Knapsack Sprayer Calibration

Knapsack Sprayer Calibration To ensure the correct amount of pesticide is applied to an area the knapsack must be calibrated in the first place. Before spraying though make sure that the problem has been correctly identified so that the correct material is to be used, confirm that the problem is at the pre-set threshold for control, or is likely to reach it shortly (check your IPM plan), and also confirm that chemical control is the last and most appropriate action to take to control the problem.

There is something else which still needs to be carried out before calibration can commence and that is to ensure the sprayer has a pre-use check to ensure there are no leaks, no damage to the lance, sprayer pump lever, nozzle, filters are clean, and that the straps are not worn etc. Set the pressure control valve to 1 bar (100kPa / 15psi) which is the typical setting for many uses.

Now we can do the calibration process, which is in two main parts, firstly gathering some data (steps 1 to 4) and then secondly doing the calibration for the desired pesticide (steps 5 to 8), along with a couple of optional steps. There are different ways in which the calculations can be carried out, but the following ones are probably the most common.

Step 1: Measure and calculate the area to be sprayed. Let us assume for this example that it is a rectangular shaped putting green of 30m length x 10m width = 300m².

Step 2: Measure out 100 metres on similar ground and then time yourself for how long it takes to walk this distance at a uniform and comfortable speed. Don't forget to wear full PPE and also the knapsack, pumping the sprayer lever as you go, and have water in the knapsack as without the added weight you will be walking at a faster and easier pace. Let us say it is 90 seconds for the equivalent of 100m.

The formula to use is walking speed = 360 ÷ time taken = x km/h, therefore 360 ÷ 90 = 4.0 km/h (or about 2.5 mph). The 360 is a constant figure required in the formula.

Step 3: Put some water into the knapsack, ensure a suitable nozzle is attached (typically an anvil or flat fan for herbicide application) and walk forward on a smooth section of concrete or other suitable dry surface and measure the width of the spray on the ground. Hold the lance so that the tip of the nozzle is 50cm above the ground. Let us say that the spray width is 1.0m.

Step 4: Now measure how much water is expelled through the nozzle into a suitable measuring container, in a defined period of time, which is 60 seconds. In this example it is 1.30 litres of water which is collected.

Step 5: Input your measured data into the standard formula to determine the required spray volume, i.e. the total mixture of water and pesticide concentrate.

Volume of mixture (in litres) required per hectare = 600 x flow rate (ltr/min) ÷ (spray width (m) x walking speed (km/hr)).

Using the example data in steps 1 to 4 the completed spraying volume per hectare is 600 x 1.3 ÷ (1.0 x 4.0) = 195 litre per hectare. The 360 is a constant figure required in the formula.

Check that this rate falls within the guidelines given on the product label. If it doesn't then you will need to change the operating pressure (which is probably unlikely for a knapsack sprayer), the nozzle to maybe a coarser nozzle opening (which might be more appropriate) or your speed (which might not be practical if a consistent and comfortable speed is to be maintained over a large area, but this might be appropriate if only a small area is to be sprayed).

Step 6: This final step is used to determine how much of the pesticide concentrate is needed within the total amount of mixture. The formula is: Application (or dose) rate (from the product label) ÷ 10,000 x area (from step 1) = x ml.

In this example the product label states that for managed amenity turf the application rate for daisy, self-heal, yarrow and plantains is 2.8 L/ha (or 2800ml) in 110 ¯ 400 litre of water per hectare, so the minimum quantity of water to use in this mixture is 110 litres per hectare.

So, completing this formula results in 2,800 (ml/ha) x 300 ÷ 10,000 = 84.0ml (or 0.084 litres, so when dealing with relatively small areas and concentrate quantities it's probably easier to work in millilitres).

Step 7: the total amount of mixture needed for the defined area to be sprayed is as follows: (Vol/ha from step 5) x (area, from step 1) ÷ 10,000 (m²). Completing the formula with the previously measured and calculated data gives 195 x 300 ÷ 10,000 = 5.85 litres.

Step 8: Calculating the amount of water in the mixture is just a matter of subtracting the amount of pesticide concentrate (step 6) from the spray volume (i.e. the total amount of mixture in step 7). In this example this is 5850ml (i.e. 5.85 litres) ¯ 84ml = 5766ml of water in the mixture.

In practice and depending on the accuracy of measuring container the water will typically be from 5750 ¯ 5800 ml, so this is a potential error rate range of (-0.3% to +0.5%), so is actually very accurate and well within any acceptable variation, which would otherwise require recalibration.


Other steps to consider:

1. If you wanted to calculate how much chemical pesticide is needed in a full knapsack then the formula to use is:

(Knapsack capacity (litres) x pesticide application rate (l/ha)) / spray volume (l/ha)

For this example, the knapsack capacity is 15-litres; the chemical concentrate application rate is 2.8 l/ha and the minimum water rate is 110 l/ha, so the calibrated spray volume is 112.8 l/ha.

15 x 2.8 / 112.8 = 0.372 litres (or 372 ml) of concentrate within the 15-litre container.

2. Calculating how much a full knapsack will cover uses the following formula:

Spray volume / knapsack capacity = number of full knapsacks needed per hectare

112.8 / 15 = 7.52 full knapsacks needed per hectare

How much one full knapsack will cover = 10,000m² / 7.52 = 1,330m².