Calibrating a Sprayer: Litres per Hectare from a Stopwatch and a Jug

Tractor speedometers read wrong when tyres wear or slip. Measure 100 m in the field you will spray, with the sprayer half full, and mark both ends with survey flags. Drive it at the gear and engine speed you will spray at, getting up to speed before the first flag, and time it. Do it twice and average. Speed in km/h = 360 ÷ seconds for 100 m. A 45 s run is 8.0 km/h.

Fill the tank with **clean water only**. Park, set the pressure you will spray at, and run the boom. Hold a measuring jug under each nozzle for exactly one minute and write down the catch. From the notebook, work out the flow each nozzle needs for your target rate, speed and spacing. Replace every nozzle more than 10 % off its rated flow — the flat-fan tips wear open with use, and a blocked one shows at once. Clean filters and nozzle strainers while you are there. Catch the replaced nozzles again. Then compute the boom's actual rate from the average catch. If it is within a few per cent of target, trim with pressure; if not, change speed or fit a different nozzle size. Write the settings on a card in the cab.

The calibration method here is the one taught by agricultural extension services: time a known distance, catch each nozzle for a fixed time, replace nozzles more than 10 % off, and compute the rate from the formula. In US units the same relation is GPM = GPA × MPH × spacing (inches) ÷ 5940, and the notebook checks the two agree. **Honest limits.** Calibrating does not make a spray safe: always follow the product label for rate, protective equipment, weather and buffer zones, and calibrate with water, never with the product. Uniform output from each nozzle does not guarantee uniform coverage on the ground: boom height, overlap between fans and wind all change it.