Most PWM spraying systems use conventional plumbing with a single boom line supplying the spray solution to electric solenoids fitted at each nozzle location across the boom. 

Flow regulation (pulse width and duty cycle)

The liquid flow rate through each nozzle is managed via a regular, very brief, shutoff of the nozzle flow at the diaphragm check valve which is controlled by an electric solenoid that replaces the spring-loaded check valve. 

For all PWM systems the flowrate is adjusted by changing the length of time that each pulse causes the solenoid to shut off flow to the nozzle. The duration that the solenoid remains open (and the nozzle sprays) is referred to as the ‘pulse width.’  

The percentage of time the solenoid is open is referred to as the ‘duty cycle.’ 

The ability to vary the duty cycle is referred to as ‘pulse width modulation.’ 

When the duty cycle is 100 percent this means the nozzle is fully on, with continuous flow.  

At 70 percent duty cycle the solenoid is open 70 percent of the time in each cycle, which means the nozzle flow is approximately 70 per cent of its capacity (this also means the nozzle flow is off for 30% of the time in each cycle).  

The period of time where the nozzle is off during each cycle are referred to as ‘skips.’ 

Pulse Frequency

Pulse frequency refers to the number of on/off cycles per second, or hertz (Hz), that a PWM system is capable of.  

Some of the early PWM systems introduced to the market operate at a fixed pulse frequency of 10hertz, which means the solenoid completes 10 cycles of open and closed (on/off at the nozzle) every second. A range of fixed frequency PWM systems are now available, which include 10Hz,15Hz, 20Hz and 30Hz systems. 

In addition to the original PWM systems with fixed frequencies, there are also spraying systems capable of changing the pulse frequency during the spraying operation. PWM combined with ‘variable frequency’ models are available, including Weed-IT (up to 50Hz) and Agrifac (up to 100Hz). These systems are also referred to as PWFM (pulse width frequency modulation). 

Both PWM and PWFM rely on duty cycle to control the flow rate, the key difference is that higher pulse frequencies reduce the distance travelled during each on/off cycle, reducing the distance travelled during the ‘off’ phase (skip). 

With higher frequency PWFM systems the uniformity of spray deposits is usually more even across the boom than fixed lower frequency systems, particularly at higher operating speeds and during turns.