Calibrating Turfgrass Chemical Application Equipment

Bulletin 817-00

Calibrating Liquid Application Equipment:
Low-pressure boom sprayers

Low-pressure boom sprayers are used frequently for applying chemicals on large areas such as golf courses and recreational areas. Calibrating a boom sprayer is not as difficult as it sounds. Although there are many methods to use, the two methods described here are the most common. Choose a method that you are comfortable with and calibrate your sprayer often, but at least once a year.

For safety, use water instead of actual chemical mixtures when calibrating. However, some carriers, such as liquid fertilizers, are much denser than water and may cause the nozzle flow rate to vary from the rate obtained with water. In this case, determine the average nozzle output with water and use a conversion factor that is usually listed in the nozzle manufacturer’s catalog.

Method I

This method is simple and requires few calculations. It is based on the spraying distance and the time needed for a nozzle to spray 1/128 of an acre. Because there are 128 ounces of liquid in one gallon, the ounces of liquid caught from one nozzle is directly equal to the application rate in gallons per acre, or GPA.

For example: If you catch an average of 80 ounces from a set of nozzles, the actual application rate of the sprayer is equal to 80 GPA. With this method, make sure that the time used to catch output from nozzles is the same as the time it takes to cover 1/128 acre. Table 1 shows the distance you must travel to cover 1/128 acre for different nozzle spacings.

Fig 3. Equipment needed to calibrate a sprayer.

To calibrate your sprayer, you need a measuring tape, a stopwatch or an ordinary watch with a second hand, and a measuring jar graduated in ounces (Fig. 3). A pocket calculator also will be handy. Follow these steps when calibrating boom sprayers for broadcast application:

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 (Fig. 4). Then measure an appropriate distance in the field based on this nozzle spacing, as shown in Table 1.
   
   
4. Drive through the measured distance in the field (Fig. 5) at your normal spraying speed, and record the travel time in seconds. You do not need to spray while traveling. 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 (Fig. 6) 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 higher or lower than the average nozzle output rate, check for clogs and clean the tip, or replace the nozzle with a new tip.
   
   
7. Repeat steps 5 and 6 until the variation in discharge rate for all nozzles is within 10 percent of the average.
   
   
8. Then, the final average output in ounces is equal to the application rate in gallons per acre:

   Average output (ounces) = Application rate (GPA)

9. If needed, convert GPA to gallons per 1,000 square feet or ounces per 1,000 square feet.

   0.023 x GPA = Gal/1,000 sq ft
   2.94 x GPA = Oz/1,000 sq ft

10. Compare the actual application rate with the recommended or intended rate. If the actual rate is more than 5 percent higher or lower than the recommended or intended rate, you must make adjustments in pressure, travel speed and/or nozzle size.
    
    
11. You can start the adjustments by changing the pressure. Lowering the spray pressure will reduce the spray delivered; higher pressure means more spray is delivered. Don’t vary from the pressure range recommended for your nozzles. (Refer to "Useful Formulas" to determine the new pressure rate.)
    
    
12. You also can correct the application error by changing the actual travel speed. Slower speeds mean more spray is delivered; faster speeds mean less spray is delivered. (Refer to "Useful Formulas" to determine the new travel speed.)
    
    
13. If these changes don’t bring the application rate to the desired rate, you may have to select a new set of nozzles with smaller or larger orifices.
    
    
14. Recalibrate the sprayer (repeat steps 5 through 12) after any adjustment.

As with any calibration procedure, this second method requires you to check the nozzle flow rate and the actual ground speed of the sprayer. Follow these steps:

Method II

Step 1. Determine the Effective Swath Width (W) per Nozzle

For boom spraying, the effective spray width of each nozzle (W) is equal to the distance in inches between two nozzles.

Fig. 4. Measruing the distance between nozles.

Step 2: Determine Travel Speed (MPH)

To determine the travel speed, measure a known distance (Fig. 5). Use fence posts or flags to identify this distance. A distance over 200 feet and a tank at least half full are recommended. Travel the distance determined at your normal spraying speed and record the elapsed time in seconds. Repeat this step and take the average of the two measurements. Use the following equation to determine the travel speed in miles per hour:

(0.68 is a constant to convert feet/second to miles/hour)

Example:

After measuring a 250-foot distance, you traveled this course and it took 24 seconds for the first pass and 26 seconds for the second pass. Find the travel speed in miles per hour.

Fig. 5. A distance of at least 200 feet is recommended to determine travel speed.

Step 3. Determine Nozzle Flow Rate (GPM)

With the sprayer parked, run the sprayer at the same pressure level and catch the output from each nozzle (Fig. 6) in a measuring jar for one minute (or collect output for half a minute and then double the ounces collected) to determine the nozzle flow rate in ounces per minute (OPM). Check for clogs and clean the tip or replace any nozzle tips having flow rates 10 percent more or less than the average of the other nozzles checked, and/or having obviously different fan angles or patterns. Repeat the above process until the variation in discharge rate for all nozzles is within 10 percent of the average.

Then, convert the final average output in OPM to gallons per minute (GPM) using the following equation:

GPM = OPM/128 (1 Gallon = 128 ounces)

Fig. 6. Use a cup graduated in ounces to measure nozzle flow rate.

Step 4. Determine the Actual Application Rate in Gallons per Acre (GPA)

Use the following equation to determine the gallons per acre application rate:

GPM: average nozzle flow rate in gallons per minute
MPH: travel speed in miles per hour
W: distance between two nozzles in inches
5,940: a constant to convert units to gallons/acre

Step 5. Compare the Actual Application Rate with the Recommended or Intended Rate

If the actual rate is more than 5 percent higher or lower than the recommended or intended rate, you must make adjustments in either the pressure or the travel speed or in both. If these changes don’t bring the application rate to the desired range, then you may have to select a new set of nozzles with smaller or larger openings. (The intended application rate may be given in gallons or ounces per 1,000 sq ft. Refer to "Useful Formulas" to convert these rates to gallons per acre and to determine the new travel speed and/or the pressure rate.)

Step 6. Recalibrate the Sprayer

Repeat steps 2 through 5 above until the application error is within 5 percent of the intended application rate.

Example:

You want to spray a golf course fairway at a rate of 1.25 gallons per 1,000 sq ft with a boom sprayer. The sprayer is traveling at 5 mph and has a nozzle spacing of 20 inches. The average flow rate of the nozzles is 100 ounces per minute at a pressure of 30 psi. What is the actual application rate and the application error of this sprayer?

First, you need to convert the flow rate of nozzles from ounces per minute to gallons per minute:

To find the application error, you need to convert 46.3 GPA to gallons per 1,000 sq ft.

Intended rate: 1.25 gal per 1,000 sq ft

Actual rate: 1.06 gal per 1,000 sq ft

How to Eliminate the Application Error

The sprayer is applying 15.2 percent less than the intended rate. Adjustments are necessary because errors over 5 percent are not acceptable. You can reduce the application error below 5 percent by trial and error. However, to save time, use the following equations to determine the new travel speed and the pressure required to increase the flow rate from 1.06 to 1.25 gal. per 1,000 sq ft.

To correct your application error, either reduce your travel speed from 5 mph to 4.24 mph, or increase the sprayer operating pressure from 30 psi to 41.7 psi.

Other Consideration

Here are some other things you should keep in mind when using a boom sprayer:

• Effective pest control is more than getting the right amount of chemicals on the ground. How the chemical is deposited on the spray target is as important as the amount deposited (Fig. 7). Make sure all nozzle tips are properly aligned. Some nozzles require overlapping adjacent spray patterns. Check the nozzle catalog to determine the overlap required for a given type of nozzle, and adjust the boom height accordingly. There are some simple and practical ways to examine the spray pattern of the sprayer boom. Special portable spray patternator tables can be used to see the uniformity of spray distribution. If a spray table is not available, add non-staining colorant to the spray mixture and spray over a test area. You should see the color on the ground the same from one end of the boom to the other.
  
  

  Fig. 7. Common application errors that cause nonuniform coverage.

  
  
• A common cause of non-uniform coverage is clogged nozzles. To detect clogging, watch the nozzles periodically while spraying. Always carry extra nozzles in your tool box, and immediately replace bad nozzles with good ones. Never blow through nozzles to clear a clog.
  
  
• In most cases, the pressure gauges on sprayers do not represent the actual pressure at the nozzle tip. Therefore, check the pressure at the nozzle tip when calibrating your sprayer.

The flow rate of nozzles, especially those made from brass, increases as they become worn. Therefore, calibrate your sprayer as often as possible.

Always look for obvious things that may indicate your equipment is not working properly. For instance, if it appears that you have obviously too much or too little spray mixture remaining after spraying an area, stop immediately. Check your measurement of the area and make sure all vital parts of the sprayer are functioning properly.

Use quick bucket checks periodically. Always carry a marked container that will collect the correct amount of spray in 10 to 30 seconds. This quick check can also be used for other liquid application equipment such as hand guns and hand pumps.