App provides snappy spray assessment

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The SnapCard app is designed to help spray operators refine their sprayer set-ups for different jobs by measuring the spray coverage obtained

water sensitive paper

Figure 1  Water-sensitive paper showing coverage when applied with various spray qualities at the same application volume.

SOURCE: Tom Wolf, AGgriculture and Agri-food Canada Researc Centre 

There are many variables that can impact on the outcome of a spray job and smart tools take some of the guesswork out of sprayer set-up to optimise spray coverage.

After product rate, the application volume and spray quality are the two main factors that drive sprayer set-up decisions and determine nozzle selection.

Screen shot of the SnapCard app

The SnapCard app is simple to download and use, and can help to measure the level of spray coverage achieved.

Water and oil-sensitive paper (WSP) is a resource that has been available to spray operators for many years to assess spray deposits (Figure 1). The WSP card has a special coating that produces a stain when a droplet lands on it.

Before the advent of image-analysis tools that could read WSP, spray operators relied on visual assessments of spray deposits and basic counts of droplets per square centimetre to compare spray set-ups. While this was useful for determining if spray was depositing where it was needed, this method does not provide useful numbers to determine which set-up provides better deposition.

Using droplets/cm2 to determine if spray coverage is adequate is only useful to compare similar droplet sizes (for example, a medium with a medium).

It becomes much more difficult to usefully compare 20 coarse droplets/cm2 to 20 medium droplets/cm2, as the amount of product deposited onto the target can be totally different. 

In trials on spray coverage and pesticide efficacy during the late 1990s, it was difficult to find a consistent relationship between droplets/cm2 and efficacy, particularly when applying products with different spray qualities.

Laboratory-based image-analysis systems that were available at the time seemed to provide a more consistent relationship between the percentage of the target covered and the level of control obtained.

SnapCard app

The SnapCard app  takes WSP to a new level.

The SnapCard app has been available for Android and Apple devices since 2014, and its use by growers and spray operators to measure spray deposition continues to increase.

The app is the outcome of a research collaboration between the entomology group at the Department of Agriculture and Food, Western Australia, and the University of WA’s (UWA) applied entomology program, which was supported by funding from the Council of Grain Grower Organisations. The GRDC also provided funding to UWA for the employment of an entomology professor.

The SnapCard app can be used to predict and measure spray coverage on WSP. While the predictions, which are based on agronomic variables, weather conditions and sprayer set-up, serve as a useful guide with which to compare measurements. Perhaps the most powerful function of the SnapCard app is the measurement tool itself, which provides the user with a measurement of the percentage of the WSP covered.

In a recent University of Queensland study comparing some of the image-analysis systems available to measure spray deposits, results obtained from the SnapCard app supported results from laboratory-based equipment, which is often expensive and difficult to use in the field.

Results obtained from this study (Table 1) show the percent coverage on WSP as measured using a range of image-analysis systems. The nozzles and operating pressures used in this study were the American Society of Agricultural and Biological Engineers reference nozzles, which are used to determine the spray-quality classification for nozzles according to standard 572.1.

While the estimates of percentage coverage measured with SnapCard were marginally lower for 110-01 nozzle outputs than those measured by other instruments, there was good agreement with other instruments for the range of droplet sizes that are likely to be used by Australian grain growers.

Table 1 A selection of the data produced comparing the percent coverage measured by four of the image-analysis systems tested.

pressure (kPa)
 Percent coverage on water-sensitive paper (WSP)*

SnapCard Image J Deposit Scan Droplet Scan
110–01 450 49 60 60 75
110–03 300 49 53 53 71
110–06 200 51 53 52 75
80–08 250 43 44 43 50
65–10 200 48 50 49 65

* As this is only a selection from the original data set the levels of statistical significance are not included.

SOURCE: University of Queensland Centre for Pesticide Application and Safety. Modified
from an article by Ferguson, JC et al (2016), Pest Management Science (in press) 

Using SnapCard

To get the best out of The SnapCard app, spray operators should plan ahead and consider what comparisons they wish to make, and how to record and store the data.

Operators should collect a set of benchmarking data about how sprays are applied as a basis for comparison when making changes to parameters in the future.

Some growers have developed their own spray-application guides for their machines by photographing the WSP cards and recording the percent coverage for each set-up they use, as well as recording the impact of changing parameters such as pressure, application volume or travel speed.

Over time this record can become a valuable tool, especially when the information in the spray records allows for direct comparison between how the application was made and the results that were obtained.

Measuring spray coverage with SnapCard to identify areas of improvement

Water sensitive paper

Figure 2  Deposits onto WSP cards at three locations in the canopy (top, mid and bottom).

SOURCE: Bill Campbell

Many operators may not have considered the number of situations where an accurate measurement of the coverage could be useful for improving the sprayer set-up. While the following examples may not include every possible use of SnapCard, they do provide a useful guide for how to look at specific situations.

Assessing penetration into crop canopies

Placing WSP cards directly onto the crop canopy using staples or paperclips at various heights throughout the canopy can demonstrate how much penetration into the canopy can be achieved with different application set-ups.

The images in Figures 2 and 3 and Table 2 are from an example of using WSP cards and SnapCard provided by Bill Campbell of Farmanco Management Consultants in Western Australia. In this example, Mr Campbell worked with one of his clients to improve the spray coverage in canola for Sclerotinia sprays.

Figures 2 and 3 show the difference in coverage on WSP cards that a change in spraying speed with the same nozzles can produce, with the results summarised in Table 2.

Table 2 Summary of spray coverage at three positions in the canopy (top, mid and bottom) at two spraying speeds. 

 Parameters to achieve 60L/ha    Top Mid  Bottom
 27km/h 2.3 ba  Coverage  11.2%  5.6%  3.5%
   Droplets per cm2  80  45  36
 22km/h 1.7 ba  Coverage  28%  18%  16%
   Droplets per cm2  120  50  40

SOURCE: Bill Campbell

Assessing spray deposits into standing stubble

To assess if the sprayer is set up to provide good spray deposition into standing stubble a comparison needs to be made between the spray deposits in the inter-row space and at the base of the standing stubble (within the stubble row). If a sprayer set-up can produce good spray coverage that is equivalent between the two locations, it generally indicates that the spray operator has selected an appropriate spray quality and application volume for stubble load and the spraying speed tested.

Water sensitive paper aligned at regular spacing

WSP aligned at regular spacing to highlight changes in spray coverage is useful for assessing the duty cycle for pulse-width modulation.

PHOTO: Bill Gordon 

Determining wake effects and wheel-track issues

Researchers zoom in on droplet behaviour

Several factors influence what a spray droplet does when it strikes the target surface. Depending on the droplet size, droplet velocity, chemical properties of the spray solution and the characteristics of the leaf surface, the droplet may adhere, bounce or shatter and possibly be recaptured.

Researchers in Australia and New Zealand (funded by the Australian Research Council and some commercial companies) are mathematically modelling what happens during the spraying of plants. Researchers from the University of Queensland, Queensland University of Technology, Plant Protection Chemistry New Zealand and SciCon Scientific Consultants have pooled their knowledge and research expertise to quantify what happens to spray droplets at the leaf surface, particularly when the parameters that influence their deposition are changed.

“It has turned out to be a lot more complicated than we first thought,” says Jerzy Zabkiewicz, from SciCon Scientific Consultants in New Zealand. “However, the main factors that influence what happens to the droplets after they impact on the leaf are the droplet velocity, spray formulation and the characteristics of the leaf surface itself.”

Combining all of these factors to produce a virtual model of the spraying process relied on sophisticated modelling of what happens from the time that the droplets exit the nozzle to when they first impact on the leaf surface, and then what happens to them in the milliseconds after impact. This research is continuing towards developing decision-support tools for industry to predict droplet retention levels on selected targets

More information:

Jerzy Zabkiewicz, SciCon Scientific Consultants,,

Placing WSP cards at different positions across the boom sprayer allows spray operators to look at the impact of changing travel speed on the amount of spray that deposits in the centre of the sprayer, adjacent to the wheels and out under the boom. This information is useful for looking at wheel-track issues and adjustments that can improve coverage.

Fine-tuning pulse-width modulation – highlighting coverage and duty cycle

Placing WSP cards at regular intervals, usually with a 2.5-centimetre gap between the cards, onto strips of timber that are at least one metre long can highlight possible coverage concerns if the duty cycle of a pulse-width modulation (PWM) system is too low to provide even coverage. Aligning the strips of timber with the direction of travel and spraying at different speeds will show the impact of low duty cycles, particularly with coarser spray qualities.

Identifying suitable travel speeds and sprayed widths with target-selectable sprayers

Placing cards on strips of timber aligned with the direction of travel, as described for PWM (see page 18), can quickly illustrate when the individual sprays turn on and off in relation to the position of a weed for target-selectable sprayers. Often it is useful to ensure the area for the trial is free of any green vegetation that might trigger the cameras, and to deliberately transplant weeds into the test area. Often when the spraying speed is not correct the spray droplets will deposit onto the WSP cards well before or well after the position of the weed.

The same timber strips, but this time aligned perpendicular to the direction of travel (parallel to the boom), with the centre of the strip aligned with the weed can show the sprayed width when the machine is operating at its usual spraying speed.

Tips for getting the best results from WSP and the SnapCard App

  • Download the SnapCard instruction manual before you start and practise using the app before commencing any trials.
  • Water and oil-sensitive paper (WSP) cards are available through TeeJet distributors such as machinery dealers.
  • Always use disposable gloves when handling WSP cards, even fingerprints and sweat can produce a stain on them.
  • Consider the position of the WSP cards in relation to the sprayer. It is a good idea
  • to set an A–B line for the sprayer, or note the wheel tracks and then place pegs in the ground where the cards are located. Even in fallow WSP cards can be hard to locate.
  • Do not try to use WSP cards when the relative humidity is above 70 to 80 per cent, as the cards will turn blue. Place out a single test card to assess this before commencing any trial.
  • Place several cards out at each location you wish to collect coverage data from. After measurements of spray coverage are completed, record an average for the group of WSP cards at that location.
  • Allow the cards to dry fully before collecting them.
  • Label the back of each card with a pencil for future reference and store them separately in snap-seal bags to avoid exposure to moisture or humid conditions.
  • After collecting all the WSP cards, go to a place out of the wind where you can use SnapCard to take individual pictures of all of the WSP cards in one session, keeping the smartphone camera height at a constant distance from the WSP cards, and under uniform light conditions.
  • Once they have been measured with SnapCard, the cards can be photographed or glued to a sheet of paper and scanned. It is a good idea to include details of each treatment and the sprayer set-up in the photograph or scan.
  • Keep the results of all of your tests in one book or folder for future reference, making sure you record all of the application parameters and conditions for each test.

More information:

Bill Gordon,
0429 976 565,


Research aims to take guesswork from spray conditions


The operators' toolbox just got bigger

GRDC Project Code BGC00002, BGC00003

Region National