Spray Tips for Turf: Flat Fan, Air Induction and Overlap

Why this matters

The tip is the last thing the product touches before the lawn, and it decides three things at once: how much liquid leaves the boom, how big the droplets are, and how evenly they land. Pick a tip that makes fine droplets on a breezy day and product ends up on the neighbor's tomatoes. Run broadcast tips at the wrong height and the lawn comes up in stripes. Swap in a worn tip and every number on the calibration sheet is wrong. None of this is a product rate question; the product label governs the rate, the site and often the droplet size. This card is about the hardware that has to deliver what the label says.

It covers boom and boomless applicators. The hose-reel spray gun most residential routes run is calibrated in 'How to Calibrate a Hose Reel Spray Gun for a Lawn'.

How a tip is named

Most flat-fan tips carry a code that reads the same way across makers: the first digits are the spray angle in degrees at rated pressure, the last two are the flow in gallons per minute times ten at 40 psi. An 8004 is an 80 degree fan passing 0.4 gpm at 40 psi; an 11004 is a 110 degree fan passing the same 0.4 gpm. A prefix or suffix names the family (extended range, low drift, air induction, even fan, off-center), and the color is standardized by flow, so every 04 tip is the same color regardless of angle or maker.

Flow follows the square root of pressure for a given tip, holding the orifice constant. Doubling flow takes four times the pressure, and dropping from 40 to 30 psi cuts flow by about 13% (the square root of 30/40 is 0.866). Pressure is a fine trim, not a way to change volume. Change the tip size for that.

Pressure also changes droplet size. Higher pressure breaks the liquid into finer droplets on any tip. That is why the tip maker's chart gives a droplet size class for each tip at each pressure, and why turning the pressure up to "get better coverage" is how a low-drift setup becomes a drifty one.

Standard flat fan: the broadcast workhorse

A standard (or extended range) flat-fan tip makes a fan-shaped sheet that is heaviest in the middle and tapers toward both edges. That taper is deliberate. On a boom, each fan's tapered edge lands on its neighbor's tapered edge, and the two add up to an even deposit across the whole width.

        tip 1       tip 2       tip 3
    --------o-----------o-----------o---------  BOOM
          /   \       /   \       /   \
        /       \   /       \   /       \
      /           X           X           \
    /           /   \       /   \           \
  /           /       \   /       \           \
/           /           X           \           \
=================================================  TURF
            |<- 20 in ->|<- 20 in ->|
            tip spacing, along the boom
 boom height: tip to turf, set from the tip chart
 each strip between two tips is hit by two fans

The drawing shows the condition the chart is built to deliver: each fan reaches the turf just wide enough to land under the next tip over, so every point between tips gets a full share from two fans. Three settings control it, and all three come from the tip maker's chart for that tip:

• Spacing. 20 inches is a common boom spacing for turf. The chart's height is only valid at the spacing it was written for.
• Height. Too low and the fans do not meet, leaving a light streak between every pair of tips; on turf that shows up a week later as thin green lines along the direction of travel. Too high and the overlap gets heavier than the chart intends, the spray travels farther through the air, and more of it drifts. Wider angle tips (110 degrees) can run lower than 80 degree tips at the same spacing, which is one reason turf booms favor them.
• Pressure. The fan only opens to its rated angle inside the tip's pressure range. Below it the fan narrows and the overlap collapses, with the same streak as running too low.

Turn the tips a few degrees off the boom line (most bodies index this for you) so neighboring fans pass each other instead of colliding in mid-air.

An even flat fan is not a broadcast tip. It puts down a uniform sheet with no taper and is meant for banding a single strip. Mounted on a broadcast boom it double-doses every overlap.

Air induction: bigger droplets, less drift

An air-induction (AI, or venturi) tip draws air into the liquid inside the tip body and releases large droplets with air bubbles inside them. The result is far fewer small droplets, which are the ones that hang in the air and move off target.

What you gain. Much lower drift at the same flow, which is why many labels now name a minimum droplet size class for applications near sensitive sites or in wind. The class names (fine, medium, coarse, very coarse, extremely coarse, ultra coarse) are defined by ASABE S572 in its current edition, the classification tip makers use on their charts. When a label says "medium or coarser", the chart is how you prove the tip and pressure meet it.

What you give up. Fewer, larger droplets mean fewer hits per square inch of leaf. For a product that moves through the plant after it lands, that rarely matters. For a product that has to cover a small leaf surface to work, the label or the product's technical sheet may favor a finer class, and there the label wins, including when it means waiting for calmer weather rather than switching tips.

What it needs. Most air-induction tips need a higher minimum pressure than a standard flat fan to pull air properly; run below it and they behave like a coarse standard tip. The chart gives the range. AI tips still need overlap, and most are flat-fan shaped, so the spacing and height rules above still apply.

Off-center and boomless tips

An off-center tip throws a fan to one side and is used at the end of a boom to reach a border, or alone on a boomless sprayer. Its pattern is less even than a boom of overlapping fans, and it throws farther through the air, so it drifts more. Use it where a boom cannot reach, not as a substitute for one.

Worked example: the front lawn beside a vegetable garden

A tech is treating 6,000 square feet of front lawn with a liquid product using a 60 inch push boom carrying three 11004 extended-range tips at 20 inch spacing. The neighbor's raised vegetable beds sit 15 feet off the property line on the downwind side, and the wind is a steady 6 mph toward them. The product label lists a wind limit and a minimum droplet class for sensitive areas; the tech reads both before mixing. This example does not involve any product rate.

Step one: what the current tips put out. At 40 psi, three tips pass 0.4 gpm each. The broadcast volume formula is gallons per acre = gpm per tip x 5,940 / (mph x spacing in inches). At a 3 mph walking pace: 0.4 x 5,940 / (3 x 20) = 2,376 / 60 = 39.6 gallons per acre. Divided by 43.56 thousand square feet per acre, that is about 0.91 gallon per 1,000 sq ft. Check the other way: 0.91 x 43.56 is 39.6.

Step two: the droplet class. The chart for this 11004 extended-range tip at 40 psi lists it as fine to medium (illustrative here; read the chart for your own tip). The label's sensitive-area instruction asks for medium or coarser. That borderline is not good enough with a garden downwind.

Step three: the swap. An air-induction 11004 passes the same 0.4 gpm at 40 psi, so the carrier volume stays 0.91 gallon per 1,000 sq ft and the mix sheet does not change. Its chart (again illustrative) lists very coarse at 40 psi, and 40 psi is inside its working range. Same spacing, same height band from the AI chart. The droplet requirement is met with margin.

Step four: what the tech does not do. Drop pressure on the old tips to coarsen the spray. At 30 psi the 11004 extended-range tip passes 0.4 x 0.866, about 0.35 gpm, which drops volume by about 13% (to about 0.79 gallon per 1,000 sq ft) and pushes the tip toward the bottom of its range, where the fan narrows and the overlap thins. Two new problems to fix one.

Step five: the border. For the 15 foot strip nearest the garden, the tech turns so the boom travels parallel to the property line and runs the first pass with the downwind edge of the boom on the line, not past it. If the label's wind limit is exceeded or the wind shifts toward the garden in gusts, the application stops and the lawn is rescheduled. A tip cannot fix a wind the label rules out.

Step six: prove it. Before the first pass the tech catches each tip into a measuring cup for 30 seconds at 40 psi. Each should read about 0.2 gallon, which is 25.6 fluid ounces (0.4 gpm for half a minute, at 128 ounces to the gallon). One tip catching more than about 10% over its neighbors is worn or damaged and gets replaced before spraying.

Wear, cleaning and what the catch test catches

Tips wear from the inside, and a worn tip passes more liquid with a worse pattern. Brass wears fastest, stainless steel and hardened plastic last longer, ceramic longest. The usual replacement trigger is flow more than about 10% over the rated figure for that pressure, found on the catch test.

Clean with a soft brush or compressed air, never a wire, a knife point or a pin, which scores the orifice and changes both flow and fan in one stroke. Wear chemical-resistant gloves and eye protection when unscrewing and cleaning tips, because a plugged tip holds pressurized product behind it, and relieve system pressure before removing one. A tip cleaned by mouth is a pesticide ingestion.

Record tip family, size and the catch readings on the calibration log; see 'The Spreader and Sprayer Calibration Log SOP'.

References

• ASABE S572, spray nozzle droplet size classification, current edition (the basis for the droplet classes on tip charts and labels)
• Tip manufacturer's chart for the exact tip family and size (spacing, height, pressure range, droplet class)
• The product label, which governs rate, site, wind and droplet requirements under FIFRA; see related: Pesticide Label Reading and Compliance
• See related: How to Calibrate a Hose Reel Spray Gun for a Lawn; The Spreader and Sprayer Calibration Log SOP