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Evan Thorhnill's Do-it-yourself Calibrator for Knapsack Sprayers

Do you have a better calibration method? Check out IPARC's Do-it-yourself Calibration Challenge

Accurate calibration of your sprayer will help you apply the right amount of spray over your crop. Too much spray and you risk causing harm to your crop, your soil and nearby waterways. Too little spray is less likely to control the pest you wish to control and may encourage the pest to develop resistance to that pesticide.

Evan Thornhill of the International Pesticide Application Research Consortium (IPARC) explains below how to make your own calibrator and use it to find out the water volume rate your sprayer will apply; the first step in making sure you apply the correct amount of pesticide over the area you are going to treat

You will need:

Items needed to create an easy-peasey calibrator (31 Kb JPEG)
Items needed to create an easy-peasey calibrator

Empty tin can; 400 g (= 400 ml) size
Clear Drinking straw (or dry stick)
Polythene bag, strong and about 500/1000 ml capacity
Elastic band or string;
Tape measure

Tin can: Ensure the tin can is clean and its capacity is 400 to 430 g]/mls [see can label].

Drinking straw: Take the straw and place it upright in the vertical tin and mark the straw with a pencil/felt pen where the top of the can reaches. Then also mark the straw at three quarters of the way up the can, half way up the can and one quarter depth. You can also use a dry stick though a straw is better.

Prepare sprayer. Check the sprayer is clean, works correctly without leaks, is fitted with the nozzle you intend to use, contains clean water and will spray the correct pattern at the intended pressure.

Polythene bag: Attach the polythene bag securely to the sprayer nozzle to trap all liquid that is sprayed. Use a strong elastic band or string.

To calculate the water volume rate

Measure out an area 10 square metres (m2):

  • If your nozzle has 0.5 metre swath (ie. will spray a swath 0.5 metres wide) then measure in a straight line and make a mark 20 metres away.
  • If your nozzle has a 1 metre swath then mark out 10 metres distance.

This will give you an area to be sprayed of 10 m2.

Spray 10 square metres (m2):  Now spray over the distance you have marked at the speed and in the manner that you would normally use, collecting all the sprayed water for that time in the polythene bag.

Measure volume sprayed out: Pour the water collected into the tin. Take the straw and hold it upright in collected water. Place your finger over the bottom end of the straw and remove the straw from the water. Note height of water in straw.

Read the calibrated water volume rate: If the can [and straw] is 

  • one quarter filled then you have sprayed at a little over 100 litres/hectare; if it is
  • half filled then you have sprayed at 200 litres/hectare; if it is
  • three quarters then you have sprayed at 300 litres/hectare; or
  • full then you have sprayed 400 litres/hectare

To calculate the calibrated water volume rate: Remember that: 1 litre = 1000ml and that  1 hectare = 10,000 m2

Calibrated water volume rate in litres per hectare  = [(can capacity (ml) x fraction can is filled) x (10,000 m2 / area sprayed) ] /1000

So if the can is one quarter full then

  • 400ml x 1/4 = 100 ml has been sprayed over 10 m2.
  • 10,000 m2 / 10 m2 = 1,000. A hectare contains 1,000 lots of 10 m2. So if you were to spray one hectare you would use:
  • 1,000 x 100 ml = 100,000 ml of water
  • 100,000 ml / 1000 = 100 litres per hectare

Now calculate the correct amount of concentrated pesticide to be added to the tank where the:

Quantity of concentrated pesticide per tank = Product rate per hectare/ (calibrated water volume rate/tank capacity)

Thus:  if 

  • the product rate (i.e. the amount of concentrated pesticide that should be applied per hectare) is 1 litre i.e. 1000ml; and your
  • calibrated spraying  rate is 200 litres/hectare and if your
  • sprayer tank has a capacity of 20 litres

then the quantity of concentrated product per tank (ml) = 1000 ml / (200/20)
 = 1000 ml / 10
= 100 ml

Our dosage wheel makes this calculation very easy - please email us if you would like us to send you one.