Compiled by J Fuls (Pr Eng)

Calibration means to measure how much chemicals will be sprayed onto the field and to adjust the sprayer to apply exactly the desired quantity. To adjust means to change the things one can change. But before we can start with the actual calibration process, a few preparatory steps have to be taken.

We thank the ARC Institute for Agricultural Engineering in South Africa, who made this article available to the readers of ProAgri Zambia.

Spraying means more than simply moving up and down a field with a machine hoping to reach the desired results. The machine cannot do it automatically; it needs to be adjusted and handled in a manner that will ensure the right results.

At the end one wants to apply the right quantity of chemicals to all the required spots in the field.

What can one adjust or change?

Change nozzle

There are various types of nozzles, and within the same type of nozzle, the hole sizes of nozzles also differ. A nozzle with a bigger hole will deliver more chemicals at the same pressure. If the nozzle is too big, you will have to pump faster to spray correctly. The tank will become empty very soon and a lot of re-filling will be needed. A smaller nozzle will save re-filling time.

The width that nozzles cover also varies. If a nozzle only covers a narrow band at a time, one would have to hold the nozzle high to get the required spray width. This may be uncomfortable for the operator and there is more chance for the wind to blow the spray away.

A wide spraying nozzle is usually the most suitable for a knapsack sprayer. One may find that a larger nozzle may now be needed to sufficiently wet the spray target.

Change the spraying pressure

A harder, quicker pumping action will increase the pressure and more water will pass through the nozzle. Too high pressure forms too small droplets, which you may not want. It also gets blown away more easily by wind. Practice and choose the lowest suitable pressure while walking.

Walk faster or slower

When walking faster, less water will be sprayed onto a spot. The opposite will of course happen when walking slower. One should however walk at a comfortable pace, to ensure that you will be able to maintain a constant walking speed and maintain good control over the spraying operation.

Change the quantity of chemicals mixed into the spraying water

Mixing more chemicals into the water means that more of it will be applied. To decide how much chemicals to mix into the spraying water, we need to do a calibration.

Measuring out the chemicals

To add the correct quantity of chemicals to the spraying water, one must be able to measure the volume of the chemicals. It is best to obtain a measuring jug, which should be available at most supermarkets.

Measuring jugs are usually marked in millilitres. Remember that it takes
1 000 millilitres to make up one litre. (One teaspoon holds about 2,5 millilitres.) If one cannot get hold of a measuring jug, plastic cold drink bottles can be used. Cold drink bottles are available in sizes of 500 millilitres, 1 litre and 2 litres: Two 500 ml bottles will fill a one litre bottle and two one litre bottles will fill one two litre bottle.

Now we can measure off 250 ml, which is half of 500 ml. (0,5 x 500 ml = 250 ml)

Repeat this again, with the bottle marked for 250 ml:

Now we can measure off 125 ml, which is half of 250 ml. (0,5 x 250 ml = 125 ml)
Next, we can fill one of these 500 ml bottles to the halfway mark, which is 250 ml.

Now we can measure 375 ml, which is 250 ml plus 125 ml. (250 ml + 125 ml = 375 ml)
Now rub over the markings with a dirty finger to make the markings stand out clear:

Next month we shall look at the careful steps you have to take in the calibration procedure. Published with acknowledgement to the ARC Institute for Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.