Recall the PWM replaces spray pressure with Duty Cycle (DC) of a pulsing solenoid as the primary means of controlling nozzle flow. The Duty Cycle is defined as the proportion of time that the solenoid is open, and for low-frequency systems, DC is more or less linearly related to flow rate.
The first rule of PWM nozzle selection is to understand that under average travel speeds, we’d like to see the duty cycle of the system at between 60 and 80%. This means that the nozzle solenoid is open about 2/3 of the time. This value also describes the flow rate as a proportion of the full capacity that nozzle.
The reason for this 2/3 duty cycle rule is to enable four key features of PWM:
- It’s ideal for turn compensation, allowing the outer nozzles to increase their flow 20 to 40%, and the inner nozzles to decrease flow about three-fold, in accordance with boom speed.
- It allows speed flexibility, providing some additional speed, but more importantly, reduced speeds should conditions require it, without a change in spray pressure.
- It compensates for pressure changes so that spray quality can be adjusted without requiring a speed change. Less pressure reduces nozzle flow, and increasing DC recoups accordingly.
- It allows for customized higher flows of certain nozzles, perhaps behind wheels, to address reduced deposition in their aerodynamic wake (available on some PWM systems).
The best tool for selecting the right nozzle size is Wilger’s Tip Wizard. This site asks for your desired average speed ( although it calls this “Max Sprayer Speed”), and reports the expected DC for a host of nozzle size solutions and pressures. It also reports maximum and minimum travel speeds and other useful information such as spray quality.
Although intended for Wilger nozzles, the site’s sizing feature works for any nozzle brand. It asks the user which PWM system they have for the purpose of calculating the documented pressure drop across the solenoid.
If you don’t have access to the site, a basic calibration chart can still work with a simple trick. Recall that we use the top row to identify the desired water volume, and the table’s interior values are speeds, as described here.
Below are two solutions for someone wanting to apply 10 gpa at 15 mph without PWM. The correct choice depends on the required pressure to produce the needed spray quality.
If you want to apply the same 10 US gpa using PWM, simply solve for a larger volume that offers the right DC. For example, choosing 13 gpa will over-apply by 3 gpa, or 30%. The PWM system adjusts by running at 100-30=70% DC. If the chart doesn’t offer 13 gpa, go nearby, to 14 gpa, as we did below:
Now solve for the same target speed, 15 mph. The solution will run at 60% DC. Again, there is more than one choice, and that will depend on the spray pressure needed.
We’ve developed a template, in US or metric units, that can be customized for any water volume. Here is the same chart with 13 gpa added:
The best solution for 10 gpa at 15 mph is the 06 size nozzle at 50 psi. This is not engraved in stone. One of the nice things about PWM is that it has inherent flexibility. Make the nozzle pressure a priority to get the correct spray quality. It really doesn’t matter whether the resulting DC is 65 or 80%, the system will still work well. Simply avoid extremes that take you below 50% or above 90%, they will limit the system’s capabilities.
The worksheet can be downloaded here. It can handle any water volume or nozzle spacing by filling in the blue cells. Two additional worksheets in the file automate the process, simply enter the desired application volume, travel speed, and nozzle spacing (yellow cells), and the solution that offers the optimal duty cycle range will be highlighted in light green.