Product Mix, Water Source and Sprayer Calibration
You are loading for a 30 ha application of a contact herbicide plus a surfactant on stubble. The label rate is 2.0 L/ha of product and 0.25% v/v surfactant in a spray volume of 80 L/ha. Your tank is 500 L. The only water available is from a farm dam with visible algae and a pH of 8.6 measured with a strip test. You have a 500 L tank, a paddle agitator, and a nozzle set that was last calibrated three weeks ago at 80 L/ha with a flow meter reading 4.2 L/min per nozzle across 20 nozzles. Today the flow meter reads 4.6 L/min per nozzle at the same pressure. The product label requires a tank mix jar test before combining with other products and states that the product is incompatible with alkaline water above pH 8.0 unless buffered.
1. Calculate the product, surfactant and water quantities for a full 500 L load, and check them against the label.
Read the worked answer
At 80 L/ha, a 500 L tank covers 500 / 80 = 6.25 ha.
Product: 6.25 ha × 2.0 L/ha = 12.5 L. Surfactant: 0.25% v/v of the final spray volume = 0.0025 × 500 L = 1.25 L. Water: 500 - 12.5 - 1.25 = 486.25 L (in practice, fill to 500 L after adding products, but the label order matters; add water first, then product, then surfactant, with agitation).
Check against the label: the product rate and surfactant concentration match the label. The number of loads for 30 ha is 30 / 6.25 = 4.8, so five loads, with the last partial. Note that the surfactant is calculated on the final volume, not on the water volume; using 0.25% of 486.25 L would give 1.22 L, a small but avoidable error.
Also verify the water: pH 8.6 exceeds the label limit of 8.0, so a buffer is required before adding the product. The algae and organic load in the dam water can also deactivate some contact herbicides and clog filters, so pre-filtration and a jar test are needed.
2. The flow meter now reads 4.6 L/min per nozzle instead of 4.2. What does this mean for the application rate, and what should you do?
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Output is proportional to flow: 4.6 / 4.2 = 1.095, an increase of about 9.5%. At the same pressure and swath, the spray volume becomes 80 × 1.095 = 87.6 L/ha, not 80 L/ha. If the pilot flies the same speed and swath as the calibration, the block will receive about 9.5% more product than the label rate, which is a label violation and increases crop damage and drift risk.
Two options: recalibrate (adjust pressure or speed to restore 80 L/ha) or recompute the mix for the new output. Recalibrating is preferable because the label rate is tied to a volume and droplet spectrum. To restore 80 L/ha at the higher flow, either reduce pressure (flow varies approximately with the square root of pressure, so a reduction to about (4.2/4.6)² = 0.83 of the original pressure would be needed, subject to the nozzle's minimum pressure for the required droplet spectrum) or increase ground speed by about 9.5%. Increasing speed changes droplet spectrum and coverage, so check the nozzle chart.
Also investigate why the flow changed: worn nozzle tips, a partially blocked filter that has now cleared, a pump or regulator fault, or a flow-meter calibration error. Do not simply accept the new figure without understanding it. Recalibrate all nozzles, check for wear, and record the new calibration.
3. What steps would you take before loading to ensure the mix is safe and effective?
Read the worked answer
- Water treatment: measure pH with a properly calibrated meter, not only a strip. Add a buffer to bring the pH to the label range (below 8.0) before adding the herbicide, and re-check. Filter the dam water to remove algae and debris that could block nozzles and inactivate the product.
- Jar test: per the label, carry out a jar test of the product, surfactant and buffered water in the proposed ratio, and observe for separation, flocculation, creaming or heat. If the mix is unstable, do not load it.
- Order of addition: water first, then buffer, then product with agitation, then surfactant. Never add surfactant before the product is dispersed.
- Compatibility: if any other product is to be added, jar-test the full combination; the label specifically requires this.
- Equipment: confirm the tank, agitator, filters, nozzles and flow meter are clean and serviceable, and that the calibration is current. Check the nozzle set against the required droplet spectrum for the product.
- Records: record water source, pH, buffer, product and surfactant quantities, jar-test result, calibration figures and weather conditions in the application log.