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The Mathematics of Reducing Drift: Part 2
by Lars Skjoldager Sorensen
• To read Part 1 of Lars' article, click here.
Last month, we investigated the concept of "the spray account" and how you can use it to reduce drift. This month, we take a closer look at individual factors that influence your ability to control drift.
When making a determined effort to control drift (and, at the same time, controlling costs) numerous factors require your attention, including droplet size, airflow control, fan performance, reducing out-of-row spraying, weather and equipment.
Droplet Size
A spray nozzle produces a range of drop sizes. The nozzle can be characterized by the average drop size based on volume (Volume Median Diameter, or, VMD).
A high VMD will dramatically reduce drift; controlling VMD is the easiest way to reduce drift. If the drop size is too big, however, your coverage will be negatively affected. The reduction of drop diameter has a dramatic effect on the number of drops produced. Each time we halve the diameter, 8 times the drops are produced.
Halving one 400 mm drop produces 8 drops of 200 mm. With these smaller drops, it is possible to cover about twice the area.
It is important to be aware of this delicate balance between drift and coverage when choosing the right droplet size. Typically, the best strategy is to find a compromise. The chemical label may give some indication. If not, the following have been traditionally been quoted:
Insecticide 200-300 mm with 130-200 drop/in²
Fungicide 100-250 mm with 320-450 drop/in²
The chemical products mode of action (systemic or contact) and whether the pest is mobile, static or concealed are other issues influencing choice of drop size.
Airflow Direction
External deflectors or spouts are very efficient ways to reduce drift. They help reduce the risk of droplets missing the target. If 2 or more deflectors or spouts are directed to the same location, they increase penetration in denser parts of the canopy. Whenever possible, deflectors should be set up to allow for a downward trajectory in an effort to reduce drift further.
Matching Fan Performance to Crop and Forward Speed
The job of the fan on any vineyard sprayer is to replace the air in the canopy with new air carrying droplets. The sprayer's ability to do this depends on 2 factors: The performance of the fan and the forward speed of the sprayer. The performance of the fan is measured in CFM, not in size or airspeed.
A parallel can be drawn from filling glasses with water from a hose; the water flow represents airflow from the fan. Move the hose too fast and you will not fill the glasses. Move too slow and you will spill the water.
The same will happen if you drive too fast or your fan is too small. You will not fill the canopy with spray-laden air. If you drive too slowly or your fan is too big, you will create drift.
The challenge if to find the fan performance that minimizes the risk of drift without sacrificing penetration.
The following formula can be used to estimate the correct size of fan:
86 x speed (MPH) x row width (ft) x canopy height (ft)
CFM
3 (factor)
| = |
approximate fan size |
(For an open canopy use factor 3.5; for a medium canopy, 3.0; for a dense canopy, 2.5.)
Remember to calculate a "worst case scenario" when choosing the right size fan.
Reduce Out-of-Row Drift
When turning the sprayer at the end of the row it is very important that all nozzles are completely turned off. Since there is no canopy present, most spray leaving the nozzles drifts. Make sure that the sprayer is equipped with valves allowing for fast and easy operation. Also make sure that the sprayer is equipped with non-drip valves at each nozzle. This will ensure that the nozzle completely shuts off every time the control is in the off position.
Assessing Good Spraying Weather
Avoid spraying on windy days. Wind drift results in losses of sprayed chemical and decreases the uniformity of distribution. It may surprise you, but you should also avoid spraying on windless days; you don't get enough circulation of spray around the target.
Temperature will influence the effect of certain chemicals so check the label for notes on upper and lower limits. Avoid spraying on hot days, as high temperature increases evaporation.
Differences in air temperature may lead to an inversion; this condition reduces deposition of the smaller drops and increase the risk of drift. Fog or early morning mist are the results of inversion, so resist spraying when these conditions prevail.
Perfect spray conditions include days that have a slight breeze (a wind speed 2-4 mile per hour is optimal) and that have a temperature range from cool to warm.
Selecting the Right Equipment
When controlling drift is your aim, having the right tool is half the job. When updating or replacing your sprayer, make sure the following features are available:
- Control of droplet size. Make sure that you can control droplet size by replacing nozzles and changing pressure.
- Correct fan size. Make sure that the airflow from the fan matches you crop, remember that airflow not air speed is important.
- Air adjustment. The more control you have over the air the better chance you will have to reduce drift.
- Sprayer controls. When the sprayer is turned off it is important that all nozzles stops spraying immediately. Make sure that each nozzle have a non drip valve.
This drift control information applies to all sprayers, and Rainbow hopes you find it helpful.
• To read Part 1 of Lars' article, click here.
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