Category: Spray Basics

How Do Hydraulic Low-Drift Nozzles Work?
Low drift nozzles have become the standard way to apply pesticides from a boom sprayer. In order to use them properly, we need to understand how they are designed and how they are intended to work.
Sprayer nozzles have three functions on a sprayer.
1. Metering flow
2. Atomizing liquid
3. Distributing liquid uniformly
Accurate metering of the flow is done through precise machining or molding of the nozzle.
Atomization of a liquid occurs by imposing some sort of force on the liquid that causes it to break up from a stream or a sheet into droplets of the desired spray quality.
Distribution is done by generating a pattern that, in combination with adjacent nozzles, produces similar dosages in appropriate droplet sizes and densities, along the target area.
All three of these functions are confirmed by the nozzle manufacturer, but the properties are likely to change with wear.
Atomization
Atomization forces could be air-shear (used in some aircraft, airblast, or twin-fluid nozzles), centrifugal energy (used in rotary atomizers), electrical energy (used in some electrostatic sprayers), or hydraulic pressure (used in the most common nozzles, the flat fan or hollow-cone tips).
Typically, the higher the applied energy, the greater the break-up of the spray. More air-shear resulting from faster aircraft or fan speeds, faster rotation of a cage, or more hydraulic pressure all have similar effects: they create finer sprays.
Most nozzles produce polydisperse sprays, comprised of a large number of different droplet sizes. For hydraulic flat fan nozzles, droplets ranging from 5 to 2000 µm can be produced. The exact distribution of the volume in these droplet sizes depends on the nozzle design, the spray liquid, and the pressure. Here are three examples, representing approximately Medium, Coarse, and Extremely Coarse sprays.
Droplet size distribution by number and volume from a Medium spray. Note the majority of the droplets are small, but the majority of the volume (dose) is in somewhat larger droplets.
Droplet size distribution by number and volume from a Coarse spray. Like in the Medium spray, the majority of the droplets are small although there is fewer of them. The majority of the volume is in intermediate sized droplets.
Droplet size distribution by number and volume from a Very Coarse spray. While the majority of the droplets are small as in the finer sprays, their overall number is sharply reduced from the finer sprays. The volume is now in the largest droplet sizes.
Let’s focus on hydraulic nozzles, by far the most common in agriculture.
Spray Pressure
Spray pressure is a useful tool for controlling droplet size from any hydraulic nozzle. Need a finer spray? Add pressure. It is also the basis for the age-old recommendation that lower pressures are a good tool for reducing drift.
We impose practical limits on the upper and lower range of recommended pressures based on several other factors, chief among them the spray pattern.
Spray patterns of a certain width, or angle, are required for proper pattern overlap. The convention is to space hydraulic nozzles at 15 or 20 inch intervals along a boom, and operate them at about 20” above the target. Boom height values will depend on the fan angle of the nozzle and the degree of overlap required. For low-drift flat fan tips, a minimum 100% overlap is best. With 100% overlap, the spray pattern width at target height is twice the nozzle spacing. With this approach, at any point under the boom, the target receives droplets from the closest two nozzle patterns.
Pattern angles are published by manufacturers, but in practice, angles often differ from those values and can vary with spray formulation. Importantly, they tend to become narrower at lower pressures. The exact pressure at which this happens depends on the tip design, but experience shows that pressures below 20 psi for conventional nozzles, and 30 to 40 psi for low-drift nozzles, result in poor (too narrow) patterns. Narrow patterns reduce overlap, resulting in poor distribution.
TeeJet AI11003 at 20 psi
TeeJet AI 11003 at 80 psi
We might also limit pressures at the upper end, based on drift potential. Most conventional flat fan nozzles, for example, drift excessively at pressures above 60 psi or so, hence that limit.
Low Drift Nozzles
Low drift nozzles were quickly adopted by applicators due to their ability to reduce drift and thereby widen the window of safe spray application. They work by using a two-stage design (often called “pre-orifice”) to reduce the internal operating pressure of the tip. The pre-orifice, the original liquid inlet, is round and sized for the nominal flow of the tip. The exit orifice is eliptical in shape and has a larger flow capacity than the pre-orifice, by about 1.2-fold to 2.5-fold. The larger exit creates an internal pressure drop, so the pattern formation produces larger droplets as though the operating pressure had been reduced. Most modern low-drift tips also introduce air into the nozzle via a built-in venturi. This further suppresses the formation of driftable droplets and introduces air into the interior of the nozzle, adding some pressure back to the system.. The Albuz AVI nozzle schematic below explains the venturi design.
Cross-section of the Albuz AVI venturi nozzle.
The tapered channel inside the nozzle is a venturi, which draws air into the nozzle via integrated ports. When low-drift nozzles are operated beside conventional nozzles at the same pressure, low-drift nozzles produce much fewer driftable fines, and also more larger droplets.
But while the two-stage design is useful for managing drift, it also conceals the actual operating pressure of the exit orifice in these tips. The exit orifice is important – it is the part of the nozzle that does the atomizing and that forms the pattern.
Let’s illustrate the pressure inside a low-drift tip by operating an air-induced low-drift nozzle at 60 psi. This nozzle has a pre-orifice size of 03 (0.3 US gpm at 40 psi, blue) and an exit orifice size of 06 (0.6 US gpm at 40 psi, grey). The operator sees 60 psi on the gauge. What is the exit orifice pressure?
The exit tip has twice the flow-rate of the pre-orifice, and therefore operates at one quarter the pressure, or 15 psi. Recall the square-root relationship between flow rate and pressure.
The relationship between spray pressure and flow rate. Doubling the flow rate requires a quadrupling of pressure
That’s not the whole story. The internal venturi is drawing additional air into the nozzle chamber, and depending on the operating pressure, this could be from 5 to 15 psi. The amount added depends on the specific nozzle, its flow rate, and its pressure. Let’s add 10 psi in this case. The exit tip is actually at 25 psi.
Now let’s assume the pressure gauge reads 40 psi, and that the venturi generates 5 psi additional pressure. The actual exit orifice pressure is now only 15 psi. This is at the lower limit at which a spray is atomized, and at which a good pattern can form.
Our general recommendation with venturi-style low-drift tips has been to avoid pressures below 30 or 40 psi for that reason. We’re trying to prevent the spray becoming too coarse for adequate coverage, and also to prevent the spray pattern from collapsing.
The upside of this design is that the same principle allows for much higher-pressure operation without creating excessive drift. These types of nozzle can, in fact, be operated at 70 to 90 psi without becoming very drift-prone because the pressure at which the spray liquid is atomized is likely only 30 or 40 psi (the actual exit pressure and drift potential will depend on the nozzle and the formulation).
Speed Range
A low-drift nozzle with a pressure operating range from 30 to 90 psi (i.e., 3-fold) would have a flow rate range of 1.73 (i.e., the square root of 3 due to the square root relationship of flow rate and spray pressure). This means that the fastest travel speed (at 90 psi) would be 1.73 times the slowest travel speed (at 30 psi).
A conventional nozzle operating between 20 and 60 psi would have the same travel speed range. So why don’t we just do that? The main reason is that the two-stage design lowers the overall amount of drift substantially, something a conventional nozzle can’t achieve even at very low pressures.
A second reason is that even at high pressures, a two-stage design will likely drift less than an conventional nozzle. This is still the case if the conventional nozzle is operating at low pressures. Any spray quality chart comparing spray qualities of conventional and low-drift tips will demonstrate that.
Pulse Width Modulation
PWM uses a solenoid to intermittently shut off nozzle flow, between 10 and 100 times per second (Hz) depending on the manufacturer. This has implications for nozzle design because the nozzle must not leak liquid during the brief off-cycle. If it does, the small amount of liquid leaving the nozzle will not only not atomize properly, it will also cause a pressure drop within the nozzle which must be replenished with the next on-pulse. This will mean the on-pulse will operate at a lower initial pressure, affecting pattern development and atomization. For this reason, venturi-style low-drift nozzles have not been recommended with PWM. The venturi provides an alternate exit for air or liquid, compromising nozzle performance.
And yet, some venturi style nozzles do, in fact, produce acceptable patterns with PWM according to the nozzle manufacturers. This goes to show that nozzle design can continue to evolve to provide the best in drift reduction technology with PWM. Design for PWM suitability should be at the top of nozzle manufacturers’ agendas.
Nozzle design continues to evolve. But in the foreseeable future, spray pressure will continue to control pattern width and droplet size. That’s why understanding the pressure limits of any specific nozzle type, and maintaining pressure within those limits, is so important in any spray operation.
April 4, 2017
How to Properly Set Up a Crop Sprayer
Article reprinted with kind permission from an original article written by Oliver Hill in the February, 2017 edition of Farmers Weekly. Photos ©Kathy Horniblow.
Crop spraying is one of the most important and highly skilled jobs undertaken on any arable farm, but it is facing increased public scrutiny. This is why it is vital that the kit you use as a means to apply pesticide to crops is in prime working order and is set up correctly to deliver the product safely and accurately to its target. Optimum sprayer set up will help to maximize the efficacy of applied products, reduce spray drift and keep machinery in good condition.
For this best practice guide to sprayer set up, Farmers Weekly teamed up with former Farm Sprayer Operator of the Year Iain Robertson. Mr. Robertson is assistant arable farm manager at David Foot Ltd, a 2,200ha mixed farm south of Dorchester in Dorset, growing wheat, barley, beans, oilseed rape and maize as forage for the farm’s three dairy herds. The machine used for this guide is a Bateman RB26 self-propelled sprayer and while most of these checks and tests are universally applicable to all sprayers, it is also important to refer to the handbook of the manufacturer of your specific machine.
Watch the video tutorial with Mr. Robertson and then see the step-by-step guide below for more detail.
Pre Start Checks
Before firing up the engine, the first thing to do is your pre-start checks – that means checking your machine’s vital fluids like fuel, hydraulic oil, hydrostatic oil, engine oil and coolant levels. If yours is a self-propelled sprayer, chances are you’ll need to get up on to the back of the machine to check some of these.
“While I’m up on the back of the sprayer I also have a quick look in the top of the tank to make sure that it is nice and clean and the tank rinse nozzles have worked properly – cleanliness is next to godliness,” says Mr. Robertson. Next, move on to the tires. Use a pressure gauge to check all tires are at the correct pressure and refer to the manufacturer’s guidelines. If you’ve got a trailed sprayer, don’t forget to check the tractor tire pressures as well.
Aim for tires to be run at the lowest pressure recommended for the load to be carried. This will help with boom height and stability and also helps tires act like a shock absorber to ride out bumps. If using a trailed sprayer, use a spirit level to ensure that the drawbar is level. Mr. Robertson says he tries to work around the machine in a methodical, clockwise manner to ensure that he doesn’t miss anything.
Coming to the pumps, check that they have got enough oil, check that any tool boxes have enough spare parts and any equipment needed and make sure you are carrying a spill kit with absorbent granules and a spade in case the worst happens and there is a spillage. Make sure all parts are lubricated daily and that any grease nipples are cleaned before and after use to avoid them collecting dirt and blocking.
Check all hydraulic hoses, spray lines and air lines for any signs of wear that could result in problems while operating.
It’s best to run the sprayer at a minimum of 5 bar to check for leaks. Also check the spray tank is fixed down securely, all straps and bolts are tight.
Boom checks
Once opened out, check the boom has good movement in the x- and y-axis. All machines are different so check with your manufacturer as to how the boom is set up. Mr Robertson’s Bateman has tie rods and stock bots that can be adjusted to set the boom up to ride well.
Check the tie rod nearest the back of the machine is slightly loose when moving and that the front rod is tight. Next, check for up and down movement by gently pushing the boom down by about 50cm and letting go. The boom should return to the central position without too much bouncing around.
“We want a little bit of movement but not excessive so that you can ride over the bumps as you go along without over- and under-dosing the crop,” says Mr. Robertson. Boom height is one of the most critical factors when spraying and the ideal height is 50cm above the crop. One of the easiest ways to work this out is by using a cable tie that is cut off at the correct length to use a visual aid from the sprayer cab.
Don’t forget to measure from the tip of the nozzle to the crop, not the spray line.
Good sprayer cleanliness is important, so make sure the system is rinsed through at the end of each day with clean water to make sure there’s no residue left in the boom. If your machine’s boom doesn’t have recirculation, remember to take the end caps off occasionally and flush out the whole boom.
Nozzle checks
Check that the nozzles are aligned both vertically and horizontally, according to the NSTS guidelines. Loosen clamps to adjust any nozzles that need realignment.
Check the nozzle output at least twice a year by running the sprayer with clean water at 3 bar pressure. Time the output of each nozzle for 30 seconds. If nozzles have been used previously, it’s best to check their output against that of a new pair. Mr Robertson advises using a measuring cylinder rather than a jug to measure the flow rate as a jug is less accurate “because you get a bigger variation over the wider surface area”.
With an 03 nozzle running for one minute at 3 bar pressure, the output should be 1.2 litres/minute as a rule of thumb but refer to the nozzle manufacturer’s output chart for the expected flow rate. “An easy way to remember this is: at 3 bar your nozzle size multiplied by four will give you your target litres/minute output. It works for all nozzle sizes.” If the output varies more than 4% of the average, or if the spray pattern visually doesn’t look correct, you need to change the nozzle set.
After checking the output, cross-reference this figure with the rate controller – you may need to adjust the flow figures to ensure that the two correlate. If a nozzle becomes blocked while spraying, Mr. Robertson says he will swap it for a new one and then clean it later using a toothbrush or airline. Never blow through a nozzle with your mouth.
Nozzle choice
The choice of nozzle is highly dependent on the sort of job you’re doing. “Timing is crucial but using the right nozzle at the right time will make the job so much easier, cut drift and mean that you’re getting more of the product where you want it to go. If you aim at it you will hit it,” says Mr. Robertson.
His nozzle of choice is an 03 size and he prefers to use the Defy 3D nozzle alternated forwards and backwards across the boom for pre-emergence work and T0 applications as well as the T3 ear spray. “In less than optimum conditions I may prefer to use the Amistar/Guardian Air, a fine induction nozzle. I would use this at T1 and T2 and also in sub-optimum conditions.” This nozzle has a 3-star Local Environmental Risk Assessment for Pesticides (LERAP) rating and is 75% drift reducing.
A water volume of 100 litres/ha is a good rate for spring fungicide application. It provides enough coverage for good disease control and allows maximum efficiency from the sprayer.
Forward speed
The third and final part of reducing spray drift is forward speed. Depending on nozzle size and water volume, aim to travel at 12kph.
Mr Robertson says he finds that this speed gives a good overall output and means you don’t get shadowing or turbulence behind the machine.
Tips and tricks
One of the biggest risk of contamination is at fill up. “A fantastic, cheap trick that I learned through Farm Sprayer Operator of the Year is to take a 200 litre plastic drum and cut it in half to create two drip trays to catch any spillages under the induction hopper and the tank overfill.” This eliminates point source contamination, he says.
“Finally, there’s a plethora of information out there on the internet, loads of good apps to download. The technology is there to help us do the best job possible and make our job as safe as possible.”
February 24, 2017
Selecting a Field Sprayer Nozzle
This article is reproduced, with permission, from Ohio State University Extension’s factsheet FABE-528.
Although nozzles are some of the least expensive components of a sprayer, they hold a high value in their ability to influence sprayer performance.
Nozzles meter the amount of liquid sprayed per unit area, controlling application rate, as well as variability of spray over the width of the sprayer boom. Nozzles also influence droplet size, affecting both target coverage and spray drift risk.
Nozzles come in a wide variety of types and sizes. The best nozzle for a given application will maximize efficacy, minimize spray drift, and allow compliance with label requirements such as application rate (gallons per acre) and spray droplet size. Selecting the best nozzle requires careful consideration of all the factors listed below:
Nozzle Type
- Sprayer operation parameters
  - Application rate, spray pressure, travel speed
- Type of chemical sprayed
  - Herbicides (soil incorporation, pre/post emergence)
  - Insecticides
  - Fungicides
  - Fertilizers and growth regulators
- Mode of action of chemical (spray coverage requirement)
  - Systemic
  - Contact
- Application type (broadcast, band, directed, air assisted)
- Target crop (field crops, vegetables, vineyard, shrubs and trees, etc.)
- Spray drift risk
Nozzle Size
Each nozzle type is designed for a specific type of target and application. For example, a nozzle designed for broadcast spraying is not good for spraying pesticides over a narrow band. Luckily, most nozzle manufacturers’ catalogues have charts showing which nozzle type will be best for a specific job. Check the websites of nozzle manufacturers to reach their catalogues. For more information, contact your county Extension office.
Nozzle manufacturers’ catalogs provide tables and charts showing application rates (gallons per acre or gpa), given a nozzle’s flow rate (gallons per minute or gpm) delivered at various pressures (psi) and travel speeds (mph). These tables are useful tools for selecting the appropriate nozzles, pressure and speed to spray chemicals at application rates prescribed by product labels. However, the charts are only for a limited number of travel speed and nozzle spacing situations. There may be situations where the charts will not provide information associated with your sprayer setup (nozzle spacing) and operating conditions (travel speed and spray pressure). The Apps developed by most of the major nozzle manufacturers can provide you the exact nozzle flow rate required for any given set of application parameters, and identify a specific set of nozzle recommendations for the given application parameters.
To find these Apps, simply visit the App Store in your smart phone or tablet and do a search under “Spray Nozzle Calculator”, or some other key words related to nozzle size selection. You may also want to do a search under the name of the nozzle company from which you are interested in buying the nozzles. However, some Apps are not user friendly and sometimes they do not take into account the droplet size requirements when recommending nozzles. Although the Apps and tables in catalogues may expedite the nozzle size selection process, it is best to understand the procedure and the maths nozzle manufacturers use to generate the values listed in tables and to recommend nozzles in their Apps. The procedure used by the nozzle manufacturers to generate numbers in tables and in their Apps is explained below. By following the steps mentioned below, you should be able to determine the exact nozzle flow rate (gpm) required for your spray application parameters.
Once the exact nozzle flow rate is determined, you can then look at the catalogue to select the nozzle that will provide you the flow rate at a practical pressure setting.
Steps to select the proper nozzle size:
The following steps must be taken to determine the nozzle flow rate (gpm):
1. Select the application rate in gallons per acre (gpa). This is a management decision you will have to make based on pesticide label recommendations, field conditions and water supply.
2. Select a practical and safe ground speed in miles per hour (mph).
3. Determine the spray width per nozzle (W). For broadcast applications, W = nozzle spacing (distance between two nozzles on the boom) in inches. For band spraying, W = band width in inches. For directed spraying, W = row spacing in inches (or band width) divided by the number of nozzles per row (or band).
4. Determine the flow rate (gpm) required from each nozzle by using the following equation: gpm = (gpa x mph x W) / 5,940 (5,940 is a constant to convert gpa, mph and inches to gpm).
5. Select a nozzle size from the manufacturer’s catalogue that will give the flow rate (gpm) determined in Step 4 when the nozzle is operated within the recommended pressure range. If a nozzle of this size is not available, change the travel speed in the equation above and determine the new flow rate required.
An Example
For example: You want to spray a pre-emergence herbicide at 15 gpa, at a speed of 8 mph. The distance between the nozzles on the boom is 20 inches. The herbicide label requires a spray quality of “Medium.” What should be the flow rate of the nozzle you will choose?
gpm = (gpa × mph × W) ÷ 5,940
Since this is a broadcast application (pre-emergence), W is the distance between nozzles (W = 20″). Filling in the variables yields the following calculation:
gpm = (15 gpa × 8 mph × 20 in) ÷ 5,940 = 0.4 gpm
This means, to apply 15 gpa at a speed of 8 mph with this sprayer setup, we need to select a nozzle with a flow rate of 0.4 gpm.
Now, we go to the nozzle catalogue, and find a nozzle that will give us a flow rate of 0.4 gpm, while operating the sprayer at an applicable pressure and travelling at 8 mph. Catalogues have charts for each nozzle, similar to the one shown on the next page. The first column gives the color code of the nozzle (which indicates flow rate), nozzle ID number, and the appropriate filter type for the nozzle. Column 2 gives the pressure range at which the nozzle should be operated. Column 3 gives the spray quality, a measure of spray droplet size (fine, medium, coarse, etc.) produced at different pressure settings. Columns 4 and 5 give the flow rate of nozzles in gallons per minute and ounces per minute, respectively, at different pressure settings. Column 6 gives gallons per acre application rate at different travel speed settings.
First, we need to find the best type of nozzle for our application. In their catalog, the nozzle manufacturer recommends a flat-fan pattern type nozzle for broadcast application of pre-emergence herbicides. Then we find a chart associated with the nozzle type recommended.
The chart shown happens to be for that type of a nozzle. Now we proceed with the process to determine the appropriate size of the nozzle.
Example of a typical nozzle rate table.
In our example above, the equation in Step 4 resulted with a flow rate of 0.4 gpm. Now, we look at Column 4 (gpm per nozzle) to determine the nozzle that provides us 0.4 gpm. Using the chart, we see that the nozzles XRC8004 or XRC11004 (shown in red) provide 0.4 gpm flow rate at 40 psi operating pressure. This nozzle also happens to provide Medium (designated with “M”) spray quality as recommended on the herbicide label. Under these operating conditions, this sprayer should apply 15 gpa at 8 mph as we expected. The validation of this is also evident on the chart. If you look at Column 6, choose 8 mph ground speed, the nozzle we selected will spray approximately 15 gallons per acre (14.9 gpa shown on the chart) at 8 mph travel speed and 40 psi spray pressure.
There may be multiple numbers of nozzles that can satisfy the 0.4 gpm flow rate requirements. However, they may not satisfy the desired spray quality and/or desired travel speed. It may be necessary to adjust pressure and/or travel speed according to nozzle selection. For example, the Brown XRC8005 nozzle is capable of producing 0.4 gpm, and achieving 15 gpa at 8 mph, if the spray pressure is reduced to about 25 psi. Similar calculations can be made using the equation below to come up with other GPM (flow rate) and PSI (pressure) combinations to satisfy the required 15 gpa application rate:
(GPM₁ ÷ GPM₂) = (√PSI₁ ÷ √PSI₂)
In this example, reducing the pressure to 25 psi alters the spray quality to “Coarse,” violating the label recommendation. When changing pressure is not an appropriate choice, the only other practical option is to change the travel speed. There is an inverse linear relationship between the travel speed (mph) and the application rate (gpa). The relationship is expressed by the equation:
(GPA₁ ÷ GPA₂) = (MPH₁ ÷ MPH₂)
or
(GPA₁ × MPH₁) = (GPA₂ × MPH₂)
Using the relationship above, we can determine that increasing the travel speed to 9.9 mph and keeping the sprayer operating at 40 psi will yield 15 gpa, as described below. The chart shown earlier indicates when using XRC11005, GPA₁ = 18.6 at 8 mph (MPH₁) at 40 psi. We want to find out what the new travel speed (MPH₂) should be to achieve 15 gpa (GPA₂). Using the equation above:
(18.6 GPA × 8 MPH) = (15 GPA × MPH₂)
so
MPH₂ = (18.6 GPA × 8 MPH) ÷ 15 GPA = 9.9 MPH
However, increasing travel speed to 9.9 mph may not be practical or safe. When changes to pressure or travel speed as dictated by the equations above are neither practical nor safe, it may be necessary to select a different nozzle.
In this example, it looks like the best nozzles to use for our application situation are XRC8004 or XRC11004, both providing 0.4 gpm at 40 psi. The only difference between these two nozzles is in the angle of spray pattern: one produces an 80 degree fan pattern (XRC8004), while the other one (XRC11004) produces a 110 degree fan pattern. Due to the difference in the angle of the spray pattern, each of these nozzles require different boom heights to obtain proper overlap between two adjacent nozzles.
Calibrate the sprayer
Selecting the right type and size of a nozzle is not sufficient to end up with accurate, effective and efficient application of chemicals sprayed. Changes in ground conditions (tilled, un-tilled, grass, wet, dry), and the topography of the field sprayed (flat, sloped) will affect the ground speed which is one of the variables used in determining the correct nozzle size. Nozzle orifices wear out with time causing larger flow rates and distorted spray patterns than when they were new. The gpm flow rate values given in catalogues or in Apps are based on spraying water only. Spraying solutions with higher densities than water (most spray solutions are) will affect the flow rates of nozzles at the same spray pressure. For the reasons mentioned above, sprayers should be calibrated frequently, especially when the field conditions change, to determine the actual application rate.
Calibration is easy, and there are many ways to do it. regardless of the method chose, three measurements will be taken:
- actual ground speed,
- the distance between nozzles, and
- nozzle flow rate for a given length of time.
One easy method is explained in an OSU Extension Publication (AEX 520) listed in the references at the end of this article.
Keep several types of nozzles on the boom
Remember that one specific type of nozzle will not be best for all applications. For this reason, it is best to have several types and sizes of nozzles on the boom so that you can switch to the “best” nozzle choice for a given spraying job. As shown in the pictures below, there are various types of sprayer components and setups you can buy to configure your boom so the new set up allows you to easily switch from one nozzle to another instantly.
Keep spray drift in mind when selecting nozzles
One of the major problems challenging pesticide applicators is spray drift, which is defined as movement of pesticides by wind from the application site to an off target site. Drift is influenced by many factors which are discussed in detail in two OSU Extension publications (Bulletin 816 and AEX-525) listed in the references at the end of this article. Equipment, especially the nozzles, used to spray pesticides play a significant role in generating as well as reducing spray drift. In nozzle catalogues, you can see a number of different nozzles of the same type, in terms of spray pattern. For example, one can find nozzles within the same “flat-fan” category classified as “low-drift.” Research conducted at Ohio State and elsewhere clearly indicate that nozzles labelled as “low-drift” significantly reduce spray drift as discussed in OSU Extension publication AEX-523 (listed in the references below). If drift is, or becomes a concern, it may be best to switch from a conventional flat-fan nozzle to a “low-drift” flat-fan nozzle with the same flow rate. Therefore, it is best to have more than one type of a “flat-fan” pattern nozzle on the boom.
Summary and conclusions
Nozzles are typically the least costly items on a sprayer, but they play a key role in the final outcome from a spraying job: achieving maximum efficacy from the pesticide applied while reducing the off-target (drift) movement of pesticides to minimum. Pesticides work well if the rates on labels are achieved during application. This can be achieved only if the right nozzle type and the proper size of the nozzles are on the sprayer, and the sprayer is operated properly.
Although the Apps and tables in catalogs may expedite the nozzle size selection process, it is best to understand the process and the math nozzle manufacturers use to generate the values listed in tables, and to generate nozzle recommendations in their Apps. This procedure, explained in this publication, hopefully will help you to determine the exact nozzle flow rate (gpm) required for your spray application parameters, while highlighting some other important parameters such as spray pressure, droplet size, spray coverage on the target, and drift, all of which should be given serious consideration when selecting the best nozzle for a spraying job.
Acknowledgments
The author thanks Mary Griffith, Agriculture and Natural Resources Extension Educator, OSU Extension; Dr. Larry C. Brown, Professor and Extension Specialist, Department of Food, Agricultural and Biological Engineering, The Ohio State University; and Dr. Robert “Bobby” Grisso, Professor and Associate Director, Virginia Cooperative Extension, Virginia Tech University, Department of Biological Systems Engineering; for reviewing this publication and for their editorial contributions.
References
1. Ozkan, E. Calibrating boom sprayers. Ohio State University Extension publication AEX-520, Columbus, Ohio.
2. Ozkan, E. New nozzles for spray drift reduction. Ohio State University Extension publication AEX-523, Columbus, Ohio.
3. Ozkan, E. and R.C. Derksen. Effectiveness of Turbodrop® and Turbo Teejet® nozzles in drift reduction. Ohio State University Extension publication AEX-524, Columbus, Ohio.
4. Ozkan, E. and H. Zhu. Effect of Major Variables on Drift Distances of Spray Droplets. Ohio State University Extension publication AEX-525, Columbus, Ohio.
December 2, 2016
How to evaluate airblast coverage
Note: While there’s nothing wrong with this article, a more recent article on this subject can be found here.
It’s nearing the end of a long morning of spraying and you just want to get it done. As the tank empties and you watch the last of the spray cloud waft through the row, you’re thinking about rinsing out and moving on… but did the spray land where you wanted?
How do you really know if you hit the target?
Maybe you’re content with the occasional “shoulder checks” you made from the cab while spraying. Perhaps you stop at the end of the row and get out of the tractor to look for wet foliage during.
Maybe you plan to return once the product is dry and look for white residue.
This early morning “shoulder check” was photographed by the operator using his smartphone. You can’t see coverage, but gaps in the spray will show if nozzles are plugged. You can also check to see if you are overshooting or blowing through the target. Photo Credit – C. Hedges, ON.
These are all good feedback practices, but a more accurate method is the use of water-sensitive paper, which turns from yellow to blue wherever spray touches it. You can easily see the distribution of the spray and the overall area covered, and it can be quantified so you can compare one sprayer set-up to another, or see the impact of weather, or even the effects of nozzle choice, pressure and water volume.
Water- (and oil-) sensitive paper: Cheap, simple and available on-line or in person from your favourite sprayer equipment store.
Draw a map
Begin by creating a simple drawing of the tree, cane, bush,vine, etc. you wish to spray. Label the drawing with unique numbers that correspond to where you are going to place the papers. Write the numbers on the back of each paper so you can see where they came from after they are collected. You should also note the pass number, so you can differentiate between each sprayer setup and corresponding pass. You might make a change and want to see how it affects coverage, and it’s very easy to mix up the papers if you haven’t record everything clearly. Plan to do this for at least two plants upwind from the sprayer to ensure you will get an accurate representation of average coverage. Be sure to wear disposable gloves and avoid dew so the papers don’t react prematurely.
Create a simple drawing of the target. Number positions on the drawing that correspond to where you plan to place the papers.
Distribute the papers
It is critical to distribute the papers evenly throughout each target canopy. They should be placed in key locations where pest damage has been an issue in the past (e.g. scab at the top of a tree, or spotted-wing drosophila at the bottom-centre of highbush blueberry), or anywhere coverage is notoriously difficult. Our preference is to place them at the top, centre and bottom of a tree canopy as well as laterally from the outer edge of the canopy beside the sprayer moving in towards the trunk.
Number positions on the drawing that correspond to where you plan to place the papers. Label the papers as well so you know where they came from. Consider writing the pass number and the position (e.g. 1-1 would be Pass 1, Position 1) so you can evaluate the changes to the sprayer settings from pass to pass. Later, all the information from the calibration can be entered into your spray records, like in this mock-up.
We use spring-back paper clips attached to alligator clips at 90 degrees to attach the papers to small branches. You can also staple them to the upper or lower face of the leaves (as long as they don’t cause leaf to droop). You can wrap them around stems for panoramic coverage or to monitor drenches. They can be stapled the trunk to show if spray is aimed into the canopy or being wasted. You can even skewer to the ground using wire flags to to illustrate poor lower-nozzle positioning and/or canopy run-off. Put them wherever you want to know about spray coverage!
This home-made double-ended alligator clip holds papers at right angles. One end for the paper, the other end to a twig or wire flag.
We typically orient them facing the alleys so their sensitive faces are square to the sprayer as it passes. We often use two in each location, oriented back-to-back facing each alley so you can resolve coverage from both sides. The important part is to ensure you are consistent. Mark the location in the canopy with some colourful flagging tape so you can find the papers after you spray, and if you wish to replace them with fresh papers to evaluate another pass, orient them the same way to make the comparison fair.
Water-sensitive papers located in five positions in an Empire apple tree. Two papers were pinned back-to-back in each position, distributed evenly throughout the canopy, facing the alleys. One paper was located at the lowest branch to determine if the lowest nozzle position needed to be on. Another paper was pinned to the ground face-up under the tree to show any excessive waste. Be consistent from pass to pass.
Spray, check and spray again
Once the papers are in place, pass by on one side with both booms open (as you would normally spray). Be sure to start spraying well before passing the target, and keep spraying afterwards to ensure the resultant coverage represents an actual application. It is very informative to get out of the cab and examine the papers before passing by on the other side. You can learn a lot about how the wind is affecting the spay.
Once papers are in place, pass by spraying with both booms open to emulate a typical spray day. Be sure to start spraying well before passing the target, and keep spraying afterwards to ensure the resultant coverage represents an actual application. It can be very informative to examine coverage at this point to see how wind is affecting the spray. Then, pass by on the other side to complete the application.
An example of the coverage obtained on water-sensitive papers placed throughout an apple tree canopy, and on the ground beneath it.
Interpret the patterns
You might notice the outer portions of larger canopies receive more spray than the inside. This is hardly surprising given that spray must pass through the outside to get to the inside. As a result, inner papers often receive proportionally less spray and should be the basis for determining if you have sufficient spray coverage. This is also why the label recommendation of “spraying to the point of runoff” is unhelpful: the outer portion of wide, dense canopies often begin to drip before the inner portion receives sufficient coverage. Further, how do you spray to the point of runoff? How do you know when to stop before it’s too late? Label language can be frustrating…
When water-sensitive paper is sprayed to the point of run-off, the blue dye will drip. This is fine for a drench (dilute) application, but excessive for a typical concentrated application like foliar fungicides and insecticides.
When assessing coverage, don’t follow the droplet counts in the small guide that comes with the paper sensitive paper kit – they haven’t been updated for a very long time and are more appropriate for field crop applications – not airblast applications. Research and experience suggest that 85 discrete fine/medium-sized droplets per square centimetre and a total coverage of 10-15% should be sufficient for most foliar insecticides and fungicides. Remember, this is only a suggested threshold and in the case of coarser sprays, focus more on even distribution and the 10-15% coverage.
It’s debatable, but 85 Fine/Medium-sized drops per square centimetre and about 15% total surface covered on a minimum 80% of all papers represents adequate airblast spray coverage for most foliar applications. It is less applicable for applications made with Coarse/Very Coarse droplets, because there are fewer of them and they generally cover more area. In this case, focus more on the even distribution of spray and the 15% coverage. An extreme example of this is a drench (dilute) application of oil where total saturation is the goal. Conversely, ultra-low volume applications employ Very Fine droplets and a better metric is uniform, high droplet density rather than area covered.
Make a change and try again
There’s no easy way to define a threshold between sufficient and insufficient spray coverage. When you retrieve and examine the papers, think about how the product is intended to work: “Is it a contact, trans-laminar or locally systemic pesticide? What are the odds that an insect or spore will come in contact with residue? Will I be spraying again soon (e.g. fungicide) and will the spray already on the leaves have residual activity?” Regarding that last thought, protectant fungicide applications are often layered, so what one spray misses, the next will catch. Quite often, “sufficient coverage” is less than most sprayer operators think.
If you are content with the coverage, record your sprayer settings to use them again in that block (in similar weather, and assuming the crop canopy doesn’t change significantly before the next spray day). If you are not content, make a change to the sprayer to improve matters, reset the papers, and go again. It can take time and some effort to get it right, but improved coverage and reduced waste are ample financial reward for your efforts.
Other methods of evaluating coverage
It should be noted that while water-sensitive paper is versatile, cheap and easy to use, it has its shortcomings. Placement and orientation of the paper is very important; it’s easy to hit papers on the outside of the canopy with the sensitive-side facing the sprayer. It’s considerably harder when they are at the very centre of the canopy, or hiding behind fruit. When the thin edge of the paper is oriented to the spray (i.e. oriented facing the ground), it presents very little surface and can be difficult to hit.
Use enough air to only just ruffle the leaves. This exposes all surfaces, however briefly, to the spray. Too much air will align leaves with the spray, exposing only their thin edge and making coverage difficult. Too much air may also cause leaves to shingle (overlap), and create shadows like on the grape leaves shown here.
Further, the papers won’t show the finest droplets (<50 µm), so there may be spray even though you can’t see it. Taken collectively with the product’s mode of action (i.e. contact or locally systemic), and any possible re-distribution by rain or dew, spray coverage becomes a good indicator for protection, but it isn’t definitive. While coverage is a good indicator, improved coverage does not always mean improved efficacy.
Some sprayer operators use other methods to confirm their coverage. Kaolin clay is an inert compound that leaves white residue when dry. Red, yellow or green water-soluble, food-grade dyes will also indicate coverage. Even fluorescent dyes such as phosphorus can be sprayed at night and illuminated under black lights.
Kaolin clay and fluorescent dies sprayed into fruit canopies give a lot of information about sprayer coverage, but are relatively inconvenient compared to water-sensitive paper.
Red food-grade dye sprayed from a horizontal boom to demonstrate downwind drift onto a white target. This was a messy experiment and my hands, and the sprayer, were pink for a long time afterwards. Photo Credit – J. McDougall, Ontario.
Take home
These methods give the sprayer operator a lot of information because they land on the actual target, not a piece of paper hung in the canopy. But, they require a lot of time and effort and are typically out of reach for most operators. Further, they do not allow multiple applications on the same canopy to compare the effect of sprayer settings on coverage – once the target is sprayed, it’s sprayed.
No matter which method you choose to use, understanding how changes to you sprayer, or the impact of weather, affect coverage is a critical piece of information. Operators should make an effort to evaluate spray coverage. Here are a few videos describing the process:
Using water-sensitive paper for airblast coverage diagnostics – thanks to Penn State, Univ. New Hampshire and Chazzbo Media (2014).
Checking water-sensitive paper in an orchard. Tower is spraying only water during a calibration run (2013).
November 28, 2016
The Pressure Gauge Shows More Than Pressure
Kim Blagborne (formally with Slimline Manufacturing) has long said that the pressure gauge on an airblast sprayer indicates more than just pressure. It can be used to diagnose a number of pump and plumbing issues… if you know what to look for. Here’s Kim’s troubleshooting guide to reading into what your gauge is REALLY telling you:
Scenario One
“As the tank empties, the pressure drops”
First, try adjusting the pressure regulator (assuming a positive displacement pump). If you can maintain the pressure up until the tank empties, your intake line may be loose and it’s sucking the bottom of the tank. Check the fitting between the suction filter and the pump. Apply a light coating of grease to the O-rings on the elbows and filter to ensure a complete seal.
Second, try stopping mid-tank (that is, turn off the tractor PTO and let the sprayer sit for a few minutes). Does the pressure gauge return to the original set pressure? If so, then the intake line inside sprayer has likely come loose entirely. Open the lid, and using a straightened-out coat hanger, hook the intake line and give a few gentle tugs – it should not be able to move. If it does, you’ll have to re-fasten the intake line so it’s not sucking the bottom of the tank.
The humble coat hanger. It opens our cars and now fixes our sprayers. Remarkable!
Scenario Two
“When I first start the sprayer, the pressure drops or fails to maintain constant pressure as the tank empties”
This might indicate improper mixing practices because the filter is probably plugging with product. Alternately, your PTO speed may be too slow to drive sufficient mechanical agitation. Check the suction filter as soon as the problem occurs (don’t finish spraying). If you wait to check when the tank is empty, the evidence of a plugged filter could be washed away before you can confirm it. This problem often happens when spraying nutrients, or when products aren’t compatible.
If that’s not it, it could be a collapsed suction valve. The pump will sound like it’s “missing” (like an misfiring engine). The suction valve might need to be replaced.
Or, perhaps you notice that you can compensate for the pressure drop by adjusting the regulator on the first tank. But it has to be dropped back down again for the second tank. In this case, the regulator might be sticking or jamming. Disassemble it and look for grit in the barrel of the regulator, then lubricate the parts.
Scenario Three
“I lose pressure when I turn my boom(s) on or off”
In this scenario, the pressure is fine as you approach the end of the row. You turn off the outside boom (or both) and finish the turn. But, when you re-engage both booms, the pressure drops. Even when you adjust the pressure regulator to compensate (assuming a positive displacement pump), the unit only gains the lost pressure slowly. In this case, the regulator might be sticking or jamming. Disassemble it and look for grit in the barrel of the regulator, then lubricate the parts.
Scenario Four
“The pressure gauge spikes when I turn off the boom(s)”
If you run a Turbomist, it could be the bypass balance. To solve this issue, head over to this article and pan down to see the step-by-step. If it isn’t the balance, then it’s likely the regulator. The issue of a spiking gauge and how to correct for it is covered thoroughly in this article by Ag mechanic extraordinaire Murray Thiessen.
Scenario Five (a positive displacement pump issue)
“My gauge pulses”
Is it more than a 20 psi range? Have you noticed that the deviation gets less as the PTO speed increases? Well, the pump pressure check-valve may have collapsed. Check the pressure check valves in the pump for broken springs on the suction valve plate.
Does the needle move rapidly through a 5 to 10 psi range? The accumulator pressure might be low. Try adjusting system pressure via the regulator and if that changes how the needle is responding, then set an air compressor to 90 psi (or manufacturer’s recommended pressure) and charge the accumulator.
Perhaps the needle movement is not affected by system pressure changes or the PTO speed. In this case the accumulator may have failed entirely and the diaphragm will need replacement.
Scenario Six
“My calibration is going farther than expected”
Sure, that sounds pretty good at first, but it may be that the gauge is stuck. With the PTO off and the spray boom on, the gauge must read “ZERO”. If it doesn’t, pony up the $50.00 and get a new one.
May 30, 2016