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> Pesticide Reference Materials
Calibration of Pesticide Application Equipment
Please read the pesticide label prior to use. The information contained at this web site is not a substitute for a pesticide label. Trade names used herein are for convenience only. No endorsement of products is intended, nor is criticism of unnamed products implied.

The challenge for any pesticide applicator is twofold: 

  • How to apply a small amount of pesticide concentrate (anywhere from quart down to ounces) to a large area (usually an acre).
  • How much pesticide is added with water in to the spray tank to spray a given area.
Usually a label will call for a certain amount of pesticide to be applied per acre. In order to physically apply a small amount of pesticide over a large area, the pesticide concentrate must be diluted. It is easier to apply 20 gallons over an acre than it is to apply a quart or a pint per acre. For this reason, it is important to know how much liquid a sprayer can apply on a per acre basis. This is known as the application rate or sprayer output and is generally expressed in Gallons Per Acre or GPA. Your GPA is based on three factors:
  • The speed at which you apply the pesticide
  • The output of your nozzles
  • The pressure of your sprayer.
To spray pesticides in a cost effective manner, there must be consistency between calibration and actual application. If you calibrate your sprayer at a certain speed or pressure, then make sure you use the same speed and pressure when you apply the pesticide.  In addition, the accurate application of pesticides at the proper dosage is dependent upon 3 major factors.
  • Determination of the proper product rate from the label - 1 pint/acre, 1 quart/acre, etc.
  • Sprayer application rate (output) in Gallons Per Acre (GPA)
  • How many acres can your sprayer covers with a given volume of liquid.

Calibration of Small Volume & Hand Held Sprayers

The procedure for calibrating a hand-held or backpack sprayer is simple. Just follow these steps:
  1. Measure out an 18 x 18 foot strip in the area similar to the one you will be spraying.
  2. Add water to your tank and in a uniform manner, spray this area with water and record the amount of seconds it takes. Do this 2 or 3 times making sure that you keep your pattern and pressure constant. Take the average.
  3. Measure the amount of water delivered to this strip by spraying into a bucket for the same amount of time as in step #2. Also keep your pressure the same as when you sprayed the strip.
  4. The amount of water collected in fluid ounces equals the output or GPA.  (Ounces = GPA)
This method works because of the relationship between a square that is 128th of an acre (18 ½  x 18 ½ = 342.25 ft2) and the fact that there are 128 ounces in a gallon.

Calibrating a Broadjet Sprayer Using the Calibration Strip Method

Broad jet or boomless sprayers enable a wide swath to be sprayed without using a series of nozzles across a boom. Swath widths vary with different brands of nozzles and can range from 10 feet on up.  Calibration of these sprayers is also easy as there are generally only one or two nozzles with which to contend. 

Step 1:  You need to do is determine the effective swath width, the width of the spray as it appears on the ground. Remember you are only using water to calibrate your sprayer. 

Step 2: Check your speedometer or other instruments to ensure that you keep a constant speed. To do this, measure a known distance and then drive the course and time yourself. Now use the following formula to determine your actual miles per hour (MPH)

MPH = Distance (feet) X 0.682
Time (seconds)

For example, suppose it took you 27 seconds to drive the 200-foot course.  Your actual speed is 5 MPH. What did your speedometer indicate? Find a field speed that is comfortable for you, easy on the equipment and one that will also give good coverage. Make sure to note all settings so that you can keep this same speed when you apply pesticides in the field.  

Step 3: Find the area of the calibration strip (200 feet x swath width), and divide it into 43,560 ft2 (square feet in an acre). This will tell you how many of calibration strips will fit into an acre.  Step 4: Use the Nozzle Collection or Refill technique to measure the amount of liquid applied per strip.

Nozzle Collection Technique

Fill the sprayer with water. For the same time it   took you to drive the calibration strip (200 ft.), collect water from the nozzle using the same  pressure as you would in the field.  Measure the amount collected.

Refill Technique

Fill the sprayer to a known mark. Spray the 200-foot course at the same speed and pressure used when actually spraying. Refill the tank to the known mark and measure the amount of water used.  

Step 5: Multiply the number of strips in an acre times the amount of liquid collected from the calibration strip. Convert to gallons if necessary.

Example: A boomless sprayer has a swath width of 30 feet. The area of the calibration strip is then 6000 ft2 (200 ft x 30 ft). There are then 7.26 strips in one acre ( 43,560 ft2 ¸ 6000 ft2). You drove the calibration strip in 27 seconds, stopped and collected from under the nozzle for 27 seconds. You collected 3 gallons. Your sprayer is calibrated at 21.78 gallons per acre or 22 GPA    (7.26 strips x 3 gallons per strip). Remember! When you started spraying, maintain the same speed and sprayer pressure as you had when you calibrated.

Calibration of Boom Sprayers With Multiple Nozzles

The calibration of multi-nozzle boom sprayers is complicated by the fact that you want a fairly uniform output across the boom. If any one nozzle is providing more or less liquid, then the effectiveness of the pesticide application may be affected. Therefore, with boom sprayers, it is important to:

  • Check the output from each nozzle individually.

  • Establish an average as a benchmark.

  •  Using the average, establish an error range in order to judge the nozzles and make a decision to replace or clean the nozzles.

Checking Nozzle Output

The flow from some nozzles may be more or less than others. This will create an uneven swath pattern.  To overcome this you should collect from under each nozzle for a standard amount of time (usually 30 seconds to one minute) and take the average flow of all the nozzles.  If the flow of any nozzle varies by more than 5% or 10%, on either side of the average, then those nozzles should be cleaned or replaced. 

HANDY TIP: To find a 10% error, simply take the average and move the decimal place one space to the left. Now divide that number in half to find a 5% error. For example, suppose we have an average nozzle output of 60 ounces per minute. Moving the decimal one place to the left is 6.0 ounces (10% of 60). Half of 6.0 is 3 (5% of 60). So the “low” side of 60 is 57 while the high side of 60 is 63 for a 5% error. This is 10% across the average. Any nozzle delivering between 57 and 63 ounces per minute falls with the acceptable 5% error range on either side of the average. Any nozzle delivering outside of this range needs to either be cleaned or replaced

Example: Suppose you had six nozzles on a boom and you collected from under each nozzle for one minute (or 30 seconds x 2).  

Nozzle #

1

2

3

4

5

6

Ounces

45

47

35

48

54

46

  • The average output of all the nozzles is 275 6 = 45.8 or 46   

  • Five percent either side of 46 is 43.7 and 48.3

  • Nozzle numbers 3 and 5 are outside of this range and need to be cleaned or replaced.            

Once you have checked all your nozzles for uniform output, the calibration process is the same as calibrating a broadjet:

  1. Find the effective swath width created by all the nozzles.

  2. Mark out a 200-foot course to verify your speed. Time yourself in seconds.

  3. Multiply your swath x 200 ft to find the area of your calibration strip.

  4. Figure how many calibration strips there are in one acre. (43,560 ft2 calibration strip ft2)

  5. Collect liquid from each nozzle for the amount of time it took to drive the strip and pour the liquid collected from all the nozzles into one container or use the Refill Technique.

  6. Multiply the number of calibration strips in one acre times the liquid collected for one strip.

Example:  Suppose we have a 6-nozzle boom and we have checked each nozzle and they all fall within a 5% error range on either side of the average nozzle output (10% across the average). 

  • Suppose the effective swath wide produced by all the nozzles is 10 feet.

  • The area of our calibration strip is 2000 ft2. (200 ft. x 10 ft)

  • There are 21.78 calibration strips in one acre (43,560 ft2 2000 ft2)

  • Suppose it takes 27 seconds to drive the 200-foot course.

  • Stop and collect from under each nozzle for 27 seconds.

  • Suppose you collect 1 gallon and 1 quart (1.25 gallons) from all the nozzles.

  • The sprayer is calibrated to 27.23 or 27 GPA (21.78 strips x 1.25 gallons per strip)

 Adjusting Output

You may have calibrated your sprayer only to find that its GPA is either to high or too low. Labels can be very specific in requiring certain volumes to be applied in order to improve pesticide performance, uptake and to prevent drift. On the other hand, the lower the GPA, the more acres you can treat with a given volume.

In order to affect output, there are three things to consider:

  1. Speed

  2. Nozzles

  3.  Pressure

To make big changes in output (GPA or GPM), you can adjust your speed. As you slow down, you apply more. As you speed up, you apply less. If you are applying 20 GPA at 3 MPH and you double your speed to 6 MPH, you will apply half as much (10 GPA). To make moderate changes in output, it is best to change nozzles unless you can fine-tune your speed with the use of speed sensors. For small changes in output, you can adjust sprayer pressure. But this is for minor changes only. In order to double your output using pressure, you will need to increase your pressure fourfold. For example if a sprayers output is 1 gallon per minute at 10 pounds per square inch (PSI), you will need to increase the pressure to 40 PSI to achieve 2 gallons per minute. Increasing pressure can lead to drift problems, the increased incidences of equipment failure and improper coverage or placement of the pesticides. It is best to use pressure to fine tune a sprayers output and use speed or different nozzles for major adjustments.

Determining GPA - Ounce Calibration Method

Multiple Nozzles
The following method has four steps and no calculations are required.  However, you must remember to first check your nozzles for uniform output.  Equipment needed: Stopwatch, container to collect nozzle discharge, a tape measure, marking flags, and a container graduated in ounces.

Step 1. Using your sprayer's nozzle spacing, select a calibration distance. Measure the travel distance in a field that is typical of the soil surface and soil conditions of the area you intend to spray.  Many tractors and sprayer will gain or lose as much as 10 percent of the desired travel speed while moving up or down slopes. If field variations exist, several speed check areas may be needed.  Remember, the time required to drive the travel distance will give the speed of the sprayer, so the measured distance and timing must be exact.  For example, if you want to travel at 5 MPH then you need to travel a 204 foot course in 27.8 seconds. .

Step 2. Drive the calibration distance and time yourself in seconds at the throttle setting, pressure setting and load used during actual spraying.  The spray tank should be ½ to 2/3  full. Repeat at least three times and average the results.  Do not change the gear or throttle setting after you have chosen a spraying speed.  A change in ground speed will change the sprayer application rate and will require recalibration. Using water, you can operate the sprayer to check for adequate coverage. Select a pressure that limits drift but also gives the coverage you desire.

 
Calibration Distances and Speeds for Varying Nozzle or Row Spacings
Nozzle or
Row Spacing

(in)
Calibration
Distance
Time in Seconds Required To Travel The Calibration Distance For Various Ground Speeds (MPH)*
3.0 MPH   4.0 MPH   5.0 MPH     6.0 MPH   7.0 MPH     8.0 MPH 
40 102 23.2 17.4  + 14.0  + 12.6  + 9.9   + 8.7   +
38 107 24.3 18.2  + 14.6  + 12.2  + 10.4  + 9.1  +
36 113 25.7 19.3  + 15.4  + 12.8  + 11.0  + 9.6   +
34 120 27.3 20.5 16.4  + 13.6  + 11.7  + 10.2  +
32 127 28.9 21.6 17.3  + 14.4  + 12.4  + 10.8  +
30 136 30.9 23.2 18.5  + 15.5  + 13.2  + 11.6  +
28 146 33.2 24.9 19.9  + 16.6  + 14.2  + 12.4  +
24 170 38.6 29.0 23.2 19.3  + 16.6  + 14.5  +
22 185 42.0 31.5 25.2 21.0 18.0  + 15.8  +
20 204 46.4 34.8 27.8 23.2 19.9  + 17.4  +
18 227 51.6 38.7 31.0 25.8 22.1 19.3  +
16 255 58.0 43.5 34.8 29.0 24.8 21.7
14 291 66.1 49.6 39.7 33.1 28.3 24.8
* 1 MPH = 88 feet per minute.
+ For times less than 20 seconds (shaded area), improved accuracy can be attained by doubling the collection time in step 3 and dividing the output collected by two.

 Step 3. While stopped, adjust the sprayer to the pressure that was used in step #2.  Catch the discharge from one nozzle for the time recorded in step #2 or catch the discharge from all nozzles and take the average.   Hopefully you have checked all your nozzles for uniformity so that discharge from all the nozzles is within at least 5% of the average output. 

Step 4. Measure the discharge in ounces with a graduated container. 

Step 5. Ounces collected now equals gallons per acre (GPA).    A point to remember!  The collection of discharge should be done with the carrier only and not with the pesticide added to the mix.  In most cases you will use only water when calibrating.  Even while collecting water you should use the proper personal protective equipment (PPE)!    

Ounce Calibration With A Boom Sprayer

Suppose you have a sprayer that has 20-inch nozzle spacing.

Step 1.  After referring to the table, you mark off 204 feet.  You then drive your sprayer through the course three times trying to maintain a constant speed and pressure for all three times. You record the time and pressure setting if you decide to spray the strip with water in order to check coverage.

Step 2.  The average time recorded was 28 seconds and the pressure was 30 PSI.  According to the table, the travel speed is about 5 MPH.

Step 3.  While stopped, you adjusted the sprayer pressure to 30 PSI and collected from each nozzle for 28 seconds. You recorded the discharge as:

Nozzles Output in Ounces for 28 seconds
14.5        
2 13.0
3 15.5
4 15.0
5 15.5  
6 15.0
7 14.5
8 15.5
9 16.0
10 15.5
Total = 150 Average = 15.0 ounces (150 ¸ 10) +5% =  15.75 ounces -5%  =  14.25 ounces

 Step 4. The average of all nozzles was 15.0 ounces so the calibration of the sprayer is 15 gallons per acres (GPA).  Notice that nozzles #2 and 6 are outside the 5% range.  These nozzles should be cleaned or replaced and then rechecked.  

Determining GPA - Ounce Calibration Method – Broadjet Sprayer

1. Remember to use water when calibrating.
2. Measure the width of the spray pattern (swath width of the nozzle).
3.  Based on the swath width, determine the length of the calibration course using the following table.
Swath Width in Feet Course Length (feet) *
35 156
40 136
45 121
50 109
* 5,460 by the swath width in feet.
Example: Course length for a 32 foot swath.
5460 ¸ 32 = 171 foot course length
4.  Measure the width of the spray pattern (swath width of the nozzle).
5. Based on the swath width, determine the length of the calibration course using the following table.
6. Drive the course in the gear and rpm you will use when actually spraying. Record the time required to drive this course. Conduct this test two or three times & take the average.
7. Park & turn on sprayer, adjust sprayer pressure & collect liquid from the nozzle equal to time required to drive course. Make note of this pressure & use the same pressure when spraying.
8. Pints of liquid collected = Gallons Per Acre (GPA)

 Determining GPA - Nozzle Volume Method

This method makes use of a formula that can be rearranged to make adjustments for:

  1. Output (GPA)
  2. Flow from the nozzles in gallons per minute (GPM)
  3. Speed in miles per hour, MPH.

The basic formula is:    

GPA = GPM x 5940
   MPH x W
GPM = gallons per minute per nozzle
GPA = the output of the sprayer
W = nozzle spacing in inches (broadcast spraying)
    = spray width in inches (boomless, band spraying, broad jets, or single nozzle)
     = row spacing in inches divided by the number of nozzles per row for directed spraying.
5940 = a constant derived by a simple mathematical cancellation.

In case you are curious how the 5940 was derived (you don't need to remember this!

GPA = gallons
minutes
X hour
miles
X       1     
width (in.)
X 60 minutes
     hour
X    mile  
 5280 ft
X 12 inches
    foot
X 43,560 ft2
    acre
GPA =
GPM x 60 x 12 x 43,560
=
GPM x 5940

MPH x W = 5280

MPH x W
 

Remember that there are 4 factors that are essential for calibration:

  1. Output in GPA
  2. Individual nozzle flow in GPM
  3. Speed in MPH
  4. Nozzle or swath width (W).

 The above formula can be manipulated mathematically so as to derive each one of these factors 

So using the basic formula:  

GPA = GPM  x  5940  
 
MPH  x   W

If you want to find GPM then rearrange the formula:    

GPM = GPA  x  MPH x W  
          5940

If you want to adjust your MPH then:   

MPH = GPM  x  5940  
 
GPA x   W

With this method, if we have a desired GPA in mind such as what is recommended by a label, then we can use the formula to determine how much we need to collect from each nozzle.

 Example:  You decide to use a herbicide and the product label recommends a minimum output of 20 gallons per acre (GPA).  We want to cover as much ground as possible so we want out sprayer to deliver 20 GPA.  We have driven over the area we want to spray and have determined that 6 MPH is a good field speed that will minimize boom flop and wear on the machinery.  The nozzles on our sprayer are 20 inches apart.  Plugging all this information into the appropriate formula we get:

GPM = GPA  x  MPH x W  
          5940
= 20  x  6  x  20 
       5940
= 2400
5940
= 0.40 GPM

Making the conversion to ounces, 0.40 GPM x 128 oz per gallon, we can see that we need to collect 51 ounces from each nozzle on our boom to achieve 20 GPA.

 Again, we want to take an average of all the nozzles across our boom.  We then determine ± 5% deviation either side of the average.  In this case, 48.45 and  53.55 are our lower and upper range.  Any nozzle either side of 48 ½ and 53 ½ needs to be changed or cleaned.

But what if we collect from our nozzles and we find that the average is 70 ounces per minute (OPM), well above the 51 OPM that we need to achieve 20 GPA.  How do we make adjustments in order to reach 20 GPA?
We can either adjust speed, use different nozzles or use pressure to make these adjustments.  As a rule of thumb, never use pressure to adjust output unless the nozzle flow is within 10% of the average.  For instance, 10% either side of 51 OPM is 46 and 56 so the amount that we collected, 70 OPM, is well above this range.  We wouldn’t even think of using pressure in this case.  It takes four times the amount of pressure to double output.  Our options would be to change nozzles or adjust speed.  It often easier to adjust speed and we can now do that by rearranging our basic formula. 

Remember we have the following information:

  • GPA  =  20 gallons per acre
  • W       =  20 inches
  • GPM  = 0.55 gallons per minute (70 ounces ¸128 oz/gallon)

To find our corrected speed we then use:

MPH = GPM  x  5940  
 
GPA x   W
MPH =   0.55 GPM x 5940   
20 GPA x 20 inches
MPH = 3267
 400
MPH = 8

This would make sense as our nozzle flow has increased we need to speed up in order to achieve 20 GPA. 

If we use the basic formula we can also determine our output if we still decided to travel at 6 MPH.

 Given:                          

  • GPM = 0.55 gallons per minute (70 ounces per minute, OPM)
  • MPH = 6 miles per hour         
  • W = 20 inches

  Plugging in the data we find that:

GPA = GPM  x  5940  
 
MPH  x   W
GPA =    0.55 GPM x 5940  
 6
MPH  x   20 inches
GPA = 3267  
 120
GPA = 27 GPA

How Much Pesticide To Add To The Tank

 The amount of herbicide formulation to be added to the spray tank is dependent upon:

  1. Volume of pesticide mixture to be used.
  2. Sprayer output in gallons per acre (GPA).
  3. The recommended product rate of pesticide.

You first need to know how much area you can treat with a given volume! 

Volume Used
Output (GPA)
= Acres Treated

Example: Your sprayer is calibrated to 25 GPA and you are going to use a full 500 gallon tank.

500 gallons
  25 GPA
= 20 acres you can spray with 500 gallons of a pesticide water mix when your sprayer is calibrated to 25 GPA

Example: Your sprayer is calibrated to 25 GPA and you are going 250 gallons of a 500 gallon tank.

250 gallons
  25 GPA
= 10 acres you can spray with 250 gallons of a pesticide water mix when your sprayer is calibrated to 25 GPA

Example: Your 2 gallon backpack sprayer is calibrated at 35 GPA and you plan on using a full 2 gallon tank.

 2 gallons
35 GPA
= 0.057 acres acres you can spray with 2 gallons of a pesticide water mix when your sprayer is calibrated to 35 GPA
Note: 0.057 acres is also 2483 ft2 (43560 ft2/acre x 0.057)

Once you determine how many acres you can spray with a given volume, you can then determine how much pesticide you need to add to that given amount of volume in the spray tank.  

Acres treated x product rate  =  How much you need to add to a given volume in the tank

For instance, a label calls for a rate of 1 pint/ acre and you can spray 20 acres with 500 gallons of water (at 25 GPA). 1 pint/acre x 20 acres = 20 pints of pesticide in 500 gallons.  

You will then be adding 20 pints to make up the 500-gallon mixture. There are 8 pints in a gallon so you will need to add 2 ½ gallons of pesticide to the tank. Fill the tank half full with water, add the pesticide, surfactants and then top off with water.

 Suppose you have that backpack that can treat 0.057 acres with 2 gallons of water (35 GPA). The label recommends a 1 pint per acre rate to control a particular pest.  One pint equals 16 ounces so: 

0.057 acres x 16 = 0.912 or 1 ounce of pesticide per 2 gallons of water.

Other Mixing Considerations

Pesticides are sold as formulated product; that is, the active ingredient (a.i.) mixed with other fillers called inert ingredients. Some recommendations are expressed in a.i. only and the challenge is then to convert the amount of a.i. that is recommended into the total amount of formulated product (both a.i. and inert ingredients) that you will apply per acre.

  • How much liquid pesticide do you add to the tank when the rate is given according to pounds of active ingredient (a.i.) per acre such as with university recommendations or label restrictions.

    Labeled rate (in a.i.) per acre   
        Amount of a.i. per gallon
      =   Amount to apply in terms of formulated gallons

    Example: A university bulletin recommends that you apply 3 lb per acre of the active ingredient (a.i.) found in Smashem EC™ insecticide.  This insecticide contains 8 lbs. of  a.i. per gallon of formulation. 

            3 lbs./acre    
     8 lbs. a.i per gallon
    = 0.375 gallons of the formulated product per acre
    0.375 gallons also equals1 ½ quarts per acre.  
    (0.375 x 4 quarts per gallon) or;
     3 pints per acre (0.375 x 8 pints per gallon).

    Another Example: You have calibrated a 300-gallon sprayer.  It can spray 7.5 acres per tank at 40 GPA.  A recommendation indicates to apply ½ pound a.i. of schnozaline per acre to control spotted knapweed. The label for schnozaline indicates that it contains 2 pounds of a.i. per gallon. How much schnozaline will you add to the tank to spray 7.5 acres?

    Step 1: Determining how much formulated product will be applied per acre

    0.50 lb a.i./acre
     2 lb a.i./gallon
    = 0.25 gallon (1 quart) of the formulated product per acre

    Step 2: Determining how much formulated product to add to the tank.

    7.5 acres/tank  x 1 quart per acre = 1.875 gallons or: 7.5 quarts (1.875 x 4 quarts) or 1 gallon plus 3.5 quarts or ½ quart shy of 2 gallons.

 

  • How much dry pesticide do you apply per acre when the rate is given as a percentage of active ingredient (a.i.). 

Recommended rate (in pounds of a.i.) per acre   
    Percentage of a.i. per pound*
  =  Amount to apply in terms of formulated pounds*

* Remember! Its a dry formulation!!

Example: A recommended rate of 0.2 lbs. a.i./acre of a 25% wettable powder (WP) is recommended. (one pound of formulation contains 0.25 lbs. a.i.).

        0.2 lbs. /acre    
25% wettable powder
= 0.80 lb formulation per acre
To convert to ounces: 0.80 lbs.  x 16 ounces/ lbs. (dry)  =  12.8 ounces per acre.

Adding Adjuvants and Surfactants

 Pesticide labels often suggest adding adjuvants to the spray mix, listing the rate of the adjuvant in terms of percentage of the spray mix, volume per acre, or volume per quantity of spray mix.

  •  When the rate is expressed as a percentage of the spray mix, use the following formula

Gal. adjuvant needed = % of spray mix   x  gallons of spray mix
         100
Example: Total spray mix = 500 gallons. Adjuvant rate is 1% of the finished spray volume.  0.01 x 500 = 5 gallons of adjuvant added along with pesticide to make a 500-gallon solution.
  • When the rate is expressed as a volume per acre, use the following formula. You will first need to determine how many acres you can spray with a given volume.

      Adjuvant needed  = adjuvant rate x acres to be treated

Example: Your sprayer is calibrated to 30 GPA and you plan on using 300 gallons of solution. An adjuvant calls for a rate of 1 pint per acre. 300 gallons ¸ 30 GPA = 10 acres x 1 pint per acre = 10 pints of adjuvant added along with pesticide to make a 300 gallon solution. 

  • When the rate is expressed in quarts per 100 gallons, use the following formula:

Adjuvant needed = rate per 100 gallons  x gallons of spray mix 
              100
Example: Adjuvant rate = 2 pints per 100 gallons. A total of 400 gallons of spray mix will be used. (2 pints/100) x 400 = 8 pints of adjuvant added with pesticide to make a 400 gallon solution.

Spraying Solutions Other Than Water

The diluent for most spray applications is water (8.34 pounds per gallon). However, some labels may require that other diluents be used.  

When selecting nozzles, you need to know either your sprayer’s output in Gallons Per Acre (GPA) or the desired nozzle output in Gallons Per Minute (GPM). Because most nozzle selection charts provided by manufacturers are based on spraying with water, the figures on the charts will be incorrect if you are using a diluent other than water.  

The following table will help you adjust the figures to fit your situation.  Multiply the values on the nozzle charts by the conversion factor from the table to determine the correct value for the solution being used.  

Weight of Solution  
(lbs. per gallon)
Specific Gravity
(lbs/g) 8.3453)
Conversion Factors
Water to other Other to water
6.6 (Kerosene) 0.79 1.26 0.79
7.0 pounds per gallon 0.84 1.09 0.92
8.0 pounds per gallon 0.96 1.02 0.98
8.34 (Water) 1.00 1.00 1.00
9.0 pounds per gallon 1.08 0.96 1.04
10.0 pounds per gallon 1.20 0.91 1.10
10.65 (28% nitrogen) 1.28 0.89 1.13
11.00 (7-27-7 fertilizer)  
11.06 (32% nitrogen)
1.32  
1.33
0.87 1.15
11.4 (10-34-0 fertilizer) 1.37 0.86 1.16
11.5 (12-0-26)  
11.7 (11-37-0) fertilizer
1.38  
1.40
0.85 1.176
12.0 pounds per gallon 1.44 0.83 1.20
14.0 pounds per gallon 1.68 0.77 1.30

Example (Water to Other): You calibrated your sprayer with water. You determine your sprayer is calibrated to 20 GPA and you are applying a pesticide with 28 % nitrogen solution. The conversion factor is 0.89. Multiply 0.89 x 20 GPA to find an actual application rate of 17.8 GPA.

Or, you collected from your nozzles and the average output is 0.42 GPM with water. The actual output with 28% nitrogen is 0.3738 GPM (0.42 GPM x 0.89). This is also equal to 47.8 or 48 ounces per minute (0.3739 GPM x 128 ounces in a gallon). So you will need to choose a nozzle that is applying 0.37 GPM.

Example (Other to Water): Your desired output or application rate is 20 GPA using 28% nitrogen as the diluent.  Multiply 20 GPA X 1.13  to get 22.6 GPA.  You should choose a nozzle size that will supply 22.6 GPA of water at the desired pressure.

Or, you need your application rate to be 22.6 GPA (with water) to achieve 20 GPA with 28% nitrogen.
View Text-only Version Text-only Updated: 04/16/07
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