Calibration Numbers That Do Not Add Up
A grower's pull-type sprayer has a 500-gallon tank, a 30-foot boom with 18 nozzles on 20-inch spacing, and is to be run at 4 mph to deliver 15 gallons per acre. The product label requires 1.5 pt/A. A one-minute catch test at 40 psi gives about 39 oz/min from 15 nozzles, 47 oz/min from two nozzles on the left wing, and 33 oz/min from one nozzle near the center. The intended field is 33 acres. Wind is forecast at 12 mph gusting to 20 mph from the west; the field borders a driveway and a neighbor's ornamental nursery on the west side.
1. Verify the setup delivers the target rate and calculate how much product goes in a full tank.
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Nozzle flow required:
GPM = (GPA × mph × swath width in feet) ÷ 5940 GPM = (15 × 4 × 30) ÷ 5940 = 1800 ÷ 5940 = 0.303 gpm per nozzle, which is 0.303 × 128 ≈ 39 oz/min - matching the 15 good nozzles.
Acres per tank: 500 gal ÷ 15 GPA = 33.3 acres.
Product per full tank: 33.3 acres × 1.5 pt/A = 50 pt = 6.25 gallons of product.
Sanity check: the field is 33 acres and one tank covers 33.3, so a single full load is correct - no partial-load arithmetic needed, and no reason to mix a second batch. Confirm the pressure gauge reads true and that all 18 nozzles are the same size and type before the tank is filled, because the rate calculation assumes both.
2. Interpret the catch test and decide what gets replaced.
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Average the good nozzles: 39 oz/min is the target.
- Two nozzles at 47 oz/min. That is 47 ÷ 39 = 1.21, about 21% high. A worn orifice flows more than its rating - these are worn out, not just dirty.
- One nozzle at 33 oz/min. That is about 15% low, the signature of a partially plugged tip or a damaged screen.
- The other 15 are within tolerance - generally ±10% of the boom average, or of the manufacturer's rating, is the working cutoff.
Replace all three. A 21% high nozzle over-applies on its swath, a 15% low nozzle under-applies, and the overlap from neighboring nozzles does not average out a 20% error across a 20-inch spacing. Uneven output shows up later as streaked weed control and crop injury, and it is invisible from the seat.
Also check: the pressure gauge, every screen and strainer, the boom height (20-inch spacing with flat fans needs consistent height and angle for proper overlap), and whether the whole set is simply due for replacement - if several tips are that far off, re-tip the boom rather than chasing individual nozzles, and recalibrate after.
3. Given the wind forecast and the neighbors, do you spray tomorrow?
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No - reschedule.
- 12 mph sustained gusting to 20 is not a drift-safe window, and it is above the wind speed many labels list as a cutoff. The gusts are the deciding factor: you can watch a steady wind, and you cannot watch a gust.
- The sensitive area is downwind. An ornamental nursery to the west with a west wind means the whole field's drift profile is aimed at the most injury-prone target on the property. Nursery stock is often damaged at rates far below the agricultural use rate.
- The driveway is an access route - equipment and people can appear in the drift path without warning.
If it must go out, the mitigations are: wait for a forecast with lower, steadier wind and a crosswind; use a drift-reduction (coarser) tip and lower pressure, accepting that the rate must be recalculated for the new tip; lower the boom to the correct height; leave an unsprayed downwind buffer as wide as the label requires (or wider); and consider a drift-retardant adjuvant only if the label permits it. With gusts to 20 mph and a nursery downwind, the honest answer is to wait.