The 15 GPA That Wasn't: Recalibrating Before the Tank Is Filled
A custom applicator agrees to spray 45 acres of dryland wheat with a 300-gallon pull-type sprayer. The boom is 24 feet wide with 12 flat-fan nozzles on 24-inch spacing. The target is 15 gallons per acre at 6 mph, and the nozzle chart lists a 0.30 GPM tip at 40 psi as close. Product rate is 12 fluid ounces per acre. Wind is 12 mph gusting to 18, blowing toward a neighbor's freshly cut alfalfa hayfield, and there are about two hours of daylight left. The boom gauge reads 55 psi. The product is a Group 4 herbicide; the label carries a 12-hour restricted-entry interval and warns that volatility increases above 85°F. The applicator says the field has waited a week and he would rather not lose another day.
1. Does this sprayer as configured deliver 15 GPA? Show the arithmetic and state what you would change before the tank is filled, plus how much product belongs in a full 300-gallon load at the correct rate.
Read the worked answer
Calculation. Use GPA = 5940 x GPM / (mph x nozzle spacing in inches).
GPA = (5940 x 0.30) / (6 x 24) = 1782 / 144 = 12.4 GPA.
He is running about 17% under target, so every acre is getting roughly five-sixths of the labeled rate - a short dose that invites poor control and, in a herbicide, selection pressure.
Options, judged in order:
- Raise pressure. Flow rises with the square root of pressure: 0.30 x sqrt(55/40) = 0.352 GPM, which gives 14.5 GPA. Still short, and finer droplets with an 85°F-plus volatile herbicide and an 18-mph gust is the worst possible trade.
- Slow down. 5940 x 0.30 / (15 x 24) = 4.95 mph. This works mathematically but stretches an already tight daylight window.
- Change tips. Best answer: install the next larger flat-fan, run it in the chart's pressure band, and verify with a timed catch from three or four nozzles rather than trusting the chart.
Load math at 15 GPA. 300 / 15 = 20 acres per tank; 20 x 12 fl oz = 240 fl oz = 1.875 gallons of product per full load. Also recheck the gauge against a known-good one.
2. Given the wind, temperature, and downwind hayfield, should he spray tonight? Justify the decision using the label, drift principles, and liability rather than convenience.
Read the worked answer
No - the load should not go out as planned.
Three factors stack rather than trade off:
- Wind: 12 mph with 18 mph gusts is outside what most labels describe as acceptable, and gusts are what actually carry droplets off target. A steady 12 mph is arguable; gusting to 18 is not.
- Temperature and volatility: the label itself flags volatility above 85°F. A Group 4 amine ester-type product applied hot and fine-dropleted can move as vapor after the droplets have already landed, so a buffer strip cannot fully solve it.
- Sensitive target: fresh-cut alfalfa downwind is forage. Beyond drift damage, many Group 4 labels restrict grazing, haying, and use of manure from treated areas - a residue problem that surfaces months later in a buyer's test.
Judgment: waiting for evening may help with wind but invites a temperature inversion as the soil cools, which traps fine droplets near the surface and drifts them unpredictably. The defensible choice is to evaluate the next morning after the surface warms, spend the remaining light calibrating and swapping tips, and pre-notify the neighbor. An applicator who sprays because the field is late owns the drift damage; “the label did not say no” is not a defense in Colorado.