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The Deposition Triangle

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Figure 1. The amount of spray applied to a canopy is affected by operating pressure, tractor speed, and nozzle selection. This trilogy can be thought of as a deposition triangle where all of them affect each other and sprayer output.

Written by: Gwen-Alyn Hoheisel, WSU Extension Regional Specialist, Benton County, WA

Now is the time to prepare your sprayers for the spring. The greatest return on your investment is to 1) change your pressure gauge, 2) change your nozzles, and 3) check your speed.  These three items cost virtually nothing but have huge impact on the output (GPA) applied. They are the deposition triangle (Fig 1). The pressure gauge is the only device telling you the psi at application.  Pressure determines the amount of liquid output and droplet size.  The higher the pressure, the more gallons per minute per nozzle and the finer the droplet. If the pressure gauge is not accurately reading the pressure, the sprayer could be delivering 10-20% more product. Nozzles that are worn not only put out more than intended but change the pattern. In my surveys of calibrating sprayers, worn nozzles have put out more than 40%, but on average 20%. Lastly, we intuitively know that speed effects output. Think of a block with large pear trees in which we might drive 2 mph to achieve 200 GPA as opposed to a trellised central leader apple block in which we drive 3 mph and lower 100 GPA. However, how we measure speed is analogous to the pressure gauge; if the tool is bad, so is the assessment. Relying on the tractor speedometer or the rate controller is not advised without confirming manually that they are accurate. Rate controllers can be off because of poor tire pressure (wheel sensor models), sensor location (gps models), or constants in the calculation of speed. A manual check may show that the speed is off as much as 0.5 mph, which depending on GPA can apply 15% more than intended.

To examine the return on the investment (ROI), let’s consider a pesticide spray that costs $60/acre and the percent overapplication that can occur if the equipment is not functioning properly.

Cost /50 acre block if…
Cost of item % over application applied correctly over application ROI
Pressure Gauge $20 10% $3000 $3300 1500%
Nozzle (20/sprayer) $130 20% $3000 $3600 460%
Speed Check (only 1hr of labor) $15 15% $3000 $3450 3000%


The return on investments are quite large because the initial investments are low. These savings could be even larger when considering a sprayer is used over multiple 50-acre blocks and many applications cost more than $60/acre.  Now is the time to change to new ceramic nozzles, new pressure gauges, and check the speed.

GAHoheiselGwen-Alyn Hoheisel
WSU Extension Regional Specialist
Benton County
Prosser office: 509-786-5609
Kennewick office: 509-735-3551



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