10.2 Calculating the correct amount of chemical

Pesticides purchased for spraying programs will come in the form of pesticide concentrate. This concentrate is very strong and must be diluted before use by mixing a small volume (amount) of the pesticide with a larger volume of water.

It is necessary to work out how much of the concentrate will be needed for the spraying job and how much water it must be mixed with. Only enough pesticide solution to fill the sprayer should be mixed at any one time.

The steps for calculating the correct amount of chemical are outlined below.
  1. Check the pesticide label to find the application rate at which the concentrate should be used. The application rate of a particular pesticide is the amount of mixed pesticide solution (chemical plus water) which is needed to treat an area of a particular size.
    Some examples are:
    • Use 10 ml concentrate per 5 L water to cover 40 square metres (40 m2)
    • Mix 5 g powder with 5 L water to cover 20 square metres (20 m2)
    • Use 1 packet of powder per 10 L water
  2. Work out the area to be sprayed. This may be the area of a floor or the combined areas of skirting boards or the combined area of external (outside) building foundations.
  3. Using the application rate stated in the instructions, calculate the amount of pesticide concentrate needed for the size of the area to be sprayed.
  4. Calculate how much water is needed to dilute the pesticide to the correct strength.
It is very important that these calculations are done correctly. If they are not done correctly the pesticide will not be the right strength for the job.
Help can be obtained from people such as the community nurse, school teacher, EHP supervisor or an EHO.

Top of Page

10.2 Mixing the chemical

Once the amount of the concentrate and the amount of water needed to dilute it have been worked out, the water and the chemical can be mixed.

This dilution exercise should be carried out carefully because the pesticide chemical is dangerous.

These are the rules which should always be followed when diluting pesticide concentrates:
  1. Always work in the open and avoid breathing the fumes.
  2. Read the label and put on the appropriate protective equipment as indicated.
      • Depending upon the type of pesticide it may be appropriate to wear a respirator.
  3. Mix water and concentrate in a large clean container, such as a 10 L bucket. This container and any measuring cups must be used only for this purpose. They should be clearly labelled ‘DANGER - POISON: DO NOT TOUCH’. When they are not being used they should be stored safely in the equipment shed.
  4. Put a small amount of water into the bucket first. Place the required amount of pesticide into the water.
    Rinse the measuring cup with clean water and add this solution to the bucket. Stir it so that it is thoroughly mixed into the water. Pour this solution into the sprayer tank and then add the rest of the water to the tank. Make sure this water is well mixed into the pesticide solution.
  5. Stir the solution carefully with a flat paddle (stirrer) and avoid splashing.
    The safest paddles are made of plastic, aluminium or steel because these materials are impervious. This means the pesticide cannot soak into them. They can be washed and used again. Wooden paddles soak up the pesticide and must be disposed of immediately after use. This must be done with extreme care. It is best to bury them along with the empty pesticide containers.

Never leave the paddles lying around after use as they will be a danger to small children and animals.
Fig.  5.37:  Concentrated pesticide should be diluted in water according to the instructions on the bottle or packet.
Fig. 5.37: Concentrated pesticide should be diluted in water according to the instructions on the bottle or packet.