Category: Rates and Calibration

For basics category

Alternate Row Spraying

Alternate Row (aka Alternate Row Middle [ARM]) spraying is an application method where the air-assist sprayer does not pass down every alley during an application. The sprayer operator is relying on the spray to pass through one or more rows and provide acceptable coverage to the entire canopy (or canopies) on a single pass.

Some state agencies promote this spraying strategy to various degrees, and many sprayer operators (whether they admit it or not) have used this method of spraying. I have advised it myself for very young and/or very sparse vineyard and orchard plantings, but never without confirming coverage. When I tell operators that I have serious reservations about alternate row spraying, they defend it. Here are the most common justifications I’ve heard over the years, and my response:

Justification	Reply
“I do not have enough spray capacity to spray every row when time is short.”	You need more sprayer capacity. Get another sprayer so you can get spray on in time or invest in a multi-row sprayer is possible.
“ARM spraying saves money and reduces environmental impact because I use less pesticide.”	Technically, if you travel every second row with a sprayer calibrated to travel every row, you have indiscriminately reduced your carrier and chemical inputs by half (or more). Without close monitoring you may compromise your efficacy.
“I only perform ARM spraying early in the season when canopies are empty, or only on young plantings.”	I grudgingly grant this one as long as coverage is closely monitored. I’ve prescribed it myself in young or sparse plantings where I couldn’t get the sprayer output low enough to prevent drenching the targets.
“The spray plume in the alley beyond the target row must mean the spray is providing adequate coverage. More is better!”	If the spray is blowing through the canopy, it isn’t landing in the canopy. Further, if the air speed/volume is too high, droplets can ‘slipstream’ past the target without impinging on them. I’ve removed water-sensitive paper from canopies with barely any spray on them despite the plume in the downwind alleys. It looks like a magic trick, albeit an unhappy one.
“Uncooperative weather doesn’t always leave me enough time to spray the entire crop, and it is the lesser of two evils to spray alternate rows than not at all. I’ll make sure I come back to spray the other rows later.”	Choosing to do half a job requires an understanding of the products’ mode of action. If you are spraying an insect at a particular stage of development, there’s no “coming back later” to get that generation – if you missed, your window has closed. If it’s a protective fungicide that offers no kick-back, then once the disease has infected tissue, the damage is done. Get the spray on as best you can, but if it washes away before it has a chance to dry sufficiently, be prepared to reapply at the earliest opportunity as long as the label allows it.
“ARM has always worked in the past.”	Would you mind picking my lotto numbers for me? You’re a very lucky person!

My reservations about ARM spraying come from published research and personal experience that show that coverage is almost always compromised when spraying from one side of a canopy. The spray must pass through the canopy to reach the far side, and the canopy filters droplets from the air as it passes through. This reduces the number of droplets available to cover the far side. In addition, high velocity spray will create “shadows” where any targets on the immediate far side of a leaf or branch become shielded and receive little if any coverage. Further still, fine droplets slow quickly as they leave the nozzle and take a long time to settle. As the entraining air slows and becomes erratic, the droplets float and change course, making their behaviour hard to predict.

The cumulative impact can be seen in this infographic I built in 2016. The orchardist was a dyed-in-the-wool ARM applicator and he was resistant to driving every row because it took so much time. I wanted to show that he could claw back some of the lost time by spraying less pesticide every row versus his current volume every second row. He would need fewer refills, and save a LOT of unnecessary pesticide. The water sensitive paper does the talking, and while I’d like to think I’ve convinced him, I’ll bet he’s still out there dicing with fate.

A very popular argument in favour of ARM spraying comes from orchardists that are shifting from semi dwarf to high-density plantings. They ask “How it is different to spray a four foot diameter tree from one side compared to an eight foot diameter tree from both sides”?

Well, we know coverage is reduced as a factor of distance. Spraying from one side gives a single opportunity to cover the middle and far side of a canopy, whereas spraying from both sides provides an opportunity for an overlap in coverage. Essentially, the centre of a canopy receives the cumulative benefit of two sprays. Coverage is therefore always improved when spraying from both sides, period.

Spraying from one side gives a single opportunity to cover the far side of a canopy. However, spraying from both sides provides an opportunity for an overlap in coverage. In other words, the centre of a canopy receives less spray than the outside, but is essentially sprayed twice resulting in a compounding effect.

Why, then, do some sprayer operators claim that alternate row applications work? Because sometimes, they do! Just because coverage is reduced doesn’t mean it isn’t sufficient to protect the crop. It simply means that the potential for poor coverage and reduced dose is dramatically increased by alternate row applications. A sprayer operator might perform alternate applications successfully for years before conditions conspire to defeat the application: unfavourable wind, poor timing, increased pest pressure, poor pruning practices, excessive ground speed, high temperatures, low humidity, insufficient spray volume, and several other factors might occur simultaneously and reduce coverage below a minimal threshold for control. This confluence of bad luck may not happen the first year, or the second, but eventually…

Product failure isn’t the only concern. Repeated reduced dosages may play a role in developing resistance. In those situations where the operator recognizes insufficient coverage, they may have to spray more often to compensate, negating any savings in time or product. Reduced dosage is a common error when a sprayer operator elects to use ARM.

If you still aren’t convinced, at least perform alternate row spraying the “right” way. Here are three situations that I’ve heard operators refer to as alternate row spraying. Situation 1 is most common, but to my mind only Situation 2 would be considered acceptable. Even then, confirming coverage is a must.

Situation 1:

The sprayer has a typical calibration for spraying every row, but only drives alternate rows. The first application (solid line) covers different rows from the second application (broken line). The operator will claim to spray more frequently, but generally does not perform the second application unless there is high pest pressure. The result is half-a-dose per hectare per application.

Situation 2:

The sprayer is calibrated for double output compared to a typical every-row situation, and the operator drives alternate rows. The result is that the hectare gets the whole dose per application, but coverage is always inconsistent.

Situation 3:

Since the sprayer will only drive alternate rows, the operator mistakenly sets the sprayer to emit half the output compared to a typical every-row situation. The first application (solid line) covers different rows from the second application (broken line). The result is a quarter-dose per application, and if the operator chooses to spray a second time, the hectare will only ever get half-a-dose. Yes, this happens.

So, my final word on alternate row applications is that they should be performed with extreme caution. I’ve used them myself in early season applications in new plantings, but never without confirming coverage with water-sensitive paper, and never in conditions that might further compromise coverage to the point that the application does not give control.

Caveat Emptor!

Well, I thought it was funny. My apologies to J. Luymes from British Columbia (pictured) and Obi Wan Kenobi (not pictured… or is he?)

May 20, 2025

Determining Airblast Travel Speed – The “Air Displacements” Method

The concept of Air Displacements was developed by Dr. David Manktelow, Applied Research and Technologies Ltd.

What is the “right” speed to drive when spraying?

Airblast sprayer operators must know their average travel speed to calculate how much pesticide and time is required to complete a spray job. Note that it’s an average, not a constant, because travel speed is significantly affected by ground surface conditions (e.g. slippage), grade (e.g. hills) and the weight of the rig (e.g. as spray mix is depleted).

The pursuit of productivity and the unchallenged status quo of traditional spray volumes, blinds many operators to the fact that travel speed is a critical factor in focusing air energy on the target canopy. As long as droplets are small enough to be entrained and directed by the air, we believe that optimizing the fit between air energy and the target canopy leads to the most frugal and effective use of spray mix and should therefore dictate travel speed. If that speed proves to be painfully slow, or terrifyingly fast, then a mismatch is revealed between the sprayer design and the operational conditions and the overall spraying strategy should be reconsidered.

This article describes a method for modelling an ideal travel speed. It can be used as a sanity check for existing operations or for those seeking to evaluate the fit of a new airblast sprayer. However, this method can only approximate travel speed. A true optimization of sprayer settings will require fine tuning using the ribbon method and, ultimately, coverage feedback from water sensitive paper (see here and an older article here). We’ll begin with how to measure average travel speed.

How to measure average travel speed

Beware the tractor speedometer or rate controller that monitors wheel rotations; both can be fooled by changes in wheel size, tire wear or slippage. GPS or radar-based speed sensors are the most accurate method.

Those that prefer a manual method can follow this classic protocol for determining average travel speed:

Go to a row that is representative of the terrain in your planting. Measure out a distance of 50 m (150 ft) and mark the start and finish positions with wire marker flags.
Fill the sprayer tank half full of water.
Select the gear and engine speed in which you intend to spray. If using a pull-behind sprayer, ensure the PTO is running or you could introduce errors.
Bring the sprayer up to speed for a running start and begin timing as the front wheel passes the first flag. This is far easier when there are two people.
Stop the timer as the front wheel passes the second flag.
Stay out of any ruts and run the course two more times.
Determine the average drive time for the three runs (i.e. the sum of all three times in seconds divided by three).
Finally, calculate travel speed using one of the following formulae, depending on preferred units:

Ground Speed (km/h) = Average drive time for 50 m (s) ÷ 13.9 (a constant)

Those that prefer a less accurate but convenient hack can download any smartphone speedometer app that can calculate an average (similar to a runner’s GPS wristwatch). Fill the sprayer tank half full and drive a representative section of your operation with the fan on and the spray off. Consult the phone for your average speed for each pass. Take a screen shot and email it to yourself as a time-stamped component of your spray records.

The “Air Displacements” method

Dwell time

Airblast sprayers use fans to move a volume of air at a certain speed, often measured in m³/hr or ft³/min. Imagine that volume of air as a three dimensional shape extending from the air outlet over a distance. Likewise, imagine the void between the sprayer outlet and the target canopy as a three dimensional shape penetrating roughly halfway into that canopy (assuming we intend to spray every row).

How long must the sprayer dwell in one spot before it pushes all the intervening air out of the way and replaces it with spray-laden air? If the sprayer drives too slowly, it will wastefully push spray through and beyond the target (i.e. blow-through). If the sprayer moves too quickly, the spray will not have an opportunity to penetrate the target canopy and most certainly not reach the highest point. This concept of focusing air energy using travel speed is called Dwell Time.

We want to calculate the volume of air the sprayer generates, compare that to the volume we want displaced, and then determine how fast we must drive to optimize the fit. We can do all this with a tape measure, an anemometer, and a partner to record the data and do a little math.

1. Measure air outlet area

With the sprayer safely off, measure the area of the air outlet(s) on one side of the sprayer. We’ll use a Turbomist 30P Low Drift Tower (below) as an example. There are two air outlets that are 5 cm wide by 150 cm high for a total area of 0.075 m² on each side. Be sure to look inside the outlet for any irregularities like baffles or obstructions intended to block air. Subtract those areas from the total. Don’t worry about small things like nozzle bodies.

For rectilinear outlets: Height (m) x width (m) = Area (m²)

For circular outlets: 3.14 x radius² (m) = Area (m²)

The air outlet on this Turbomist 30P Low Drift tower sprayer is 5 cm wide by 150 cm tall for a total area of 0.075 m².

2. Measure air speed

First, a few safety warnings: High speed air is loud and can carry debris, so always wear ear and eye protection and respect the hazards inherent to working with air-assist sprayers. Only use an anemometer rated for at least 160 km/h (100 mph) (e.g. here). Do not use a handheld weather meter such as a Kestrel because the impellor could be destroyed and become dangerous shrapnel.

Use an anemometer rated for at least 160 km/h (100 mph) (e.g. here). Do not use a handheld weather meter such as a Kestrel because the impellor could be destroyed and become dangerous shrapnel.

Bring the fan up to speed and holding the meter about 25 cm (10 in.) from the outlet, measure the air speed at several locations along the air outlet both vertically and horizontally. We calculate an average speed because many air outlets do not produce uniform air speed or volume along their outlets. For this example, we measured four locations along the air outlet on both sides of the sprayer and saw significant differences. We did this both in low and high gear (see table below).

	High Gear	High Gear	Low Gear	Low Gear
*Location Along Outlet*	*Left Side (m/s)*	*Right Side (m/s)*	*Left Side (m/s)*	*Right Side (m/s)*
Top 1/4	41.1	80.3	42.9	24.6
Upper	34.9	32.2	26.4	30.8
Lower	30.8	30.0	24.0	26.4
Bottom 1/4	33.5	40.2	26.8	31.3
Average	35.1	45.7	30.0	28.3

Anemometer readings from the low drift tower sprayer outlets, on left and right side, in high and low fan gear. Four readings from bottom to top to determine the average. Readings taken 25 cm from edge of outlet and PTO set to 540 rpm.

Multiple air outlets

Before we continue with the method, let’s change sprayers to this Turbomist 30P Grape Tower (below). The design is intended to spray adjacent rows from the vertical outlets (5 cm x 150 cm = 0.075 m²) along the tower. The upper, inverted outlets (10 cm x 63.5 cm = 0.0635m²) throw spray over the adjacent rows and cover the outside rows. The intention is to improve productivity by covering four rows of grape (or possibly three) per pass.

The Turbomist 30P Grape Tower Sprayer is a multirow system intended to drive every third or fourth row.

Lower, vertical ducts are 5 cm x 150 cm = 0.075 m²

Upper, inverted ducts are 10 cm x 63.5 cm = 0.0635m²

However, when we consider this design through the Air Displacement lens, it’s almost like having two sprayers performing two jobs simultaneously. The vertical outlets and the upper, inverted outlets are different shapes. Further, their position (distance and angle, as the top outlets are angled back more aggressively) relative to their respective target canopies are significantly different. How fast must this sprayer drive to optimize the fit? Do we have to compromise coverage and incur drift and waste from one set of outlets to accommodate the other set? The manufacturer has worked to address this potential issue by partitioning the majority of the air energy to the top outlets, but let’s see how that affects travel speed.

3. Total volumetric flow

Having already measured the outlet area, we then measured average air speed (see table below).

	High Gear	High Gear	Low Gear	Low Gear
*Location Along Outlet*	*Left Side (m/s)*	*Right Side (m/s)*	*Left Side (m/s)*	*Right Side (m/s)*
Top Outlet	27.0	26.5	27.0	26.0
Bottom Outlet	12.0	13.0	10.5	12.5

Average anemometer readings (n=4) for top and bottom outlets, on left and right side, in high and low fan gear. Readings taken 25 cm from edge of outlet and PTO set to 540 rpm.

Now we can use these two values to determine how much air the sprayer generates by calculating total volumetric flow. We first have to convert air speed from m/s to m/h to make the units work, so just multiply it by 3,600. Then we multiply that by the outlet area and we get the table below.

Average air speed (m/s) x 3,600 (a constant) = Average air speed (m/h)

Average air speed (m/h) x Outlet area (m²) = Total volumetric flow (m³/h)

	High Gear	High Gear	Low Gear	Low Gear
*Location Along Outlet*	*Left Side (m³/h)*	*Right Side (m³/h)*	*Left Side (m³/h)*	*Right Side (m³/h)*
Top Outlet	6,172.0	6,058.0	6,172.0	5,944.0
Bottom Outlet	3,240.0	3,510.0	2,835.0	3,375.0

Total volumetric flow for top and bottom outlets, on left and right side, in high and low fan gear, with PTO at 540 rpm.

4. Target volume to displace

Now that we know the volume of air the sprayer generates, let’s determine the volume of air we need to replace with that spray laden air. This is really the only tricky bit because you have to picture a cross section and then measure the shape. See the illustration below.

For the bottom outlet, it’s simple. The outlet is 81 cm from the grape panel and the grape panel is 112 cm high. We calculate the area of a rectangle by multiplying length by width, so:

Length (cm) x Width (cm) = Area (cm²)

However, the sprayer design makes the top outlet’s job trickier to figure out. This isn’t a rectangle, it’s a “quadrilateral”. We get this odd shape when either the sprayer outlet or the target canopy are significantly taller than the other. Fortunately this one has a right angle so we don’t have to brush off our high school trigonometry textbooks. Instead, we can lean on the internet using this link and plug in the values. As we can see below, the cross sectional areas spanning from the outlets and the middle of the target canopies are 0.9 m² for the bottom outlet, and 2.35 m² for the upper outlets.

This gives us a cross sectional area, but we need to convert that to a volume so we can compare the air generated to the air needed. To do that, we multiply the cross sectional area by 100 m, representing how much air would be needed over 100 m of row length. The formula and the results are presented below.

Cross sectional area (m²) x 100 m of row length = Target displacement volume (m³)

*Outlet*	*Target Displacement Volume (m³)*
Top Outlet	235.0
Bottom Outlet	90.0

Target displacement volume for each outlet over 100 m of canopy row.

5. Displacement rate

We see the target displacement volumes for each outlet are significantly different. Assuming the air from the upper outlet maintains its integrity and reaches its target canopy without being blown off course, it must produce enough air energy to fill more than twice the displacement volume of the lower outlet. We can see from the earlier calculations that it does produce almost twice the total volumetric flow. But is it enough? To know we must calculate the Displacement Rate for each outlet. Let’s just focus on the left side of the sprayer in high gear.

Total volumetric flow (m³/h) ÷ Target Volume (m³) = Displacement Rate ( displacements/h)

*Outlet*	*Displacement Rate (displacements/h) for left side of sprayer* *in high gear*
Top Outlet	26.25
Bottom Outlet	36.0

Displacement rates for the outlets on the left side of the sprayer in high gear.

So we see that the outlets at the top of the sprayer, if stationary, could displace the target volume of air 26.25 times an hour. However, the lower outlet would displace its target volume 36 times in that same hour. We see that we might have a problem. But this is for a stationary sprayer and not a sprayer in motion. The last step gives us what we came here for.

6. Ideal travel speed

We can now determine the ideal travel speed for this sprayer using that same 100 m row length.

[Displacement rate (displacements/h) x 100 m of row length] ÷ 1,000 (a constant) = Ideal travel speed (km/h)

*Outlet*	*Ideal travel speed (km/h) based on left side of sprayer*
Top Outlet	2.6
Bottom Outlet	3.6

Ideal travel speed for each outlet on the left side of the sprayer in high gear.

As we stated at the beginning of this article, this is only a model. It doesn’t account for canopy density and assumes the spray laden volume of air produced by the sprayer can reach the target intact over a given distance. However it does indicate that there is a potential issue that will lead to either over spraying the adjacent row (slower travel speed) or under spraying the distant rows (faster travel speed) which could lead to waste, drift and poor coverage.

In the image below, we chose to drive close to 2.6 km/h in high gear. No effort was made to adjust the liquid flow (i.e. change the nozzles) so there was too much spray volume here, but we can see the losses on the left (upwind) side, and the blow-through three rows over on the right (downwind) side. Leaving aside the excessive liquid volume, we could drive faster or reduce the fan gear to reduce the blow-through on the adjacent rows, but we may go too fast (or reduce the rate of air displacement) for the upper outlets to reach the target. We can already see the integrity of the upper-left outlet breaking down as it sprays into the wind.

Testing a travel speed. No effort was made to adjust liquid flow, which is excessive here. Cross wind was from the left to the right in the image. Photo by Corey Parker (Instagram: _parkerproductions)

Take home

An ideal travel speed for an airblast sprayer is more than just being productive. The spray must reach and penetrate the target. If this requires dangerously high speeds, or if you simply can’t move slowly enough, it suggests a problem with the spraying strategy. Changes will have to be made to the sprayer, the target canopy, or even the weather conditions you’re willing to spray in. Getting the job done quickly should not compromise the quality of the job. Use this method to re-evaluate your practices, or to assess the capabilities of candidate sprayers if you’re considering a new purchase. Be sure to confirm what this model is telling you using some coverage indicator, such as water sensitive paper.

Happy spraying.

August 20, 2024

Basic Sprayer Math Demystified
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!
May 7, 2024
Calibrating a Plot Sprayer for Airblast Crops
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:
1. The area you wish to treat (e.g. bush, grape panel, tree, etc.), including it’s share of the alley (in m²).
2. The emission rate from the calibrated plot sprayer (in US gal./min.)
3. 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.
PM001GL201 – 40V max XGT Brushless Cordless 15L Backpack Mist Blower (8.0Ah x2 Kit)
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:
1. Calibrating a Knapsack Sprayer (www.weedfree.co.uk – 2008)
2. Don’t Overlook Backpack Sprayers (John Grande, Rutgers)
3. Hand Sprayer Calibration Steps Worksheet (Bob Wolf, Kansas State University – 2010)
4. Sprayer Calibration Using the 1/128th Method for Motorized Backpack Mist Sprayer Systems (Jensen Uyeda et al., University of Hawai’i – 2015)
Pro Tip: To maintain a consistent boom height without a wheel, coil a measured length of wire from a plot marker flag to guide you.
May 12, 2023
Calibrating a Plot Sprayer
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:
1. 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:
1. 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:
1. 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
2. Use the flow from the calibration and a set walking speed to arrive at an application volume;
  
  or
3. 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.
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May 8, 2023