine (F)
• Medium (M)
• Coarse (C)
• Very Coarse (VC)
• Extremely Coarse (XC)
• Ultra Coarse (UC)

Many labels specify a droplet class, commonly as a minimum such as "Medium or coarser" or "Coarse or coarser" to limit drift. The label prevails.

Coverage versus drift

Smaller droplets give more droplets per litre and better coverage, which matters for contact products and small targets such as insects, fungal spores or narrow grass leaves. But they fall slowly, evaporate quickly and are carried far by the wind.

Larger droplets fall quickly and resist drift and evaporation, but give fewer droplets per unit area. They can bounce off hard-to-wet leaves, and coverage on small or vertical targets may be poorer.

As a general pattern, extension guidance associates finer droplets with contact fungicides and insecticides that need thorough coverage, and coarser droplets with systemic herbicides, which move within the plant. The label's recommendation comes first.

Matching droplet class to the weather

• Wind. As wind rises, move to a coarser class. Fine and Very Fine sprays are suited only to light wind. See wind speed for spraying.
• Delta T. When Delta T is high, droplets shrink in flight, so a coarser starting size helps them arrive. In the marginal 8–10 °C band, guidance usually calls for coarser droplets. See Delta T.
• Inversion. A coarser class reduces the fine fraction but does not make spraying under an inversion acceptable.

Nozzles and pressure

For a given nozzle, higher pressure generally produces finer droplets and lower pressure coarser ones. Air-induction nozzles draw air into the liquid and produce larger, air-filled droplets with fewer fines. Check the manufacturer's chart for the class at your operating pressure, and remember that worn nozzles change both output and pattern. Drones use their own nozzle or rotary atomiser systems; the droplet size is set in the drone's settings or by the nozzle fitted, according to the manufacturer.

Further reading

• Best Practices for Effective and Efficient Pesticide Application — Ohio State University Extension
• Selecting the Best Nozzle for the Job — Ohio State University Extension

In PulveCast

PulveCast uses six of these classes — Very Fine, Fine, Medium, Coarse, Very Coarse and Ultra Coarse — to rate drift risk against the forecast wind. In automatic mode it starts from your crop preset's ideal class (Medium for most, Coarse for cotton, Fine for coffee) and suggests a coarser one when the wind requires it.

More guides

• Delta T for spraying: what it is and why 2 to 8 °C
• Delta T chart: how to read it
• Thermal inversion and spraying
• Spray drift: causes and how to reduce it
• Wind speed for spraying
• The best time of day to spray
• Humidity and dew point for spraying
• Rain and spraying: rainfast periods and forecasts
• Temperature and spraying
• Weather for drone spraying
• Kp index, RTK and GNSS for spray drone operators
• Pre-spray checklist