Fixed pressure or pulse-width modulation?

The original optical spot spraying systems use constant pressure in the spray line, so spray quality remains constant, unless the pressure or the nozzles are changed by the operator.

Constant pressure systems generally operate between 2 and 3 bar pressure, but always follow the specific recommendations for each manufacturer and the label requirements for spray quality for the particular nozzles fitted to the machine.

Systems using solenoid valves to rapidly turn nozzles on and off will change the applied rate if the spraying speed varies from the intended or calibrated speed.

Some newer optical spray systems may utilise pulse-width-frequency modulation (PWFM), typically at 50 hertz (cycles per second) to control the nozzle output, so the application rate and droplet size remain constant even if the spraying speed changes (provided it remains within the limits of the duty cycle of the system)..

If using PWFM ideally run high duty cycles (close to 100 per cent) when operating towards the upper end of your normal spraying speed, as this will provide a buffer as the machine slows down. It is important to note that as the duty cycle decreases, the area covered by coarse (or larger) spray droplets may be impacted as they do not readily redistribute in the same way as fine or medium spray qualities.

PWFM offers the opportunity to apply a blanket rate across the whole paddock (e.g. at 50 per cent duty cycle) for small weeds and to use the sensors to apply a spot rate (100 per cent duty cycle) to larger weeds. Before considering this approach to spraying, careful consideration must be given to the nozzle type fitted to the machine, as even nozzles are less than ideal for blanket applications. The operator also needs to carefully consider the registered or permitted rates of the selected products, as well as the potential for resistance should less-than-robust rates of product be applied.

In almost all situations it is better to have two separate tanks and spraying systems: one for broadacre applications and another for target-selectable sprayer applications.

Find out more

For more information, see Module 17: Pulse-width modulation