Application Equipment and Calibration
Calibration fundamentals
The goal is to deliver the labeled rate uniformly. The core relationship is: GPA = (GPM × 594) ÷ MPH × W, where GPM is nozzle output per minute, MPH is ground speed, W is swath width in inches, and GPA is gallons per acre. You can calibrate by measuring nozzle output, by the 1/128-acre method, or by catching output over a measured course. Whatever method you use, calibrate with the same nozzle, pressure, and speed you will use in the field.
Nozzles and drift
Nozzle type controls droplet size. Flat-fan nozzles are standard for broadcast herbicide and are drift-prone at high pressure; air-induction and drift-reduction nozzles produce coarser droplets. Smaller droplets drift farther. Pressure changes output: doubling pressure increases flow by about 1.4× (square root relationship), not 2×. Check nozzles for wear; worn nozzles over-apply and pattern-test poorly.
Traps in questions
- Changing speed without recalibrating - speed is the most common cause of misapplication.
- Assuming doubling pressure doubles output - it does not; flow rises with the square root of pressure.
- Mixing incompatible products - always jar-test and add in the correct order (carriers, then dry, then liquid, then adjuvants).
- Ignoring wind - drift is a legal and environmental issue, not just a performance one.
Worked example
A sprayer with 0.4 GPM nozzles, 20-inch spacing, at 4 MPH: GPA = (0.4 × 594) ÷ (4 × 20) = 237.6 ÷ 80 ≈ 2.97 GPA. If the label calls for 3 GPA, you are on target. If you speed up to 5 MPH, output drops to about 2.4 GPA - recalibrate or adjust pressure.