What is Spray Quality, Part 4 – An Important Change in S572.3 and ISO25358

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. Lechler and Arag are both reporting using ISO 25358. But Agrotop and 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.