GuideCrops
Calibrating a knapsack or boom sprayer, step by step
A sprayer that puts out more or less water than you think puts out the wrong dose of every product. Here is how to measure your real spray volume per hectare and work out how much product goes into each tank.
Herbicides, insecticides and fungicides are registered at a rate per hectare. The label tells you how much product to put on a hectare, and often a recommended water volume too. But the product only lands at that rate if you know how much water your sprayer actually applies per hectare. Nozzles wear, walking speeds differ and pressure drifts, so the only way to know is to measure. This is called calibration.
Before you start
- Use clean water only for calibration.
- Check that the sprayer does not leak, the filters are clean and the nozzles are the type the label recommends for the job (for example, flat fan nozzles for many herbicides).
- Wear gloves: even a rinsed sprayer can hold residue.
- Calibrate on ground similar to your field, since walking or driving speed changes on rough or soft soil.
Knapsack sprayer: the 100 square metre method
- Mark out an area of 100 m², for example 10 m by 10 m, on ground like the field you will spray.
- Fill the knapsack to a known level with clean water.
- Spray the whole area as you normally would: the same walking speed, the same pumping rhythm and the same nozzle height above the target.
- Measure how much water you used by refilling to the starting level with a measuring jug.
- Litres used on 100 m² × 100 = litres per hectare.
Example: you used 2.5 L to cover 100 m². Your spray volume is 2.5 × 100 = 250 L per hectare. Repeat the test two or three times and take the average. If the results differ a lot, your walking speed or pumping is not steady; practise until they agree within about 10 percent.
How much product per knapsack
Product per tank = label rate per hectare × tank volume ÷ spray volume per hectare. If the label rate is 1 L per hectare, your tank holds 16 L and you apply 250 L per hectare: 1 × 16 ÷ 250 = 0.064 L, or 64 mL per tank. One full tank covers 16 ÷ 250 = 0.064 ha, which is 640 m².
Boom sprayer: nozzle output method
For a tractor-mounted or trailed boom, you need three measurements: the output of each nozzle, the nozzle spacing on the boom and your forward speed.
- Measure forward speed: time how long the tractor takes to cover 100 m in the field, in the gear and engine speed you will spray at, with the tank about half full. Speed in km/h = 360 ÷ seconds taken.
- Park the tractor, run the sprayer at working pressure and catch the output of each nozzle for exactly one minute in a measuring jug.
- Replace any nozzle that differs from the average by more than about 10 percent, or that is far above the manufacturer's rated flow for that pressure. Worn nozzles usually spray more, not less.
- Litres per hectare = (average nozzle output in L/min × 600) ÷ (speed in km/h × nozzle spacing in m).
Example: nozzles average 0.8 L/min, you drive at 7 km/h and nozzles are 0.5 m apart. Spray volume = (0.8 × 600) ÷ (7 × 0.5) = 480 ÷ 3.5 = about 137 L per hectare.
| If you… | Spray volume… |
|---|---|
| Drive or walk faster | Goes down in proportion to speed |
| Increase pressure | Goes up, but only slightly; doubling pressure raises flow by roughly 40 percent and makes finer, drift-prone droplets |
| Fit larger nozzles | Goes up; the best way to make a big change |
| Let nozzles wear | Creeps up unevenly across the boom |
Change volume mainly by changing nozzle size or speed, not by turning the pressure up or down a lot, because pressure also changes droplet size and spray pattern.
Product per boom tank
The same sum applies: product per tank = label rate × tank volume ÷ spray volume per hectare. A 600 L tank at 137 L per hectare covers about 4.4 ha; with a label rate of 1 L per hectare, that tank needs about 4.4 L of product. Work out how many tank loads you need for the field, and mix the last load only for the area that is left.
Good spraying habits
- Recalibrate at the start of every season, whenever you change nozzles and whenever a different person sprays.
- Spray in calm conditions, avoid the heat of the day and never spray when drift could reach neighbouring crops, homes, water sources or beehives.
- Keep the boom at the height the nozzle maker recommends, usually around 50 cm above the target for common flat fan nozzles.
- Triple-rinse empty containers and dispose of them as the label and local regulations require.
- Record the date, field, product, rate, spray volume and weather in your farm records.
Key points
- Measure your real spray volume per hectare with clean water before mixing any product.
- Knapsack: litres used on 100 m² × 100 = L/ha. Boom: (L/min × 600) ÷ (km/h × spacing in m) = L/ha.
- Product per tank = label rate × tank volume ÷ spray volume.
- The registered label is the law for rates, crops and safety; follow it every time.
This guide gives general information. Conditions differ from farm to farm, so confirm recommendations with your local extension officer, agronomist or veterinarian, and always follow the registered product label.
