Application Equipment and Calibration
The calibration relationship
Spray rate depends on nozzle flow, ground speed, and swath width. A useful formula is GPA = (GPM × 5940) ÷ (MPH × nozzle spacing in inches). Calibrate with clean water before mixing product. Check all nozzles for wear and pattern; a worn nozzle can increase flow and overapply. Keep filters clean and agitation adequate, especially with wettable powders and suspensions.
Pressure, speed, and pattern
Flow through a nozzle is not linear with pressure. Flow varies with the square root of pressure, so doubling pressure increases flow by only about 41%, not 100%. Changing speed changes GPA inversely: doubling speed roughly halves the rate per acre. Boom height affects overlap and drift; follow the nozzle manufacturer’s spacing and height recommendations.
Calibration methods
The 1/128-acre method is common for broadcast sprayers: measure a calibration area of 340 square feet, spray it at operating speed and pressure, and collect ounces from one nozzle. The ounces collected equal gallons per acre. For large equipment, catch-test several nozzles and average them.
Traps
Questions often assume pressure and flow are directly proportional. Another trap is calibrating with product in the tank or forgetting that nozzle wear increases output. Always recalibrate after changing nozzles, speed, or pressure.
Worked example
A nozzle delivers 0.2 GPM at 4 mph with 20-inch spacing. GPA = (0.2 × 5940) ÷ (4 × 20) = 1188 ÷ 80 = 14.85 GPA, about 15 GPA.