By: Dr. Jason S.T. Deveau, Application Technology Specialist, OMAF and MRA

Walking a boom to see if the nozzles are emitting an unbroken pattern is a good habit. This will tell you if the tips are on or off, and may tell you if there is a minor blockage distorting the spray quality. The problem is, unless the tip is emitting more than +/-50% than it’s neighbours, you will not be able to tell. Even then, you need a very good eye to see the difference.

We recently calibrated a trailed ginseng field sprayer. The nozzles on this sprayer are not conventional nozzle tips. They are the assemblies found on the ends of spray guns (see figure 1). Nevertheless, they terminate in a ceramic disc with a 1.5 millimetre oraface, very much like a conventional nozzle.

Figure 1 – Trailed ginseng field sprayer with ARAG Microjet® nozzles

The grower explained that this sprayer applied 100 US gallons / acre. He knew this because of where the tank emptied in his fields. We filled it with water, confirmed that the pressure gauges were accurate by comparing them to a known standard, and brought the boom up to 100 psi.

Each nozzle featured a ball valve that could be adjusted about 20° to make minor changes to the output and the spray pattern. Each appeared to be approximately 10° to the right of centre, and each nozzle appeared to be emiting the same rate… until we performed a timed output test on each of them (see figure 2). Obviously, there were problems.

Figure 2 – Output in US G/min at 100 psi from each of 21 nozzles

The grower had no idea this was happening, because the difference from nozzle to nozzle cancelled each other out. Nevertheless, some portions of the boom were over-applying, and others were under-applying. And, you couldn’t tell without the timed output test.

There are a few different methods for performing a timed output test, but the two easiest (and most accurate) are:

1)      With a digital scale and a pitcher, collect the output from a single nozzle for 30 seconds. Weigh the volume collected in grams using the scale, and subtract the weight of the pitcher. Enter the results into this formula: US Gallons/minute = (2 x weight in grams) / 3,785.4. Do this at least twice.

2)      Use a Spot-On SC-1 or SC-4 calibration tool from Innoquest Inc. (www.innoquestinc.com) which quickly and accurately calculates the output for you.

When nozzle output deviates from what is expected, spray operators have choices, depending on the situation:

1)      If all nozzles are worn by the same amount on a field sprayer, common practice is to reduce pressure to compensate. This is only acceptible when the spray quality is not compromised and should not be done when nozzles exceed 5% more than the ideal. Otherwise the droplet size may change significantly.

2)      If all nozzles are worn on a sprayer, whether by the same amount or not, it is best to replace them all. There are usually no more than 20 and they pay for themselves quickly in saved pesticide and improved coverage.

3)      Nozzles should be replaced as a set. The only time a single nozzle should be replaced is when it deviates significantly from the rest. In this case, it is likely damaged or plugged. It should be cleaned, replaced and re-checked. If it is still off, replace it.

The critical value when you choose to replace them all is up to you, but if you can detect 5% wear on all tips, consider what 5% of your pesticide costs over the season versus the cost of new tips. It is generally worth it.

The take home: Don’t check nozzle output by eye – perform timed output tests.

For more information on sprayer calibration and optimizing coverage, please register for the Ginseng Sprayer Workshop to be held June 19 at 10:00 am. For details click here.