The job of selecting the right nozzle is easier for researchers than for farmers. Researchers have special tools like high-speed photography with laser lighting.
Their imaging technology lets them count and measure the diameter and speed of individual droplets in the pattern. Every tiny adjustment at the tip is documented and studied.
If you’ve ever wondered how valid the published nozzle recommendations are, take a look at the images the experts use. While producers may not have direct access to imaging technology, they certainly take full advantage of this information each time they study a nozzle chart.
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But researchers also use a low-tech method of documenting droplet size on water sensitive paper.
For about $1 per sheet, producers can buy the blotters and set them on the ground under the nozzle to get an accurate picture of how their sprayers and nozzles perform in the real world.
The paper is placed on the ground and a single sprayer pass is made. Every droplet that hits the paper turns the spot blue. It doesn’t work in high humidity and also tends to get saturated at volumes above 10 US gallons per acre because droplets overlap.
Tom Wolf, research scientist with Agriculture Canada, says the cards are valuable to anyone who wants a visual assessment of spray coverage.
“It’s especially recommended for applicators who are making a change to a new nozzle or spray volume, so they can compare the new spray technique to what they had before.
“I always tell chemical companies that instead of their usual promo material, they should give farmers these cards.”
Wolf says water sensitive paper is available from Spraying Systems Co. or through distributors like Sprayer Parts Warehouse or Even Spray.
He says the cards have become more relevant because the industry now uses a standardized system to describe droplet size from very fine to extremely coarse.
Water sensitive paper shows the category the spray droplets fall into when they hit the target. If a certain nozzle doesn’t perform as it should on the sprayer, it might be time to go back to the catalogue.
As tips evolve and new nozzle concepts come to the market, producers are faced with a growing list of choices, spray patterns, droplet size and droplet speed.
Wolf says the tip trend is driven by faster ground speeds and the associated requirement for higher volumes to go with those speeds. At the same time, there is increasing pressure to reduce drift for legal, environmental, efficacy and cost reasons.
“When sprayers were primarily pull type, average travel speeds were five to 10 m.p.h.,” said Wolf. “This speed did not need to change in the course of spraying a field.
“Today’s sprayers travel at faster speeds. A sprayer going 15 m.p.h. encounters conditions where it needs to slow down for turns and going around or over obstacles.
“Because it is well suspended and powerful, such a sprayer may also speed up when time pressure is greater. As a result, nozzles experience a range of pressures and they may not operate at their optimum at all these pressures.”
Wolf says conducting quality assurance with water sensitive cards is a good idea when changing to a new application system. Compare the droplet deposit to this plate that shows droplet sizes (figure 1.)
“Aim for the middle. The coverage at eight (US gallons per acre) and a coarse spray is ideal for most applications.”
Fine droplets that are much smaller are more likely to cover the target, but they are also more likely to drift.
Large droplets don’t drift, but they concentrate too much product in one shot. If it misses, the droplet is wasted and the target gets away.
Water sensitive paper can come into play here. Once all the checks are done on the sprayer, producers can make the adjustments they expect to use in the field and use the paper to do a realistic check on the volume median diameter (VMD) they will be putting out.
As an example, Wolf says an extremely coarse spray with a 550 um (micrometre) VDM has low drift risk, but it also has low coverage because of the large droplet size. He says that’s why it’s not recommended for many applications (see figure 2.)
At the other extreme, a fine nozzle with a 150 um VDM has plenty of droplets for good coverage, but at the expense of a high drift risk and fast evaporation. For those reasons, it is also not recommended.
For grasses and broadleaf weeds, Wolf recommends medium to coarse spray qualities. Medium sprays are 250 to 350 um VMD. Coarse nozzles are 350 to 450 um VMD.
“The water sensitive paper sample chart shows the features of various spray qualities. Because sprays that are either too fine or too coarse are not suitable for specific tasks, coarse has become the modern standard,” Wolf says.
“If you can manage drift, medium sprays can be used. The majority of the spray volume should be in intermediate sized droplets between 200 um and 500 um.”
For more information, contact Tom Wolf at 306-956-7635 or email tom.wolf@agr.gc.ca.
In next week’s paper, Wolf will review some of the low drift nozzles designed to meet the need for high speed, high pressure and high volume.
