Read time: 4 minutes

What’s the best PWM set-up for summer spraying?

with Andrew Freeth, sprayer operator and GOA director, Collie

Pulse Width Modulation (PWM) sprayer technology gives operators greater control over pressure and droplet size to suit various spray jobs.

Andrew Freeth, a grain grower and PWM sprayer operator at Collie, NSW, says it can take time to fully understand how PWM works. However, getting the set-up right is essential to avoid poor application results, including underdosing that could contribute to herbicide resistance.

“The key advantage of PWM is that spray quality remains constant, irrespective of ground speed, because the sprayer maintains constant pressure in the spray line,” he says. “The pulse delivery from paired nozzles mixes spray droplet sizes to produce even coverage of the target.”

There are specialised nozzles for use on PWM sprayers. Like conventional sprayers, selecting the correct nozzle for the spray job is an important first step in the setup.

“To avoid spray drift, it is important to choose nozzles that will deliver the largest spray quality possible while maintaining the efficacy of the products in the tank,” says Andrew. “When 2,4-D is applied, choose the nozzle and pressure to deliver very coarse or coarser spray quality.”

The WeedSmart Big 6 strategy includes tactics to maximise spray efficacy and minimise the impact of herbicide resistance on farm profitability.

How does PWM work?

PWM technology uses a system of paired nozzles, with each pair controlled by a solenoid. The solenoid ‘fires’ the nozzles alternately to maintain a steady pressure in the spray line. This means the operator does not need to adjust the pressure when slowing down to turn, avoid obstacles or travel over rough terrain.

To maintain the pressure, the solenoids open and close the nozzles to manage the flow rate. This is called the ‘duty cycle’ and ranges from 0% (no flow) to 100% (continuous flow). The higher the duty cycle, the more accurate the spray delivery to the target. At 50% duty cycle, the nozzle is open for half the cycle duration and closed for the other half. At 75% duty cycle, the nozzle is open for 75% of the cycle and closed for 25%. Ideally, the sprayer setup will maintain a high duty cycle (80% or higher) at the intended ground speed for the spray job. The duty cycle must not exceed 100% (i.e. both nozzles open simultaneously), as the result is underdosing due to the machine travelling too fast at the set pressure.

The different PWM systems vary in the frequency of cycles per second. Lower frequency systems (e.g. 10 cycles per second or 10 Hz) need to operate at a higher duty cycle than systems with higher cycle frequency (e.g. 15/30/100 Hz). The higher frequency systems allow for a wider range of operating ground speeds.

Maintaining a steady pressure means the spray coverage and application rate remain the same throughout the spray job. This is important for maximising efficacy, minimising spray drift and minimising low dosing of target weeds.

See the GRDC factsheet below for more technical details.

What parameters should I consider when setting up?

When setting up the PWM for a spray job, first calculate the flow rate (L/min/nozzle) based on water volume (L/ha), working speed suitable for the paddock being sprayed and the nozzle spacing on the PWM rig.

Next, determine the operating pressure required to achieve a 100% duty cycle and the minimum pressure necessary to keep the sprayer operating effectively (e.g. above 70% duty cycle at the lowest expected speed). Operating at low duty cycles can produce poor spray coverage and results in skips (or tiger striping).

The final step is to select the appropriate nozzle for the required pressure range and droplet spectrum.

Using the pressure and application rate parameters, you can determine the optimal ground speed range to operate within during the spray job. Typically, a PWM sprayer can operate effectively within a 5–6 km range while maintaining the necessary duty cycle to achieve a consistent spray quality.

Use water-sensitive paper to assess the spray coverage before heading out to do the spray job. Then, the operator can be confident they will meet label requirements for spray quality and application rates while operating within the ground speed range. 

What should I keep in mind while doing the spray job?

When the spray rig slows down, the duty cycle also drops. Plan the spray job to minimise the area covered at the lower duty cycle. Avoid increasing the pressure setting on-the-go as this will lower the duty cycle and could compromise spray quality.

When applying coarse droplets, it is recommended to work at a higher boom height (700 mm rather than 500 mm), but remember that the drift-able fraction of droplets will travel further from the target.

Where possible, park a PWM sprayer in a shed to protect the solenoids from excessive exposure. If a solenoid ceases to work, the PWM system will revert to standard sprayer operation.

The solenoids on a PWM sprayer allow operators to use these rigs as spot sprayers if a weed map is available for the paddock.

More resources

GRDC Factsheet – Pulse Width Modulation (PWM) sprayers 

SOS-NSW website  

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