There are limits to what the operator should expect from a single nozzle size on a PWM system. Selection of the most appropriate nozzle size/s requires an understanding of how the nozzle orifice size and other factors can impact on duty cycle and spray deposit uniformity. 

Nozzle orifice size, duty cycle and skips

Duty cycle is adjusted by the PWM rate controller in response to spraying speed, based on the nozzle orifice size selected, the chosen operating pressure, and the application volume L/ha).  

Impact that various changes will have on duty cycle: 

• decreasing the spraying speed will reduce duty cycle 

• Increasing  the pressure in the spray line during the spraying operation (with the same nozzle) will reduce the duty cycle 

• changing to a larger orifice size (without changing application volume, pressure or speed) will reduce duty cycle 

• decreasing the application volume (without changing nozzle or pressure) will reduce the duty cycle.  

Reducing duty cycle increases the distance travelled while each nozzle is off (therefore results in larger skips).  

Example: 

Using a PWM system with a 50cm nozzle spacing to spray 80 L/ha at 18km/h and 3 bar line pressure, this could be achieved with 025 orifice nozzles operating at 100 duty cycle (nozzle continuously on).  

If the same PWM sprayer was fitted with 05 orifice nozzles, it would operate at 50% duty cycle to match the flowrate required for 80L/ha (at 18 km/h and 3 bar).  

At 18 km/h (5 m/sec) using a 10Hz PWM system each nozzle will complete one on/off cycle for each 50cm of forward travel.  At a duty cycle of 50% each nozzle will be on for 25cm (green) and off for 25cm (pink) (see Figure 1 below to compare the impact of spraying speed on on/off distance travelled per cycle at 10 Hz).  

With a PWM system we could then elect to increase application rate by increasing the duty cycle from 50% to 70%. Increasing the duty cycle also reduces the time each nozzle is off in each cycle.  

Compare Graph 1 (50% duty cycle) and Graph 2 (70% duty cycle). The total distance travelled per cycle is the same for both at equivalent spraying speeds, but the skips time the nozzle is off are smaller at higher duty cycles.  

Increasing pulse frequency reduces the distance travelled per cycle = smaller skips. 

Graphs 3 and 4 below show the distance travelled by a single nozzle during on and off phases at different speeds for 15Hz and 30Hz systems operated at 70 % duty cycle (and can be compared to Graph 2 for a 10Hz system). Note the reduction in distance travelled per cycle for each nozzle as system frequency is increased.  

Spray pattern ‘blending’ from adjacent nozzles

For the majority of PWM systems which operate a single nozzle per outlet, the impact of skips on spray uniformity depends on the distance travelled while each nozzle is off, the operational nozzle height, the droplet size and the contribution of adjacent nozzles to the overlap of the spray patterns.   

With lower system frequencies (e.g. 10Hz), low duty cycles, and higher spraying speeds, skips may impact efficacy, particularly when using larger droplets or when the boom height is not correctly adjusted to allow for blended pulse to occur. 

‘Blended pulse’ occurs when droplets from adjacent nozzles are relied on to fill the spray pattern while a particular nozzle is off during each cycle, helping to reduce the impact of skips on the uniformity of the spray deposits across the boom. Typically, when one nozzle is ‘off’ the adjacent nozzles on either side should be ‘on’.  

Blending of nozzle patterns is more likely to be achieved with smaller droplets (e.g. Medium spray quality) that will move around across spray fan overlap. Very large droplets will tend to move more in a direct line upon leaving the nozzle and there will be less ‘blending’ of spray patterns. PWM systems rely on blending when some of the nozzles are in the ‘off’ position, so extra care needs to be taken when using PWM with Very Coarse or larger spray quality. 

Systems that use two PWM controlled nozzles at each nozzle spacing (e.g. one nozzle located behind the other) reduce the requirement for blended pulse from adjacent nozzles across the boom to improve overlap (as skips are typically aligned with direction of travel, rather than across the boom). One of these options with dual nozzles at each location is the John Deere ExactApply system, which is able to operate 2 nozzles per outlet (known as A&B pulsing mode, where each nozzle pulses at 15Hz). 

The impact of boom height on pattern overlap and spray uniformity.

For a conventional boom, the minimum recommended overlap of nozzle patterns to gain a uniform distribution of spray is 100% overlap (see Figure 5), where any position across the boom is covered by two nozzles. While 100% overlap is effective when the boom is level and the terrain is smooth, decreases in boom height, uneven target height or gusts of wind can easily impact the overlap and reduce uniformity of spray deposit. 

On a boom fitted with 110 degree nozzles at 50cm nozzle spacing, the boom height is typically set so that the nozzles are around 50cm above the target to achieve 200% overlap. An overlap of 200% occurs when the spray pattern at any point across the boom is covered by three adjacent nozzles (see Figure 5 as an example of various percentage overlaps at different heights above the target). The intent of maintaining 200% overlap of nozzle patterns is to improve uniformity of the spray deposits across the boom to compensate for situations where the boom may suddenly change height or level, the target height is not uniform, or the spray patterns may be temporarily impacted by wind. 

For a conventional boom 200% overlap does not mean doubling of the application rate. 

As boom height rises the percentage overlap increases, and more nozzles will be covering any given point on the ground. This may improve uniformity of coverage, although will increase the potential for spray drift (especially where smaller droplets are used).

The volume output from each nozzle remains constant regardless of release height. So as percentage overlap increases, the same volume per nozzle is spread over a wider area, but more nozzles will be covering that same area. The net result is typically the same volume or product per unit area.

However, if overlap drops below 100% then there is potential for less than the target application rate in some areas, i.e. missed strips.

Changing the fan angle of the nozzles requires a change in nozzle height to maintain overlap (i.e. changing from 110 degrees to 80 degrees requires an increase in nozzle height to achieve the same percentage overlap). Similarly, a change in nozzle spacing will also require a change in boom height to ensure adequate overlap of the nozzle patterns is maintained.  

For PWM systems with a single nozzle per outlet that rely on alternating nozzles for blended pulse, nozzle spacing is effectively doubled when half of the nozzles are briefly in the ‘off’ phase of each cycle (see Figure 6 which illustrates the effect of increasing nozzle spacing on overlap).  

Narrower nozzle spacings (e.g. 25cm spacing) will have increased overlap at equivalent boom height and hence reduce the total boom height required to ensure the uniformity of the spray deposits. 

Interaction of nozzle height and duty cycle on nozzle pattern overlap

Nozzle height, and therefore boom height, is critical to maintain sufficient overlap of spray patterns across the boom. For PWM systems, nozzle height can interact with duty cycle and skips to impact the uniformity of spray deposits not only across the boom, but also in the direction of forward travel.  

For single nozzle PWM systems, when all nozzles are ‘on’ the PWM system will be achieving satisfactory overlap i.e. at least 200% if 110 degree nozzles are used on 50cm nozzle spacing and boom height is at least 50cm above the target, or 300% overlap if boom height is greater than about 70cm. 

However, with this same setup (single nozzle PWM systems using 110 degree nozzles on 50cm boom spacing) the percent overlap will reduce when half of the nozzles are ‘off’. For this brief period within the PWM cycle the boom sprayer is effectively operating with 1m nozzle spacing.  

If the boom height for this system is at least 70cm above the target then 100% coverage should still be achievable across the boom when half the nozzles are ‘off’ (i.e. all points covered by 2 nozzles) and this should maintain adequate coverage provided travel speed is not excessive, droplet size is not too large and there is not too much wind interference. 

Where the boom height is lower e.g. 50cm above the target, when the PWM system is in the ‘off’ phase and every second nozzle is not operating, there will be some areas across the boom that will be still achieving 100% overlap, while other areas will have less than 100% overlap i.e. the area will only be covered by a single nozzle. This area with less than 100% overlap will also receive less than the target pesticide application rate.  

To reduce this variability with PWM systems it is important to ensure a minimum boom height is maintained. For 50cm nozzle spacing with 110o nozzles, ensure boom height is at least 70cm. For 25cm nozzle spacing with 110o nozzles the boom needs to be at least 50cm above the target. Keeping the duty cycle as high as practically possible will reduce the amount of time, and therefore distance on the ground, when half of the nozzles are ‘off’. Reducing travel speed will also reduce the area of ground covered when half of the nozzles are ‘off’, which will also reduce the size of each individual spot treated with less than 100% coverage and a reduced application rate. 

At duty cycles below 50%, it is also possible with single nozzle PWM systems that all nozzles across the boom could be ‘off’ at the same time. Should this occur, there is the potential for complete missed strips across the paddock. 

Duty cycles less than 40% coupled with incorrect boom height can lead to skips across the whole boom.  

The impact of skips on spray deposits can be significant at higher spraying speeds, low pulse frequencies, boom heights too low for blended pulse, and when using larger droplet sizes (typically very coarse or larger). 

Ways to increase spray uniformity across the boom: 

• Set appropriate nozzle heights to maintain consistent nozzle pattern overlaps. As a minimum, the boom height must allow for the spray pattern from to reach the second nozzle along the boom. For 110o nozzles on 50cm boom spacing this is likely to be a minimum of 70cm above the target.  For 110o nozzles on 25cm boom spacing this is likely to be a minimum of 50cm above the target. 

• Avoid nozzle orifice sizes and pressures that result in a low starting duty cycle (ideally settings should allow nozzles to run above 70% duty cycle when operating at the most common paddock speed and spray volume) 

• Consider systems with narrower nozzle spacings (i.e. 25cm) to increase pattern overlap 

• Spray systems that can operate two PWM controlled nozzles per outlet (at each nozzle spacing) reduce the reliance on adjacent nozzle patterns across the boom for blended pulse. 

Ways to increase spray uniformity in the direction of travel:  

• Avoid high spraying speeds and low duty cycles (less than 50%) that may produce large skips, especially if using very coarse or larger spray quality  

• Systems with increased pulse frequency reduces the distance travelled per cycle which reduces the potential size and impact of skips. 

• With lower fixed frequency systems (e.g. 10 Hz), select nozzles, pressure and speeds that maintain higher duty cycles (e.g. 80% or more) 

 

Single Nozzle Section control and Turn Compensation

One of the features of PWM systems is the ability to turn individual nozzles on and off without affecting the flow rate and pressure across the boom.  

Where available, single nozzle section control can greatly reduce the potential for over-dosing, particularly in irregular shaped paddocks or paddocks where there are many obstacles, where boom patterns regularly overlap with previously sprayed areas. 

The ability to adjust the pulse width at the nozzle allows flow to be increased or decreased as the boom extremities speed up or slow down when the boom is turning (within the limits of the orifice size and duty cycle for the selected application rate).  

For variable frequency PWM systems, the distance travelled in each on-off cycle can be adjusted across the boom to maintain uniformity of the spray deposits (see Figure 14).  

Fixed frequency systems can only adjust the duty cycle, so the distance travelled between each skip will be related to the travel speed. Where the outer parts of the boom travel at higher speeds during a turn, the skips on the outer parts of the boom will be larger.   

There are limits to how much any system can compensate for the speed at the outer boom tip, so a useful feature is a speed warning for the operator when the turn speed is too high to maintain reasonable spray deposits.