PWM is gaining popularity, and there is an ever-increasing number of first-time users that need to make nozzle selections for their system. We’ve written about it here, here, and here.
Recall the PWM replaces spray pressure with Duty Cycle (DC) of a pulsing solenoid as the primary means of controlling nozzle flow. The solenoid shuts off the flow to the nozzle intermittently, between 10 and 100 times per second depending on the system. The Duty Cycle is defined as the proportion of time that the solenoid is open, and for low-frequency systems, DC is more or less linearly related to flow rate.
The first rule of PWM nozzle selection is to understand that under average travel speeds, we’d like to see the duty cycle of the system at between 60 and 80%. This means that the nozzle solenoid is open about 2/3 of the time. This value also describes the flow rate as a proportion of the full capacity that nozzle.
The reason for this 2/3 duty cycle rule is to enable four key features of PWM:
It’s ideal for turn compensation, allowing the outer nozzles to increase their flow 20 to 40%, and the inner nozzles to decrease flow about three-fold, in accordance with boom speed.
It allows speed flexibility, providing some additional speed, but more importantly, reduced speeds should conditions require it, without a change in spray pressure.
It compensates for pressure changes so that spray quality can be adjusted without requiring a speed change. Less pressure reduces nozzle flow, and increasing DC recoups accordingly.
It allows for customized higher flows of certain nozzles, perhaps behind wheels, to address reduced deposition in their aerodynamic wake (available on some PWM systems).
The best tool for selecting the right nozzle size is Wilger’s Tip Wizard. This site asks for your desired average speed ( although it calls this “Max Sprayer Speed”), and reports the expected DC for a host of nozzle size solutions and pressures. It also reports maximum and minimum travel speeds and other useful information such as spray quality.
Fig 1: The Tip Wizard is a useful tool for sizing nozzles on any PWM system. Sizing information applies to any nozzle. Spray quality information is for Wilger ComboJet nozzles only.
Although intended for Wilger nozzles, the site’s sizing feature works for any nozzle brand. It asks the user which PWM system they have for the purpose of calculating the documented pressure drop across the solenoid.
Fig 2: Tip Wizard results for the Wilger SR11006 tip at 10 gpa and 15 mph. Look for a solution that provides 60 to 80% Duty Cycle (DC).
If you don’t have access to the site, a basic calibration chart can still work with a simple trick. Recall that we use the top row to identify the desired water volume, and the table’s interior values are speeds, as described here.
Below are two solutions for someone wanting to apply 10 gpa at 15 mph without PWM. The correct choice depends on the required pressure to produce the needed spray quality.
Fig 3: A conventional calibration chart, solving a 10 gpa application for 15 mph.
If you want to apply the same 10 US gpa using PWM, simply solve for a larger volume that offers the right DC. For example, choosing 13 gpa will over-apply by 3 gpa, or 30%. The PWM system adjusts by running at 100-30=70% DC. If the chart doesn’t offer 13 gpa, go nearby, to 14 gpa, as we did below:
Fig 4: By pretending to require 14 gpa instead of the actual 10 gpa, the conventional calibration chart is tricked into solving for a nozzle size that will work with PWM at 60% Duty Cycle.
Now solve for the same target speed, 15 mph. The solution will run at 60% DC. Again, there is more than one choice, and that will depend on the spray pressure needed.
Fig 5: Two possible solutions for achieving 10 gpa at 10 mph. An 06 nozzle at intermediate pressure or an 08 nozzle at low pressure.
We’ve developed a template, in US or metric units, that can be customized for any water volume. Here is the same chart with 13 gpa added:
Fig 6: A conventional calibration chart with the 13 mph speed added.
The best solution for 10 gpa at 15 mph is the 06 size nozzle at 50 psi. This is not engraved in stone. One of the nice things about PWM is that it has inherent flexibility. Make the nozzle pressure a priority to get the correct spray quality. It really doesn’t matter whether the resulting DC is 65 or 80%, the system will still work well. Simply avoid extremes that take you below 50% or above 90%, they will limit the system’s capabilities.
It can handle any water volume or nozzle spacing by filling in the blue cells. Two additional worksheets in the file automate the process, simply enter the desired application volume, travel speed, and nozzle spacing (yellow cells), and the solution that offers the optimal duty cycle range will be highlighted in light green.
We’ve heard it often: calibrate your nozzles to be sure your boom output is uniform across its entire width. The downside of poor uniformity is obvious: strips of over- or under-application causing problems with pest control or crop tolerance. A graduated cylinder held for 30 s under each nozzle is the approach of choice. Several electronic versions exist to make the job easier, for example the Spot On.
But there’s more to the story. Nozzle calibration only ensures volumetric uniformity from nozzle to nozzle. It serves to identify worn, plugged, or damaged nozzles, and little else.
After release, the spray is atomized and distributed across a wider area with a properly developed pattern. An operator adjusts boom height or spray pressure to generate proper overlap for a given fan angle at the target height. Unfortunately, the uniformity of this pattern can’t be measured with a graduated cylinder, so we’ve traditionally used a “patternator”, a flat collector placed under a few nozzles that uses a series of channels to show the peaks and valleys of the volumetric distribution. Both calibration and patternation are done with a stationary spray boom. Nozzle manufacturers employ both methods to ensure their products meet international uniformity standards before marketing.
A spray patternator determines the uniformity of a stationary boom’s spray distribution (Photo: TeeJet)
Burt even that isn’t enough. We can have good volumetric distribution but still have inconsistent coverage in places. To identify those regions, we need a way to measure small amounts of spray deposit under a moving boom, ideally in the canopy we intend to treat. Here we have a few options. We can place a tracer (dye, salt, etc.) in the tank, and collect spray on small collectors placed throughout the area to be treated. We collect the samples, wash them, and analyze the solvent for the tracer. This requires special equipment and takes time. It’s useful, but only measures dose, not droplet size or density.
Plastic straws can act as collectors of sprays under field conditions.
Monofilament strings can be used to collect spray over long distances.
A faster way is to use water-sensitive paper, about which we’ve written here and here. Using WSP is fast and easy, and it can provide additional information such as the number of droplets per unit area, or the total percent of the area covered, or even the size of the deposits, with the right equipment. We call this “coverage”, and believe this to be one of the two components of good pest control (the other being “dose”, the total amount of material deposited). Because the world isn’t fair, WSP isn’t great at quantifying dose.
Water-Sensitive paper provides a quick visual indication of the deposit, not just amount but also qualitative aspects such as droplet size and distribution.
The industry has done a good job of identifying the dose required for good control, and this is reflected in the rate recommendations on a label. But there are a few gaps. They don’t tell us, for example, what “good coverage” is, despite often telling us to “ensure” it.
Back to Deposit Uniformity
We quantify deposit uniformity by calculating the Coefficient of Variation (CV) of a series of measurements. The CV is defined as the standard deviation of these measurements, expressed as a percent of the mean value.
Because it’s hard to measure, it’s easy to ignore. But here are a few basics our research has told us: (In the first three examples, deposits were measured under a spray boom using petri plate or drinking straw samplers. There was no interference from a canopy. The last example was taken from within a canopy.)
When measuring the deposited dose, the CV under a boom tended to rise with increased wind speed. This is no surprise, as it reflects that more wind has a greater chance to displace spray from its intended destination.
Spray deposit uniformity, observed during various spray drift studies, tended to decrease with higher wind speeds.
Higher booms and increased travel speed also tended to increase deposit CV.
Faster travel speeds during spray drift studies tended to decrease uniformity.
Finer sprays tended to increase deposit CV. This makes sense, as the finer droplets are more easily displaced by air movement.
Coarser sprays created more uniform deposits possibly because they were more resistant to turbulent displacement.
Deposits were reduced and became more variable deeper in a broadleaf canopy. Again this makes sense, as there are a lot of obstacles to clear and canopies themselves are by no means uniform.
Deposit amount was lower in the canopy, as expected. But the lower deposit was also more variable.
Also note that the CV in the canopy was quite a bit higher (40 – 60%) than for the exposed targets (10 – 20%). That’s another challenge.
To recap, the best uniformity was achieved with low booms (as long as patterns overlap sufficiently), slow speeds, low winds, and coarser sprays. It’s easy to see that current spray practice isn’t always conducive to uniform deposits.
Deposit variability as captured by a 2 mm diameter string with two sprayer configurations.
So What?
Why does uniformity matter? It matters because more variable deposits are less efficient. They require higher doses for the same effect as uniform deposits. Here’s why:
The figure below shows a typical dose response curve for a herbicide. On the y-axis, we see weed biomass, on the x-axis herbicide dose. At low pesticide doses, not much happens. (In fact, we often see a slight increase in biomass with very low herbicide doses.) As we increase dose, biomass begins to decline, and as dose increases further, the effect begins to taper off. At a certain dose, no further biological response is possible.
A typical dose response curve for a herbicide.
In the next figure, we see that application of a uniform dose “a” results in biomass “y”, about 20% of untreated.
A dose response curve represents the weed biomass that resulted from any applied dose.
Next, we apply the same average dose, but we do it non-uniformly. At some locations under the boom, the deposit may be 40% higher or lower than average. The result is response “z”. Weed control is worse, as bad as it would have been at a lower uniformly applied dose (effective dose “b”).
A variable dose across a field results in many individual weed biomasses because of deposit variation. The net result is lower control.
This effect only happens when the effective dose is near the lower inflection point of the dose response curve. Perhaps we’re shaving rates. Perhaps the weather is challenging the herbicide’s performance. Or perhaps the weed is difficult to control. Under those conditions, any gain in performance with a higher dose is less than the penalty from a lower dose.
There are two ways to correct this performance loss. One is to apply a higher herbicide rate. It’s commonly done, as insurance against – you guessed it – variability, and it’s one reason why label rates have some flexibility. The second way is to improve deposit uniformity. In effect, better uniformity allows for rate reductions.
Label rates are typically in the flat region of the dose response curve to allow for variable conditions in weed susceptibility, weed growth stage, growing conditions, and deposit variability.
Take Home Message
Uniform spray deposition improves overall control. Our examples used herbicides, but the same is true for fungicides and insecticides. It’s true for field crops as well as fruit and vegetable sprays.
Uniformity is especially important when the application is done under adverse conditions in which the pesticide performance is challenged. It’s a fundamental part of good application practice.
It’s not always easy to improve uniformity. But at least it should be measured. Without measuring it, an applicator may never know how much product is being wasted. Have a look at the Crop Adapted Spraying approach Jason is using, it’s a template for all sorts of applications.
What can you do? The easiest task is to record the flow from each nozzle. The results might be surprising. Ensuring proper and consistent boom height is also important. Using water-sensitive paper to visualize the quality of the job would be icing on the cake. And adjusting application method, with uniformity as a goal…that gets you a gold star.
Spot spraying promises to dramatically cut herbicide use. Data from Green-on-Brown (GoB) sprays suggest at least 50% and possibly 90% savings are possible, depending on weed density and the system employed. These savings are significant. But system performance depends on the nozzle selection even more than for broadcast sprays. What are the issues?
Pattern Width
Spot sprays represent a unique mix of single nozzle banding and multiple nozzle broadcasting on the same boom at different times and locations, depending on what the weedy spots require. Both need to be optimized to get the best performance and savings out of such a system.
Even (Banding) Nozzles
Let’s say the spot spray boom has a spacing of 10” (25 cm) and is carried by wheels to ensure consistent height. An operator would want the spray pattern to have a very similar width as the nozzle spacing. A 30 degree even fan angle would create a band of about 10” wide at a boom height of 19” (48 cm, download a worksheet that solves this for any fan angle and boom height here). Assuming a travel speed of 12 mph (20 km/h) and a pressure of 40 psi (2.75 bar), an 03 sized nozzle would apply 14.9 US gpa (139 L/ha) in these 10” wide bands.
But most applicators would be uncomfortable with zero overlap, and would prefer to raise the boom to allow, say, 20% overlap. This would ensure targetting of taller weeds that appear exactly between two sprays, for example. At 22” (56 cm) boom height, the pattern would be about 12” (30 cm) wide and affording 1” overlap on either edge.
Spot spray booms activate any number of nozzles depending on the weed locations.
Because the application is diluted by the extra pattern width, the applied volume is now 12.4 US gpa (116 L/ha), about 20% less than before. This change is easily accommodated by mixing the product more concentrated in the tank. The downside is that the overlap in banded sprays receives twice the dose, and this is less than ideal.
Tapered (Overlapping) Nozzles
A possible solution is to employ tapered flat fans that are the standard type on broadcast booms. These produce more of their volume in the centre, diminishing at the edges, to allow for overlapped patterns and thus functioning better when more than one nozzle is activated. In addition, the extra coverage from a wider pattern is not as wasteful as it is from an even pattern type since it comprises less volume. A single nozzle spray, however, would have a higher dose in the centre than at the edges, since a single pattern has a bell-shaped volume distribution. (note: a single nozzle moving through air loses some of its volume from the centre and places it at the edges, due to aerodynamics of the fan shape. That levels out the bell shape somewhat.)
Broadcasting
When more than one nozzle is triggered by the sensor, the spot spray of that region is just a small section of a broadcast boom. The average dose is now related to the nozzle spacing, not the actual band width as it was for a single nozzle. The wider the section of nozzles that are activated simultaneously, the less inefficiency a wider individual pattern creates because it’s only wasted on the outside edges of the outside nozzles.
Clearly, a sprayer that sometimes functions as a single nozzle spot spray, and at other times as a broadcast boom requires some compromises. Monitoring the activation of nozzles and learning from the relative frequency of single vs multiple nozzle activations will be useful to optimize the configuration. But when boom height is constant, a good compromise solution is possible.
Suspended Booms
A more challenging situation arises from suspended booms that do not hold a consistent height. Let’s assume a boom height variance of 10” (5” in either direction), and a wish to retain 20% overlap at the lowest height to avoid misses from a 30 degree nozzle. The lowest height would have about a 12” pattern width, achieved at 22”. The boom would be set 5” higher, 27” (69 cm). At this height, a 30 degree fan would produce a band width of 14.5” (37 cm), producing a 45% overlap. If the boom sways up to 32” (81 cm), the pattern width would be 17.1” (43 cm).
For multiple adjacent nozzles, boom height determines overlap, and a minimum overlap must be achieved even when the boom sways low.For single nozzles, boom height determines band width and therefore dose.
This is where it gets tricky. At suboptimal heights, the difference between a single band and a section of overlapping patterns increases. Do we calculate the tank mix for the rate a single nozzle delivers within its band, or for a set of nozzles activated simultaneously? If we knew that the majority of activations are for a set of two or more nozzles, we could opt to assume an application rate of a boom section with 10” spacing. An 03 nozzle at 40 psi and 10” spacing would apply 14.9 gpa (139 L/ha). But when a single nozzle is activated, the application volume in the 14.5” band is just 10.2 US gpa (95 L/ha), and the plants that triggered just a single nozzle would be under-dosed.
At the top of the sway (32”), a single nozzle’s wider pattern would deliver about 8.7 gpa (81 L/ha) , another 16% less spray volume than at 27”. At the low end of its sway, the band is 12” wide, applying about 12.4 gpa (116 L/ha) , 23% higher than the 10.2 gpa rate at the 27” boom height.
It’s clear that to take advantage of the potential savings of spot spraying, and to ensure good success with single nozzle activation, consistent and accurate boom heights are essential. I’m not sure how much more obvious a development priority can be.
Band Length
Spot sprays allow the user to select the length of band that the spray is activated for. Shorter band lengths require more targetting certainty. If booms and travel speeds are both low, an individual detected weed can be targetted accurately with relatively short band lengths because relatively little can happen to displace the spray during its short journey. But as booms and travel speeds are higher, the time that the spray arrives at the target is more difficult to predict and longer band lengths need to be programmed. For example, wind can push the spray off its target. Or the faster speeds impart more of a horizontal vector to the spray, causing it to land further away from the point of release.
The variances in where the spray lands along the direction of travel depend on droplet speed and boom height. A conventional flat fan nozzle produces an initial droplet velocity of about 20 m/s. These droplets slow at a rate dependent on their size and whether they’re entrained in the spray plume. At 45 cm below the nozzle, larger droplets are still moving at 10 m/s. Smaller droplets are only moving at 1 to 2 m/s.
Droplets take time to reach their target, and the spray band length must accommodate variance in this time arising from different from boom heights or droplet speeds.
Let’s assume an average droplet speed of 10 m/s for the journey. At that speed, the spray takes about 0.05 s to travel the 0.7 m (27”) from nozzle to target. During that time, the sprayer going 12 mph (5.6 m/s) moves about 0.25 m forward, as do the larger droplets from the released spray. If the boom sways down to 22” or up to 32”, the distance travelled by the sprayer is 0.2 and 0.3 m, respectively. In other words, the band length would need a buffer of 10 cm to accommodate the variability of the beginning and end of the band.
Overall Efficiency
Are these numbers such a big deal? You might say that we’re already cashing in on some big savings here, so why sweat the details?
It’s the principle and the resources. If we’re talking about individual nozzle band width and its change with boom height, accommodating boom sway means applying more than necessary on average to avoid under-dosing when booms sway high. The examples used here show a potential dose variance of 40% with a boom sway of 10”, a modest assumption. That’s a big number to leave on the table. If we had a constant boom height, we could decide what overlap we wanted and minimize these losses.
One of the features on most spot sprayers is to turn on all nozzles of a section that exceed a certain boom height. While this prevents under-dosing and ensures an area is treated even when the sensor is outside of its optimal range, it is possibly an unnecessary use of product.
If we’re talking band length, adding 10 cm to a band length of 50 cm is 20% over-application. That can also add up.
The key to being efficient with spot sprays is accurate and consistent boom height. We know we can do that with a wheeled boom. But show me a suspended boom that can deliver on this, and I see an instant industry leader in spot spray application.
North American built boom sprayers have nozzle spacings of 20” (50 cm in the rest of the world), but other spacings such as 15” (37 cm) and 10” (25 cm) also exist. What are the reasons for these alternative spacings and do they offer any inherent advantages?
Why spacing matters
Nozzles are spaced along a boom to allow their fans (patterns) to overlap sufficiently at the target. In broadcast spraying, a uniform distribution of spray volume gives us the best chance for consistent coverage along the boom. Since flat fan nozzles produce a tapered pattern (i.e. the volume is highest in the centre and diminishes towards the edges), approximately 100% overlap (i.e. 50% from each neighbour) will produce a uniform swath.
Figure 1: Tapered flat fans that require some overlap are the default pattern type for agricultural boom nozzles. This is true of conventional and low-drift styles. Note that the flat fans are turned 15° to prevent the spray patterns from interfering with one another.
The 100% overlap isn’t just for volumetric distribution. Flat fan spray patterns tend to have more and finer droplets in the centre and fewer and coarser droplets at the edges. All droplet sizes contribute to coverage in different ways, so the overlap ensures both number and sizes are evenly distributed along the entire boom.
Figure 2: 30% overlap may achieve volumetric uniformity. But because the centre of the pattern contains the majority of the smaller droplets, low overlap may result in low coverage in the overlap regions, resulting in striping.Figure 3: Consistent droplet number distribution along the boom requires at minimum 100% overlap (50% from each neighbouring nozzle). This blends those regions of the patterns with high and low droplet densities.
The generic 20” spacing arose from long-held conventions about boom height, fan angle, and travel speed. Specifically, this spacing required a boom height of 20” to obtain good overlap of the once-dominant 80° fan angle. Combined with 0.15 to 0.3 US gallon per minute (gpm) nozzles and travel speeds of 6 to 8 mph, operators were able to apply 5 to 15 US gallons per acre (gpa) volumes. Using nozzles with smaller flow rates would generally result in nozzle blockages.
But what if we want to change any of those variables? How does this affect nozzle spacing? Figuring out the pros and cons of an alternate spacing requires a little math and some contingency management.
Boom Height Math
First the math. If the boom has 20” nozzle spacing and we need 100% overlap, the width of the spray pattern at target height must be two times the nozzle spacing, which is 40″. You must calculate the required fan angle and boom height to achieve this. Most nozzle catalogues have tables to help with this, or you can download a handy spreadsheet to calculate your own scenarios here.
For today’s standard 110° fans, a minimum boom height of 14” is needed to achieve 100% overlap. For 15” spacing, the height is reduced to 11”. For 10” spacing, we drop to a mere 7”. However, consider that most modern suspended booms are not operated at heights less than 24” to allow for sway. At that height, there’s plenty of overlap to go around for 20″ nozzle spacing. For those booms that are able to operate at a consistent height, narrower spacings permit lower heights that will reduce drift potential significantly. Every time we halve boom height, we also halve drift potential.
Figure 4: Using 110° tips with 20″ spacing, the theoretical height at which we achieve 50% overlap is 11″ above target.
By tilting the nozzles forward or backward from the vertical, we can reduce the boom height somewhat further and still get the same overlap. For example, for 20 and 15” spacings, angling nozzles forward or backwards by 30° allows us to drop the boom another 2” closer to the target.
Contingencies
A suspended boom hardly ever stays at a uniform height; It sways up and down with field conditions, topography, etc. This is why many operators set their booms above the minimum height – to prevent striping when the boom sways low. The penalty is that this increases the distance droplets need to travel, increasing drift potential and any turbulent displacement problems arising from the moving boom.
Assuming a 110° flat fan at 24” boom height, each nozzle achieves a theoretical pattern width of about 70”, which is an overlap of 70÷20=3.4-fold or 240% on 20” nozzle spacing. Given a minimally-acceptable overlap of 50% (25% from each neighbouring nozzle), the boom could be as low as 11”. For 15” spacing, the minimum height for 50% overlap is 8”, and for 10” spacing it’s 5”. This means the narrower spray patterns gain 3” to 6” in allowed downward boom movement.
Figure 5: Using 110° tips on 15″ spacing, the height for 50% overlap is 8″ above target.
A second contingency is that spray patterns are rarely the exact value that the nozzle catalogues specify. A so-called 110° nozzle may operate at only 90°, or up to 150°, depending on the nozzle model, the spray pressure, and the tank mix. Learn more here and here. Patterns also don’t continue to grow at their rated fan angle, as droplets slow due to air-resistance and fall more vertically due to gravity. For that reason, a visual check is recommended to ensure the expected overlap is achieved.
Figure 6: Fan angles indicate initial trajectories of droplets at the edge. With distance, gravity pulls these droplets downward, narrowing the pattern width from that achieved theoretically (figure adapted from image in TeeJet catalogue).
A third issue to consider is less related to boom height but nonetheless affects spray distribution. Small droplets move with air currents, and the turbulence created by large, fast sprayers creates enough turbulence to move these droplets significantly. A perfect pattern under static conditions can look quite different at a fast travel speed with a modest side wind. Low booms may help prevent some of this displacement because droplets spend less time in flight, and their average velocity is faster.
Figure 7: Spray deposition onto a 2 mm string to measure deposit uniformity for a fast travel speed and high boom and a slow speed, low boom configuration.
Flow Rate Math
Flow rate requirements per nozzle change whenever we equip a boom at an alternate spacing. The basic formulae are shown below.
Moving from a 20″ to a 15″ spacing would require a nozzle with 0.75 of the flow rate, approximately from a 02 to 015 size, or 03 to a 025 size, or 04 to 03 size, etc.
Pulse Width Modulation
The use of Pulse Width Modulation (PWM) has increased the overlap requirement. With PWM, alternate nozzles are on a 180° timing offset from their neighbours. This means that when running >50% duty cycle, when one nozzle is temporarily off, its neighbours are on. These neighbours’ patterns must now span the gap, and 100% overlap is the absolute minimum to achieve this. PWM users therefore select the wider pattern angles and some opt for >100% overlap.
Figure 8: Pulse Width Modulated booms require 200% overlap so that the entire boom receives proper coverage when the alternate set of nozzles is off. For 110° fans at 20″ spacing, the minimum boom height would be 21″
PWM Considerations
High flows (greater than 1 US gpm at the nozzle) that are common for fertilizer top-dressing may require higher-flow PWM valves.
Narrow spacings reduce the individual nozzle flow rates and can therefore support higher application rates before triggering a larger valve requirement.
PWM valves aren’t cheap and for example 15″ spacing compared to 20″ spacing adds 24 valves on a 120′ boom.
Banding
We noted that 20” nozzle spacing is a standard because it corresponds to what has traditionally been achievable with available boom heights and spray pattern angles. But things can change.
Narrower spacings such as 15” originate with row crops and planter row spacings of 15” or 30”. These spacings exist so the spray pattern can be placed either over the top of a crop row, or in between the rows for banding. Using narrower fan angles and/or lower boom heights, together with “even” (as opposed to “tapered”) fans, banding sprays can be applied over the top of, or between crop rows. Or drop hoses can reach between the rows for top-dressing or directed sprays into the canopy.
Canopy Penetration
With narrower spacing, it can be argued that a greater proportion of the boom length has spray directed directly downward (corresponding to the centre of the pattern). Whether or not this translates into better penetration of a canopy is a fair question. In laboratory trials, use of 10” or 20” spacing did not improve penetration into a broadleaf canopy. But if the lower boom height afforded by the narrower spacing was utilized, some improvements in the deposit of angled sprays onto vertical targets was observed.
Adjusting to Narrower Spacings
As we showed earlier, use of 15” or 10” spacing booms for broadcast sprays requires a smaller nozzle size to achieve the same spray volumes as the 20” spacing. If boom height remains constant, narrower spacings result in greater pattern overlap which provides more latitude for sway. Alternately, lower boom heights can be used.
Using smaller nozzles on narrower spacing presents some challenges. Generally, smaller nozzle size means finer spray quality. If an operator wants to retain the spray quality they had on a 20″ spacing, they may opt to use lower pressure (not advisable for non-PWM systems) or swap to different nozzle design that can produce the desired spray quality at the lower flow rate.
Smaller nozzles are more prone to plugging, so that needs to be managed with filtration, filling practices and water sourcing. Be aware of the the product formulations and their requirements for filter mesh size. Most dry products specify a 50 mesh filter (or coarser). Also, check size options for nozzles. The smallest size for most nozzle models is 015, but certain PWM-specific nozzles are only available in 03 or larger.
The marriage of narrow spacings with individual nozzle shutoff can result in a versatile system capable of producing high resolution banded sprays in narrow seeded crops. For example, consider a boom with a 10” nozzle spacing spacing that matches the seeder row spacing. The operator can shift from 10” to 20” or 30” from the cab if the valve control software allows it. With accurate guidance and good boom levelling, topdressing foliar products (e.g. nutrients, fungicides) can follow the crop row precisely.
Spot Sprays
Spot sprays present a situation where compromises are needed. Some, such as WEEDit, utilize narrower nozzle spacings to allow better treatment resolution and increase product savings. Any one nozzle or sets of adjacent nozzles may be triggered by the sensor. For single nozzle activation, to preserve the value of the better resolution a uniform, narrow band of spray needs to be created. This means a 30° or 40° fan angle from a banding nozzle will be necessary. For example, a 24” boom height will result in a 13” band with a 30° fan, and an 18” band with a 40° fan. In the latter case, the dose would be diluted by 80%, wasting much of the potential savings.
Figure 10: Boom height is critical for banded sprays and for spot sprays. Too wide a pattern on a single nozzle reduces dose, too narrow creates misses.
Frequently, a patch of weeds will trigger several adjacent nozzles. Now these individual bands need to work together to create a uniform swath. This will inevitably require some overlap to avoid gaps, but too much overlap will result in bands where twice the dose will be applied. A tapered fan may suit this situation better. As a result of these varying needs, tolerances for spot spray boom height are even more strict than for broadcast spraying. More thoughts on spot spray nozzle selection are here.
Conclusions
Narrower nozzle spacings on a broadcast boom allow somewhat lower boom heights and these can in turn reduce drift and improve deposition of sprays. Lower flow nozzles will be needed with narrower spacings, requiring management of plugging and potentially a more drift-prone spray quality. The value of narrower spacings depends on the availability of booms that control sway, allowing them to operate at uniform, low heights.
This is the final part of our three-part article discussing methods for digitizing and processing water sensitive paper. You can read part one here and part two here.
Morphological operations
We can now move on to the larger shapes, or “morphology” of the objects in our binary image. Our goal is to quantify deposits by interpreting these shapes. Once again, these operations are powerful processing tools, but we must acknowledge three overriding limitations:
1. Inconsistent stains
Sometimes deposits do not create a consistent blue colour – they can get lighter or take on a greenish-yellow hue towards the perimeter of the stain. During thresholding, the outer edge can be accidently eroded, leaving behind an object with a jagged edge. This may lead us to underestimate the percent area actually covered. In the case of tiny stains, it might eliminate them entirely and lead us to underestimate deposit density.
2. Overlaps
It can be difficult to determine if an object represents a stain from a single droplet or is the result of multiple, overlapping deposits. This becomes significant when the surface of the WSP exceeds ~20% total coverage. The resulting objects may or may not have hollow centres where droplets do not overlap entirely. Misidentifying overlaps leads us to falsely conclude that an object is the result of a single, coarser droplet rather than multiple finer droplets.
3. Ellipses
Non-circular stains are formed when droplets scuff along the surface. Two droplets with the same volume encountering a paper at different angles can create stains with significantly different areas. We may wrongly conclude that the droplets that created them were coarser than they truly were. One approach is to use Feret’s Diameter (aka Caliper Diameter) by measuring the widest spans on the X and Y axes and taking the average. Another approach is to interpret the ellipse as a series of circular stains. Or we can decide to only acknowledge these objects when calculating percent area covered, but omit them when calculating deposit density or predicting original droplet size. Each strategy is a compromise, so it is important to be consistent and transparent when reporting results.
Three common problems when analysing water sensitive paper.
We’ll explore two morphological operations that can help us separate fact from fiction: Granulometry and Dilation-and-Erosion. We’re introducing these operations as part of the processing and detection step, but they may also overlap with the measurement step in our three-step process.
Granulometry
We can estimate the range of object sizes and get a sense of how they are distributed on the paper by filtering or “sieving” the image. Imagine pouring a mixture of sand and rocks through a series of ever-finer sieves. Doing so allows you to separate particles based on size exclusion. A granulometry function compares each object to a series of standardized objects with decreasing diameters. This isolates objects of a similar size and bins them in that size range. This is a powerful operation, but accuracy is lost when stains overlap to form larger objects. In this case, we move on to Dilation and Erosion.
Dilation and Erosion
Think of dilation as adding pixels to the boundary of an object. This makes tiny objects bigger, fills in any interior holes and can cause objects to merge. The number of pixel-wide dilations required to make objects contact one another can be used as a measure of deposit density.
Erosion removes pixels from the outer (and sometimes inner) boundaries of an object. This eliminates tiny artifacts that may not actually represent stains. It can also split non-circular objects into multiple parts before shrinking them into multiple nuclei (aka centroids). These last-remaining points are not necessarily the centre of a stain, but the pixels furthest away from the original boundary.
When a non-circular shape has more than one nucleus, they likely represent individual droplets that combined to form the larger stain. We can then use these nuclei to measure deposit density, such as in a Voronoi partition which triangulates each nucleus in relation to the two closest neighbours.
Many image processers use both these operations sequentially. When an image is eroded and then dilated (a process called “Opening”), smaller objects are removed, leaving the area and shape of remaining objects relatively intact. Dilating and then eroding (a process called “Closing”) fills in small holes and merges smaller objects, once again leaving the area and shape of remaining objects relatively intact. We can use both of these functions to help smooth an image prior to measurement.
(Top) Opening operations erode and then dilate the image. Moving left to right, the smaller objects tend to disappear. (Bottom) Closing operations dilate and then erode the image. Moving left to right, smaller objects either disappear or merge and holes are filled in
Distance Transformations
Distance transformations are advanced operations specifically used to separate objects that are densely packed. While not typically used when analyzing WSP, distance transformations are another means of identifying object nuclei. They are another means for teasing apart objects that are likely the result of overlapping deposits and then mapping their relative sizes and positions.
Measurement
The calculation of the area covered by deposits is straightforward. The pixels belonging to objects (the deposits) and those belonging to background are summed and then the fraction is converted to percent area covered. Research has shown that the image resolution does not significantly impact percent coverage assessments and has suggested that all image analysis software tends to produce similar results (+/- 3.5% observed when the same threshold was applied to multiple papers). This is acceptable because it’s within the variability inherent to spraying.
We ran a similar experiment wherein we analyzed the same piece of WSP using four methods. Here are a few facts about the software we used:
DropScope produces images between 2,100 and 2,300 DPI. Currently, it ignores ellipses and doesn’t count anything spanning less than ~35 µm (3 pixels).
We set ImageJ to ignore any object spanning less than 3 pixels, which at 2,400 DPI was 30 µm in diameter.
We are unaware of Snapcard’s processing methods except that the software was benchmarked using ImageJ. Developers note it will underestimate the percent area covered if the image is out of focus. (Note: As of 2026, this app may no longer be supported by the GRDC).
The images shown in the figure below were cropped from screenshots produced by each method. The actual ROI analyzed was ~3 cm2 for SnapCard, 3.68 cm2 for DropScope and 2.0 cm2 for both Epson/ImageJ methods. Our results indicate an +/- 4% difference in percent area coverage. This variability reflects the results of a 2016 journal article that compared SnapCard with ImageJ and other leading analytical software. That study claimed no statistically significant difference in percent coverage detected (standard deviations were about 20%). However, the ImageJ results tended to trend several percent higher than SnapCard. We saw this as well. And so, while resolution may not have a significant impact on percent area covered, there does appear to be some correlation.
Percent area covered as reported by three image analysis systems. Only a minor difference was observed when resolution was doubled using the Epson/ImageJ method.
Resolution definitely affects deposit counts. Particularly in applications that employ finer droplets. Consider the difference between detecting or missing 1,000 30 µm diameter objects. It may only amount to a fraction of a percentage of the surface covered, but +/- 1,000 objects on a 2 cm2 area is significant in terms of deposit density.
Output
Once a WSP image (or set of images) has been scanned, pre-processed, processed and measured, we will receive some manner of output. Some software packages create an attractive report with images, graphs and key values. These reports include percent coverage and many provide droplet density. Deposits may be binned by size, or spread factors are used to calculate the original droplet diameters and even estimate the volume applied by area. Other software packages provide raw data that can be imported into a statistical program or spreadsheet program like Excel for further analysis. Some software packages provide both.
How far can we take this?
Blow-by-blow data analysis is beyond the scope of this document, but how much weight should we give to coverage data obtained using WSP? The answer depends on the metric in question, but in all cases we must first acknowledge the three overriding caveats. Take it as said that they apply to everything that follows:
Different brands (and even different production runs) of WSP can produce significantly different coverage metrics. When conducting experiments, use a single brand of WSP. Better still, use papers from the same production batch whenever possible.
The same of piece of sprayed WSP can produce significantly different results depending on the software and protocol used to analyze it. When conducting experiments, use the same software and assessment protocol and be transparent about the process when communicating results.
WSP coverage may not reflect the coverage achieved on an actual plant tissue surface. It is suitable as a relative index (I.e. papers can be compared to papers, but not to tissues) but the spread factor changes with surface wettability and the surface tension of the liquid sprayed. Note the differences in percent area covered in the following experiment with an organosilicone super-spreader:
Difference in deposit spread on water sensitive paper versus a leaf surface using an organosilicone super-spreader and UV dye. The same volume was applied in each case and while the area increased two-fold on WSP it increased ~10-fold on an actual leaf. Image reproduced from work by Robyn Gaskin, Plant Protection Products, New Zealand.
Recall that we started this document by listing the four pieces of information commonly sought using WSP. They were listed in order of reliability, and now we can explain why.
The percent surface area covered: We have established that this is the most reliable piece of data. Droplets do not spread on WSP the way they do on plant surfaces, so it will underestimate actual coverage. The results vary by analytical method, but it’s likely not dependent on resolution and still falls within the variability inherent to spraying. This metric gives us valuable and actionable information. We can say whether or not we hit a target, and evaluate whether a sprayer change resulted in more or less deposit.
The density of deposits on the target area: We have established that that there are limits to the reliability of this metric. It is affected by the analytical method used and can be greatly underestimated when resolution is poor or when deposits overlap in high numbers. Also, it will never reliably reflect deposits under 30 µm. Nevertheless, under controlled conditions this information does have value and is of great interest in enquiries about drift and contact fungicides.
The size of the droplets that left the stains: This metric is highly questionable except under controlled conditions. The many assumptions about surface tension, droplet speed, and droplet evaporation make it impossible to make definitive statements about spray quality. Finer droplets are greatly underestimated in this equation. Therefore, while there may be some value in using WSP as a relative index, this metric is a crude indication at best.
The dose applied to the target surface: This metric has not been discussed up to this point, but is quickly and easily dismissed. Let’s assume that a droplet with a high concentration of an active ingredient will leave a stain that is the same area as another droplet with a lower concentration. This will lead some to suggest that as long as the original concentration is known, we can back-calculate the dose (which is the amount of active on a given area). However, one droplet has the same volume as eight droplets that are half it’s diameter. This cubic relationship means that if they all deposit, the larger droplet will cover roughly 1/2 the surface area as the eight smaller droplets. Therefore, the smaller droplets spread the same amount of active over a greater area. Spread factor muddies this a bit, but ultimately it means that dose cannot be estimated from area covered. Dose is better assessed using collectors that permit the residue to be removed, such as Petri dishes, Mylar sheets, pipe cleaners, alpha cellulose cards, or glass slides.
And so, the image analysis process described here is powerful and effective when used with water sensitive paper as long as the limitations are acknowledged. The same process can also be used with dyes and specialized collectors such as Kromekote to permit even greater resolution. But that’s another story.
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SnapCard website. University of Western Australia and the Department of Primary Industries and Regional Development, Western Australia. (Note: As of 2026, may no longer exist).
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