Author: Jason Deveau

  • Testing and Correcting Airblast Pressure

    Testing and Correcting Airblast Pressure

    The role of pressure is often underappreciated in spraying. Many airblast operators (still) don’t use rate controllers, so the only way to monitor sprayer pressure is using a single liquid-filled pressure gauge located near the pump… and it may not be trustworthy. An inaccurate pressure gauge may cause you to spray more or less product than you intended. That translates to wasted resources and potentially higher residue levels. Conversely, spraying less than intended may lead to reduced efficacy and the need to re-apply. Many operators use budget pressure gauges on their sprayers and have never tested or replaced them.

    Testing pressure gauges

    Here are a few clear indications that your pressure gauge should be retired:

    • Gauge has an opaque or unreadable face
    • Mineral oil leaking or mostly gone
    • Needle does not rest on zero pin when sprayer is not under pressure (it has likely spiked)

    Sometimes a gauge is not obviously in need of replacement. To test it, you need to apply a known pressure to see if it is reading accurately. One way to do this is using a commercial manometer.

    AAMS-SALVARANI Gauge tester
    AAMS-SALVARANI manometer

    These systems work well, but they can be an expensive proposition if you only use them once in a while. In a past sprayer workshop, one participant had a great suggestion for testing gauges. His idea was to use an air compressor (which most farms have) and some simple plumbing to create a homemade manometer. Be sure to vent the gauges before testing.

    The Pressure Gauge Tester. The “true” gauge is in the elbow and can be compared to the suspect gauge in the tee. Concept from K. Voege, Ontario.
    The “Pressure Gauge Tester”. The accurate gauge is in the elbow and is compared to the suspect gauge in the tee. Concept: K. Voege, Ontario.

    This tool allows you to test your suspect gauge (set in the tee) against an accurate gauge (set in the elbow) for less than $75.00 CAD. Construct your own “Pressure Gauge Tester” using the following parts (valve optional):

    PartApprox. Price (CAD)
    ¼” by 3” Galvanized nipples (x 2)$3.50
    ¼” Galvanized 90º elbow$3.50
    ¼” Galvanized Tee$3.50
    ¼” Ball valve (threaded)$10.00
    *Plug Air Connector (A over ¼”)$4.00
    Teflon pipe tape$3.00
    †300 psi liquid-filled gauge$40.00
    *Depending on the quick-connect fitting on your compressor 
    †The range of the accurate gauge should match your existing gauge. The range of your existing gauge should be twice as much as your typical operating pressure. 

    As a public service announcement, be aware that many budget, liquid-filled gauges are inaccurate right off the shelf. A 5% variance is typical. When replacing a worn gauge, or buying the “accurate” test gauge for your homemade manometer, buy a few and save the receipt. Test them in different combinations to ensure they all agree with one another. Return the extras and let the dealer know if you discover an inaccurate gauge. I’m sure they won’t put it back on the shelf for the next person… *ahem*.

    Gauges should be rated twice as high as your average operating pressure. For example, if you typically spray at 150 psi, your should have a gauge rated up to 300 psi. That way, you can see small changes in pressure more clearly. Plus, if your needle is pointing straight up, a quick glance confirms the ideal operating pressure.

    Another way to confirm pressure gauge accuracy is to install a second in-line. They’ll keep one another honest. This may be difficult if the gauge set into a molded plastic tank, or located under the chassis next to the pump where it is not visible from the tractor.

    Two gauges keep each other honest – this GB (Italian-made Good Boy) is sporting a home-made assembly that cost ~$50 to assemble, including the second gauge. The silver spray paint on the black pipe prevents rust and makes it look pretty darn sharp.
    Two gauges keep each other honest – this GB (Italian-made Good Boy) is sporting a home-made assembly that cost ~$75 CAD to assemble. The silver spray paint on the black pipe prevents rust and makes it look pretty darn sharp. Note that they should be the same range, but are not in this photo. The one on the right is the correct range for this operating pressure.

    Measuring and Correcting for Pressure Drop

    Boom pressure can sometimes be less than the desired operating pressure (a phenomenon known as “pressure drop”) and must be accounted for. Pressure drop is affected by hose diameter, hose fittings, and the distance from the pump. You’ll find it at the far ends of boom sections on field sprayers and it’s an issue that plagues many low-pressure, tower-style sprayers. Dress appropriately because you’re going to get wet performing this diagnostic.

    1. Fill a clean sprayer about half-full with water.
    2. Install a liquid-filled test gauge in the highest nozzle position of one of the booms. The image below shows how the nozzle cap or entire nozzle body may need to be removed for this step. For Metric fittings, contact your sprayer dealer – they can be hard to find.
    3. With the tractor parked, bring up the rpms and get the lines to the desired operating pressure.
    4. Open the boom(s) and measure the pressure at the nozzle farthest from the pump. All nozzles on all booms should be open during this test. That’s why you are wearing PPE.
    5. For positive displacement pumps, adjust the main pressure regulator until the test gauge reads the desired pressure. For centrifugal pumps, it is possible to make small changes to the pressure, but more important to note any pressure differential for later considerations regarding nozzle output and spray quality.
    There are many ways to install a gauge onto a nozzle body. Here are three examples of common fittings.
    There are many ways to install a gauge onto a nozzle body. Here are three examples of common fittings.

    Switching between multi and single boom operation

    When sprayers that employ a positive-displacement pump are switched to one-sided operation (E.g., border spraying or during turns), the pressure can change considerably. Most units will experience a pressure increase, thereby increasing the boom output. This is typically an indication of a faulty relief valve, which is positioned between the pump and nozzles. It’s actuated by a spring-loaded piston or diaphragm, opening and closing in response to changes in pressure. The operator sets the desired pressure and any additional pressure forces the valve open, diverting excess flow back to the tank via a bypass.

    Spraying from one boom. This operator checked to make sure the pressure didn’t increase when he closed the second boom. High pressures or sudden spikes could indicate a faulty regulator valve.
    Spraying from one boom. This operator checked to make sure the pressure didn’t increase when he closed the second boom. High pressures or sudden spikes could indicate a faulty relief valve.

    This problem can be greatly reduced by properly sizing the regulator (specifically the spring) to the typical operating pressure. Many sprayers come equipped with regulator springs matched to the maximum pressure range of the pump (often 600 – 900 psi). These springs are unable to respond to changes when operating at lower pressures (E.g., 100-200 psi, which is typical of applications to moderately-sized canopies).

    The springs are so stiff that the liquid pressure is unable to act on the spring and the valve essentially acts as a flow control (throttling) valve rather than a pressure control valve. Liquid pressure is difficult to control using a throttling valve; it is unable to compensate if the tractor engine speed drops while driving uphill and sprayer output is subsequently reduced. Further, this phenomenon can cause pressure gauges to spike.

    Valve springs and seats wear out, such as in this regulator assembly. Check yours each season.
    Valve springs and seats wear out, such as in this regulator assembly. Check yours each season. If you spray using moderate pressures, be sure your regulator spring can compensate for small changes.

    Some sprayer designs attempt to compensate for excess flow during single-boom operation. They employ an additional throttling valve to shunt the volume that would normally would be spraying out through the closed boom. The result is that the pressure should remain constant when a single boom is shut off. If your sprayer has this feature, here’s how you set the valve:

    1. With PTO at application speed and both booms open, adjust regulator to calibrated operating pressure.
    2. Close one boom.
    3. If pressure increases, open throttling valve to achieve calibrated operating pressure. If pressure decreases, close throttling valve to achieve calibrated operating pressure.
    4. Repeat process for the other boom, and find a compromise position for the valve.

    Some operators elect to remove the handle from the throttling valve once it is set so they don’t accidentally bump it later. That’s fine, but further adjustments may be required when transitioning between dilute and concentrated volumes, so don’t lose the handle.

    Here’s an oldie-but-a-goodie filmed in New Hampshire in June, 2014. It’s something to keep in mind when you’re getting your sprayer ready for spring service. Thanks to Chazzbo Media and Penn State Extension for making an unscripted and spur-of-the-moment concept into a polished video.

  • Spray and Soil Fumigant Buffer Zones in Canada

    Spray and Soil Fumigant Buffer Zones in Canada

    Spray buffer zones are no-spray areas required at the time of application between the area being treated and the closest downwind edge of a sensitive terrestrial or aquatic habitat. Spray buffer zones reduce the amount of spray drift that enters downwind, non-target areas.

    Sensitive Terrestrial Habitats

    Sensitive terrestrial habitats can include hedgerows, grasslands, shelterbelts, windbreaks, forested areas and woodlots. Crops and private properties adjacent to treated areas are not considered to be sensitive terrestrial habitats and do not require spray buffer zones. However, labelled spray buffer zones are a good indicator of potential damage to adjacent vegetation. Applicators are responsible for ensuring their spraying programs do not adversely affect neighbouring properties.

    Sensitive Aquatic Habitats

    Sensitive aquatic habitats can include lakes, rivers, streams (channelized or natural), creeks, reservoirs, marshes, wetlands and ponds. Temporary bodies of water resulting from flooding or drainage to low-lying areas are not considered sensitive aquatic habitats. Nor are aquatic drainage ditches or seasonal water courses that are dry at the time of application. Water body depth will determine the buffer zone distance, as indicated on the pesticide label. Downslope open water may also require a vegetative filter strip .

    The pesticide label will indicate when a spray buffer zone is required. The distance will depend on the product used, the method of application and the crop being sprayed. In some cases, the buffer zone may be modified using Health Canada’s Spray Buffer Zone Calculator . When provincial and label restrictions differ, or label restrictions differ between tank mix partners, use the greatest distance.

    Buffer zones or No-Spray zones physically separate the end of the spray swath for the nearest downwind sensitive area.
    Buffer zones or No-Spray zones physically separate the end of the spray swath for the nearest downwind sensitive area.

    Spray Buffer Zone Calculator

    Unless forbidden by the pesticide label, Health Canada’s Spray Buffer Zone Calculator may permit applicators to reduce the size of the spray buffer zone specified on a pesticide label. To be eligible, the product label must specify a field or aerial spray quality coarser than “Very Fine” and finer than “Very Coarse”. All airblast spray qualities are applicable.

    Modifications are based on meteorological conditions, sprayer configuration and the application method at the time of application. If modified spray buffer zone distances are less than provincial or municipal distances, use the greater distance.

    Applicators that choose to use the calculator must retain a copy of the summary page for at least one year following the application to demonstrate compliance with label directions.

    Vegetative Filter Strips

    A vegetative filter strip is a permanently vegetated strip of land that sits between an agricultural field and downslope surface waters. Vegetative filter strips reduce the amount of pesticide entering surface waters from runoff by slowing runoff water and filtering out pesticides carried with the runoff.

    Pesticide labels may require a vegetative filter strip, or recommend one, as a best management practice. They must be at least 10 metres wide from edge of field to the surface water body and be composed primarily, but not exclusively, of grasses.

    Spray buffer zones do not apply to vegetative filter strips unless there is a pre-existing sensitive terrestrial habitat within them. Therefore, vegetative filter strips may overlap spray buffer zones when open water is both downslope and downwind (see illustration). In this case, the minimum 10 metres vegetative filter strip distance must be observed, but the set-back can be larger based on spray buffer zone, provincial or municipal restrictions.

    Soil Fumigant Buffer Zones

    Soil Fumigant Buffer Zones are mandatory, untreated perimeters surrounding the treated field. They limit user exposure and increase the protection of workers, bystanders and the environment. The distance will depend on the application method, product rate and field size, as indicated on the pesticide label. An Emergency Response Plan is required when residences or businesses are located within 90 metres of the buffer zone perimeter.

    Soil fumigant buffer zones have a time component. This Buffer Zone Period begins at the start of the application and ends a minimum 48 hours following the application. Respiratory protection and stop-work triggers, as specified on the pesticide label, will apply to anyone present in the buffer zone area during the buffer zone period.

    Buildings and residential areas within the soil fumigant buffer zone must be unoccupied during this period. Unless in transit, non-handlers (including field workers) must be excluded from the soil fumigant buffer zone during this period. Entry is permitted for fumigant handlers with appropriate certification, emergency personnel and local, provincial, or federal officials performing inspection, sampling, or other similar duties.

    Image from www.onspecialitycrops.ca

    Soil fumigant buffer zone signage must be posted within 24 hours prior to the application and remain posted until the buffer zone period expires. Signage must include, but is not limited to, the date and time the buffer zone period ends and the name, address, and telephone number of the applicator. Soil fumigant buffer zone signage must be located at the outer perimeter of the buffer zone, at all entrances to the field, and along likely routes where people not under the owner’s control may approach. Soil fumigant buffer zone signs are in addition to, and do not replace, fumigant application block signage .

    Applicators must develop a written Fumigation Management Plan prior to the start of any application. The plan outlines key steps to ensure a safe and effective fumigation, including site conditions, buffer zones and emergency response planning. Both the owner/operator of the fumigated area and the fumigant applicator must retain signed fumigant management plans as well as a summary of Post-Application Procedures for two years following the application.

  • Three Manageable Factors that Affect Spray Drift

    Three Manageable Factors that Affect Spray Drift

    In 2014 one of our OMAFRA summer students designed a short-and-gritty demonstration using a backpack sprayer, a variable-speed fan and some water-sensitive paper positioned downwind at 1.5 metre intervals. The intent was to illustrate how sprayer operators could reduce the potential for off-target drift by recognizing and accounting for three factors:

    • Apparent wind speed (i.e. the sum of wind speed and travel speed)
    • Boom height (i.e. release height)
    • Droplet size (i.e. nozzle spray quality)

    Apparent Wind Speed

    Spray operators know they should not spray when the air is calm or when the wind is too high, but they often forget that the nozzles experience “apparent wind speed” which means driving 10 km/h into a 10 km/h headwind is essentially spraying in a 20 km/h wind.

    The result of spraying with a Medium spray quality in 10 km/h and 15 km/h wind: water-sensitive papers indicated that there is more downwind drift in higher winds.

    Boom Height

    Spray operators raise their booms to ensure their nozzles clear the crops, but this contributes to off target drift and greatly reduces coverage – particularly when using twin-fan style tips. Dr. Tom Wolf explains how to set your boom height here, or you could watch one of our Exploding Sprayer Myths videos on the subject.

    The result of spraying with a Medium spray quality in a 10 km/h wind at 50 cm and 100 cm from the ground: water-sensitive papers indicated that downwind drift increases as the boom gets higher.

    Droplet Size

    The coarser the spray quality, the less likely the spray will drift off target. Remember, for a given volume, shifting to larger droplets means fewer droplets. Application volumes may have to increase to compensate for potentially reduced coverage.

    The result of spraying with a Medium spray quality versus spraying with an Extremely Coarse spray quality: water-sensitive papers indicated that there is more downwind drift from smaller droplets.

    Take-Home

    This demo used percent coverage as a metric, which is convenient but greatly underestimates drift. So even when the spray window is small and the spray has to go on, take a moment to drop the boom, use a coarser droplet size and if it’s too windy, just don’t spray.

    WUR Drift Calculator

    There are many drift calculators available for home use. Some require more expertise than others to get a reliable result. This free downloadable calculator from Wageningen University & Research was made available in 2021. It can quantify spray drift deposits onto surface waters and non-target terrestrial areas near a sprayed field or orchard

    The calculator uses statistically obtained regression curves to calculate spray deposition next to the sprayed field. The spray drift curves are based on the latest experimental data for field crops, fruit orchards and avenue tree nurseries.

    Download your copy here.

     

  • Spraying Ginseng with Arag Microjets

    Spraying Ginseng with Arag Microjets

    In June 2013 we ran a ginseng spraying workshop and we learned as much as the growers did. Ginseng is notoriously difficult to spray:

    • It is highly susceptible to pathogens given the high humidity and still conditions generally found under the shade structure.
    • It forms a solid ceiling of leaves that resist spray penetrating to the stem and crown below and makes under-leaf coverage very difficult to achieve.

    Many growers have (wisely) walked away from the old Casotti sprayers, which have been shown to give erratic coverage at best. They have adopted the Arag Microjet system with it’s characteristic orange shields. The >$80.00 CAD price tag for each nozzle is due to the brass mixing valve and swivel joint, as well as import costs from Italy. Contrary to popular belief, it does not use air-assist, or air-induction – it is strictly hydraulic. It does tend to create a ‘wake’ of air movement at high pressure. This phenomenon is called air entrainment and it is caused by large droplets travelling at high speed.

    Classic Arag microjet nozzles.
    Classic Arag microjet nozzles.

    This nozzle is essentially the business-end of a spray gun. The way it is used in ginseng it works more-or-less like a hollow cone disc-core assembly. This begs the question “Why not use the cheaper and more readily available ceramic disc-core?” We set out to compare the two options using water sensitive paper set within the canopy. These yellow, paper targets turn blue when sprayed, clearly showing spray coverage.

    Location of water-sensitive papers in the ginseng canopy.
    Location of water sensitive papers in the ginseng canopy.

    Determining rates

    The first step was to determine the output rate for each nozzle. Generally, nozzle manufacturers provide rate tables showing how much volume a nozzle emits by time (e.g. US gallons per minute) at a given pressure. Finding these tables for the 1.5 millimetre Arag Microjet proved difficult. When we finally found one, it was discovered the rates were established for 200 to 850 pounds per square inch. This is excessively high pressure for a typical boom sprayer, so tables had to be developed for lower pressures.

    Classic Arag microjets have a mixing valve that opens the spray up into a hollow cone, or collapses it into a tight stream. This also changes the rate. It can never be shut off completely, and it's hard to adjust consistently.
    Classic Arag microjets have a mixing valve that opens the spray up into a hollow cone (valve handle left or right), or collapses it into a tight stream (valve handle middle). The valve position also changes the rate. It can never be shut off completely, and it’s hard to adjust consistently.
    Determining nozzle rate using the Innoquest Spot-On SC-4.
    Determining nozzle rate using the Innoquest Spot-On SC-4.

    Further, given the odd design of the mixing valve, it was determined that moving the handle ~10 degrees left of centre, or ~10 degrees right of centre, gave a difference of as much as 60%. The table below  shows the outputs for a 1.5 millimetre nozzle with the handle in both positions and the two graphs show the results… well… graphically. Outputs were determined using the Innoquest Spot-On SC-4, but the frothing effect created by the nozzles may have created minor errors. Each rate is the average of a minimum of three samples.

    Valve SettingPressure (psi)Avg Output (gpm)Pressure (bar)Avg Output (L/min)
    10 degrees left401.022.763.86
    10 degrees left501.13.454.16
    10 degrees left601.254.144.73
    10 degrees left701.254.834.73
    10 degrees left801.385.525.22
    10 degrees left901.46.215.3
    10 degrees left1001.456.895.49
    10 degrees left1101.67.586.06
    10 degrees left1201.758.276.62
    10 degrees left1501.8710.347.08
    10 degrees left2002.213.798.33
    10 degrees right400.652.762.46
    10 degrees right500.73.452.65
    10 degrees right600.84.143.03
    10 degrees right700.854.833.22
    10 degrees right800.95.523.41
    10 degrees right900.96.213.41
    10 degrees right10016.893.79
    10 degrees right1101.077.584.05
    10 degrees right1201.18.274.16
    10 degrees right1501.2510.344.73
    10 degrees right2001.3713.795.19
    Average 1.5 mm ARAG Microjet output at a range of pressures and two valve settings in US Imperial units.
    Average 1.5 mm ARAG Microjet output at a range of pressures and two valve settings in US Imperial units.
    Average 1.5 mm ARAG Microjet output at a range of pressures and two valve settings in Metric units.
    Average 1.5 mm ARAG Microjet output at a range of pressures and two valve settings in Metric units.

    Comparing nozzles

    Using the grower’s typical ground speed of 5 km/h (~3 mph) and operating pressure of 6.9 bar (100 psi), we found four TeeJet disc-core combinations that emitted a hollow cone pattern and approximately the same output as the Arag Microjets. The five nozzles sets tested were:

    1. ARAG Microjet® 1.5 mm = ~0.95 US g/min avg at 100 psi
    2. TeeJet® D8-DC25= 0.97 US g/min at 100 psi= ~97° cone
    3. TeeJet®D7-DC45= 0.97 US g/min at 100 psi= ~81° cone
    4. TeeJet®D4-DC46= 0.88 US g/min at 100 psi= ~33° cone
    5. TeeJet®D6-DC45= 0.93 US g/min at 100 psi= ~81° cone

    We did not use nozzle drop hoses (aka drop arms or hose drops) because it has already been firmly established that they are absolutely required to achieve under leaf coverage See OMAFRA factsheet 10-079 and this article.

    Observations

    While there were some complications with setting up the papers for the demo, we observed the following:

    1. The output of each Microjet nozzle can be as much as 50% more or less than expected without being visually detectable and output for each nozzle must be confirmed before spraying. Therefore, outputs should be confirmed before every application.
    2. Microjets at 100 psi emitting ~890 L/ha (~95 US gallons per acre) gave satisfactory coverage on all upward facing targets, but unsatisfactory under-leaf coverage. This has been demonstrated many times before.
    3. The TeeJet D7-DC45 combination emitting a similar rate gave satisfactory coverage on all upward facing targets, but unsatisfactory under-leaf coverage. They may be a viable alternative to the Microjets.
    4. Nozzle drops are advised to achieve under-leaf coverage.

    The demo also raised some questions:

    1. Did the TeeJet disc-core push the canopy apart as much as the Microjet? The audience noticed there was some leaf-shadowing where the cards did not get complete coverage using disc-core. This might have been coincidence, or it may not have. This question will be addressed in a research trial next season, but for now, the D7-DC45 appeared to give similar coverage to the Microjet.
    2. Can nozzle drops be avoided if pressure is raised to 27.5 bar (400 psi)? Thanks to one grower trying this experiment in his garden after the demo, we saw some under-leaf coverage is possible at such high pressures, but this occurred at the cost of a lot of noise, diesel fuel and considerable wear on the ceramic Microjet discs. The grower tested these tips and discovered they needed replacement after only two years of use. Nozzle drops are cheaper, easier and result in considerably more spray in the under leaf positions.
    3. We saw what minimal and excessive foliar coverage looked like, and determined how much variability there was from one nozzle to another. A significant question was “How much spray can be saved when using a more accurate application?” and the answer is yet to be determined, but could be well in excess of 10% of the typical spray volume. Given that this crop can be sprayed more than 100 times over it’s 3 or four years before harvest, this represents significant savings in pesticides and refill time.

    Additional – Newer ARAG Microjet Design

    Since this work was performed, growers have been exploring a newer option from ARAG.

    They are an improvement over the older version insofar as they are more easily calibrated and held at a given rate thanks to a lock nut. They still employ a 1.5 mm diameter ceramic disc, but this can be changed for a 1.0 or 1.2 quite easily. They are still somewhat finicky when trying to set a consistent spray quality and rate from nozzle to nozzle, but are better than the mixing-valve option.

    Learn more in this article.

    Custom-made ginseng sprayer. A standard design.
    Custom-made ginseng sprayer. A standard design with newer, cheaper and easier-to-use ARAG microjets.

    Special thanks to Syngenta Canada for providing lunch, to C&R Atkinson Farms Ltd. for hosting, to TeeJet for supplying the disc-cores and water-sensitive papers, and to Dr. Sean Westerveld, Dr. Melanie Filotas and OMAFRA summer student Megan Leedham for contributing to the workshop.

  • Assessing Water Sensitive Paper – Part 3

    Assessing Water Sensitive Paper – Part 3

    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:

    1. 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.
    2. 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.
    3. 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.

    References (Further reading)

    Bankhead, P. 2014. Analyzing fluorescence microscopy images with ImageJ.

    Cunha, J.P.A.R., Farnese, A.C., Olivet, J.J. 2013. Computer programs for analysis of droplets sprayed on water sensitive papers. Planta Daninha, Viçosa-MG. 31(3): 715-720.

    Ferguson, J.C., Chechetto, R.G., O’Donnell, C.C., Fritz, B.K., Hoffmann, W.C., Coleman, C.E., Chauhan, B.S., Adkins, S.W. Kruger, G.R., Hewitt, A.J. 2016. Assessing a novel smartphone application – SnapCard, compared to five imaging systems to quantify droplet deposition on artificial collectors. Computers and Electronics in Agriculture. 128: 193-198.

    Ledebuhr, M. 2016. Small Drop Sprays.

    Marçal, A.R.S., Cunha, M. 2008. Image processing of artificial targets for automatic evaluation of spray targets. Trans. of the ASABE. 51(3): 811-821.

    Moor, A., Langenakens, J., Vereecke, E., Jaeken, P., Lootens, P., Vandecasteele, P. 2000. Image analysis of water sensitive paper as a tool for the evaluation of spray distribution of orchard sprayers. Aspects of Applied Biology. 57.

    Panneton, B. 2002. Image analysis of water‐sensitive cards for spray coverage experiments. Applied Eng. in Agric. 18(2): 179‐182.

    Salyani, M., Zhu, H., Sweeb, R.D., Pai, N. 2013. Assessment of spray distribution with water-sensitive paper. Agric. Eng. Int.: CIGR Journal. 15(2): 101-111.

    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).

    Syngenta. 2002. Water‐sensitive paper for monitoring spray distributions. CH‐4002. Basle, Switzerland: Syngenta Crop Protection.

    Turner, C.R., Huntington, K.A. 1970. The use of a water sensitive dye for the detection and assessment of small spray droplets. J. Agric. Eng. Res. 15: 385-387.