- Lab droplet measurements varied due to instruments, settings, and methods, creating inconsistent droplet size results across laboratories.
- Reference nozzles let each lab use its equipment while defining spray quality by where measurements fall between agreed boundary nozzles.
- ASABE and ISO standards evolved: S572 series added categories, corrected boundary conditions, and aligned international boundary nozzles by S572.3 and ISO 25358.
- Labs compute DV0.1, DV0.5, DV0.9 against references, label spray quality, and manufacturers publish qualities to aid nozzle selection despite broad category ranges.
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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 (DV0.1, DV0.5, and DV0.9) 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.
