How To: Calibrate a sprayer

Too often, a spray application doesn't work quite as well as you hoped. There are many possible reasons why this might happen, but improper calibration is one of the more likely.

Poor results shouldn't be what finally motivates you to calibrate your sprayer. Calibration should be a regular part of your operations, just as changing the oil should be a regular part of your equipment maintenance. Sprayer output changes over time due to nozzle and pump wear, among other reasons, so it's good to periodically check this.

There's no escaping a bit of math when you calibrate. However, the good news is that the calculations are relatively simple. Several methods of calibration have been devised, but they all accomplish the same result — they help ensure you're putting the right amount of pesticide onto the application site.

You must account for several variables during calibration. Each is important. Calibration of a self-propelled or pull-behind sprayer involves determining sprayer output, sprayer speed and application width, and then using these to determine how much pesticide should be added to the spray tank to achieve the proper rate. Handheld sprayers are calibrated somewhat differently, but with the same goal.

Let's look at how you can achieve an accurate, effective application through proper calibration.

Before you begin…

Initially, you'll need to make sure your sprayer is operating properly, with no leaks, plugged filters or screens, kinked lines or other problems. Therefore, a quick check should include an inspection of lines, fittings, pump seals and all filters to make sure flow is not obstructed and that no leaks are allowing material to escape. An additional step you should take at this point is to make sure all the proper nozzles are installed — it's easy to accidentally insert the wrong nozzle.

It may sound obvious, but perform this inspection with water only! You do not want to discover a problem that needs fixing when you've got a tankful of pesticide.

Calibrating a boom sprayer

One method of calibrating a boom sprayer entails the following steps:

1 Determine output.

To measure sprayer output:

• Fill the sprayer with water (and colorant).

• Run the sprayer pump to operational pressure and turn on the nozzles.

• Hold a measuring container under a nozzle for 1 minute, collecting all of its output.

• Determine the volume and make a record of it.

• Repeat this for each nozzle and add all the values to find the total output.

In reality, most nozzles will have a slightly different output, but the variance shouldn't be too great. This is a good time to ensure that each nozzle is working properly. A frequently used standard is 10 percent variance. Nozzles that emit 10 percent more or less than their design specs should be replaced (manufacturers typically supply nozzle specs that tell you the flow rate to expect at a given pressure). This is important to check periodically because nozzle orifices gradually wear, changing their output.

EXAMPLE OF STEP 1:

Assume that each nozzle puts out 0.5 gallons per minute (gpm). If there are 10 nozzles, then the sprayer's output is 5 gpm.

0.5 gpm × 10 nozzles = 5 gpm total output

2 Determine sprayer coverage.

The next step is to determine how much area your sprayer will cover in a given amount of time. This depends on rate of travel and width of spray coverage.

• Measure the spray width by running the sprayer to operational pressure. Quickly turn the nozzles on and off. Then measure the width of the wetted spray pattern.

• Next, determine your sprayer's speed and coverage. You can do this by running the sprayer at a fixed rpm and gear, and measuring how far it travels in 1 minute.

EXAMPLE OF STEP 2:

Your sprayer travels 200 feet in 1 minute. With a spray width of 15 feet, that means you are spraying 3,000 square feet per minute.

15 ft. × 200 ft. × 1 minute = 3,000 sf/minute

3 Determine application rate (see illustration, previous page).

• Divide output (as determined in Step 1) by coverage (Step 2) to determine application rate.

If you need to determine gallons per acre (gpa) instead (labels vary and may provide application rates in gpa or gal./1,000 sf), multiply gal./1,000 sf by 43 (an acre is roughly 43,000 square feet).

EXAMPLE OF STEP 3:

With an output of 5 gpm, and coverage of 3,000 sf/minute, your application rate is 1.66 gal/1,000 sf.

5 gpm ÷ 3,000 sf = 1.66 gal/1,000 sf

4 Determine dilution or mixing rate. Simply divide the label rate by the application rate of the sprayer to find the dilution rate.

Now, you can determine how much to add to an entire tank. For example, if you have a 200 gallon tank, you know you will need 250 ounces for the whole tankful. (1.25 ounces/gal. × 200 gal. = 250 ounces.)

EXAMPLE OF STEP 4:

2 ounces/1,000 sf [the label rate] ÷ 1.6 gal./1,000 sf [the sprayer's application rate] = 1.25 ounces of pesticide per gallon of water [the dilution rate].

Many lawn applications are made with high-volume handheld or “showerhead” sprayers. An easy method of calibrating such sprayers is as follows:

• Mark off an area of 1,000 sf (10 × 100 feet, or 20 × 50 feet, etc.).

• Fill the sprayer with water and colorant.

• At normal operational pressure, spray the area to achieve complete coverage without over-applying. Use your normal walking speed and application technique (i.e. apply as you would during normal operation).

• Measure how much water you used by refilling the tank. This amount is your application rate in gallons/1,000 sf.

• Divide the product's label rate (in ounces/1,000 sf, for example) by the sprayer's application rate to get the proper dilution rate.

If the dilution rate was 0.6 ounces/gallon, you would load 60 ounces of pesticide into a 100 gallon tank.

CONVERTING TO GPA:

1.6 gal./1,000 sf × 43 = 68.8 gpa (round up to 69)

USING GPA INSTEAD OF OUNCES/1,OOO SF:

A label rate of 8 pounds/acre is divided by the sprayer's application rate of 69 gpa (determined in example in Step 3) for a dilution rate of .116 pounds (1.86 ounces) per gallon of water.

3 ounces/1,000 sf [the label rate] ÷ 5 gallons/1,000 sf [the sprayer's application rate] = .6 ounces of pesticide per gallon of water [dilution rate].

AN ALTERNATE CALIBRATION METHOD

Another method of calibrating a boom sprayer is as follows:

Divide 4,000 by the width of the spray pattern. In our example, the width is 15 feet, so 267 is the result (4,000 ÷ 15 = 267). This should be the length, in feet, of your calibration area.

Fill the sprayer with water and colorant. At operational speed and pressure, run the sprayer for 267 feet. Then refill the tank with water, measuring how much it takes.

Let's say it takes 20 gallons to refill the sprayer. Now divide this by 4 to obtain the sprayer's application rate in gal./1,000 square feet (sf). In this case, it is 5 gal./1,000 sf. (20 ÷ 4 = 5)

Knowing this, you can determine the correct dilution rate by dividing the label rate by the sprayer's application rate. In this case, let's say the label rate is 2 ounces/1,000 sf. Dividing 2 ounces/1,000 sf by 5 gal/1,000 sf = 0.4 ounces per gallon dilution rate. Thus, if you had a 100 gallon tank, you would add 40 ounces to 100 gallons of water (0.4 × 100 = 40).

ALL OUNCES ARE NOT THE SAME

Ever wonder why you measure fluid ounces with a measuring cup and not a scale? Okay, probably not, but it's still worth pointing out that a fluid ounce is not the same as a “regular” ounce. In fact, a fluid ounce actually is a measure of volume, not weight. Don't believe it? Consider that 1 gallon of water weighs 8.33 pounds. That's 133.28 ounces. Yet, there are 128 fluid ounces per gallon, and that doesn't change no matter what type of fluid you're measuring. It's helpful to measure fluids this way because different liquids have different densities. Some pesticide rates are given in gallons, some in fluid ounces. It's not difficult to convert from one to the other if you remember that a gallon is equal to 128 fluid ounces.

But an acre is an acre

Also remember that 1 acre is approximately 43,000 sf (43,560 to be exact). That means that 43 is a handy multiplier (or divisor) for converting rates from “per 1,000 square feet” to “per acre” (or vice versa). So, for example, a rate of 2 ounces/1,000 sf is equal to 86 ounces per acre (2 × 43 = 86). Conversely, 1 gallon (128 fluid ounces) per acre is equal to about 3 fluid ounces/1,000 sf (128 ÷ 43 = 3).

CALIBRATION TIPS

• Constant speed and pressure are important for proper calibration and application. However, a full sprayer may be slower than an empty one, so some experts recommend using a half-full tank when calibrating a sprayer. This should provide a good average between full and empty sprayer speeds.

• Perform your calibration runs on level ground, for the same reason. Hills can cause your speed to vary in ways that may skew your calibration.

• When you make a calibration pass, mark a line at the start and end of the run. Make sure the sprayer is up to speed when you cross the starting line, and continue at that speed past the ending mark. Simply turn the nozzles on and off as you pass the starting and ending points.

• Colorant added to the water is an excellent calibration aid, because it helps you see spray patterns more clearly.

• Finally, remember that changing the variables of the “calibration equation” will require you to perform another calibration. If you change nozzles or sprayer pressure, for example, you must perform another calibration.

Technical credit: PBI/Gordon Corp.; Dr. David Kopec, University of Arizona; Brad Fresenburg, University of Missouri Turfgrass Research Center; Spraying Systems Co.

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