10.2 Nozzles, Droplets, and Coverage

Why the tip is half the sprayer

40 CFR 171.103(c)(6) pairs equipment with use and maintenance; nozzle choice is how a hydraulic sprayer actually delivers product. The same gallon of mix can be a covering fungicide spray or a drifting fine mist depending on the tip, the pressure, and the boom height. Oklahoma wind, low humidity, and inversion mornings make that choice a legal and practical issue. Chapter 11 will turn output into gallons per acre. Here you learn what the tip does to pattern and droplet size.

A nozzle is an orifice plus a geometry. It meters flow and shapes the spray. Two tips with the same catalog flow number are not interchangeable if one is a tapered flat fan and the other is a flood or a cone. Core questions often hide in that difference: banding versus broadcast, airblast pecans versus a wheat boom, or a drift-reducing venturi tip used on a contact insecticide that needed coverage.

Nozzle families you must recognize

Flat-fan nozzles are the standard boom tip for broadcast herbicides on wheat, pasture, and turf. The pattern is tapered: thinner at the edges, heavier in the center. Adjacent fans must overlap so the field does not get stripes of underdose between nozzles and overdose in the centers. Flat fans are pressure-sensitive: they have a rated band where the fan stays whole. Below that band the pattern streaks; well above it the spray goes finer and more driftable.

Even-fan (even-spray) tips put a more uniform band under a single nozzle. Use them for banding over a row, for directed strips, or for a single-nozzle wand where you do not intend overlap. Putting even-fan tips on a broadcast boom with the usual tapered-fan overlap overdoses the overlap zones. That is a method error, not extra insurance.

Flood (flooding-fan) nozzles run at relatively low pressure, throw a wide angle, and produce coarser droplets. They suit some soil-applied herbicides and fertilizer mixes on Oklahoma pasture and row-crop ground. Pattern edges are less crisp than a new flat fan, and boom height still matters. Flood tips are not a free pass to spray in high wind; large droplets still move if the boom is high and the wind is strong.

Cone nozzles—hollow-cone and full-cone—are the usual choice for airblast pecans, directed foliar insecticides, and some handheld wands. Hollow cones produce a ring of smaller droplets that wrap foliage. That wrap is why they appear on orchard and some vegetable jobs. They are not automatically the right boom tip for a pasture herbicide, where a coarser flat fan or flood may be specified.

Air-induction (venturi) tips draw air into the droplet. The resulting spray is coarser and more drift-resistant, and it is often preferred for systemic herbicides when the label allows that spray quality. Air-filled droplets can bounce or give weaker coverage on a contact insecticide or protectant fungicide if you treat them as identical to a fine flat fan. Raindrop and other pre-orifice drift-reduction tips also shift the spectrum coarser. "Drift-reduction" is not a synonym for "always better."

Handheld 7A wands use the same families in miniature: a pin-stream or crack tip for crevices, a fan for surfaces, a coarse low-pressure tip when staining and drift matter. Switching to a fine cone "for better kill" along a foundation on a windy day is still a droplet decision.

Spray quality: ASABE fine, medium, coarse, extra coarse

The American Society of Agricultural and Biological Engineers (ASABE) spray-quality classification describes the droplet spectrum relative to reference nozzles. Core teaching uses class names including fine, medium, coarse, and extra coarse (finer and coarser classes exist on the full scale). The point is not to memorize a single magic micron number. The point is that finer classes increase the fraction of droplets that stay airborne, and coarser classes fall faster, drift less, and may bounce or miss the underside of a leaf.

Labels increasingly specify a spray-quality class or prohibit fine spray. When a wheat herbicide next to grapes or a homestead calls for coarse or extra-coarse spray, that instruction is about off-target movement, not about saving money on tips. When a foliar fungicide or contact insecticide needs coverage, a medium or finer class—if the label allows it—may be required for the product to work. Always read the product's spray-quality statement.

The chart below is a teaching picture of how volume median diameter generally rises as you move from fine toward extra coarse. Actual classification uses ASABE reference nozzles and test conditions, not a pocket micrometer on your boom.

Pressure, pattern, boom height, and overlap

Pressure changes both flow and droplet size. Raising pressure through a given orifice increases output (the exact curve is calibration math in Chapter 11) and shifts the spectrum finer. Lowering pressure coarsens droplets but can collapse a fan into streaks if you drop below the tip's rated range. Set pressure at the boom or gun, with a working gauge, inside the manufacturer's recommended band for that tip. Doubling pressure does not double droplet control in a useful way; it often buys drift.

A practical Oklahoma example without doing gallons-per-acre math: a turf boom set at a pressure that makes a crisp 110-degree fan will throw a different pattern if the operator later "bumps" pressure to push the gun farther into a bunker. The boom may now be producing more fines over the fairway while the gun is a different rate. Treat boom and gun as two systems that happen to share a tank.

Boom height and overlap determine whether tapered flat fans produce an even blanket. For common 80-degree and 110-degree flat fans operated at the height recommended for their spacing, National Core teaching uses about 30–50% overlap of adjacent patterns. Too low: strips of underdose between nozzles. Too high: a wider pattern, more hang time, and more drift. Do not "fix" a low boom by cranking pressure into a fog, and do not "fix" missed strips by driving faster—speed belongs with calibration in Chapter 11, and it does not rebuild a collapsed fan.

110-degree fans usually run lower than 80-degree fans at the same spacing because the wider angle meets the neighboring fan sooner. If you swap 80-degree tips onto a boom that was set for 110-degree tips and leave the height unchanged, overlap falls and stripes appear. Height is part of nozzle selection, not a separate habit.

Worn nozzles and mixed tips

Worn nozzles increase flow. The orifice erodes, especially with wettable powders, suspension fertilizers, and grit. Output climbs while the pattern distorts—often heavier in the center or with streaks at the edge. A tip that looks only slightly oval can put on well over the intended rate. Core expectation: compare output to catalog flow (Chapter 11) and discard tips that exceed the wear limit your manual uses (commonly about 10% over new-tip flow). Clean with a soft brush, toothpick, or compressed air. A pocket knife, wire, or brass pick ruins the orifice and creates a permanent streak.

Never mix nozzle types, sizes, or spray-quality classes on the same boom unless a manufacturer has designed that configuration (for example a documented pulse-width system with specified tips). A flood tip next to a fine flat fan gives two rates and two droplet spectra in one pass—streaked control and unexplained complaints from the next field. Replace the whole set when you change jobs. Color-coded tips help only if every body on the boom actually matches.

Section valves that are clogged, a strainer that bypasses, or one boom end that sags will mimic "bad nozzles." Inspect plumbing and boom level before you blame the catalog.

Drift reduction versus coverage

Drift reduction and coverage pull in opposite directions. Contact insecticides and protectant fungicides need droplets that deposit on the pest or the leaf surface; too coarse a spray can fail even when the gallons per acre are later shown to be "correct." Systemic herbicides that move in the plant after a reasonable deposit often allow a coarser, more drift-resistant quality when the label says so—important when an Oklahoma wheat field sits beside grapes, gardens, or pecan. Buffer that choice with wind speed, boom height, shields if used, and whether a temperature inversion is sitting over the prairie at dawn.

Preventing pesticide loss into the environment is also a Core application-method standard at 40 CFR 171.103(c)(7)(iv). Nozzles are one control, not the only one. A coarse venturi tip on a high boom in a gust still loses product. A fine hollow cone at a labeled pressure still drifts if you mist-blow a foundation toward a neighbor's vegetables.

If a 7A technician switches tips to "get better kill," the extra mist can move off the three-foot adjacent strip that the credential even allows (Section 10.3). That is not a nozzle upgrade. It is a droplet and placement decision that must still respect the label, the wind, and the site.