Module 20: Optical spot sprayers

20.5 Timing and sensitivity

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Published 24 January 2025 | Last updated 20 January 2025

Optical spray systems have the ability to adjust sensitivity to detect smaller or larger weeds. However, increasing the sensitivity too much may lead to an occasional ‘misfire’.

Systems have the ability to manipulate the length of time that the nozzle is ‘on’ which directly corresponds to how much of the ground and target will be sprayed (before and after the weed).

While reducing the length of time the nozzle is ‘on’ will save chemical, this will be relatively small in comparison to the overall saving a optical spot sprayer will provide, when compared to a conventional boom ‘blanket spraying’ a whole paddock. Reducing the duration the nozzle is ‘on’ too much risks missing some weeds, particularly in situations of higher wind speeds and higher spraying speeds.

Assessing spray coverage in relation to the target weed

Water-sensitive paper (WSP) is a valuable tool for setting up the timing and determining the sprayed width for optical spot sprayers.

Using long strips of water-sensitive paper, or a number of cards clamped to timer strips will help to highlight where the spray is depositing in relation to a target weed

Using water-sensitive paper to assess spray deposits

In this example the sprayer was travelling from the bottom of the image to the top using a pre-orifice nozzle, which delayed the droplets arriving at the target at the correct time. Source: Bill Gordon (53.7 KB JPG)

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Determining sprayed width for L/ha applications

Measuring the actual sprayed width in the field, under operating conditions, is critical to determining the application rate.

WSP cards are useful for assessing the sprayed width. Once this has been determined the following formula can be used to determine the spray volume rate (L/ha) in the applied spot:

L/ha = litres per minute per nozzle x 600 ÷ speed (km/h) ÷ width (m)

While WSP is a useful tool for determining where the droplets are landing, it will not indicate how many droplets will actually stay on the target weeds. When using high- speed, coarse droplets there will be a lot of droplet bounce and shatter from the leaf surface, which may not be picked up when using WSP.

To assess actual spray deposits it may be useful to consider using a fluorescent dye and black light blue (UV) torch to assess how much product is actually depositing on the target weed, and where on the weed the droplets are landing.