Calibration Drift Mid-Season
An applicator calibrated a boom sprayer at the start of the season to deliver 20 gallons per acre at 4 mph with 8003 nozzles spaced 20 inches apart at 40 psi. Over six weeks the applicator replaced two worn nozzles with new ones of the same size but did not recheck output, and also switched from a wettable powder to a flowable formulation that the label allows. The applicator now notices uneven weed control in strips behind certain nozzles and calculates that a 300-gallon tank is covering fewer acres than expected. A spot check shows one nozzle delivering 0.55 gpm while a new nozzle of the same tip delivers 0.40 gpm at the same pressure.
1. Diagnose the likely causes of the uneven control and the reduced acres per tank, and explain the relationship between nozzle flow, speed, and carrier volume.
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Two issues are interacting:
1. Worn nozzles. The 0.55 gpm nozzle is delivering about 37% more flow than the 0.40 gpm new tip at the same pressure. Worn tips enlarge the orifice, increasing flow and producing coarser, more uneven patterns. Mixed old and new tips create strips of over-application (behind worn tips) and under-application (behind new tips), which matches the strip pattern of poor weed control.
2. Carrier volume shift. Gallons per acre depends on nozzle flow, nozzle spacing, and speed:
GPA = (GPM x 5940) / (MPH x nozzle spacing in inches)
Using the original 0.40 gpm: (0.40 x 5940) / (4 x 20) = 2376 / 80 = 29.7 - this does not match the stated 20 GPA, so the applicator's original calibration figures or speed assumption were already inconsistent. The key point for the exam: if actual average flow rose because of worn tips, GPA rises and each tank covers fewer acres, exactly what the applicator observed. The flowable formulation change can also affect mixability and suspension but does not by itself change nozzle output.
A worn-tip average above the calibrated flow means more carrier per acre, fewer acres per tank, and uneven deposition.
2. What should the applicator do before resuming, and how should the new flowable formulation change the calibration and mixing procedure?
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Before resuming:
- Stop and recalibrate. Replace all worn nozzles (do not mix old and new tips on the same boom), then measure output from every nozzle with a calibrated container or flow meter.
- Verify speed over a measured course with a partial tank, since speed errors compound volume errors.
- Recompute GPA using the measured average flow and actual speed, and confirm it matches the label's required carrier volume for the flowable.
- Check pattern with a spray table or pattern check to ensure even overlap.
Formulation handling:
- Flowables are suspensions; they settle and require thorough agitation. Follow the label's mixing order (typically fill partway, add product with agitation, then finish filling).
- Recheck compatibility if tank-mixing and use a jar test if uncertain.
- Because flowables can be abrasive and can clog screens, check and clean nozzles and screens more often.
- Confirm the labeled rate is expressed per acre and adjust for the corrected GPA so the active ingredient per acre stays on label.
3. The applicator wants to compensate for the shortfall by simply adding more product to the tank. Evaluate this approach.
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Adding product to 'stretch' the tank is the wrong fix and risks an off-label application. The shortfall is a carrier volume problem (worn nozzles delivering too much water), not a product deficit. If the applicator increases product concentration to compensate for fewer acres per tank, the actual active ingredient applied per acre may exceed the labeled rate, which is a label violation and can injure the crop or create residue and environmental problems.
The correct sequence is: fix the equipment (replace worn tips), recalibrate to the labeled carrier volume, then mix product at the labeled rate for the corrected GPA. Concentration adjustments are only permissible within what the label explicitly allows, and most labels do not permit arbitrary rate increases. The applicator should document the recalibration and the corrected GPA.