Application Equipment, Calibration, and Application Techniques
Calibration items are arithmetic, but the exam also tests nozzle choice, droplet size, and technique - whether the spray reaches the target and stays there.
The core formulas
- GPA = (GPM × 5940) ÷ (MPH × nozzle spacing in inches). If you use total boom width, divide total GPM by the number of nozzles first.
- Flow scales with the square root of pressure: new GPM = old GPM × √(new psi ÷ old psi).
- Acres per tank = tank size ÷ GPA; product per tank = rate per acre × acres per tank.
Nozzle and droplet decisions
Flat-fan for broadcast, hollow cone for penetration, air-induction or low-drift for coarse droplets, even flat-fan for banding. Coarser droplets cut drift but can reduce coverage and increase runoff; finer droplets improve coverage and drift. Replace nozzles when flow exceeds the new-nozzle value by 10%, because worn tips overapply and distort the pattern.
Technique
Set boom height so patterns overlap correctly (commonly twice the nozzle spacing for 110° tips), hold speed constant, verify the pressure gauge, and clean strainers. Droplet size and boom height are equipment decisions you control before wind becomes a factor.
Worked example
20-inch nozzle spacing, 10 nozzles, target 20 GPA at 5 mph. Required output per nozzle = (20 × 5 × 20) ÷ 5940 = 0.337 GPM. Measured output is 0.37 GPM at 30 psi. Pressure correction: new psi = 30 × (0.337 ÷ 0.37)² = 30 × 0.83 ≈ 25 psi. Recalibrate and check three nozzles across the boom rather than guessing.
Granular, airblast, and aerial
Calibrate granular applicators by weighing output over a measured distance and check for skips. Airblast requires the right volume, air speed, and droplet size for the canopy. Aerial work requires a licensed pilot and business, and the swath and buffers must be flown as permitted.
Traps that cost marks
Putting swath in feet into the 5940 formula, which takes inches; forgetting to divide by nozzle count; changing speed without recalibrating; and assuming more pressure always means better coverage.