Tank Mix Order and Calibration Drift on a Rights-of-Way Crew
A roadside vegetation crew is mixing a 300-gallon tank for a rights-of-way application. The planned mix: a wettable powder, an emulsifiable concentrate, a water-soluble liquid, and a surfactant, plus a drift-control adjuvant. The crew's sprayer was last calibrated a month ago; a nozzle swap was done since then, and one boom section now delivers visibly more output. Wind is 8 mph gusting to 14 mph, blowing toward a 30-foot-wide planted median and a residential fence line 50 feet downwind. The crew plans to spray at 40 psi.
1. State the correct tank-mix addition order and why it matters.
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General order for this mix: fill tank ~half full of water → water-soluble liquid → wettable powder (pre-slurried) → emulsifiable concentrate → surfactant/adjuvants last, with agitation throughout. Rationale: powders need dispersion before oils are added; adding EC before WP can cause clumping or incompatibility; surfactants go in last to avoid foaming and to let you check compatibility first. Always do a jar compatibility test before committing 300 gallons, and check the label for product-specific order.
2. The sprayer was not recalibrated after a nozzle change and one section over-delivers. What must happen before spraying?
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Stop and recalibrate. A nozzle swap and an over-delivering boom section mean the application rate is unknown and likely uneven - over-application on one section (injury, off-target load) and under-application elsewhere (poor control). Steps: replace/verify all nozzles are the correct size and type, check for wear, measure output per nozzle, recalculate GPA, and adjust pressure/speed. Do not 'eyeball' it. Document the calibration. Only then mix and spray.
3. Given the wind and downwind median/fence, what drift-management decisions apply at 40 psi?
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Wind at 8 gusting to 14 mph toward a planted median and a residential fence 50 ft downwind is a drift risk. 40 psi produces finer droplets that drift farther - lower pressure within the label's effective range, use low-drift/low-pressure nozzles and a drift-reduction adjuvant (which is in the mix), keep boom height low, and maintain a buffer to the median and fence. If gusts exceed the label's wind-speed limit, stop. Consider spraying when wind is calmer or in a direction away from the fence. The median is a sensitive site - protect it with a setback and coarse droplets, and document the conditions.