Ch.15: Calibrating Application Equipment

2. Two Variables You Adjust (hydraulic sprayers)

• (1) Ground speed and (2) nozzle flow rate.
• Ground speed = ground covered in a set time. Spray rate is inversely proportional to ground speed — double the speed → half the output per area.
• Flow rate is set by 3 factors: (1) orifice size, (2) spray pressure, (3) viscosity of the spray mix.
  • Larger orifice → more spray mix out in the same time, and larger droplets.
  • Spray pressure affects both droplet size and flow rate; higher pressure → more flow and droplets broken into more, smaller ones. Pressure is for small flow changes, not large ones.
  • Doubling pressure does NOT double flow — you must increase pressure four-fold (4×) to double the flow rate.
  • Viscosity = thickness of the mix. Water-based ≈ water; a thicker carrier (oil) sprays out more slowly.

3. Three Key Terms

• Swath width = area treated by one pass of the equipment.

4. Calculating Area

• Large ag/turf → rate per acre; small landscape → rate per 1,000 ft².
• Use one consistent unit — convert first if measurements differ (e.g., 100 yards = 300 ft).
• 1 acre = 43,560 ft². Square feet → acres: ÷ 43,560. Acres → square feet: × 43,560.

• Common errors to avoid: using diameter instead of radius (or not squaring it) on circles; forgetting the ½ on triangles; adding sides instead of multiplying on rectangles.

5. Liquid Spray Batch Math

• Amount per tank depends on the label product rate and the area treated per tank. Same math for a 5-gal backpack or a 1,500-gal tank — only the units differ. Large booms use GPA.
• Acres per tank = tank capacity (gal) ÷ spray rate (GPA).
• Pesticide per tank = acres per tank × product per acre.
• Worked example: 1,500-gal tank at 15 GPA → 1,500 ÷ 15 = 100 ac/tank; label 2 pints/ac → 100 × 2 = 200 pints = 25 gal (8 pints/gal) → final mix = 200 pints + 1,475 gal water = 1,500 gal.

6. Granular Calibration

• Calibrate with the actual product you'll apply; apply over a tarp to recover and reuse it.
• Calibrate frequently — orifices enlarge over time from abrasion/corrosion, changing the rate.
• Metered opening (gate) = the granular version of a nozzle orifice.
• Rate is affected by: granule size/density/type, gate size, agitator/rotor speed, swath width, ground speed, site slope/bumpiness, flowability, and temperature/humidity (which change how granules flow or stick).
• Info sources: the equipment manual, plus local Extension or pesticide safety training programs.
• Rotary (a.k.a. centrifugal, spinner, cyclone, broadcast): product passes a metering plate, strikes a rotating disk, and is thrown to the front and sides. Effective swath width ≠ machine width (it throws farther).
• Drop spreader: product falls through the metering gate straight down; swath width = the width of the spreader itself.

7. Control Systems: Still Calibrate

• Even the newest GPS/automated systems can be fouled up in simple ways — truth-check with calibration; don't rely totally on the technology.
• "Garbage in, garbage out": wrong input numbers → over- or under-application. Systems rely on accurate operator input.
• What fails: plugged/faulty nozzles; speed sensors caked with mud or knocked out of position; rate controllers that don't account for how pressure affects nozzles; and pumps that can't reach the higher pressures a controller calls for.
• Boom-only monitoring: a failed nozzle goes unnoticed → streaks or over-application.
• Real benefits when used right: better control, more efficient distribution, reduced chemical cost.

Key Vocabulary