Calibrating a Boom Sprayer Before a 22-Acre Pass
You are calibrating a pull-behind boom sprayer before a 22-acre herbicide pass on a fallow field. The boom is 20 feet wide with 8 flat-fan nozzles spaced 20 inches apart. You want 20 gallons per acre at 4 mph. The 300-gallon tank is full of water, the tractor's speedometer agrees with a timed 200-foot run, and the pressure gauge holds 40 psi. You collect from each nozzle for 30 seconds and get, in fluid ounces: 18.0, 17.5, 17.2, 19.8, 17.0, 17.4, 17.1, 16.9. The label rate for the product is 1.5 pints per acre, and it is a 4EC formulation. You have a graduated cylinder marked in fluid ounces, a stopwatch, and a calculator.
1. What is the target output per nozzle, and what should each 30-second catch measure?
Read the worked answer
Use the standard boom calibration relationship:
GPM per nozzle = (GPA x mph x nozzle spacing in inches) / 5940
= (20 x 4 x 20) / 5940 = 1600 / 5940 = 0.269 gpm per nozzle
Convert to a 30-second catch:
- 0.269 gal/min x 128 fl oz/gal = 34.5 fl oz/min
- x 0.5 min = about 17.2 fl oz per nozzle in 30 seconds
So if the sprayer is set up correctly at 40 psi and 4 mph, every nozzle should catch roughly 17.2 fluid ounces in 30 seconds. If the average is off by more than a few percent, adjust pressure or speed before adding any product.
2. Evaluate the catch data. What is the actual application rate, is any nozzle out of spec, and what do you fix?
Read the worked answer
Average catch: (18.0 + 17.5 + 17.2 + 19.8 + 17.0 + 17.4 + 17.1 + 16.9) / 8 = 140.9 / 8 = 17.61 fl oz per 30 seconds
Convert back to GPA:
- 17.61 x 2 = 35.2 fl oz/min = 0.275 gpm
- GPA = (0.275 x 5940) / (4 x 20) = 1634 / 80 = 20.4 GPA
That is about 2% above target - within normal variation. No pressure or speed change is needed; simply calculate the product for the actual 20.4 GPA.
Nozzle check: nozzle 4 caught 19.8 oz versus the 17.61 oz average = +12.4%. Replace any nozzle more than about 10% above the average or above its new-nozzle rating - a worn or damaged tip over-applies, streaks the field, and can cause crop injury. Pull the screen first to rule out debris, replace the tip, then re-run the 30-second catch to confirm every nozzle is within tolerance. Record the calibration.
3. How much product goes into the full 300-gallon tank, and how many loads will the 22 acres take?
Read the worked answer
Acres per full tank: 300 gal / 20.4 gal per acre = 14.7 acres
Product per full tank: 14.7 acres x 1.5 pt/acre = 22.1 pt, round to 22 pt = 2.75 gallons (352 fl oz)
Loads for 22 acres: 22 / 14.7 = 1.5 tanks, so two loads:
- Load 1: full 300 gallons, 22 pt of product, covers 14.7 acres.
- Load 2: 22 - 14.7 = 7.3 acres remaining. Water = 7.3 x 20.4 = about 149 gallons. Product = 7.3 x 1.5 = about 11 pt (1.375 gal).
Do not fill a full 300-gallon tank to finish a 7-acre remainder - you would be mixing product you cannot legally or safely put out. Also check the label for maximum seasonal or per-acre limits before the second pass, and confirm the product's label rate is per broadcast acre, not per treated acre, since this is a fallow burndown.