Agricultural Herbicide Spray Nozzles: Spec for Drift Control Without Losing Coverage

Herbicide drift is usually decided before the sprayer leaves the yard. The agricultural herbicide spray nozzles on the boom set the droplet size, and droplet size decides how far the product travels. A fine mist looks like perfect coverage. It is also the spray most likely to end up on the neighbour’s crop. The numbers are unforgiving. Droplets under 150 µm can stay airborne for minutes and travel hundreds of metres. A 500 µm droplet falls from boom height in about a fifth of a second. This guide covers the selection chain in order: droplet classes, nozzle types, pressure and wind. Every section ends with a number you can use at the parts counter.

Droplet size sets the drift budget

Small droplets are light, and light droplets follow the air. Terminal velocity is the physics that matters. A 100 µm droplet falls at about 0.25 m/s. A 500 µm droplet falls at roughly 2.3 m/s, about nine times faster. From a boom at 0.5 m, the 500 µm droplet reaches the crop in about 0.2 seconds. The 100 µm droplet takes about two seconds. At a 4 m/s crosswind that means under a metre of drift for the coarse droplet and roughly eight metres for the fine one.

Below about 150 µm the behaviour changes again. Evaporation accelerates as droplets shrink, because surface area grows relative to volume. A fine droplet can lose a meaningful share of its mass before touchdown. The size that lands is smaller than the size that left the nozzle. The smallest fraction, roughly below 100 µm, stops behaving like falling rain entirely. It follows turbulence, hangs in the air, and shows up hundreds of metres downwind. Drift-control programmes therefore track one number: the volume fraction of the spray below 150 µm, often called driftable fines.

The consequence is a hard design rule. For herbicides, droplet size is chosen on purpose, never accepted as a side effect of the cheapest tip on the shelf.

ASABE S572: the droplet language

Everyone in spraying talks about droplet classes, and the vocabulary is ASABE S572. The standard sorts spray quality into named classes from Very Fine to Ultra Coarse. Each class is anchored to reference nozzles run at set pressures, so the classes are comparable across brands. Major tip makers print the class on the box, which means agricultural spray tips from different manufacturers can be compared directly. In the field, most operators work from the simplified micron bands below.

Droplet class	Approx. VMD (µm)	Drift risk	Typical herbicide role
Very fine	< 150	Very high	Avoid for herbicide work
Fine	150–250	High	Coverage-critical work, calm air only
Medium	250–350	Moderate	Balanced coverage and drift
Coarse	350–450	Low	Systemic herbicides, standard choice
Very coarse	450–550	Low	Drift-sensitive fields
Extremely coarse	550–650	Very low	Buffer zones and sensitive boundaries
Ultra coarse	> 650	Minimal	Rare; leaf coverage suffers

The class names matter more than the micron numbers, because the label speaks in classes. Many herbicide labels state a minimum droplet class, for example Coarse or coarser. That instruction is a nozzle spec, not a suggestion. Run below it and the product label no longer covers the application. Run at or above it and you have a defensible record if a drift complaint arrives later.

One more reading rule: a class rating is only valid at the pressure it was measured at. The same tip can rate Medium at 1.5 bar and Coarse at 3 bar. Always match the rated class to the pressure you actually run, not the pressure on the pump gauge when the tank is full.

Flat fan nozzles: the herbicide workhorse

The agriculture sprayer nozzle types that cover most herbicide work are the tapered-edge flat fan, the even flat fan and the air-induction family. For most of the season the standard flat fan is the right tool. It throws a tapered-edge sheet of spray that overlaps with the neighbouring nozzle to give even coverage. The geometry rules are simple. At 50 cm nozzle spacing, a 110° flat fan runs with the boom at about 50 cm above the target. An 80° tip needs more height, roughly 75 to 80 cm, to close the pattern. Overlap in the 30 to 50% range is where even coverage lives.

Two flat fan facts save most of the trouble on a sprayer. First, flow scales with the square root of pressure: Q2 = Q1 × √(P2/P1). Doubling pressure from 2 to 4 bar raises flow by about 41%. Pressure is therefore a flow dial, not a coverage dial. The fan angle stays essentially constant across the working pressure range. Second, tip size follows the ISO 10625 colour code: blue 01, red 015, yellow 02, green 025, orange 03, lilac 04, brown 05. The number is the flow in US gallons per minute at 2.8 bar. A 03 tip flows about 0.3 gpm, roughly 1.2 L/min, at that pressure.

The even flat fan is the variant for band spraying. Its edges are uniform instead of tapered, so a 25 cm band stays 25 cm instead of fading at the edges. It costs more and wears the same way, but for row-crop banding it is the only tip that holds the dose across the band.

Air-induction nozzles: cutting the driftable fraction

Air-induction, sometimes called venturi, nozzles are flat fans with an internal rebuild. Liquid passes through a pre-orifice into a chamber, where it pulls in air, then exits through a second orifice. Each droplet leaves as a shell that carries air inside. The droplet is larger and heavier for its volume, so it falls faster, and it carries far less fine material.

The measurable effect is on driftable fines. A standard flat fan at working pressure can put 10 to 20% of its volume into droplets under 150 µm. An air-induction tip at the same flow typically stays under 5%. That difference is the whole drift story. Cut the fine fraction and the spray stops travelling.

Nozzle type	How it controls drift	Typical VMD	Driftable fines (< 150 µm)	Best for
Standard flat fan	No drift control	250–350 µm	10–20%	General work, no sensitive boundary
Air-induction flat fan	Pre-orifice plus venturi air entrainment	400–600 µm	< 5%	Herbicides near sensitive crops
Even flat fan	Uniform edge for banding	250–350 µm	10–20%	Banded herbicide over rows
Air-induction even fan	Banding plus air entrainment	400–600 µm	< 5%	Drift-safe banding

Two practical limits apply. Air-induction droplets are bigger, so they cover less leaf surface per litre and they can bounce or run off waxy leaves. Compensate with water volume. Moving from a Medium to a Coarse class usually means raising the carrier. Go from about 100 L/ha to 150 L/ha or more to hold the same deposit. And air-induction tips still need pressure inside their design band, typically 2 to 4 bar. Below that, the venturi stops pulling air and the drift control quietly disappears.

Coverage vs droplet size: the tradeoff in numbers

Drift control pushes droplet size up. Coverage pushes it down. The geometry is exact, so the tradeoff is easy to quantify. At a fixed application rate, droplet count scales with the inverse cube of diameter. Halve the diameter and the same litre makes eight times more droplets.

The table below assumes a uniform droplet size at 100 L/ha and no retention losses. Real sprays spread across a spectrum, so treat the numbers as a scale, not a forecast.

Droplet VMD	Drops per cm² at 100 L/ha	Surface coverage	Drift risk
100 µm	~1,900	~15%	Very high
200 µm	~240	~7.5%	High
350 µm	~45	~4%	Low
500 µm	~15	~3%	Very low

What does a herbicide actually need? Systemic products, which move inside the plant, work with 20 to 30 droplets per cm². Contact products, which must hit the leaf surface directly, want 50 to 70 droplets per cm². The table shows why Coarse classes need more water. At 100 L/ha, a 500 µm spray lands about 15 droplets per cm², below the systemic threshold. Raise the carrier to 150 L/ha and that becomes about 22 droplets per cm², back inside the band. Water volume is the dial that keeps coverage alive when the droplet class moves coarse.

Pressure: stay inside the 2–4 bar band

Pressure does two things at once, and both matter. It sets the flow, and it sets the droplet size. The flow law is Q2 = Q1 × √(P2/P1). The droplet law is softer but consistent: droplet diameter scales roughly with pressure to the power of minus 0.3. Doubling pressure from 2 to 4 bar therefore shrinks droplet diameter by about 20%. A tip that ran Medium at 2 bar can sit in Fine territory at 4 bar, and the driftable fraction climbs with it.

The working rule for herbicide work is to stay in the 2 to 4 bar band and let the nozzle type do the work. Low pressure favours coarse classes and drift control. High pressure is only justified when the label or the coverage target demands it. If the spray looks fine, the fix is not more pressure. It is more water volume or a coarser tip. Cheap agri spray nozzles hold their class only at the pressure they were tested at. Buy from a maker that publishes the class across the band.

Pressure also drifts across a boom. A worn pump or a long boom section can leave the far end 10 to 20% down on pressure. That shifts both dose and droplet class in the same pass. Check pressure at the boom ends, not just at the pump. It is part of nozzle selection, not an afterthought.

Nozzle flow and travel speed: the calibration math

The rate on the field comes from four numbers: rate per hectare, travel speed, nozzle spacing and nozzle flow. The sizing formula ties them together: L/min = (L/ha × km/h × spacing in m) ÷ 600. At 150 L/ha and 12 km/h with 50 cm spacing, each nozzle must deliver 1.5 L/min. Speed is the number operators change most.

Travel speed	Required flow per nozzle
8 km/h	1.0 L/min
10 km/h	1.25 L/min
12 km/h	1.5 L/min
14 km/h	1.75 L/min
16 km/h	2.0 L/min

Speed changes shift the rate in proportion. Dropping to 10 km/h raises it by 20% to 180 L/ha; climbing to 14 km/h cuts it to about 129 L/ha. Pressure compensates within limits, since flow scales with the square root of pressure. To lift flow by 10%, pressure must rise 21%. Small corrections work, but pressure moves the droplet class. The fix for a speed change is usually a different tip, not a different gauge reading.

The fastest field check is the 1/128 acre catch test: 340 ft², about 63 m (207 ft) of travel at 50 cm spacing. Drive that distance at spraying speed and time it. Hold a jug under one nozzle for exactly that time. Ounces caught equal gallons per acre. Millilitres caught times 0.316 give L/ha, so 475 ml means about 150 L/ha.

Tip size follows the ISO 10625 colour code, and the code number is the flow at 2.8 bar in US gallons per minute.

Colour	Code	Flow at 2.8 bar
Blue	01	0.10 gpm / 0.38 L/min
Red	015	0.15 gpm / 0.57 L/min
Yellow	02	0.20 gpm / 0.76 L/min
Green	025	0.25 gpm / 0.95 L/min
Orange	03	0.30 gpm / 1.14 L/min
Lilac	04	0.40 gpm / 1.51 L/min
Brown	05	0.50 gpm / 1.89 L/min

Each size runs about 5% under its rated flow at 2.5 bar. The TeeJet nozzle chart guide shows how to read class and flow data across manufacturers.

Wind windows and buffer decisions

Wind speed decides which classes are legal to run. The table below is standard field guidance.

Wind speed	What to run
Under 5 km/h (1.4 m/s)	Any class, including Fine if the label allows
5 to 10 km/h (1.4–2.8 m/s)	Medium to Coarse
10 to 15 km/h (2.8–4.2 m/s)	Coarse and air-induction only
Over 15 km/h (4.2 m/s)	Stop. No class saves this pass

Wind speed alone is not the full picture. Temperature inversions are the silent drift killer. In an inversion the air near the ground is cooler than the air above, so spray droplets hang instead of mixing. Fog, dust hanging in the air and smoke drifting sideways are the field signs. Never start a pass in an inversion, even with the coarsest tip on the boom. Evaporation is the second hidden factor. Above about 28 °C with humidity under 40%, the fine fraction shrinks in flight and drift rises. Spray in the early morning or evening instead.

Buffer zones follow the same logic. The coarser the class, the narrower the buffer you need, because less product is free to travel. Some label programmes allow a reduced buffer when an air-induction tip is run at a Coarse class or coarser. Check the label before relying on it, and log the tip, the class, the pressure and the wind on every sensitive pass.

Matching the nozzle to the herbicide job

Herbicides fall into groups with different coverage needs, and the nozzle follows the group. The table maps the common jobs.

Herbicide job	Droplet class	Nozzle family	Water volume
Pre-emergence, soil-applied	Coarse to Very Coarse	Flat fan or air-induction	100–150 L/ha
Post-emergence, systemic	Coarse	Air-induction flat fan	100–150 L/ha
Post-emergence, contact	Medium to Coarse	Low-drift flat fan	150–200 L/ha
Banded row treatment	Coarse	Even flat fan or air-induction even	Hold the dose across the band
Burndown near sensitive boundaries	Coarse to Extremely Coarse	Air-induction	150 L/ha or more
Shielded or hooded pass	Coarse	Flat fan inside the hood	100–150 L/ha

Soil-applied pre-emergence work is the most forgiving on droplet size, because the target is soil, not a leaf. Drift control wins, so Coarse and air-induction are the norm. Post-emergence contact products are the hardest case: 50 to 70 droplets per cm² from a finer spray or more carrier. Running a Coarse class on a contact product means lifting the water to 150–200 L/ha. Systemic products sit in the middle: Coarse classes work at 20 to 30 droplets per cm², and air-induction tips keep the drift budget small.

Weed stage changes the water decision. Small weeds at the two to three leaf stage expose little leaf area, so 100 L/ha can carry the dose. Large, established weeds need more carrier, typically 150–200 L/ha. Pressure is not the lever here. More water is.

Adjuvants deserve a caution. Some additives change droplet size, up or down, and a class rating measured without them may not hold in the tank. If you add a new adjuvant, recheck the class on a clean water test before trusting the box.

For the full flat fan and air-induction picture, see the agricultural nozzle overview.

A five-step selection sequence

Here is the sequence in one place.

Step 1: read the label. Find the minimum droplet class and treat it as a spec.

Step 2: fix the rate and the speed. Example: 150 L/ha at 12 km/h with 50 cm spacing.

Step 3: size the nozzle with the standard formula. L/min = (L/ha × km/h × spacing in m) ÷ 600. The example gives 150 × 12 × 0.5 ÷ 600 = 1.5 L/min. At 2.5 bar, a lilac 04 air-induction tip flows about 1.5 L/min, which fits.

Step 4: set the boom height from the tip angle. A 110° tip at 50 cm spacing means about 50 cm above the target.

Step 5: verify in the field. Catch-test one nozzle per section, check the pattern for streaks, and log the class, pressure and wind before you open the valve.

The sequence holds for any herbicide and any boom. Change the label, the rate or the spacing, and only step 3 changes.

Seasonal maintenance keeps the class honest

A worn nozzle changes the class without anyone touching the boom. Orifice erosion enlarges the hole, flow rises, and the spray coarsens unevenly across the boom. The industry rule of thumb is simple: replace the tip when flow exceeds 10% above the value when new. A 1.5 L/min tip that delivers 1.7 L/min is worn out, not “a bit fast”.

Check	How	Replace when
Flow	Catch test against a new tip	+10% or more
Pattern	Spray onto a flat board	Streaks or a heavy centre
Screen	Inspect mesh, rinse with water	Blocked or torn
Check valve	Watch for drips at boom ends	Leaks at shutoff

Material choice sets the wear clock. Brass wears fastest and can lose its class within a season or two on abrasive mixes. Stainless lasts several seasons. Ceramic inserts hold flow within a few percent over years. For herbicide booms running wettable powders or suspensions, ceramic or stainless tips pay for themselves.

Screens protect the orifice. Fifty mesh is the common default for flat fans, and small orifices take finer screens, typically 100 mesh. A blocked screen starves the tip, while a torn screen lets grit straight through.

Cleaning rules are non-negotiable. Never poke the orifice with wire, a pin or a blade. A metal probe enlarges the hole, and the drift class is gone. Use a soft brush, compressed air or a tip-cleaning solution, from the outlet side.

Storage matters more than it looks. Rinse boom and tank with clean water after each day, then drain. Water left to freeze in a tip distorts the orifice and kills the pattern. Store tips dry, off the boom. Fit fresh check-valve diaphragms before the season. On drift-critical sections, replace every tip older than one season.

Keep the class, keep the coverage

The pattern repeats on every field. Hold a coarse class on purpose. Keep pressure between 2 and 4 bar. Raise water volume when you move coarse. Stop when the wind or the inversion says so. The agricultural herbicide spray nozzles that earn their place on a boom hold their class across the pressure band. They do not drift fine the moment the throttle opens. The BoreJet agricultural nozzles range lists air-induction and flat fan tips with their droplet classes and pressure bands. Send the application team your herbicide, your sensitive boundaries and your boom spec. They will match the tip and class that keep the product on your field. For the mechanics of why droplets drift and how air-induction tips change the picture, see our herbicide drift control guide.