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A new version of standard ASABE S572.3 (adopted 2020) and its international equivalent, ISO 25358 (adopted 2018) addresses a limitation of the previous standards. In ASABE S572.2, the nozzle that defined the category boundary between the XC and UC sprays was the TP8015 SS at 4.22 L/min, corresponding to 150 kPa. Only very few nozzles produced UC sprays, and those that did only did so at the lower pressures and higher flow rates. For those nozzles, the categories worked.

The emergence of a host of new low-drift nozzles designed to meet the regulatory requirement for drift protection for Xtendimax and other dicamba formulations in soybeans revealed the limitation of this boundary nozzle choice. In essence, all the new dicamba-compliant low drift sprays were UC, regardless of nominal flow rate or spray pressure. Any effect of, say, higher pressure that would create a finer spray, was not revealed. It was decided that new boundary nozzles were needed so that the coarser sprays’ response to flow rate, pressure, and formulation or adjuvant choice, could be better described.

S572.3 therefore lowered the defining flow rate of the XC/UC nozzle from 4.22 to 3.41 L/min, with a new spray pressure of 100 kPa (from 150 kPa). This made the new boundary spray coarser. Because this change made the adjacent spray quality class, the XC, broader, the VC and C categories were each also widened somewhat, by lowering their respective boundary nozzle flow rates and pressures (Table 1).  

Table 1: Change in spray pressure of three coarsest boundary nozzles from ASABE S572.2 to S572.3

For XF, VF, F, and M spray classes, there is no change with this new standard because the boundary nozzle conditions were not changed. But the C, VC, XC, and UC classes became coarser.

In order to signal the change to consumers, the colour coding of the Coarse and Very Coarse sprays was switched. With S572.2, C was blue and VC was green. With S572.3, C is now green and VC is blue (Figure 1). 

The change means that low-drift sprays categorized using S572.2 can no longer be accurately compared to nozzles categorized using S572.3. For example, if a researcher conducted efficacy studies and concluded that ASABE S572.2 Coarse sprays provided acceptable efficacy, but S572.2 Very Coarse sprays had lower performance, they might recommend Coarse sprays. But would that recommendation be safe for nozzles classified with S572.3? In that standard, the C category extends into the territory formerly occupied by VC, deemed too coarse by the research.

A similar problem exists for spray drift risk assessments. Let’s say a regulator recommends a 25 m buffer zone for ASABE S572.2 VC sprays based on modelling the drift from that C/VC boundary nozzle. What will the buffer zone be for the new S572.3 VC spray? Since the newer VC spray is coarser at its lower boundary, drift, and therefore the buffer zone, would be less. Determining how much so would require a new spray drift dataset for the new boundary nozzle.

In fairness, most of the risk assessments are done on finer sprays such as Medium and Coarse, and those would remain unchanged because to be conservative, the regulator would assume the finest spray within each category (i.e., the lower boundary nozzle). But when a registrant requests that risk assessment be done with a coarser spray quality such as VC, XC, or UC, the regulator would need a new dataset.

Another problem is that not all spray quality data reported by manufacturers use the new standard. Of the major North American manufacturers, Spraying Systems started reporting their products using the S572.3 standard in 2023. Pentair Hypro, and by extension John Deere, as well as Wilger Industries have not yet followed suit.

The European manufacturers are well on their way. Agrotop (Greenleaf in the US), Lechler and Arag are both reporting using ISO 25358. But Billericay Farm Systems (BFS) are still reporting using the older standard.

It may have been some years since nozzle manufacturers last tested the spray quality of their nozzles. If they now measure their sprays using the new standard, some other changes have also have ocurred in the interim. For example, new laser measurement systems may have been acquired. New scanning techniques may have been developed. New staff are operating the systems. And while the ASABE and ISO reference nozzles are intended to negate these differences, we are still seeing some discrepancies. It is also possible that changes to the manufacturing of the nozzles has also altered the spray quality they produce.

For example, compare Figure 2, the TeeJet AIXR nozzle spray quality chart, developed using ASABE S572.2 and published in 2015, to Figure 3, the same nozzle whose spray quality was categorized using ASABE S572.3 and published in 2023.

First, let’s compare the portion of the chart that reports Medium spray qualities in Figure 2 to that in Figure 3. These should be the same given that the same boundary nozzle and pressure was used in both. But under the new testing, the region reporting a Medium spray has significantly expanded towards the lower pressures. The Medium sprays begin at 60 and 75 psi in the old standard, but at 50 psi for the new standard.

The region occupied by the Coarse category has shifted towards lower pressures, as expected with the coarser upper (C/VC) boundary nozzle. It would be wider, as was the intent, had the M catergory not encroached.

The region occupied by VC and XC have grown towards the lower left, more or less as expected. The VC category in particular, now dominates in the lower pressure region, a space previously occupied by the XC category. The small region which the UC category occupied under S572.2 in Figure 7 was replaced entirely by the XC category.

Looking at it another way, the commonly used AIXR11004 size at pressures between 30 and 90 psi would have produced XC, VC, and C spray qualities in the old catalogue. The same nozzle now apparently produces VC, C, and M sprays at the same pressure range.

These changes may or may not extend across all nozzles and manufacturers to the same extent as this example illustrates. But they add another level of complexity, and perhaps confusion, to applicators striving to be compliant with label directions. I am hopeful that the sector can quickly move to adopt the changes universally so that the mixed messaging is short lived and mistakes are minimized.  

In particular, the regulatory bodies that rely on these spray quality classes to communicate drift potential and buffer zones must recognize that their risk assessments will need to reflect these new standards as more nozzle manufacturers begin communicating them.

Droplet size continues to explain most of what matters in spraying, from deposition uniformity, to coverage and spray drift. Hopefully this series of articles explaining the measurement of spray droplet sizes is useful and sheds some light on how the industry measures and communicates it.

When scientists with laser droplet sizing instruments compared their results with other labs using the same nozzles, the numbers didn’t agree exactly. Laser equipment type and age, optics, operating settings, scanning techniques, and software versions were important. Everybody had their own numbers that they trusted more than their colleagues’ in another lab. In order to communicate droplet sizes to the end user consistently, a way to resolve these differences needed to be found.

Scientists ushered in a new approach, that of using reference nozzles.

Rather than argue about whose instrument or measurement method was correct, experts decided to allow each lab to use their equipment but with measurement protocols that met certain standards for scanning accuracy. The way the systems were compared was with a set of agreed-on reference nozzles. These reference nozzles defined the boundaries of spray qualities and were measured by the lab at the same time they tested other nozzles. When a nozzle of interest produced a spray whose parameters fell between two reference nozzles, the spray quality was thus defined. In this way, the peculiar results of a certain measurement approach became irrelevant. All that mattered was where the result lay with respect to the reference nozzles tested by the same lab.

Credit is due to members of the British Crop Protection Council, who pioneered this result in the UK in the mid 1980s, and on which today’s droplet size classification standards are based. Their initial standard was first proposed in this paper:

Doble S J, Matthews G A, Rutherford 1, Southcombe E S E (1985). A System for Classifying Hydraulic and Other Atomizers Into Categories of Spray Quality. Proceedings British Crop Protection Conference – Weeds, 3, 1125-1133.

Thus the term “Spray Quality” was born, and after various incremental revisions, sprays were named Extremely Fine, Very Fine, Medium, Coarse, Very Coarse, Extremely Coarse, and Ultra Coarse and given a colour code for easy identification.

The American Society of Agricultural and Biological Engineers (ASABE) published the first Spray Quality Standard, S572, in 1999. It specified reference nozzles that formed boundaries from VF/F to VC/XC. It was updated to S572.2 in 2014 with the addition of the Ultra-Coarse category.

Figure 1: Symbols and colours used to designate spray qualities according to ASABE S572.1 and S572.2

In order to better represent nozzles that produced finer and coarser sprays, boundary nozzles for XF/VF and XC/UC were introduced with ASABE S572.1 in 2004. ASABE S572.2 corrected an error in the flow rate of the XC/UC in 2014.

In 2020, ASABE S572.3 was introduced, following on the heels of ISO 25358 that changed the operating conditions for the C/VC, CV/XC, and XC/UC boundary nozzles.  S572.3 and ISO 25358 share the same boundary nozzles, allowing for similar implementation of the spray qualities across borders.

Table 1: ASABE S572 spray quality categories and their respective boundary nozzles and operating parameters.

The newest standards changed the boundary nozzle conditions for C/VC, VC/XC, and XC/UC as shown in Table 1. This was done in order to create more equally sized spray quality categories for the coarsest sprays.

How the Standard is Implemented

A laboratory that measures spray qualities would start by running the ASABE reference nozzles on their instrument. They would then calculate the three parameters we described in Part 2 (D_V0.1, D_V0.5, and D_V0.9) and plot them as shown in Figure 2. Next, the lab would atomize various nozzles, pressures, adjuvants, etc. of interest to see where their parameters fall with regard to the reference nozzles and make a spray quality designation. In the example we’ve been showing, the Wilger SR nozzle is in the “Coarse” category using this approach.

Figure 2: ASABE S572.2  reference nozzles showing spray quality boundaries. Note that the example Wilger SR 11005 tip falls between the Medium/Coarse and the Coarse/Very Coarse border nozzles, making the spray quality “Coarse”.

Nozzle manufacturers have a responsibility to publish the spray qualities of their nozzles, and they do so on their web sites or catalogues. This makes it much easier for applicators to select the right nozzle for the job. With spray quality, the nozzle brand becomes less important. In fact, nozzles that produce the same spray quality can be expected to have very similar performance. Various nozzle models still have important distinguishing features, such as pressure range, fan angle, pattern tilt, etc. But a 110 degree air-induced nozzle that produces a “Coarse” spray quality should perform very similarly to a different manufacturer’s 110 degree air-induced nozzle that also produces a “Coarse” spray quality.

Figure 3: Spray qualities of the TeeJet AIXR nozzles according to standard ASABE S572.2. Note the sensitivity of spray quality to both spray pressure and nozzle size. Source: TeeJet catalog 51A.

Note that the spray quality categories are quite broad. Within any given spray quality range, we might see a two-fold range of drift potential, for example. It remains important to have a sense of where one is operating withing the spray quality range.

As these standards change, there are implications for how they can be used. An important change occurred with the latest version of the spray quality standard, and that will be discussed in Part 4.