By

Dr. Erdal Ozkan

Pesticide Application Technology Specialist

Extension Agricultural Engineer

The Ohio State University

ozkan.2@osu.edu

Have you calibrated your sprayer this year before the spraying season started? If yes, great! If not, there is still time to do that. You may ask: Why do I need to calibrate my sprayer? The answer is simple: If you don’t calibrate your sprayer, it’s as if you were driving your car with a speedometer that doesn’t work. You assume you know what speed you are traveling at from habit, but you are not really sure. The problem with a sprayer is that nozzles may be plugged or worn out, and application rates change with different field conditions which affect traveling speeds. Many growers don’t take these factors into account.

Applying pesticides at the proper rate is essential to achieving satisfactory weed, disease, and insect control. The directions on the container label tell what application rates give the best results. However, proper application rates will be attained only if sprayers work well and are calibrated correctly.

Calibrating a boom sprayer is not as difficult as it sounds. It usually doesn’t take more than 30 minutes, and only three things are needed: a watch (or smart phone) showing seconds, a measuring tape and a jar those measures in ounces. The ultimate goal is to calculate the actual rate of application in gallons per acre to check for accuracy, or make adjustments as needed

Here is an easy method to calibrate a boom sprayer:

There are several ways to determine the actual application rate, but the one outlined below is perhaps the easiest, and do not require difficult calculations:

1. Fill the sprayer tank with water.
2. Run the sprayer, inspect it for leaks, and make sure all vital parts function properly.
3. Measure the distance in inches between the nozzles. Then measure an appropriate distance in the field based on this nozzle spacing. A table available at http://ohioline.osu.edu/factsheet/fabe-520 shows various nozzle and row spacings and the distance you must travel. For example, the travel distance for a 15-inch nozzle or row spacing is 272 feet, for 20-inch nozzle or row spacing is 204 feet; for a 30-inch nozzle or row spacing, the distance is 136 feet.
4. Drive through the measured distance in the field at your normal spraying speed, and record the travel time in seconds. Repeat this procedure and average the two measurements.
5. With the sprayer parked, run the sprayer at the same pressure level and catch the output from each nozzle in a measuring jar for the travel time required in step 4.
6. Calculate the average nozzle output by adding the individual outputs and then dividing by the number of nozzles tested. If an individual sample collected is more than 10 percent lower than the flow rate of the same nozzle when new (check the nozzle catalogs or web site) check for clogs and clean the tip. If this does not help, then you should replace the nozzle. If an individual sample collected is more than 10 percent higher than the flow rate of the same nozzle when new, this indicates the nozzle orifice is worn out. In this case, replacement of the nozzle is needed.
7. Repeat steps 5 and 6 until the variation in discharge rate for all nozzles is within 10 percent of the nozzles when they were new.
8. The final average output in ounces you get is equal to the application rate in gallons per acre.
9. Make adjustments if necessary. Compare the actual application rate determined above resulting from the calibration process with the intended application rate.  If the difference between the calculated actual rate and the intended rate is more than 5% of the desired rate, you need to make adjustments to bring the error below 5%. You can start by changing the pressure. Lowering the spray pressure will reduce the spray delivered; higher pressure means more spray delivered.  Do not exceed the pressure rate recommended for the nozzles when adjusting the pressure rate. Remember that changes in pressure will result in changes in droplet size. So, don’t go to extreme pressure settings just to reach the desired output from nozzles. You can also correct the application error by changing the travel speed.  Slower speeds mean more spray delivered, faster speeds mean less spray delivered.  If changes in either pressure or travel speed, or both do not bring the application rate to the desired rate, then you may have to select a new set of nozzles with smaller or larger orifices. Learn how to make these adjustments and additional information about calibration at http://ohioline.osu.edu/factsheet/fabe-520
10. Recalibrate the sprayer often. Calibrating a sprayer once a year is not enough.  It should be calibrated several times throughout the season to compensate for wear in pumps, nozzles, and metering system; and when spraying in a different farm than the one where the previous calibration was made. Changes in the soil surface characteristics and topography cause changes in travel speed which will directly affect the application rate.

In summary, properly maintained spraying equipment is critical to pest control and user safety.  A properly calibrated sprayer saves you thousands of dollars in chemical cost; improves yield; reduces the chance of damage to your crop as a result of over application and potential risk of contamination of the environment with pesticides. Safety is extremely important.  Use water instead of chemical mixtures when calibrating your sprayer, and wear gloves and protective clothing.