Honey, don’t shrink the droplets too much.
That’s one piece of advice from spraying expert Gary Moffat, who urged attendees at a recent seminar to think small – but not too small – when they take their sprayers on the field.
“It is droplet density that is most important for control,” Moffat said at his seminar at the Northlands Farm and Ranch Show.
“The most important factor in spraying is understanding droplets.”
The goal of spraying is to apply a product in a way that is effective, efficient and environmentally friendly, said Moffat, owner of Specialized Spray Systems in Lethbridge. The number of droplets per square centimetre can vary considerably, and you’re wasting money if droplets, which are measured in microns, are too big or too small.
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“An average human hair is 100 microns in size and a staple used to hold paper together is 420 microns in size,” Moffat said.
“Droplet sizes below 200 microns do not have enough weight to be pulled to the target by gravity, and increase the potential for drift and evaporation. Experts say that droplets between 220 microns up to 600 microns are the ones that have enough weight to be carried to the target.”
But droplets that are too large will be so heavy they will run off the plant before having any effect. Droplets can also be measured by their median volume diameter or MVD, and you want to have droplets that are similar in size.
When you switch to a nozzle that cuts a droplet’s diameter in half, it will actually split it into eight droplets, Moffat said. These smaller droplets can cover twice the surface area of a large one.
The newest technology on the market is the air-induction nozzle, which has been on the market for nearly two decades. These nozzles greatly reduce drift, and emit fewer than two per cent droplets under 200 microns.
Air-induction tips result in 10 per cent more surface area covered when spraying, but not all air-induction nozzles are the same, said Moffat.
“Each manufacturer produces varying droplet spectrums,” he said.
New standard
The ASAE standard, which measures and defines sprays, was created about four years ago. Prior to this, the British standard chart was used, so there’s still a lot of confusion between the two. According to the ASAE standard, fine sprays are between 100 and 250 microns, medium sprays are between 250 and 400 microns, coarse sprays are between 400 and 600 microns and very coarse sprays are between 600 and 900 microns. Any spray with a micron range of between 250 and 400 is preferable and gives the most coverage, said Moffat. Run-off can occur when droplets are between 400 to 900 microns.
Moffat uses a program called Droplet Scan to test sprayers used by his clients.
“This is the first program where you can actually measure droplet sizes in the field,” he said.
It’s actually a simple system which uses water-sensitive paper and a computer program to track droplets and measure sprayer coverage. Knowing what comes out of the nozzle allows you to improve performance, said Moffat.
“First we start the sprayer up and see how it’s spraying. Then we hang a set of jugs on there and measure the flows,” he said.
By measuring the flows, he can see whether or not they are uneven and if the nozzles need replacing. The water-sensitive cards are then placed on markers and the sprayer is run in the area, so droplets and coverage can be monitored.
Factors that affect droplet size include speed, turbulence, dispersal orientation (or angle of the nozzle), rates, pressures, temperature, humidity and additives, he said. Ideally, nozzles should be facing 15 degrees back in order to gain a five per cent increase in coverage. Air-induction tips are good for reducing pressure.
“A low-pressure air-induction nozzle reduces pressure from top to bottom,” said Moffat. “A medium-pressure air-induction nozzle reduces it by 50 per cent and a high-pressure air-induction nozzle, which you won’t even find on the market today, reduces it by 75 per cent.”
Temperature and humidity also affect spraying. High humidity and temperatures above 20 C are best for spraying.
“Make sure the humidity is up if it’s that hot,” said Moffat. “If it’s hot and dry, it might be better to keep the sprayer in the shed.”
He said he has done a lot of testing with additives, but isn’t convinced they are worthwhile.
