University of Vermont

UVM Fruit

Tree Fruit: Integrated Pest Management (IPM)

Sprayer Calibration


A dilute application is when the volume of spray water is sufficient tomake all foliage surfaces wet enough so that the water begins to drip offthe leaves. If the spray will not be a dilute application (as is usuallythe case), then the amount of pesticide formulation to add to each 100 gallonsof tank&shyp;mix depends on the spray concentration.

Spray concentration is the ratio between number of gallons of water peracre needed for a dilute application, compared to the number of gallonsof water that are actually sprayed per acre.

This section describes how to calculate the dilute gallons of water peracre for each group of similar orchard blocks, how to choose or calculatespray concentration, and how to set up the sprayer to deliver a specifiedvolume of water per acre. By using this simple "Gallons per Minute"method, you know precisely at what concentration you are spraying. Withthis method, the accuracy of pesticide dosage does not depend on estimatinghow many acres are being sprayed per tankload.

Another way to estimate the amount of pesticide to add to the tank is toestimate how many acres of orchard will be sprayed by a tankload. This "Acresper Tank" way of estimating how much spray material to use is subjectto error if the acreage is not accurately measured. The number of acresto be sprayed per tankload is then multiplied by the amount of pesticideto be used per acre. The amount of pesticide to use per acre depends onthe size of trees. As with the "Gallons per Acre" method, accuratepesticide dosage using the "Acres per Tank" method depends onadjusting the dosage to tree size. For example, if the tree size and spacingis such that the dilute gallons per acre is 250 gallons, then (in most cases)the amount of a pesticide to use per acre would be 2.5 times therate per 100 gallons. If you feel that orchard and application conditionsallow it, pesticide dosage can also be further adjusted for concentratespray application.

Tree Row Volume

Orchard blocks with different size trees and row spacings have differentrequirements for dilute gallonage per acre, and thus pesticide dosage peracre. By making a couple of simple measurements and using the tree row volume(TRV) formula, growers can estimate how much water would be needed to makea dilute spray to the block. By comparing the actual spray volume to theestimated dilute spray volume, growers have measure of the pesticide concentrationto use. For example, if esti mated dilute gallonage is 4 times greater thanthe volume of water sprayed per acre, then the amount of a pesticide toadd to each 100 gallons of tankmix would be 4 times the dilute rate per100 gallons.

This concept is not new, most growers already take tree size into accountas they mix and apply pesticide sprays. The tree row volume method is muchmore accurate than making rough estimates of spray concentration by comparingspray volume per acre to a fictitious "standard" of 300 or 400gallons. A grower who sprays at 50 gallons per acre and assumes that a dilutespray would require 300 gallons would calculate that he/she is sprayingat 6X (1/6 dilute volume). Thus, the grower would put roughly 6 times therecommended dilute pesticide rate in each 100 gallons of spray mix.

Many orchards are now planted on semi-dwarf and dwarf rootstocks, and comparingthe actual spray volume to a hypothetical dilute volume of 300 gallons peracre is no longer accurate. For example, if the tree size in the block issuch that a dilute application would only require 200 gallons water peracre, then an application made at 50 gallons per acre is a 4X spray, nota 6X spray. If the grower had used the 6X rate, he/she would have applied50% more pesticide than necessary.

For larger trees, 300 gallons per acre may be an underestimate of the dilutegallonage.

Accurate dosage for pesticide, thinning, growth regulator, and calcium spraysis too important (and mistakes too costly) to leave to guesswork. With afolding carpenter's ruler or tape measure, and a pocket calculator, a growercan have a relatively precise estimate of the dilute gallonage requiredfor a block in just a few minutes.

Definition of Terms

(All distance measurements are in feet)

  1. Tree Height = Distance from the ground to the top of the canopy.(Except for trees trained to high clearance.)
  2. Tree Width = Looking down a row from one end, the average maximumtree width.
  3. Row Length per acre = 43,560 square feet divided by distance betweenrows.
  4. Tree Row Volume (TRV) = Tree Height x Tree Width x Row lengthper acre.
  5. Dilute Gallons per acre (DG/A) = TRV x 0.7 gallons divided by1,000.
  6. Concentrate Gallons per acre (CG/A) = Dilute gallons per acredivided by Concentration (or "X") factor.

If the trees are 13 feet high, 12 feet wide, and plantedwith 20 feet between rows, then:

  • Row Length = 43,560/20 = 2,178 feet/acre.
  • TRV = 13 x 12 x 2,178 = 339,768 cubic feet/acre.
  • DG/A = TRV x 0.7 gallons divided by 1,000
  • = 339,768 x 0.7/1,000 = 238 gallons/acre.
  • If you want to spray at 3X, then: CG/A = 238/3 = 79.3 = 80 gallons per acre.

Comments: A pocket calculator will never replace grower judgment.Where tree size is variable, TRV should be estimated for the average ofthe larger trees. For late summer sprays or very dense cano&shyp;pies, theestimated dilute gallonage may need upward adjustment. Unless experience has proven otherwise, estimate the dilute gallonage per acre to be at least150 gallons even if TRV calculations indicate a lower amount. Be aware thatsterol inhibitor fungicides (Nova, Rubigan) and possibly other pesticideshave a minimum dose per acre, regardless of tree size.

Even though the TRV method is highly recommended, growers should experimentwith one or two blocks to gain experience before using an unfamiliar methodfor the whole orchard.

Sprayer Setup

Being able to calculate the appropriate spray volume per acre is only usefulif the spray equipment can be set to deliver that volume. This takes a littlemore time than figuring TRV and DG/A, but is still based on fairly simplecalculations.

Every sprayer should be calibrated before the beginning of each season.A mid-season calibration checkup is also recommended. Here are a few itemsfor a preseason calibration checklist:

  • Check tractor operation and PTO mechanism.
  • Have a current nozzle chart and owner's manual for your sprayer.
  • Clean sprayer thoroughly, flush out tank and lines to remove rustparticles and sludge, clean or replace screens or filters. Check and replaceworn parts as necessary, especially nozzles.
  • Check pump and pressure regulator. Have an extra pressure regulatorand pressure gauge on hand. Pressure gauges often go bad, usually by readingtoo low.
  • Obtain a stop watch, and use a tape measure to measure off an 88 foot long tractor course.

Before setting up the sprayer and calibrating it, figure out how much waterper acre is needed to treat individual blocks at 3X and 6X (or whateverspray concentrations you will use during the season). This can be done bycalculating the TRV and DG/A for each block. For example, the 3X spray volumeequals DG/A divided by 3. In choosing the desired spray volume(s) for eachblock, take into account cultivars and what you know about pest problemsin that block.

Now you are ready to begin calibrating. With a moving start, and while pullinga tank half filled with water in the gear and at the RPM used for normalspraying speed, time how long it takes to cover the 88 foot course. It ismuch more accurate to determine this by timing the sprayer over the 88 footcourse than to rely on a speedometer reading. Repeat the time trial goingthe other way to check your results, take the average of the two runs.

Definition of Terms

  1. Travel Speed (TS) in miles per hour = 60 divided by the num&shyp;berof sec&shyp;onds re&shyp;quired to travel 88 feet.
  2. Swath (S) (for spraying every row) = the dis&shyp; tance betweenrows. (For alternate row spraying, swath would be twice the row spac&shyp;ingif you want the full pesticide rate per acre. It would be only the dis&shyp;tancebetween rows if you want half of the full pesti&shyp;cide rate per acre,applied more frequent&shyp;ly.)
  3. Gallons per minute (GPM) = the gallons of water pumped out of thesprayer each minute.
  4. Gallons per minute (GPM) = (Concentrate gallons per acre xSwath x Travel Speed) divided by 495.
  5. GPM = (CG/A x S x TS) divided by 495.

A grower is calibrating the sprayer which will be used totreat the block described in Examples A. It takes an average of 25 secondsto run the measured tractor course at normal operating speed.

  • TS = 60/25 = 2.40 miles per hour
  • S = 20 feet
  • CG/A = 80 gallons/acre (see Example A).
  • GPM = (80 x 20 x 2.40)/495
  • = (3,840)/495 = 7.76 gallons per minute

For high pressure sprayers, use the nozzle chart for your sprayer, choosea pump pressure and design a nozzle set up where the total output of thenozzles at the pump pressure is as close as practical to 7.76 gallons perminute (in this example).

Designing a Nozzle Setup for a High Pressure Sprayer

A general guideline that has been used in some orchards is: For trees over10 feet high, direct 2/3 of the sprayer output to the upper half of thetree, and 1/3 of the output to the lower half of the tree. However, be awarethat a general guideline such as this may not be appro priate for everyorchard. Adjust nozzles and air vanes to obtain the desired spray pattern.ater sensitive paper is a useful tool to evaluate the spray deposit deliveredby your sprayer to differ&shyp;ent areas of the canopy.

High pressure nozzles deliver the appropriate range of droplet sizes at200 psi, and possibly down to 100 psi. The proportion of superfine droplets(those less than 100 microns diameter) increases with nozzle pressure over200 psi. Superfine drop lets are prone to off-target drift and overly quickevaporation. Pressure above 200 psi not only increases the amount of spraylost to drift, it also puts more wear on the pump. Another benefit of settingpressure no higher than 200 psi is to reduce nozzle clogging. This is becausefor the same out put, a larger nozzle orifice is used at a lower pres sure.

Two ways to use flip over nozzle bodies are:

  1. Set up the sprayer for applications of the same tankmix to two groupsof blocks that have different spray volume and spray pattern requirements,or
  2. Set up the sprayer for applying tankmixes of different concentrationto the same blocks (e.g. one set for 3X applications, the other set for6X applications).
Measuring Sprayer Output

Once the nozzles are in place, measure the actual sprayer output in gallonsper minute. Relatively inexpensive flow meters for both high and low pressuresprayers are commercially available. When pressed onto a nozzle (or manifoldline for low pressure, air shear sprayers), the meter indicates the flowrate in gallons per minute. While not as easy as using a flow meter, individualnozzles can also be checked by collecting their output into a measured containerfor a measured time. Checking individual high pressure nozzles will revealthose that are misfiring because of a partial plugging or be&shyp;causeof a worn orifice. A nozzle that varies by more than 10% from its ratedoutput at a given pressure should be replaced because as the orifice enlargeswith wear, spray pattern and droplet size can also be affected.

To measure overall output, fill the tank to the top with water, and operatethe sprayer for 3 minutes. After 3 minutes, turn off the sprayer and measurehow much water it takes to refill the tank. A 5 gallon bucket marked likea measuring cup is useful for this. Calculate the gallons per minute output.For large adjustments, change cores or nozzles. For small adjustments (tovary gallons per minute by roughly 10% or less), set the pump pressure upor down as needed to bring the total output as close as possible to thedesired gallons per minute.

How close is close enough? Consider this: if the yearly expense for pesticide,foliar nutrients and growth regulators is $250 per acre for a 50 acre orchard,then improving calibration accuracy by 10% represents $1,250 in spray materials.The benefits of precise calibration include: better crop protection; reducedrisks from poor control, russeting or overthinning; lower applica&shyp;tioncosts by getting the job done right the first time; accurate estimationof spray costs per block; less chance of neighbor problems and proof thatyou are a skilled applicator; less guesswork and more peace of mind.

As a final check, fill the sprayer with water and spray a known acreage.Measure how much water is needed to replace the water sprayed out, calculatethe rate per acre, and compare it to the desired output per acre.

Adjustments for Different Blocks

Often the same sprayer and tank&shyp;mix is used to treat orchard blockswith different tree size and row spacing. When moving from one block intoanother one that has different spray pattern and spray volume requirements,the sprayer can be quickly adjusted to provide optimal spray delivery forthe second block. While each block is unique, for simplicity of planningand adjustments, blocks with similar spray require&shyp;ments can be lumpedinto groups. As long as the range of difference among blocks combined ina group are minor, the spray requirements for the group can be defined bythe block with the highest spray volume requirement. The following exampleshows how a grower plans the adjustments to make for a block (or group ofsimilar blocks) called Block C.


The grower with the 13 foot high trees described in ExampleA is planning the sprayer adjustments that will be made so that the samesprayer and tankmix can be used to treat Block C. The trees in Block C are16 feet high, 15 feet wide, and with 23 feet between rows.

  • Row length= 43,560/23 = 1,894 feet
  • TRV=16 x 14 x 1,894 = 424,256 cubic feet
  • DG/A=424,256 x 0.7 gallon/1,000 cubic feet
  • =297 gallons per acre.

The block in Example A was sprayed at 3X, so in order to use the same tank mix for Block C, it will also have to be sprayed at 3X.

  • CG/A = 297/3 = 99 gallons per acre.

Because the trees in Block C are taller than in Block A, the grower willturn on an additional nozzle on each side of the manifold. For this example,let's say that the added nozzles are rated at 0.60 gallons per minute atthe operating pressure chosen for Block A. By adding the two nozzles, thetotal output becomes 7.76 (see Example B) + 1.20 = 8.96 gallons per minute.

The grower knows that after spring calibration, the sprayer output withthe top nozzles on will be close to 8.96 gallons per minute.To calculatewhat the travel speed will have to be to get the desired 99 gallons peracre in Block C, the grower uses this formula (for DEFINITION OF TERMS see Example B):

  • Travel speed = (Gallons per minute x 495) divided by (Swath x Concentrategallons per acre)
  • TS = (GPM x 495) / (S x CG/A)
  • TS = (8.96 x 495) / (23 x 99)
  • TS = (4,435) / (2,277) = 1.95 miles per hour.

Thus, by adding the two top nozzles and slowing the travel speed to 1.95mph, the grower can use the same tank-mix to make 3X applications to bothBlock A and Block C.

Flow monitors for sprayers are commercially available. They account formanifold output, travel speed, and row spacing to give a running reportof the gallons per acre being sprayed. This can help the driver detect calibrationerrors, plugged nozzles, and other problems during the application.

There are several ways to vary sprayer output for different block. Methodscan be combined to get the desired result. Adding or subtracting nozzlesis the simplest and most common method. As long as the sprayer operatorknows what happens to pesticide delivery by adding or subtracting nozzles,it is also probably the best. Other ways include: adjusting travel speed,adjusting pump pressure (within a limited favorable range of roughly 100to 200 psi on high pressure sprayers), changing the manifold flow metersettings (low pressure, air shear sprayers), or by switching nozzle sizeor disc/core selection at one or more nozzle positions. Caution is advisedin adjusting travel speed. Research trials have indicated that going slowerthan 1.5 mph or faster than 2.5 mph can cause problems with airblast spraypenetration into tree canopies and affect spray coverage. For power take-offdriven sprayers, the gear and RPM settings needed to properly operate thesprayer may limit the options for adjusting travel speed.

Continuing with the example of Block C, what if the grower wants to go noslower than 2.1 mph? The grower can use the Gallons per Minute formula fromExample B to calculate the necessary Gallons per Minute output if the sprayerwill be used to make a 3X spray to Block C at 2.1 mph.

  • GPM = (CG/A x S x TS) divided by 495
  • GPM = (99 x 23 x 2.1) / 495
  • GPM = (4782) / 495 = 9.66 gallons per minute

In this case, the grower will have to boost output from 8.96 to 9.66 gallonsper minute, either by adjusting pressure and/or substituting one or morepairs of nozzles.

Sprayer Setup Chart

With this preseason planning, one result can be a sprayer setup plan whichrecords the following: Blocks to be sprayed with this setup, Spray concentration,Nozzle positions used, Nozzle orifice/disc/core selection for each position,Pump pressure, Travel speed.

It is very important to verify the output of each setup by doing apreseason sprayer calibration, find the adjustments needed to get the desiredoutput, and record them.

Finally, the different setups can be recorded in a small durable notebookkept with the tractor. This will insure that the person operating the sprayerwill have the information they need to make quick adjustments to the sprayerwhen moving between blocks with different spray requirements.

Last modified April 30 2014 10:18 AM