October 2024

PIETMAN BOTHA, INDEPENDENT AGRICULTURAL CONSULTANT |

SPRAYER CALIBRATION MUST BE DONE TO ENSURE THAT YOU ARE SPRAYING THE CORRECT AMOUNT OF LIQUID (WATER AND HERBICIDE) ON TO THE DESIRED AREA. FARMING IS ABOUT PROFIT, AND THE CORRECT APPLICATION OF HERBICIDE OR PESTICIDE WILL IMPROVE YOUR PROFIT.

A once-off calibration is not enough – you must calibrate it before the spraying season begins, and recalibrate it frequently throughout the spraying season. If you know the field size, compare the applied mixture with the mixture calculated to apply. For instance, if you spray 300 ℓ of water per hectare and the field is 10 ha, you should have applied 3 000 ℓ of water.

You only need a stopwatch or timer, which most cellphones have, a measuring jug (marked in ml) and a measuring tape.

**START THE PROCESS**

Although many methods can be used, the method described here is simple and requires only a few calculations. In principle you want to calculate how much water is sprayed per hectare. To do it, is very easy. Before you start, read your operation manual.

It is very important to clean your sprayer tank and the various filters and nozzles. Fill the tank with water and open the sprayer boom. Let the tractor run at the required revolutions/min (rpm) (e.g. 2 500 rpm). Engage the pump and let the sprayer spray. Make sure all the nozzles are the same type and colour and that the sprayer pressure is in line with what is needed for the nozzles.

**METHOD**

Measure a distance of 100 m on the farm road. Use the tape and place a dropper at the beginning and end of the 100 m.

- Fill the tank of the sprayer with clean water.
- Decide which gear will be used and at which rpm (e.g. fourth gear at 2 500 rpm). Remember it is difficult to drive a tractor with a sprayer accurately, so don’t go too fast. First or second gear is fine.
- Using that gear and the correct rpm, measure the time that it takes for the tractor to cover the 100 m – for example, 60 seconds.
- Run the tractor in a stationary position, at the agreed 2 500 rpm, and check the spray cover that you are achieving on the ground.
- Set the pressure on the sprayer to give the desired effect (e.g. 3 bar).
- Using the same time that it took the tractor to cover the 100 m, engage the spray and measure the amount of water produced by catching the water delivered by one separate nozzle. Normally a few clean 2 ℓ milk bottles can be used to catch the water delivered by letting the nozzle spray into the bottle – for example, 1 200 ml/1,2 ℓ for 60 seconds.
- Use the amount of liquid produced by one nozzle (for example, 1 200 ml/1,2 ℓ) and multiply this with the number of nozzles (for example, twelve nozzles) so that you know how many litres you are spraying out over 100 m.
- With all the nozzles in this case: 28 x 1,2 ℓ = 33,6 ℓ.
- Measure the width of the boom (for example, 14 m). Now you know the width that you are spraying, and you know the amount of water you are spraying over that width over the 100 m. The area covered by the sprayer over the 100 m: 100 m x 14 m = 1 400 m
^{2}. - On this area, 33,6 ℓ of water is used. An area of 1 ha is equal to 10 000 m
^{2}(100 m x 100 m). The volume to be used on 1 ha will be 10 000/1 400 = 7,14 x 33,6 ℓ = 240 ℓ/ha. - To make sure your calculation is correct: If your nozzles are 500 mm apart, take the delivery of one nozzle and multiply it with 200. In this case, 1,2 ℓ x 200 = 240 ℓ/ha, and this shows that the calculation is correct.

The label on the chemical container will indicate the amount of water required per hectare, as well as the number of litres of herbicide required per hectare.

**CALIBRATING A KNAP-SACK SPRAYER**

Source: PGP Farmer Development Training Manual

- Using a measuring wheel or tape measure, measure 50 m.
- Walk up and down the 50 m and measure the time it takes to cover the distance (50 m + 50 m = 100 m). Remember to walk at a steady pace, as you will have to keep up that speed. The faster you walk, the less liquid will be sprayed onto a particular area.
- Spray liquid from the knapsack sprayer into a measuring jug for the same number of seconds that it took you to walk the 100 m. You will now know how much liquid you would have sprayed out onto the 100 m. Example: Suppose you sprayed 1,75 ℓ of water into the jug. You know then that you will use 1,75 ℓ of water for every 100 m walked.
- It is now necessary to calculate the number of rows in 1 ha. As you know, 1 ha is 100 m by 100 m. Assuming the row width is 0,9 m, there are 111 rows in 1 ha.
- If the liquid in the jug was 1,75 ℓ (step 3), you will use 1,75 ℓ on one row. There are 111 rows in 1 ha, which means to cover the whole hectare, you will need 111 x 1,75 ℓ of liquid = 194 ℓ/ha in total.
- The chemical label will indicate the amount of the specific chemical, which should be applied per hectare or per 194 ℓ of water.
- The chemical rate (per hectare) ÷ the number of litres used = the concentration of chemical per litre of water.

You now have the following options:

- Fill a container with 194 ℓ (a standard large drum is 200 ℓ) and pour in the chemicals required per hectare. Stir well and continue stirring regularly to avoid the chemicals settling. You can then use this premix to fill your knapsack or spray tank.
- If you are using several smaller water containers, for example 20 ℓ containers, or even if putting chemicals directly into the knapsack sprayer, multiply the concentration of the chemical per litre (step 7) by the number of litres that the container holds.

**Publication:** October 2024

**Section:** Pula/Imvula