Application Equipment, Calibration and Application Techniques
Calibration means matching output to the label rate. For boom sprayers:
GPA = (GPM per nozzle × 5940) ÷ (MPH × nozzle spacing in inches)
Worked example: nozzle output 0.20 GPM, speed 4 mph, nozzles 20 in apart → (0.20 × 5940) ÷ (4 × 20) = 1188 ÷ 80 ≈ 14.9 GPA, roughly 139 L/ha.
Calibration steps: inspect nozzles for wear and pattern (a nozzle flowing 10% above rated output is worn - replace it; also replace streaked or distorted patterns), set pressure from the nozzle chart, measure a known course at the intended gear and RPM, catch and average several nozzles, then calculate. Recalibrate whenever nozzles, pressure, speed or product change.
Backpack/handheld: measure output for one minute, then your walking speed and effective swath width over a measured area; or use the 1/128-acre method (spray 1/128 acre per nozzle; ounces caught equals gallons per acre).
Drift and coverage: smaller droplets drift further. Use low-drift air-induction tips, correct pressure, boom height just high enough for pattern overlap (commonly 45–60 cm above target), and spray in light, steady wind (roughly 1–10 km/h) - never in a calm inversion. Keep speed constant, shut off at headlands, avoid double overlaps that double the rate, and triple-rinse between products, especially across herbicide groups.
Traps: calibrating once and never again; using pressure to compensate for a worn nozzle (it changes droplet size and drift); and assuming a bigger nozzle means a stronger mix - concentration and volume per hectare are separate decisions.
Rule of thumb: check, don't guess - a five-minute nozzle check before each job prevents most rate errors and complaints.