Key points

• Pulse width modulation (PWM) controls the flow rate by rapidly turning on and off a solenoid located just prior to the nozzle, causing the nozzle to ‘pulse’  

• PWM systems maintain constant pressure in the spray line so that spray quality (droplet size) does not substantially change when flow rate (pulsing) changes in response to spraying speed 

• The duration of each pulse (time that the nozzle is ‘on’) is referred to as the ‘pulse width’ 

• The percentage of time the nozzle in ‘on’ (compared to off) is referred to as the ‘duty cycle’ 

• The number of on/off cycles per second is known as the frequency, reported as hertz (Hz).  

• Some PWM systems have a fixed frequency (e.g. 10 hertz). Other systems can vary both the pulse width and pulse frequency (up to 50 Hz or 100Hz) for greater control of the nozzle outputs 

• Rapid on/off control at each individual nozzle allows for options such as single nozzle section control, turn compensation, boom recirculation and variable rate application 

• The principle of PWM systems means that there will be times when all nozzles on ‘on’ and other times only some nozzles will be ‘on’. This will mean that there can be different application rates applied across the paddock. Boom height, travel speed, duty cycle and nozzle selection require careful consideration, particularly with lower fixed frequency systems to avoid the duty cycle becoming too low for the preferred spraying speeds and application volume.  Low duty cycles, particularly when associated with fast travel speed and large droplets, can impact on spray coverage and efficacy