Calibration with Knapsack Sprayer

Calibration of a knapsack sprayer prior to spraying is recommended in knowing how much chemical to mix in a given tank load. Spraying technique is operator dependent and can also vary from one occasion to the next even with the same operator. This is why a little time should be spent performing this activity each time one sprays a given area with a chemical.

The Backpack sprayer should be checked for leaks and the filters cleaned prior to use :

(above) fan nozzle filter

First of all, the flow rate of the sprayer is measured five times and then averaged. Output is in litres per minute. The readings we recorded were 1.2L, 1.0L, 1.0L, 0.92L, 0.92L, giving an average of 1.01 L per minute.

(above) measuring output by spraying into a measuring cup.

Next, the distance covered in one minute for the operator whilst spraying at a normal constant operating speed is measured:

This was measured to be 46m. The swath width during this time was measured also - this means that the operator should keep the spraying arm at a comfortable and constant position. Our result was 0.51m. This gives a total area sprayed in one minute as 46 x 0.51 = 23.46 metres square.

The Output/Area is now calculated to give the amount of litres per square metre. This is multiplied by 10,000 to give L per hectare (most chemical manufacturers provide these units for commercial spraying).

ie. 1.01/23.46 = 0.043052 x 10,000 = 430.52L/Ha (the output of the unit with that operator on that particular day)

Then to calculate the total quantity of chemical to add to a given tank load :

Amount to add = tank capacity/output of unit x label directions of how much to add (L/Ha)