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Written by Margret McCoy, WSU Ph.D. Student, Horticulture/Viticulture
This time of year many growers are exchanging their daily use machinery away from orchard management tools and into harvest equipment. This is also an optimum time of year to change out sprayer components that may be worn or broken before putting this piece of equipment away for the winter. One factor of sprayer maintenance that often gets overlooked or is misunderstood is where pressure is measured within the sprayer. Pressure is one of the more important components when calibrating for applications, but pressure can fluctuate throughout the closed system of a sprayer. Many people equate pressure with the type of pump mounted on a sprayer, but pumps create flow, not pressure. Pressure is actually the resistance to the pump’s flow. This is why different types of pumps can produce different ranges of pressure based on the mechanism of how the pump operates.
When operating the sprayer, it is likely that the gauge is visible from the seat of the tractor. This is a convenient way for the operator to monitor how the sprayer is functioning by watching the gauge for changes. A pressure gauge is an indirect representation of what the pressure is at the nozzle where the tank mix is being applied to your target. The actual pressure at the nozzles could be a much as 10-20% less then what is being displayed on the gauge viewable from the tractor’s seat. The percentage of pressure drop at the nozzles also increases as flow rate increases, so higher application rates can also lead to a reduction of pressure. Making sure the pressure gauge is reading correctly is the first line of defense in avoiding issues with application rate, unwanted change in droplet size, and keeping the machinery in an appropriate working pressure range.
Pressure gauges used on sprayers should always be filled with an oil to reduce the noise, or bouncing, of the needle from the vibration of the machinery so ease of being able to read the pressure. Make sure the gauge can be seen easily or without obstruction, and that it is in the appropriate units for operation. Gauges can be purchased to show the pressure in bars or psi, so make sure to check the units before purchase and to stick with whichever unit is preferred. There is also the decision of how high a gauge should read when being used on a sprayer. It is suggested that gauges go to double the psi that the sprayer will be operating at during an application. The difference between a pressure gauge used on a weed sprayer and an airblast can be a big jump, so make sure the appropriate gauge is on the sprayer when making applications.
Checking a pressure gauge periodically to ensure correct readings is always suggested since gauges can read incorrectly directly out of the box. Any damage to a gauge, including a smashed faceplate, should result in its replacement. If the needle does not return to the zero, this is also an indication for replacement. Without a correct pressure gauge, the sprayer calibration will be incorrect, which results in over or under application of products. Take the few minutes to replace the pressure gauge on a sprayer to keep from being under pressure.
WSU Regional Extension Specialist, Benton Co. Director
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