Boom Calibration That Lands Outside the Label's Carrier Volume
You are calibrating a boom sprayer before a pre-emergent herbicide application to a 6-acre commercial landscape bed. The boom carries 8 flat-fan nozzles on 20-inch spacing, each rated 0.20 GPM at 40 psi; you plan to drive 4 mph with the boom 18 inches above the soil surface. The 150-gallon tank is filled with clean water for calibration. The label states: "Apply 1.5 pints per acre in 20 to 40 gallons of water per acre. Do not apply when wind exceeds 10 mph." The account manager reminds you that the crew is behind schedule and the customer wants the job finished today.
1. Calculate the sprayer output in gallons per acre for the planned setup. Is the machine usable as configured?
Read the worked answer
Using the broadcast nozzle-spacing formula:
GPA = (5940 × GPM per nozzle) ÷ (mph × nozzle spacing in inches)
GPA = (5940 × 0.20) ÷ (4 × 20) = 1188 ÷ 80 = 14.85 gal/acre
- Total boom flow = 8 × 0.20 = 1.6 GPM; swath = 8 × 20 in = 160 in ≈ 13.3 ft.
- No, it is not usable as configured. The label requires 20-40 gal/acre; 14.85 is below the floor. Running it would be a label deviation and, practically, would put the pre-emergent on in too little carrier - streaking, poor soil activation, and uneven weed control.
- For reference only: at 14.85 GPA a 150-gal tank would cover about 10.1 acres, but that number is irrelevant until carrier volume is fixed.
2. Show how you bring the machine into the label range and recalculate the product per full tank.
Read the worked answer
Raise carrier volume without exceeding the label rate:
- Slow down: at 3 mph, GPA = 1188 ÷ (3 × 20) = 19.8 - still just under 20, so this alone does not comply.
- Larger orifices: 0.30 GPM tips at 4 mph, 40 psi → (5940 × 0.30) ÷ 80 = 22.3 GPA, comfortably inside 20-40. (0.25 GPM at 3.5 mph = 21.2 GPA also works.)
Product per 150-gal tank at 22.3 GPA:
- Acres per tank = 150 ÷ 22.3 = 6.7 acres
- Product = 6.7 × 1.5 pt = 10.1 pt ≈ 1.25 gal (about 1 gal + 1 qt)
Important: do not compensate for low carrier volume by raising the product rate. Rate per acre is fixed by the label; the fix is more water per acre, not more chemical.
3. During the pre-job nozzle check, 30-second catch samples from two tips read 14.6 and 14.8 fl oz. What do you do, and what happens to the rate if the operator actually drives 4.5 mph instead of 4?
Read the worked answer
Nozzle check. Expected catch from a 0.20 GPM tip in 30 seconds:
- 0.20 gal/min × 0.5 min = 0.10 gal = 12.8 fl oz
- Readings of 14.6-14.8 fl oz are +14% to +16%, beyond the common 10% wear/replacement limit.
Replace those tips. Worn tips also distort the pattern (streaking at the edges), so check the full boom on a pattern table and replace any tip more than 10% off rating. Never "adjust" for a worn tip by lowering pressure.
Speed error. If the operator runs 4.5 mph instead of 4:
- With the original 0.20 tips: 14.85 × (4 ÷ 4.5) = 13.2 GPA
- With the new 0.30 tips: 22.3 × (4 ÷ 4.5) = 19.8 GPA - back outside the label range.
Fix by verifying speed on a measured 200-ft course and holding the correct gear and RPM. Do not chase GPA with pressure - higher pressure means more fines and more drift. Recalibrate any time speed, tip size, spacing, or pressure changes, and note the calibration in the job file.