Boom Sprayer Calibration and Load Planning
You are calibrating a 60-foot boom sprayer with 20-inch nozzle spacing (36 nozzles) and a 500-gallon tank. The target is 15 GPA at 12 mph. The pressure gauge reads 40 psi, which matches the nozzle chart for the tips installed. A timed catch over a 200-foot course - about 11.4 seconds at 12 mph - gives an average of 17 ounces per nozzle. Individual tips range from 15 to 19 ounces. The field to be treated is 80 acres. The label requires a minimum of 15 GPA carrier and the product at 1.5 pt/A.
1. Is the sprayer delivering the intended 15 GPA? Show the arithmetic and state the actual rate.
Read the worked answer
Required output per nozzle:
GPM = (GPA x MPH x nozzle spacing in inches) / 5940 GPM = (15 x 12 x 20) / 5940 = 3600 / 5940 = 0.61 GPM
In ounces per minute: 0.61 x 128 = 78 oz/min Expected catch over an 11.4-second pass: 78 x (11.4 / 60) = about 14.8 oz
Measured: 17 oz, which is 17 / 14.8 = 1.15, or about 15% high.
Converted to rate: 15 GPA x (17 / 14.8) = about 17.2 GPA.
So the sprayer is over-applying by roughly 2.2 GPA. On 80 acres that is 1,376 gallons instead of 1,200 - about 176 extra gallons of mix, and 15% more product than the label rate the grower is paying for. It is not a minor rounding issue.
2. Plan the loads for the 80-acre field.
Read the worked answer
Acres per tank at the actual 17.2 GPA: 500 gal / 17.2 GPA = 29.1 acres per full load
Loads needed: 80 / 29.1 = 2.75, so three loads, with the last one partial.
- Load 1: full 500 gal, 29.1 acres (acres 1–29.1)
- Load 2: full 500 gal, 29.1 acres (acres 29.1–58.2)
- Load 3: 80 − 58.2 = 21.8 acres, requiring 21.8 x 17.2 = about 375 gallons
Product per full load: 500 gal / 17.2 GPA = 29.1 acres x 1.5 pt/A = 43.7 pt, about 5.5 gallons of product. The partial load takes about 32.7 pt.
Do not forget: sprayer cleanout and rinse water volume between products or at the end of the day, the overlap you will create on headlands and point rows (which locally exceeds label rate), and the fact that a partial last load is the right place to absorb any shortfall - running out mid-field means a skip.
3. The gauge and the nozzle chart both check out. What is the most likely cause, what is the best correction, and how do you verify it?
Read the worked answer
Cause: with correct pressure and correct tip size on the chart, the extra output is almost certainly nozzle wear - worn orifices flow more at the same pressure. The tip-to-tip spread of 15 to 19 ounces (a range of about ±12% around the 17-ounce average) confirms it; the common standard is to replace a tip that deviates more than 10% from the average, and here several are at or past that line.
Correction: replace the full set as a set, not the worst three. A mixed set of new and worn tips produces an uneven pattern and uneven rate across the boom, which is a worse outcome than a uniform 17 GPA. Re-check that the replacement tips are the same capacity and spray angle.
Verify: re-run the timed catch at the same 200-foot course and the same 40 psi, and confirm the average is within a few percent of 14.8 ounces. Only then redo the acres-per-tank calculation and the load plan.
Do not "fix" it with speed or pressure. Bumping speed to about 13.8 mph would get the GPA back to 15, but with worn tips it would leave a streaky pattern, and the label minimum carrier volume assumes a proper pattern, not just an arithmetic average.