Calibration Math and the 'Just Turn Up the Pressure' Mistake
A ground rig has a 20-ft boom with 8 nozzles spaced 30 in apart. The applicator collects 20 fl oz from one nozzle in 60 seconds while driving 4 mph. The label for the target herbicide requires 15–20 GPA and warns against pressures above 40 psi to limit drift. The tank is 200 gal, and the field is 40 acres. Nozzles are 11002 flat-fans, one nozzle looks worn, and the applicator wants to 'just turn up the pressure' from 30 to 60 psi to double output.
1. Calculate the current sprayer output in gallons per acre and compare it with the label requirement.
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Convert the catch to gallons per minute: 20 fl oz/min ÷ 128 fl oz/gal = 0.15625 gpm per nozzle. Use GPA = (gpm × 5940) ÷ (mph × nozzle spacing in inches). GPA = (0.15625 × 5940) ÷ (4 × 30) = 928.1 ÷ 120 = 7.7 GPA. That is less than half the label minimum of 15 GPA. Applying at 7.7 GPA would under-dose the herbicide and could also reduce coverage. The worn nozzle adds uncertainty, so the whole boom should be checked, not just the one nozzle that was caught.
2. Evaluate the plan to double pressure from 30 to 60 psi. Will it double output? What other problems does it create?
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No. For a fixed orifice, flow varies roughly with the square root of pressure. Going from 30 to 60 psi doubles the pressure, so flow increases by about √2 = 1.41, not 2. Output would rise from about 7.7 to roughly 10.9 GPA - still below the 15 GPA minimum. It would also exceed the label's 40 psi drift warning and produce more fine droplets, raising drift risk. Doubling pressure is therefore both ineffective for the calibration target and inconsistent with the label.
3. Recommend practical ways to reach the label rate and calculate the tank mix for the 40-acre field at 15 GPA.
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Two safer options are: (1) slow down until the output is in range - at 2 mph the same nozzles give about 15.5 GPA, though that may be impractical; or (2) install larger nozzles and recalibrate at a pressure at or below 40 psi. Replace the worn nozzle and verify every nozzle's output. At 15 GPA over 40 acres, total spray volume = 15 × 40 = 600 gal. With a 200-gal tank, that is 3 full loads. Add the herbicide at the label rate for each 200-gal load, not for the whole 600 gal at once. Recalibrate after any nozzle, speed, or pressure change.