14.2 Nozzle Selection and Droplet Size

Quick Answer: Pick the orifice for the GPM the 5940 formula needs, then pick the nozzle type and pressure for the droplet spectrum the label names. ASABE classes run XF → VF → F → M → C → VC → XC → UC. Flow ∝ √pressure — doubling psi does not double GPM. A label that says medium or coarser is mandatory under N.J.A.C. 7:30-10.2(a). Finer spray is a particle-drift setup under 10.2(f).

40 CFR 171.103(c)(6)(ii) is calibration and the use of the equipment you calibrated. 40 CFR 171.103(c)(7)(iv) is prevention of drift and pesticide loss. The controllable hardware for particle drift is the nozzle, the pressure, and the boom height. Vapor drift is a formulation and temperature problem; a coarser tip will not stop a volatile ester from leaving a hot parking lot. Chapter 10 already covered inversions. This section is the chart, the square-root law, and the New Jersey duty to use the spectrum the label printed.

ASABE droplet categories (XF through UC)

The American Society of Agricultural and Biological Engineers (ASABE) S572 standard classifies spray quality by comparison with reference nozzles, not by a single micrometer number you must memorize for PACER. Manufacturer charts print the class at each pressure. Learn the order and what it means in the field:

Volume median diameter (VMD or Dv0.5) is the droplet diameter that splits the spray volume in half: half the volume is in smaller drops, half in larger. Charts may list VMD, but the legal language on modern labels is usually the ASABE class. If the Directions for Use say apply as a medium or coarser spray, Fine is illegal even if your GPA is perfect. That sentence is mandatory labeling, not a suggestion. N.J.A.C. 7:30-10.2(a) incorporates it. 10.2(e) reasonable precautions and 10.2(f) foreseeable nontarget deposit still apply if you chose Fine next to a school playground on a 12-mile-per-hour wind.

Finer droplets: more drops per gallon, better coverage of small insects or leaf undersides, longer hang time, more evaporation in New Jersey summer heat, more particle drift. Coarser droplets: less drift, fewer drops, possible skipping on a dense canopy if you do not raise volume or slow down. Coverage problems are fixed with GPA, angle, and boom height, not by violating a coarse-spray mandate.

Reading a nozzle chart

A chart row is a tip; columns are pressures. Each cell is GPM and often a spray-quality letter. Standard flat-fan tips are commonly rated at 40 psi. Air-induction (venturi) tips are often rated at 40–80 psi and produce a coarser class at the same orifice because they mix air into the drop.

ISO 10625 color codes (industry convention, not a New Jersey statute) at the usual 40 psi flat-fan reference:

A red 04 is 0.40 GPM at 40 psi, which is the nozzle behind the 19.8 GPA example in 14.1. It is not “0.40 GPA.” It is not legal just because it is red. Match orifice + pressure + type to the GPM you calculated and to the ASABE class on the label.

Spray angle (80° vs 110°) changes the boom height needed for overlap. A 110° fan can run lower, which shortens drop flight time and cuts drift. Too low, and you get stripes; too high, and the pattern thins and drifts. Tapered flat fans overlap about 30–100 percent on a broadcast boom. Even (E) fans are for banded work without overlap. Hollow cones make a large Fine fraction — a poor default for outdoor herbicides next to a nontarget. Flood tips are coarser and common on turf booms. Air-induction tips are the usual way to stay Coarse to Ultra Coarse without starving GPM.

Worn orifices over-apply. A 04 that should be 0.40 GPM and measures 0.48 GPM is a 20 percent over-application at the same speed. Industry practice is to replace when output is about 10 percent over the chart; that 10 percent is not a number printed in N.J.A.C. 7:30. 9.9(a) still requires proper maintenance, and 9.9(b) still requires the equipment you use to be calibrated. A lying gauge is a 9.9(a) problem: you cannot know pressure, class, or GPM.

Pressure versus flow: the square-root law

GPM₂ = GPM₁ × √(PSI₂ / PSI₁)

Flow is not proportional to pressure. It is proportional to the square root of pressure.

Doubling pressure does not double flow. √2 ≈ 1.41. A 0.40 GPM tip at 40 psi becomes about 0.56 GPM at 80 psi — a 41 percent increase, not 100 percent. GPA at 6 MPH and 20-inch spacing goes from 19.8 to about 28 GPA, not 40 GPA. If you needed to double output you would need four times the pressure (40 psi → 160 psi), which is outside most fan ratings, shreds droplets toward Fine/Very Fine, and is not a calibration method.

Worked square-root checks:

• 0.40 GPM at 40 psi → 0.50 GPM: 0.50 / 0.40 = 1.25 = √(P / 40) → P / 40 = 1.5625 → P = 62.5 psi. Then re-read the chart for ASABE class at 62 psi. Many 04 extended-range tips move from Coarse toward Medium as pressure rises.
• 0.40 GPM at 40 psi → 20 psi: √(20/40) = √0.5 ≈ 0.707 → 0.28 GPM. Pattern may collapse (striping) even if the class looks coarser. Stay inside the manufacturer’s pressure band.

Use pressure as a fine trim after the orifice is in the right family. Use orifice size and speed for large GPA changes. Use nozzle type (standard vs air-induction) for droplet class. Using pressure as the only knob is how a 3B crew in Burlington “fixes” coverage by creating a Fine mist that 10.2(f) will treat as foreseeable drift onto the next patio.

Boom height, overlap, and New Jersey 10.2

Lower boom (within the angle’s overlap window) = less hang time = less particle drift. Bounce at high speed raises the effective height even if the gauge says 20 inches. 10.2(e) reasonable precautions include setting height, pressure, and class so off-target movement is not reasonably foreseeable. 10.2(d) still forbids directly spraying a nontarget. A nozzle that streaks onto the sidewalk is not “drift philosophy”; it is a direct deposit.

Drift-reduction adjuvants can help when the label allows them. They do not legalize a Fine spectrum the label forbids, an inversion, or wind above the labeled maximum.

Category 11 awareness (aerial nozzles)

40 CFR 171.103(c)(7)(ii) requires extra certification for aerial application. New Jersey Category 11 is that extra exam, taken with Core and a related use category. 40 CFR 171.103(d)(15) — the federal aerial standard — still belongs on Core radar because the same physics appears in the Core equipment items:

• Inspect the system before the load.
• Interpret a nozzle flow-rate chart.
• Determine how many nozzles you need from flow, aircraft speed, and swath width.
• Place nozzles to compensate for wingtip vortices, rotor wash, and propeller turbulence.
• Place nozzles to produce the labeled droplet size.
• Verify flow with timing, a known distance, or a flow meter.

N.J.A.C. 7:30-10.6(d) then requires that aerial equipment be properly calibrated to the manufacturer’s specification for the specific type of application, with proof kept by the aerial pesticide applicator business. Section 14.3 works that proof rule. For this section: an aerial “fine mist for coverage” that the product label forbids is the same 10.2(a) mistake as on a turf boom, with a larger swath.

Worked New Jersey scenarios

Scenario D — Fine on a coarse label. A 3B herbicide in Middlesex County says “apply as a coarse or coarser spray.” The crew installs standard 04s and runs 70 psi to “push through” the thatch. The chart at 70 psi is Medium or Fine. GPA may still be near 20. The spectrum is still a label violation and a 10.2(f) drift setup next to vegetable gardens.

Scenario E — doubling psi to double GPA. A 6B operator at 20 GPA wants 40 GPA, so he doubles 40 psi to 80 psi and leaves the 04s in. Flow becomes 0.56 GPM, GPA about 28, droplets finer. He did not double GPA. He created a different, illegal-if-labeled-coarse application. Larger orifices or slower MPH are the tools.

Scenario F — color as GPA. An operator says “we always spray red, that’s 4 gallons.” Red 04 is 0.40 GPM at 40 psi. At 6 MPH / 20-inch it is 20 GPA. At 10 MPH it is 12 GPA. Color is an orifice reminder, not a rate.

High-yield traps

• Doubling pressure ≠ doubling flow; flow ∝ √pressure.
• ASABE class on the label is mandatory language.
• XF–F raise particle drift; they do not cause vapor drift.
• ISO color is GPM at a reference psi, not GPA.
• Worn tips over-apply; a bad gauge falsifies every chart cell.

Official resources

• 40 CFR 171.103(c)(6)–(7) and (d)(15)
• N.J.A.C. 7:30-10.2 pesticide use
• N.J.A.C. 7:30-10.6 aerial application
• N.J.A.C. 7:30-9.9 application and safety equipment