Tank size

It is always difficult to predict how much product will be used by an optical spot sprayer, so it is hard to anticipate how large the spray tank for the system should be. But, as a guide, expect to use less than 20 per cent of the total spray volume that a conventional boom may use on the same field.

Alternatively, it may be more important to consider how long it will take to empty the tank, to avoid tank mixes ‘going off’, especially where high concentrations of herbicide mixtures may be used.

Carefully consider the width of the machine and the spraying speed, for example a 9m system spraying at 16km/h will cover approximately 15 hectares per hour.

If the application volume is calculated to be about 100L/ha, at a maximum of 30 per cent weed coverage, you may expect to apply about 30L of the tank mix over a hectare of paddock.

So the sprayer output could be about 30L/ha x 15ha/hr = 450L/hr, hence a 1000L tank would allow for at least 2 hours of spraying. For this same setup, if the average weed density is only 10 per cent of the paddock, then it will take approximately 6 hours of spraying to empty this same tank.

With experience, many users report that they quickly get a feel for the rate of spray volume required relative to visual weed density in the paddock.

Agitation requirements of the tank mix

It is important to ensure that the tank mix is well, but not excessively, agitated for the tank size. Excessive agitation can lead to foaming with some products or mixtures.

Product limitations

Always consult the product label or APVMA-approved permits to determine the application rate that a particular product can be applied using an optical spot spray application.

Always consult the sprayer’s instruction manual to see if there are limits to the types of formulations that can be used, or potential impacts from the solvents that may be contained in certain products on the components used in the optical spot sprayer.

Operating limitations

The sensors or cameras on optical spot sprayers are not very different to the human eye: if something obstructs our view we are not able to see everything that is in front of us.

Anything that obstructs the field of view of the sensor may impact on the performance of the optical spot sprayer.

Other factors that impact on performance include high spraying speeds and high wind speeds. High spraying speeds may impact on the droplets’ ability to stay on the target or may increase shadowing and deposition onto one side of the target weeds. High wind speeds will tend to displace the spray away from the target weed.

If purchasing an optical spot sprayer, anticipate that the spraying speed will not be more than 18km/h.

Find out more

For information about the agitation requirements for a particular tank size, see Module 11: Pumps, plumbing and components