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Control more with more control: Pulse-width modulation

Agriculture.com Staff 07/07/2010 @ 9:10am

In the January 2004 Illinois Pesticide Review, I wrote an article about spray-droplet size measurement and classification. That article provides important background information for the subject of this article. Please refer to that issue if you are unfamiliar with spray-droplet classification.

Pesticides labels will eventually require specific droplet sizes for applications, meaning the applicator will have to control the droplet size category created by the sprayer during applications. When selecting your nozzle size based on the flow rate (in gallons per minute) needed to get the correct gallons per acre (gpa), you will also need to select a nozzle type, size, and operating pressure to create the droplet size category required by the label. The nozzle will then have to be operated within a pressure range that maintains that droplet size category.

What happens to spray-droplet size if pressure increases? Droplet size decreases. The opposite is also true. If pressure decreases, then droplet size increases. During an application, if you increase the pressure too much, your droplet size category will decrease. Decrease the pressure too much, and your droplet size category will increase. In either situation, you will be making an application with the wrong droplet size category, thus, you will be going off label.

This brings us to the subject of spray rate controllers. Spray rate controllers maintain a constant gpa as you change speeds. If we think about the three factors that determine the gpa, which are nozzle flow rate, ground speed, and effective sprayed width (nozzle spacing), we realize that if we change the speed, the rate controller has to adjust one of the other two factors to keep the gpa constant. Because there is no way for the controller to change nozzle spacing, it has to change the nozzle flow rate. There are two ways to change the nozzle flow rate: operating pressure or orifice size. The controller has no way of switching your nozzles for you, so it adjusts the pressure.

Here is a scenario to consider, keeping in mind the calibration equations (I'm sure you have these memorized). Let's say you're making an application for which the primary speed is 8 mph with nozzles spaced 20 inches apart. The label requires a spray application rate of 10 gpa and a very coarse or coarse droplet size category. You are using TT11003 nozzles, which at a pressure of 32 pounds per square inch (psi) give the required flow rate of 0.27 gallons per minute (gpm). You are using a spray rate controller. At times during the application, the speed is increased to 10 mph. At this speed, the flow rate needs to be increased to 0.34 gpm to maintain 10 gpa. To increase the flow rate from the TT11003 nozzle, pressure must be increased to 51 psi. This pressure changes the droplet size category from coarse to medium. You are now off label because the droplet size is too small.

Is there any way to maintain a constant gpa while changing speeds without changing the droplet size category? Technology called pulse-width modulation (PWM) can provide this control.

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