Calibration Drift Mid-Season
An applicator calibrated a boom sprayer at the start of the season to deliver 20 GPA at 4 mph with 11003 flat-fan nozzles spaced 20 inches apart and 30 psi. Halfway through the season, the applicator notices uneven spray patterns and replaces four worn nozzles with new ones of the same size but notices the new nozzles flow noticeably more. The applicator also increases ground speed to 5 mph to cover more acres per day and raises pressure to 40 psi to compensate for perceived poor coverage. The target herbicide requires 20 GPA for effective weed control and the label prohibits exceeding labeled rates. The applicator plans to apply 1.5 pints per acre of product.
1. Calculate the effect of the speed and pressure changes on actual gallons per acre delivered, and determine whether the applicator is over- or under-applying product.
Read the worked answer
Baseline: 20 GPA at 4 mph and 30 psi.
Speed change (4 → 5 mph): GPA is inversely proportional to speed at constant pressure. New GPA = 20 × (4/5) = 16 GPA.
Pressure change (30 → 40 psi): Flow varies with the square root of pressure. Flow multiplier = √(40/30) = √1.333 ≈ 1.155. GPA at 40 psi = 16 × 1.155 ≈ 18.5 GPA.
Net effect: Actual delivery is about 18.5 GPA, below the required 20 GPA. The applicator is under-applying by roughly 7.5% relative to the labeled 20 GPA requirement.
Product implication: At 18.5 GPA, the applicator would need to adjust the mix concentration to still deliver 1.5 pints per acre. If mixed for 20 GPA, actual product applied per acre is 1.5 × (18.5/20) ≈ 1.39 pints/acre - under the labeled rate, risking reduced control and resistance selection.
2. Explain why replacing four nozzles with same-size nozzles changed the flow, and what the applicator should do before spraying.
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Why flow changed: Worn nozzles have enlarged orifices and eroded tips, which actually increase flow and distort pattern. New nozzles of the same nominal size flow at the rated rate. If the new nozzles appear to flow more than the old ones, the old nozzles were likely partially clogged or the new ones are a different flow rating than assumed. Either way, mixed old and new nozzles create uneven application across the boom - some sections over-apply, others under-apply.
Required actions before spraying:
- Replace all nozzles at once, not just the worn ones, so flow is uniform across the boom.
- Verify nozzle size and flow rating against the manufacturer's chart.
- Recalibrate the entire sprayer after nozzle replacement.
- Check for clogged screens and worn seals.
- Confirm the new nozzles are appropriate for the target spray quality.
3. Describe a correct calibration procedure to restore the 20 GPA target at the new 5 mph speed.
Read the worked answer
Procedure:
- Fill the tank with clean water and mark a known distance (e.g., 200 feet).
- With the sprayer at operating pressure, measure the time to cover the test course at 5 mph.
- Catch output from several nozzles for a measured time and average the flow.
- Calculate GPA using the standard formula: GPA = (GPM per nozzle × 5940) ÷ (mph × nozzle spacing in inches).
- Solve for the nozzle size or pressure needed to hit 20 GPA at 5 mph and 20-inch spacing:
- Required GPM per nozzle = (20 × 5 × 20) ÷ 5940 ≈ 0.337 GPM.
- Select nozzles rated near 0.337 GPM at the chosen pressure, or adjust pressure within the nozzle's rated range.
- Re-verify by catching output again after adjustment.
Key point: Pressure changes should stay within the nozzle's rated range; do not use pressure as a substitute for correct nozzle selection.