The short answer
What is boom sprayer calibration?
Calibration means measuring how many litres of spray your machine puts on each hectare. The product label gives a rate per hectare. You can only hit that rate if you know your sprayer's output, because the output decides how much product goes into each tank.
A boom sprayer is a tractor-mounted or trailed sprayer with a horizontal bar of nozzles, used to spray field crops, pastures and fallow land. The method below works for any boom width and any nozzle type.
What do you need?
- A tape measure
- A stopwatch or phone timer
- A measuring jug marked in millilitres
- Clean water in the tank, at least half full
- Two markers, such as pegs, to mark out 50 m
How do you calibrate a boom sprayer, step by step?
- Measure the boom width. Measure from the outer nozzle on one end to the outer nozzle on the other, then add one nozzle gap. Each nozzle sprays half a gap past its neighbour on both sides, so the end nozzles spray beyond the boom tips.
- Count the working nozzles. Count only nozzles that are switched on for the job.
- Time 50 m. Mark out 50 m in the field you will spray. Drive it in your spraying gear and engine speed, already moving when you pass the first marker. Write down the seconds. Do it twice and use the average.
Tip: time five gears, not one.
- Set the engine revs so the PTO turns at 540 rpm. The PTO (power take-off) is the shaft that drives the sprayer pump, and rpm means revolutions per minute. Most tractors mark this setting on the rev counter.
- Choose a gear that gives 6 to 8 km/h at these revs. Over 50 m, that is about 23 to 30 seconds.
- Time 50 m in this gear, in the field.
- Keep the engine revs the same. Time 50 m in each of the two gears below and each of the two gears above. Time each gear twice and use the average.
- Write down the time for all five gears.
With the PTO at 540 rpm and the pressure unchanged, each nozzle delivers the same flow in every gear. Only your ground speed changes. If your output is off target, enter the time for another gear into the calculator. A lower gear gives more litres per hectare, a higher gear gives fewer. Pick the gear that comes closest to your target. You do not need to time the sprayer again.
- Measure nozzle discharge. Discharge is the litres one nozzle delivers in a minute. Run the sprayer at working pressure, hold the jug under one nozzle for 60 seconds and read it in litres. Check nozzles at both ends and the middle, and use the average.
- Work out litres per hectare. Use the formula below, or enter your figures in the calculator.
- Mix the tank. Divide tank size by litres per hectare to get hectares per tank. Multiply that by the label rate to get product per tank.
What is the formula?
Speed (km/h) = 180 ÷ seconds to cover 50 m
The 600 converts minutes, kilometres and square metres into hectares.
Worked example. A 12 m boom with 24 nozzles. Each nozzle gives 1.2 L/min, so the boom gives 28.8 L/min. The tractor covers 50 m in 30 seconds, which is 6 km/h.
Output = 28.8 × 600 ÷ (12 × 6) = 240 L/ha
A 1 000 L tank covers 1 000 ÷ 240 = 4.17 ha. At a label rate of 2 L/ha, add 4.17 × 2 = 8.3 L of product per tank.
How do you change your output?
Change speed first. Slower gives more litres per hectare, faster gives fewer. If speed alone cannot get you to the rate you want, change to a larger or smaller nozzle size.
Write down the pressure on the gauge each time you calibrate, so you can set the sprayer up the same way next time.
What are the common mistakes?
- Boom width measured tip to tip. Measure nozzle to nozzle and add one gap, or your output figure will be wrong.
- Timing on a road. Tyres slip less on a hard road, so you drive faster there than in the field. Time in the field.
- Empty tank. A light tank runs faster than a full one. Time with the tank at least half full.
- Worn nozzles. Old nozzles pass more liquid and spray an uneven pattern.
- Mixing to the label without knowing L/ha. The label rate is per hectare, not per tank. Without your output figure, the tank mix is a guess.