Boom Sprayers Calibration

By Tim Smith, WSU Extension,
400 Washington Street,
Wenatchee, WA 98801
PRINTER-FRIENDLY VERSION (PDF FILE)

After harvest, but before leaves fall, is the best time of season to apply soil residual herbicides. All boom sprayers need a calibration and adjustment check prior to use, even if they were operating correctly the last time they were used. The following procedure is quick, easier than it looks, and will ensure an even application of a precise rate of product per acre:

EQUIPMENT NEEDED: A wide-mouthed container for catching spray, a tall narrow container calibrated in ounces for measuring fluids, a tape measure and a calculator.

STEP 1:

Clean out the sprayers’ tank, remove all the nozzle tips and strainers and flush out the boom. Use a soft toothbrush to clean the nozzles and strainers. Check to see that the tips are all the same size, and strainers are the same style and mesh (100 or 50). After rinse water is clean, reassemble the boom.

STEP 2:

Partially fill the sprayer tank with clean water. Run the sprayer for a few minutes, adjusting the pressure to your preferred PSI. (30 – 35 PSI is plenty high). If your pump is “pulsing” you may need to pressurize the diaphram a few pounds more.  Observe the spray pattern at the level of the target (weed height or soil height).

Are the flat fan spray patterns overlapped so that about 1/3 of each fan pattern overlaps with the spray from the neighboring nozzle? If not, adjust the HEIGHT of the boom until the proper overlap is achieved. If the boom is either too high or too low, the rate of application per acre will be uneven across the spray swath, even though it may look O.K. Measure the WIDTH of the swath the sprayer is covering. Measure from the edge of the spray pattern near the tractor to a point about 2/3 across the final (boom end) nozzles’ spray pattern. (Usually about 4 to 6 inches in from the end of the spray pattern). Record this in feet and tenths of feet.

EXAMPLE: 3 FEET 7 INCHES = 3.6 FEET.

(ARITHMETIC TIP: Divide total inches across the spray width by 12 to equal feet and tenths of feet).

STEP 3:

Catch each nozzle output for one minute. Are the nozzles applying a similar rate per minute? Are they putting out about the rate they should, relative to the pressure? (See * below) If not, replace the ones that are putting out more than they should, and reclean those that are putting out too little. Keep up the checking, nozzle replacement, and cleaning until all of the nozzles are similar in their output.

*NOTE: 8002 nozzles should put out 18 ounces per minute at 20 PSI, 20 OZ./min. at 25 PSI, 22 OZ./min. at 30 PSI, 24 OZ./min. at 35 PSI, and 26 OZ./min. at 40 PSI. See a nozzle chart for other “should be” rates/min.

If you are certain your pressure gage is accurate, and the nozzle is putting out 2+ ounces/min. more than the above examples, the nozzle tip should be replaced. They are likely to be excessively worn, wasting product and putting out an uneven pattern.

ARITMATIC TIP:

Bring a calculator, they’re faster than fingers.

STEP 4:

Once you have the boom and nozzles adjusted, total the per minute output for all nozzles. Catch each nozzle for one minute, and total the output. Divide total ounces caught by 128 to determine the gallons applied per minute.

STEP 5:

Estimate the speed you wish to drive. (about 2 to 2.5 MPH is best for even application). Then fill in the following formula:

Multiply 495 by the gallons applied per minute, then divide the result by the tractor speed multiplied by the width of the spray swath.  The result equals the gallons sprayed per surface acre.  (Not “orchard acre, you will probably be spraying 2.5 to 3 orchard acres with each surface acre sprayed.)

XXXXXXXXXXXXXXXXXXXXXX

EXAMPLE:

You caught 102 total ounces = 102 divided by 128 = 0.8 gallons per minute.

Your swath width is 3.6 feet.

You want to drive 2 miles per hour, so…….

STEP 6:

You can now adjust your gallons per acre to your desired amount, such as 50 GPA, by reducing your boom output, increasing your speed, or both.

(ARITHMETIC TIP: How much to drop the ounces per minute? Divide the rate per acre you want by the rate per acre you have calculated from your present set-up. Then multiply that number by the ounces you caught when totaling the boom. The result is the number of total ounces per minute you need to equal the proper rate per acre.)

EXAMPLE:

OUNCES YOU CAUGHT: 102 X 0.91 = 92.8 OUNCES/ MIN FOR 50 GPA.

To change your output per minute slightly, you can raise or lower the boom pressure, but don’t go over 40 PSI or under about 20. In this example, We could lower the boom output by the 10 or so ounces by reducing pressure about 5 or 6 PSI. 5 PSI equals about 2 ounces per minute for each of the four nozzles on our hypothetical boom. If you change your boom pressure more than 5 pounds or so, recheck the nozzle overlap. The nozzle spray angle may have changed, and you may have to readjust the boom height.

Original calibration speed, multiplied by the adjustment factor equals the proper new speed to deliver the desired rate per acre.

2 MILES PER HOUR X 1.1 = 2.2 MILES PER HOUR.

Keeping your original boom pressure, drive at 2.2 MPH for an application rate of 50 gallons per acre.

Now You’re Ready:

Once you get the boom output to fit your desired spraying speed, you may begin application. Hope the wind isn’t blowing by this time. Maybe you should have done the calibration and adjustment the evening before you planned to start the spraying?