Application Equipment, Calibration, and Application Techniques
Nozzles and droplet size
Flat-fan nozzles suit broadcast herbicide and fertilizer sprays on booms; even/deflector tips reduce drift near sensitive areas; hollow cones give smaller droplets and better penetration for insecticides and fungicides; full cones cover soil-directed sprays; air-induction tips produce coarse, drift-resistant droplets. Higher pressure and smaller orifices mean smaller droplets and more drift; larger droplets reduce drift but increase runoff potential. Maintain correct boom height so adjacent flat-fan patterns overlap properly - raising the boom increases both drift and streaking.
Calibration
Calibrate with the product you will actually spray (or correct for density), using the speed and pressure you will use in the field, then recheck periodically and whenever nozzles, speed, or product change. Measure several nozzles, not just one - the average governs. Replace any nozzle whose output exceeds the new-nozzle rating by 10 percent.
Key formulas and numbers:
- 1/128 acre = 340 sq ft (18 ft 6 in × 18 ft 6 in).
- GPA = (GPM per nozzle × 5940) ÷ (MPH × nozzle spacing in inches).
- Flow varies with the square root of pressure: quadrupling pressure only doubles flow.
Worked example: 0.4 GPM nozzles on 20-inch spacing at 4 mph: (0.4 × 5940) ÷ (4 × 20) = 2376 ÷ 80 ≈ 29.7 GPA. To apply 30 GPA you are essentially set; to apply 20 GPA you would raise speed to about 6 mph or drop to lower-output tips.
Trap: changing ground speed changes GPA inversely but pressure changes it by a square root - a small pressure tweak will not fix a rate error caused by the wrong speed.
Rule of thumb: calibrate, then verify - a sprayer that was right last season is not necessarily right today.