There are many factors that affect the work rate of an airblast application. If an operator can improve their work rate, without compromising spray efficacy or safety, they improve operational efficiency and save money.
But how does each variable factor in? Is it worth the cost of a tender truck and operator to fill more efficiently? Should you upgrade to a multi-row sprayer? Should your next planting have longer rows? We have a simple calculator that can help you make these decisions. You can build and compare multiple scenarios to explore the relative impact of small changes to your typical spray program. We recommend making only one change for each scenario so you can better understand the results. Print the comparison page for your records.
Whether you’re a sprayer operator, or a manager of sprayer operators, this exercise will help you see your spray program in a whole new light. Download a copy of the Airblast Budget and Work Rate Calculator and explore your productivity. You must have Excel to run the spreadsheet, and you must permit the use of macros (you’ll be prompted to accept).
Spoiler: It’s amazing how changes to travel speed have only a marginal impact on work rate. Often less than 60% of the total spray job is spent actually spraying!
If you’d like to see just how productive you can be, check out this rare (possibly unique) sprayer from Ed Oxley Farms in Michigan. Built on an OXBO 7550, this sprayer is the fourth iteration of a concept developed over the last 20 years by Ed Oxley Farms and ag engineers from Michigan State University.
Capable of spraying five rows at a time, this self-propelled beast is a hybrid wrap-around and targeting-tower system that uses CurTec spray heads equipped with tangential fans and wire-mesh basket rotary atomizers.
That’s not dribbling – that’s purging the boom prior to spraying.
It sprays a mere 150 L/ha (~ 15 gallons/acre) at a ripping 13 km/h (~8 mph), as seen on the Ag Leader monitor below.
When row spacing and turn time are accounted for, that means it’s capable of covering almost 15 hectares (~40 acres) per hour.
And, when not spraying grapes, the boom can be swapped to make it a high-clearance corn sprayer. It doesn’t get much more efficient than this.
The following videos will show the view from inside and outside the cab. Note that the row that’s straddled is sprayed from an overhead spray head mounted to the centre rack behind the sprayer. The two adjacent rows are covered from one side from vertical spray heads mounted on the chassis. Finally, the boom holds two more overhead spray heads for the outer-most rows.
Ideally, the boom-mounted spray heads would be suspended vertically inside the row, but it makes for such a wide turn radius that it would take too long to turn… assuming there was enough headland to allow it. They’re also swept-back to minimize the turn radius and reduce the amount of airborne spray that deposits on the sprayer itself.
A clever design that makes a few compromises to ideal coverage in order to improve productivity. The balance works for them and this sprayer might be a sign of things to come in horticultural crop production systems. Want to see how your sprayer stacks up? Download the calculator and see where you might be able to make improvements.
Sprayer math can be intimidating, but the effort gives solid value. When combined with a calibrated sprayer you reap the following benefits:
Estimate how long a job will take.
Estimate how much spray mix is required.
Estimate how much crop protection product must be ordered for the season.
Populate spray records which allow you to review practices, respond to enquiries and satisfy traceability requirements.
There are many ways to perform sprayer math, and you need only look to local pesticide safety courses, industrial catalogues, and extension resource centres for examples. If you’re already comfortable with your current method, don’t mix and match with others. Sprayer math is a series of related calculations that employ constants to keep the units straight. It’s all or none.
Walkthrough
Let’s start with the classic, US Imperial formula for calculating the required nozzle output. In other words, you want to know which nozzle size you need to get the volume-per-planted area you’re aiming for. This is the bread-and-butter formula that seems to be needed most often, so that’s why we list it first.
In order to determine nozzle size (gallons per minute), you’ll need to know your target volume (gallons per acre), your average travel speed (miles per hour) and your nozzle spacing (in inches). The number “5,940” is a constant that handles all the unit conversions. It is what it is.
GPM = [GPA x MPH x W] ÷ 5,940
Of course, this formula can be adjusted to allow you to solve for any factor, as long as you’re only missing one piece of information. Algebra is all about solving for X, or in other words, determining some unknown variable. I know, it’s been a long time since you learned this in school and it doesn’t come easily to most. I propose brushing up on the basics using a series of three great YouTube videos from “Mathantics“
As we noted earlier, you can do a lot more with sprayer math than just pick the ideal nozzle. But before we continue, a warning: If you live where units are strictly US Imperial, or strictly Metric, then Canada’s odd hybrid “Mock-tric” units can get a little confusing. The rest of this article attempts to be globally-relevant by including examples of both Metric and US Imperial formulae, but watch out for unit conversions. If at any time you don’t see the units you’re looking for, you can consult our exhaustive list of unit conversion tables.
Grab your calculator or favourite smart phone app – it’s math time!
Don’t be intimidated. With a little practice, sprayer math gets easier and it’s always worthwhile. The real trick is navigating unit conversions.
Step 1 – How large is the area you need to spray?
Multiply the length of the area you plan to spray times the width. If you are using metres, then divide the product by 10,000, which is the number of m2 in a hectare (ha). For feet and acres, divide by 43,560 which is the number of ft2 in an acre (ac):
Step 2 – How much product is needed to spray the area?
Consult the rate(s) shown on the label. In Canada, rates are often based on planted area (E.g. hectares). In Australia and New Zealand, they may be based on row length (not covered in this article). If you measure your area in acres, you’ll have to convert the rate by multiplying by a constant: 0.4.
Now multiply the area you want to spray (step 1) by the rate (step 2).
Step 3 – How far can you go on a full tank?
You know your sprayer output (determined through calibration) so you divide that into your tank size. Watch your units:
Step 4 – How much pesticide per tank?
Multiply the area that can be sprayed per tank (Step 3) by the pesticide rate (Step 2). Again, watch your units:
Step 5 – How much area is left to spray?
Just subtract what you’ve already sprayed from the total area.
Step 6 – How much pesticide in the last, partially-full tank?
Multiply the area you have left to spray (Step 5) by the pesticide rate (Step 2). Yes, watch your units:
Step 7 – How much spray mix will I need for the partial tank to finish spraying the total area?
Multiply the area you have left to spray (Step 5) by the sprayer output (determined through calibration). Guess what? Watch your units:
Sample problems
Time to test your knowledge. Let’s suppose you want to apply a product rate of 3 L/ha to your blueberries. You calibrate your sprayer and determine your output to be 50 L/ha. Your tank holds 400 L of spray mix. Your planting is 500 m long and 200 m wide.
Q1 – How large is the area you need to spray?
Q2 – How much product is needed to spray the area?
Q3- How much area can be sprayed on one tank?
Q4 – How much product should be added to a full tank?
Q5 – After the tank is empty, how much area is left to spray?
Q6 – How much product to add to the last, partially full tank?
Q7 – How much spray mix will be needed to finish spraying?
Exceptions
Certain situations aren’t covered in this article. If you are spraying a greenhouse, the math is different. If you are performing a banded application, the math is different. And, if you’re an airblast operator trying to reconcile why a pesticide label uses planted area rather than canopy volume for its rates, you’re in for a lot of additional reading. Some of that latter process can be summed up in this infographic:
When you find a method that works for you, write it down and keep it with your spray records. Happy spraying!
This article has been modified with kind permission from an article on Retrofit Parts.
Liquid pressure in an agricultural sprayer must be controlled so as to apply the correct volume of spray per hectare.
For sprayers that use positive-displacement pumps driven by the tractor’s power take-off, higher flow rates are produced than are actually required. Part of the pump’s output is therefore returned to the tank through a bypass line (aka return line), and spray pressure is controlled by a regulating or relief valve in this bypass line.
Ordinary regulators can maintain a constant pressure if the volume of liquid does not vary too much. But when it’s necessary to shut off part of the spray-boom in order to avoid double-spraying, (or in the case of an airblast sprayer, shut off one side), there is a large increase in the bypass flow and this causes the pressure to rise unless special compensating valves are fitted and correctly set.
Ramsay (or Ramsay Nocton) pressure-sets automatically maintain constant pressure over a far wider range of variation in flow-rate; for example the bypass flow through Retrofit’s model can increase from 1 to 100 L/min with an increase in pressure of only 0.2 bar (~3 psi). Lechler also offers 1 1/4″ and 2″ thread models called “AirPress” with different flow and pressure metrics. Each model allows the spray boom to be shut off with ordinary diaphragm check valves and also allows the operator to change gear (and so change the pump speed) without affecting the pressure.
Ramsay pressure-sets are non-electric, and the only moving part is a flexible diaphragm. The working pressure is set by inflating the pressure-set with air.
It is often desirable to vary the pressure while spraying, either automatically or manually. This can be done by a small electric air compressor and two valves which respectively increase or reduce the air pressure behind the diaphragm. These valves may be actuated by an electronic ground-speed-related system, or controlled manually by the operator.
How it Works
When the pressure in the IN chamber exceeds the air pressure it pushes the diaphragm away from the holes (as shown below) allowing some of the liquid to pass across to the OUT chamber, thus relieving the pressure (A backing plate, not shown in the diagram, prevents the diaphragm from being stretched too far).
A Ramsay valve with the diaphragm closed. Pressure in the IN chamber does not exceed the air pressure behind the diaphragm.
In practice, the diaphragm opens just as far as it needs to for the liquid pressure and the air pressure to come into equilibrium. When a boom is shut off and more flow needs to return to the tank, the diaphragm opens wider and remains in equilibrium in its new position.
A Ramsay valve with the diaphragm open. Pressure in the IN chamber exceeds the air pressure behind the diaphragm and can flow past to the OUT chamber.
Consequently, as long as there are no restrictions in the line back to the tank, the pressure remains constant even when there are large changes in pump speed or spray speed-boom demand. Pressure varies only slightly as the compression of the air in the air-reservoir varies with the movement of the diaphragm.
The calibration of handheld plot sprayers is an important part of agricultural research, and this article already covers all the bases… as long as you are spraying broadacre or row crops. But what happens when you are trying to emulate an airblast sprayer and treating a tree, bush, cane or vine?
The key difference is that spraying a two dimensional area requires the operator to pass the boom over the target at a uniform height and pace to achieve consistent coverage. But, a three-dimensional target requires the operator to circle the target, or spray from both sides, until it has received the required dose (or volume).
In order to scale down a typical airblast carrier volume for small plot work, we need to know three things:
The area you wish to treat (e.g. bush, grape panel, tree, etc.), including it’s share of the alley (in m2).
The emission rate from the calibrated plot sprayer (in US gal./min.)
The airblast carrier volume you wish to scale down (e.g. L/ha).
The illustration below shows two options for calculating the treated area. Option A requires you to measure from the outermost edges of the canopy (imagine if the canopy was wet and dripping – the dripline is that outermost point). It is less consistent than the preferred Option B, where the area is determined from row centres and planting distance.
Two options for scaling down an airblast carrier volume for small plot work. Both produce the same treated area, but Option B is the preferred method.Use the average planting distance and row spacing in metres. For a panel of grapes, use the centre of each panel as the planting distance.
If you are using a CO2 powered hand wand (preferred over a manual pump) with one or more hydraulic nozzles, then you can calibrate it using the methods in this article. There are battery-powered options from Jacto and Petra Tools, the latter offering a battery powered ULV system as well. Makita also has a battery entry (image below). However, if you are using a backpack mistblower, which better approximates an airblast sprayer compared to a hydraulic hand boom (see this article), it requires a different approach. Plus, you get to look like a Ghostbuster, which is a win in my book.
Follow along in the following images as we explore how to calibrate a backpack step by step:
When transporting a mistblower, use a loop of nylon cord to secure the boom in an upright position.For calibration, fill the completely empty sprayer with a known volume of water. If the boom is gravity-fed, be sure the feed valve is closed so the water doesn’t run out of the boom.With the sprayer on the ground, brace it with your foot. Step on the metal frame, not the motor housing or tank. Follow the operating instructions to pull start the motor.Being cautious of the hot exhaust, set the sprayer on a tailgate, or other elevated surface to facilitate strapping it on.
Be aware that most mistblowers use gravity to feed the spray mix from the tank to the boom. A pressure pump kit is recommended for applications where the spray tube is held upward more than 30 degrees to maintain a consistent discharge rate. A hip belt is also recommended to reduce fatigue. Examples are shown below are for Stihl-brand sprayers. Some may or may not require the pump (e.g. Tomahawk) but they are primarily intended for mosquito control and in that case a consistent rate over a vertical plane may not be as important.
If your sprayer does not have a pump kit, pointing the boom upward will cause spray to slow or even stop. This greatly diminishes your ability to reach high targets and achieve consistent coverage. In this case, attach the deflector (which comes with the sprayer) before proceeding with the calibration.
Deflectors angle the spray upwards without having to lift the boom. This is easier on your shoulder and keeps the rate consistent.
Set the flow rate to the preferred setting (usually a dial at the end of the boom), and using a stopwatch, time how long it takes to spray the entire volume. Be sure to move the boom exactly as you would when spraying the target, either side-to-side or up-and-down, to capture possible rate changes from the gravity feed. Convert the output to US gal./min.
When timing output, move the boom as you would when spraying the target.
Alternately, some people will stand on a bathroom scale with the backpack full. Then get off and spray for a period of time. Then get back on the scale. One millilitre of water weighs one gram, so you can calculate the flow from the weight difference.
Now you know the area and the emission rate. You should have a target carrier volume in mind (e.g. L/ha). Using the following example, let’s determine how long you need to spray the target:
A sample calibration.
In this example, an ideal airblast Carrier Volume [C] for the orchard is 400 L/ha. We want to scale this down to determine the Volume for Treated Area [V]. First, divide [C] by 100 to convert it to 40 mL/m2. Then, because in Canada our nozzles are in US units, we do an ugly conversion: Since 1 mL = 0.000264 US gallons, [C] becomes 0.0106 US gal./m2.
The Treated Area [A] measures 3.5 m by 2 m = 7 m2.
The Emission Rate [R] is the rate the plot sprayer sprays. While we prefer using a mistblower, many still use a hand wand with no air assist. In this case let’s suppose we are using a hand wand with two 8002 flat fan nozzles operating at 40 psi. According to our calibration, we confirm it sprays 0.4 US gal./min.
[C](US gal./m2) × [A](m2) = [V] (US gal.)
0.0106 US gal./m2 × 7 m2 = 0.074 US gal.
We know we want to spray the target with 0.074 US gal., and we also know [R] which says our boom emits 0.4 US gal./min. We convert this to seconds by dividing by 60, so [R] = 0.0067 US gal./sec. From this we can calculate how long [T] we must spray the target.
[V](US gal.) / [R](US gal./sec.) = [T](seconds).
0.074 US gal. / 0.0067 US gal./sec. = approximately 11.0 seconds.
So, we know that to spray the target with an equivalent 400 L/ha, we must achieve consistent coverage from all sides by spraying it for a total of 11 seconds. Pro tip: Always mix a little more spray volume than you will need to account for priming.
This is only one way to calibrate a backpack sprayer for spot spraying. If it’s isn’t quite what you need, check out these resources:
It’s the rite of passage of many agricultural summer students across the world: applying experimental treatments to field plots using a research sprayer. The results of these experiments may be the basis of new product use registrations, or provide clues into future scientific studies. Needless to say, the application method needs to be bullet proof to ensure the results are reliable. Here are a few guidelines, starting with some tips:
Pro Tips:
When assembling a hand-held boom, ensure the threads are properly sealed using Teflon tape. More or less tape can be used to create a snug fit at the right part of the thread rotation.
2. Choose nozzle bodies with diaphragm shutoff valves. These valves stop flow below 10 psi and prevent dripping of the nozzles after shutoff, without pressure drop during operation.
3. Avoid the use of older style “check-valve strainers”. Although these also prevent drips, they create a pressure loss of about 5 psi which creates uncertainty around the actual spray pressure.
4. Install a trusted pressure gauge on the handle of the sprayer in clear view for the operator. This provides important information. Don’t believe the gauge on the regulator. Ours, for example, is stuck at 30 psi.
5. For hand-held booms, rotate the booms so that the nozzles point down, for each application. Different size people or height of crops will change this angle and make accuracy more difficult.
6. Set the boom height so that you achieve 100% pattern overlap. This means that a nozzle’s pattern width should be twice the boom’s nozzle spacing. Boom height will be close to 50 to 55 cm above target, depending on fan. Too low, and the pattern may cause striping. Your supervisor will see that all year long and think of you.
7. You can test the spray pattern by applying water to a concrete pad. At the right boom height, the entire boom width should dry at a similar rate.
8. Install a visual guide for boom height. For example, place a wire flag at the end of the plot, at the correct height. This will provide a handy reference of boom height as your arms get weary. Or hang a wire, zip tie, or chain from a spot that doesn’t interfere with your spray pattern (thanks ACC).
9. Minimize weight by using smaller bottles of CO2. We use 20 oz paintball bottles, they are much lighter, last long enough, and can be legally refilled with liquid CO2 or topped up with gas from a nurse tank in the field.
10. Spray out leftover mix in a designated part of the plot area. Do not pour any mix on the ground. Please. Consider a biobed on your research farm.
11. When completing a treatment, spray the boom completely empty so air comes out of each nozzle. This provides certainty that the next liquid at the nozzles is from the next bottle, be it water or another treatment.
12. When spraying dose responses of the same product, always start with the lowest dose. Again, spray out in a designated place until the boom produces air, no need to flush.
13. Construct a boom hanger from electric fence posts and coat hangers. Nozzles face down and can be serviced. The boom should never lie on the ground.
14. Use nozzle screens to prevent time delays due to plugging. Usually 50 (blue) or 80 (yellow) mesh is sufficient. Any finer mesh may interfere with some dry formulations. Note: Beware old screens – ISO mesh colours have changed. Learn more here.
15. It’s very useful to apply research sprays with low-drift nozzles. Air-induction tips are most effective. These reduce drift, and are also closer to the commercial spray quality used by producers.
16. 01 size (orange) air-induced nozzles are available from Albuz (AVI Twin and AVI), Arag (CFA, CFAU, AFC), Billericay (Air Bubble Jet), Greenleaf (AirMix and TurboDrop XL), Lechler (ID3 and IDK). No other major manufacturer produces this small size of tips in air-induction.
17. 015 size tips (green) and larger are produced by the above, as well as Albuz (CVI Twin and CVI), Hypro (GuardianAIR or ULD) and TeeJet (AIXR, AI, and TTI), within both manufacturers listed in order of increasing coarseness.
18. Always carry several other nozzles of the same size and type already on the boom. Should a nozzle plug, replace it, don’t clean it. Clean it later.
19. If a nozzle plugs and there is no extra nozzle, use compressed air to clean it. Compressed air electronics cleaners are available in most electronic stores.
20. If a plugged nozzle can’t be cleaned, simply place it at the end of the boom and continue. Plot ratings and yields are usually taken from the centre. Remind your supervisor of this.
21. Always de-pressurize a sprayer before disconnecting any liquid hoses. You can’t rely on check valves. If two people work together, make sure you practice and communicate this with each other.
Calibration:
Assemble the sprayer and run water through it to ensure it’s free from silt or residue. Repair leaks.
2. Install nozzles and ensure none are plugged and the pattern looks good.
3. While spraying water, set pressure to what you intend to spray with. (Note: boom pressure will be lower than regulator (attached to CO2 canister) by a few psi, hence the separate pressure gauge on the boom. Also note that the set pressure will always be higher when the system is at rest.)
4. Obtain four containers of similar size that can hold about 500 mL, and place on ground at nozzle spacing. Using stopwatch, emit spray directly into all four for a set time, say 30 s.
5. Expected spray volume at 40 psi: 01 tip, 380 mL/min; 015 tip, 570 mL/min; 02 tip, 760 mL/min. In other words, from a 2 L bottle you’ll not get much more than 30 s spray time from 4 tips.
6. Measure collected volume from four tips using the same graduated cylinder.
7. Repeat, for total of three times.
8. Average three reps for each nozzle and convert to mL/min. Make sure all nozzles are within 5% of the average flow. Replace those that aren’t or place worst offender on outside edge of boom.
9. Advance to “Calculations”, but be prepared to conduct another calibration
Now for the fun part.
Calculations
There are three options for applying the correct amount. We’ll be using metric in these examples:
Use the average nozzle flow from the calibration (mL/min) and the target application volume (L/ha) to calculate the necessary walking speed (km/h);
or
Use the flow from the calibration and a set walking speed to arrive at an application volume;
or
Use a set walking speed and a set application volume to calculate a required calibrated flow.
Option 1:
Walking Speed = (60*flow)/(Volume*nozzle spacing)
If your nozzle flow was 330 mL/min and you wanted to apply 100 L/ha using a sprayer with 50 cm nozzle spacing, your required walking speed is 60*330/100/50 = 3.96 km/h
Option 2:
Application Volume = (60*flow)/(Speed*spacing)
If your nozzle flow was 330 mL/min and you wanted to walk 5 km/h using a sprayer with 50 cm nozzle spacing, your application volume is 60*330/5/50 = 79 L/ha
Option 3:
Required flow = (Speed *Volume*spacing)/60
If your speed is 5 km/h and you wanted to apply 100 L/ha using a sprayer with 50 cm nozzle spacing, your required flow is 5*100*50/60 = 417 mL/min
If you selected Option 3, you now need to return to your sprayer and find a nozzle, or a pressure, that delivers an average of 417 mL/min. You can use math to get into the ballpark with the nozzle you already have:
New Pressure = (required flow/calibrated flow)2*calibrated pressure
If your required flow is 417 mL/min and the calibrated flow is 330 mL/min, and you calibrated at 30 psi, then you should be close to your required flow at (417/330)2*30 = 48 psi
Now, return to your sprayer, set the pressure to 48 psi, and confirm this estimate.
We use Option 3 when comparing nozzles of the same size but from different manufacturers. It’s not uncommon for these to have slightly different outputs. Rather than adjusting our walking speed slightly, which is very difficult to do accurately, we change pressure slightly so all nozzles produce the same flow. This is also useful when comparing water volumes by switching to a larger nozzle.
Travel Speed:
The last step is to confirm travel speed. Say you want to walk at 5 km/h. The best way to calibrate walking speed is to measure a known distance (m) in the field you’ll spray. Wearing the gear and carrying the sprayer you will use to spray, walk this distance. Use a wire flag to mark the start and end points; when the boom hits the flags, start and stop the timer. Repeat until comfortable.
Time needed to walk distance:
Time (s) = Distance *3.6/required speed
Say your walking distance is 10 m, and you need to walk 5 km/h.
10*3.6/5 = 7.2 s
A simple spreadsheet that can be used for the calculations can be found here.
Congratulations! You’re done. Happy spraying! Remember to not worry too much about a 5% deviation from your expected application. That’s definitely an acceptable error, as long as you don’t allow too many of those to add up.
Low Volume Research (Aerial)
Some product uses are by air, and the label volumes for those are often 30 to 50 L/ha. Registrants need to provide efficacy data at those volumes. Ground application can be accepted as a surrogate for aerial as long as the volumes are correct.
Since the spray nozzles aren’t typically available below the 01 (orange) size and if they are, they usually plug so easily and make such a fine spray that they’re frustrating to use. The alternative, to travel faster, is also problematic on research plots.
We recommend that Turbo TeeJet nozzles be used for this purpose. They produce such a wide fan angle that a 100 cm spacing is justifiable. Simply cap off every second nozzle body. Booms need to be elevated to ensure overlap, for uniformity. The value of the small nozzles and wider spacings is the low total application volume that is now possible.
The TT tips can also be used at fairly low spray pressures (say 20 psi) further reducing their output.
Spray Quality of TeeJet Turbo TeeJet (ASABE S572.1). This tip is available in smaller sizes and, due to its wide fan angle, can be used at 40″ (100 cm) spacing, therefoe applying low water volumes.
