Author: Jason Deveau

Thermal Inversions for Sprayer Operators
In April 2014, NDSU extension published an excellent factsheet explaining what thermal inversions are, how to detect them and how they affect pesticide spray drift. That factsheet inspired this article.
The Atmosphere
The Earth is surrounded by a layer of air called the atmosphere. Think of it as a sheet of liquid percolating and flowing over the Earth’s surface. Seems a bit precarious, doesn’t it?
We define “layers” of atmosphere based on their distance from the Earth’s surface (see image below). We’ll focus on the lowest part of the Earth’s atmosphere: the Surface Boundary Layer. As it drags along the Earth’s surface it experiences rapid changes in wind speed, temperature and humidity (on a time scale of an hour or less).
The Earth’s Atmosphere. The illustration of the Earth is to scale, but obviously the landscape is not. Our focus in on the Surface Boundary Layer.
Atmospheric temperature
In relatively calm, clear and dry conditions (e.g. a nice afternoon), air cools with elevation at a rate of about 1°C per 100m. This change is called the Adiabatic Lapse Rate and it’s caused by pressure changing with elevation. If your ears have popped when driving down a steep hill, you’ve experienced pressure change with elevation; there is more atmosphere overhead and the weight pushes down.
With higher elevation, there is less atmosphere overhead. Less weight means less pressure and this gives air room to expand. Expansion takes work and work costs energy, which creates a cooling effect. See how simple thermodynamics are?
In the graph below, the red line shows the Adiabatic Lapse Rate of air cooling with elevation. The blue line indicates wind stirring and homogenizing the atmosphere, reducing the degree of temperature change with elevation (more on that later).
Day and night
When we add the effect of daytime solar heating and nighttime cooling, the rate of temperature change is affected. Let’s consider how this works on a clear, relatively calm day:
Early morning
The morning sun emits short wave radiation, which is absorbed by the Earth’s surface. The surface conducts some of this energy deeper into the ground and also heats the air near the surface. This creates a temperature gradient wherein the surface is warmest and the air gets relatively cooler with elevation (remember the red line in the graph above).
As the air near the surface warms, that energy causes air molecules to vibrate and push away from one another. Parcels of air become less dense and rise just like the gloop in a lava lamp. The cooler air around it falls to fill in the space left behind, and air begins to circulate in a Convection Cell. The rising parcel of air will eventually cool and shrink as it rises through the relatively cooler air above it.
These convection cells create Thermal Turbulence, which is a very effective way for airborne particles, such as pesticide vapour, to be rapidly diluted. This is also how the atmosphere disperses pollution. More on the process of dispersion, later.
Mid to late afternoon
As the sun passes over and the wind starts to rise, the convection cells get disrupted by the wind and experience mechanical turbulence (remember the blue line in the graph above). So, mechanical turbulence also mixes warmer air near the ground with cooler air above it, but suppresses thermal turbulence.
Mid-afternoon to night
As the energy from the sun lessens, the soil begins to cool and so does the air next to it. Once the air cools enough to be colder than the air above it, we have the beginning of a Radiation Inversion, which is a specific kind of Thermal Inversion (see the green line in the graph below). It is called that because we now have the reverse of the typical day-time temperature profile. The height of the inversion (the ceiling) grows with time, and can reach a maximum of about 100m by sunrise. Within the inversion layer (before the green line bends back at 100m), turbulence is suppressed. We have a stable air mass. More on that below.
How inversions affect dispersion
The rising portion of a convection cell carries whatever particles are in the air with it. Suspended particles become much less concentrated at ground level thanks to the thermal turbulence.
Thermal Turbulence allows particle-laden warm air to rise and clean cool air to fall. This disperses air-borne particles like dust or pollution.
Now let’s imagine we are in a thermal inversion. The cooler, particle laden air near the ground cannot rise and the cleaner air above, which is now relatively warmer, cannot sink. Thermal turbulence is suppressed, and so is any vertical dispersion.
Thermal Turbulence is suppressed during a Temperature Inversion. Particle-laden cool air at the surface cannot rise, and warm, clean air cannot fall. No dispersion occurs, and the concentrated, particle-laden air tends to move downhill or laterally with light winds.
When spraying, the smallest spray droplets fall slowest, staying airborne for long periods of time. If spraying occurs during an inversion, those particles accumulate beneath the inversion layer. Remember we said our atmosphere behaves like a liquid? The colder, denser (pesticide-laden) air drains downhill into low-lying areas. It can also move laterally over great distances, in unpredictable directions, when light winds begin.
Clouds
If the morning were overcast instead of clear, the clouds would intercept much of the sun’s short-wave radiation, absorbing or reflecting it back into space. The Earth’s surface would still warm, but more slowly, suppressing thermal turbulence. As an aside, if clouds form in the evening, they reflect long-wave radiation from the Earth’s surface back down. This Greenhouse Effect is why overcast nights are warmer than clear ones.
Therefore, extended periods of mostly clear skies in the evening or night means a high probability of strong temperature inversions. Conversely, cloud cover usually means a near-neutral atmosphere, so no strong inversion.
Wind
Inversions are only mildly affected by light wind (e.g. 6 to 8 km/h), but as the wind increases and mechanical turbulence mixes the air, the strength of the inversion will be reduced and the atmosphere will approach a neutral condition (see the blue line). In this condition, airborne particles are not dispersed by thermal turbulence, but some mixing will occur. So, there may not be a thermal inversion, but spraying would still be inadvisable if the wind got too high.
Humidity
Inversions form more rapidly when there is less water vapour in the air to absorb radiation. Once humid air has cooled to the dew point, water condensation gives off energy and warms the air a little. This slows the formation of the inversion. Be aware that inversion conditions can exist long before fog, dew or frost forms, so they are not a good indicator for the beginning of an inversion – you’re already in one!
If you see fog, dew or frost, you’re already in an inversion. The air has become cold enough to condense or even freeze water.
Soil conditions and topography
This is a complex issue, but soil conditions that make inversions more intense include low soil moisture, freshly tilled soils, coarse soils, heavy residue and closed crop canopies. Topography matters, too. We’re discussing radiation inversions in arable regions, and the kind that form on mountains or deep valleys. Nevertheless, inversions in shaded areas (e.g., behind windbreaks) start sooner, and last longer. See the NDSU factsheet for more detail.
Spray timing
Inversions, once formed, persist until the sun rises and warms the Earth’s surface, or until winds increase and mix the stationary layers of air together, re-establishing a more neutral temperature profile.
Sunset is not a good indicator of the beginning of an inversion – it can start a few hours before. Therefore, evening spraying may be just as risky as night spraying. Very early mornings (e.g. around sunrise) are not much better. Remember, at sunrise, the inversion will be at its maximum height.
The rising sun will warm the earth and create turbulent conditions, starting near its surface (e.g. a few metres). Most inversions will have dissipated two hours after sunrise, which may be the best choice for spraying.
Detecting an inversion
The only sure way to know if you are in an inversion is to take two air temperature readings: one near the ground and one about three metres higher. If the surface air temperature is cooler, you are in an inversion. The magnitude of the difference indicates how strong the inversion is.
Accurate measurements are difficult to manage with conventional thermometers, but SpotOn now makes a hand-held detection unit. If you have one, be sure to let it acclimate before you use it. Leaving it in a hot, or cold, truck or sprayer cab prior to use means it may give a false reading.
Inversion forecasting is getting better, but it’s still location-specific and not entirely reliable. Sprayer operators should learn to watch for the following environmental cues:
- Large temperature swings between daytime and the previous night.
- Calm (e.g. less than 3 km/h wind) and clear conditions when the sun is low.
- Intense high pressure systems (usually associated with clear skies) and low humidity where you intend to spray.
- Dew or frost indicating cooler air near the ground (fog may be too late).
- Smoke or dust hanging in the air or moving laterally.
- Odours travelling large distances and seeming more intense.
- Daytime cumulus clouds collapse toward the evening.
- Overnight cloud cover is 25% or less.
Note: If you suspect a temperature inversion, don’t spray.
For more information on how weather affects drift, download this pamphlet from the Australian Government Bureau of Meteorology.
March 26, 2020
Reading Airblast Nozzle Tables
Airblast operators should know how to read a nozzle table. They are found on dealer and manufacturer websites as well as in their catalogs. Table layout varies with brand, but they all relate a nozzle’s flow rate to operating pressure. The better tables also provide the spray angle and the median droplet size (i.e. spray quality).
Operators need this information to complete calibration calculations (aka sprayer math) and when deciding how to distribute nozzle rates, angles and spray quality along a boom relative to the target canopy.
This article focusses on hollow and full cone nozzles, which are commonly found on airblast sprayers. For more information on flat fan nozzle tables (e.g. for banded under-canopy or, vertical booms or broadcast applications from horizontal booms), refer to this article.
Reading the table
Let’s use the table below to determine a nozzle’s flow rate for a given pressure. First, find the nozzle colour in the top row. Second, find the operating pressure in the left-most column. Finally, the flow rate is indicated in the cell at the intersection between the row and column. For example, a red ATR hollow cone nozzle operated at 9 bar will emit a flow rate of 1.83 L/min.
Perhaps you want to determine which nozzle will give a specific flow rate. Find the rate in the body of the table and trace the column and row to determine which nozzle/pressure combination will achieve it. For example, if we want a flow rate of ~1.00 L/min, we can use a Yellow at 10 bar or an Orange at 5 bar. Yellow is the better choice since the Orange would have to be operated at the bottom of its pressure range (more on that later).
This Albuz nozzle table for 60 and 80 degree molded hollow cones gives flow rates in litres per minute.
Note: Do not to confuse TeeJet’s ISO-standardized TXA or TXB nozzles with TXVK or ConeJet nozzles. They may be the same colour, but their outputs are very different.
Higher flow rates or full cone patterns can be achieved using combination disc and core (or disc and whirl) nozzles. Depending on the manufacturer, the disc plate is defined by it’s diameter in 64th’s of an inch. The core or whirl plate might be described by the number of holes (e.g. 2-hole, 3-hole, etc.), or some other manufacturer-specific nomenclature (e.g. 45’s, 25’s etc.).
Using the table below, we see that a D2 disc and a DC35 core will emit 0.34 gpm at 80 psi. By continuing along the row, we see that the spray angle for this combination will be 47 degrees at that pressure.
This TeeJet nozzle table gives the flow rate for a disc (D#) and core (DC#) full cone combination nozzles in US gallons per minute.
Pressure problems
Do not choose a nozzle at the extreme of their flow or pressure range. A trailed PTO sprayer will experience pressure changes from driving on hills, or rate controllers will create pressure changes in response to changes in travel speed. In either situation, coverage will be compromised if the nozzle is pushed outside its optimal range.
Note: Use pressure to achieve small changes in flow, but for more extreme changes, switch nozzles. Remember, it takes 4x the pressure to get 2x the flow. Stated differently, it takes 1/4 the pressure to get 1/2 the flow.
You may not find a nozzle/pressure combination that emits the rate you are looking for. When your desired rate or pressure falls between the figures listed in the table, you can take the average. When nozzling an entire boom with different nozzle rates, get each position as close as you can to achieve the overall boom rate for a given pressure. It’s always a compromise – don’t stress over it.
Looking up nozzle rates during a spring calibration. The operator was running at 190 psi, but the catalogue only listed 180 psi and 200 psi. When the increment is only 20 psi, it’s reasonable to approximate the output. When the span is 50 psi increments, it is more difficult to determine the rate without testing the output (it’s not a linear relationship). This issue usually occurs at pressures above 200 psi, and that’s far too high for cane, bush, vine and high-density orchards. In these situations, consider using a lower operating pressure.
Different nozzles, same rate
Different disc core combinations, or molded nozzles at different pressures, can produce similar flow rates. However, their spray quality and spray cone angles can be very different (see last three columns in the TeeJet table above).
The angle of the spray cone can have a big impact on spray coverage. When the target is far away from the corresponding nozzle (e.g. the tops of nut trees), or the canopy is very, very dense (e.g. citrus canopies), consider tight-angled full cones under high pressure. This is inefficient and can give variable coverage, but it is sometimes the only option in extreme situations.
Oops! Two hollow cone nozzles on top and five full cone nozzles below is the exact opposite of how things should be. Note the lack of spray overlap with the full cones for the first few meters. Spray from the top two positions will likely not reach the intended target.
When the target is very close to the sprayer, full cones do not overlap and create undesirable striping or banded coverage. Creating a full, overlapping spray swath that spans the entire canopy is a function of nozzle spacing, distance-to-target, and sprayer air-settings. It can also be affected by humidity, wind speed and wind direction at the time of spraying.
Confirm your settings by parking the sprayer in the alley between crops. With the air on, spray clean water while a partner stands a safe distance behind the sprayer to look for gaps in the swath. The partner will see things the operator’s shoulder check will not reveal.
Here’s what the operator sees. But, shoulder checks may not show you what’s really happening. Have someone stand a safe distance behind the sprayer while spraying clean water to see the nozzle spray overlaps sufficiently to span the entire canopy.
Here’s what the partner standing behind the sprayer sees. Take a picture with a smartphone to show the operator.
Nozzle tables can be wrong
Sometimes nozzles do not perform per the nozzle table. We have discovered errors in published tables, worldwide. Here are the big three:
- Conversion errors. Manufacturers publish catalogs in Metric and in US Imperial, but we have found many errors in the conversions.
- Spray angle errors. When nozzles are operated at the extremes of their pressure ranges, spray angles deviate from those listed in the tables.
- Flow rate errors. When tables are not updated to reflect changes in nozzle design, or the manufacturing process, actual flow rates deviate from those listed in the tables.
Perhaps it’s not the table, but the nozzle itself. Most nozzle manufacturers accept a flow variability up to +/- 2.5% for new nozzles, but we have seen higher. It depends how they are made (machined, stamped, printed) and the material they are made of.
Validate flow rate and pattern
When errors are discovered and reported, the manufacturers can be slow to issue corrections and the errors will persist in old tables. Yes, even apps (which are often based on tables) can be wrong. So, predicted flow rates can prove unreliable. This is why it is important to double check by observing nozzle overlap and validating flow rate when you replace nozzles – even when they are brand new.
Thanks to Dr. David Manktelow (Applied Research and Technologies, Ltd., NZ) for input into this article.
March 2, 2020
The Pressure-Spray-Coverage Relationship
Pressure is integral to nozzle performance. Reducing hydraulic pressure reduces nozzle flow rate, increases median droplet size, and typically reduces spray fan angle. Increasing pressure increases nozzle flow rate, reduces median droplet size and typically increases spray fan angle.
You can watch this Exploding Sprayer Myths video to learn how pressure, boom height and nozzle spacing interact. In extreme cases, too low a pressure can collapse the fan angle enough to reduce overlap and compromise coverage, as explained in the video at the end of this article.
Using a flat fan nozzle as an example, a lower pressure increases the median droplet diameter, reduces the droplet count, reduces the nozzle flow rate and typically reduces the spray angle. Alternately, a higher pressure decreases the median droplet diameter, increases the droplet count, increases the nozzle flow rate and typically increases the spray angle.
Always plan to operate a nozzle in the middle of its recommended range so it can handle small changes in pressure during spraying (such as from a rate controller, or when changing PTO speeds on hilly terrain). Don’t operate an air induction nozzle below 2 bar (30 psi), even if it’s rated lower in the manufacturer’s nozzle table. Most AI nozzles perform best at >4 bar (60 psi).
Pressure can be used on-the-fly to make minor changes to flow rate while spraying. This is how rate-controllers work to compensate for changes in ground speed and maintain a constant overall rate per planted area.
However, pressure should not be used to make significant changes to flow rate. It takes a 4x change in pressure for a 2x change in flow rate, so it’s inefficient. Operating pressures at the upper or lower limit of a nozzle’s range can have undesirable impacts on nozzle wear, median droplet size and swath uniformity.
For a more in-depth discussion of the relationship between spray pressure and nozzle performance, and how rate controllers work, check out this article.
Note: It is far better to simply switch nozzles when a significant change in flow rate is required.
In 2015, we ran demonstrations at Ontario’s Southwest Agriculture Crop Diagnostic Days. The 20 minute sessions were designed to explain:
- The phenomenon of pressure drop,
- how to measure pressure drop,
- how to correct for pressure drop, and
- how low pressure affects coverage.
Although manufacturers of air induction nozzles often rate their performance as low as 15 psi, such a low pressure collapses the spray pattern and the resulting gaps reduce coverage. Additionally, the spray quality at such low pressures is coarser than at higher pressures, reducing the number of droplets available. This further reduces coverage potential.
This video covers the key speaking points from that demonstration.
February 22, 2020
Airblast Maintenance Inspection – the Morning Walkaround
An airblast sprayer inspection is part of preventative maintenance. This daily activity identifies small problems before they become big ones. You can do it at the filling station, so it’s fairly convenient.
Don’t think of it as stealing time from your spray day… it’s part of your spray day. Don’t skip it. If time is tight there are many other ways to improve your work rate.
This spray plane was left on the runway with the engine exposed for less than four hours. When the owners returned they found a precocious bird had built a nest! Perform regular sprayer inspections – you never know what you’ll find. Photo Credit – S. Richard, New Brunswick.
Note: Always wear appropriate personal protective equipment (as indicated on the product label), including hearing protection.
Inspection steps
Follow this generic inspection process. If your sprayer manufacturer or manager advises additional steps, be sure to perform them.
Before filling
1. Work with a rinsed sprayer parked on level ground (e.g. the filling station).
2. Check lines/hoses and fittings for signs of wear or cracking. Leaks or bulging may only become apparent under pressure (see Test spray).
3. Filters, screens, strainers and nozzles are clean and unbroken. Leaks may only become apparent under pressure (see Test spray).
As a plastic suction filter ages, it can warp or become brittle. When this happens, the O-ring may no longer sit correctly and the unit may allow air to be drawn into the lines. They should be cleaned and inspected when the sprayer is rinsed.
4. Engage each nozzle shut-off valve or nozzle body flip position. They can seize or loosen with time.
Begin filling
5. Begin filling the sprayer 1/2 full with water.
6. For PTO-driven sprayers, confirm universal joint(s), sprayer-tractor hitch and all connections are clean, lubricated and secure.
7. Check that all guards (e.g. PTO shaft shield) are in place and intact.
8. Ensure fan blades are unbroken and scraped clean. Intake grill(s) must also be clean and unbroken.
9. When 1/2 full, stop filling and check tire pressure (tractor and sprayer).
Test spray
For multi-row sprayers, you may have to move the sprayer off the fill pad for the test spray; it’s easier with the air off, if possible. Perform the following steps:
10. Open the manifold valve to fill the lines and begin spraying clean water.
11. Ensure each nozzle sprays correctly. Get out of the cab to inspect, don’t just shoulder-check. This gives the opportunity to double-check for line-bulges and leaks.
12. Ensure the agitation / bypass system is functioning properly.
13. Check that the tank is secure on the chassis and both crack and leak-free.
Complete filling
Continue filling. Once the sprayer is back up to 1/2 full, mix products per usual. If your sprayer manufacturer advises contrary or additional steps for a sprayer inspection, be sure to perform them.
Checklist
Download Walkaround Checklist
Sprayer inspections become repetitive, so it’s easy to accidentally miss things. Have you ever driven home while preoccupied, only to discover you don’t remember how you got there? Download our checklist to keep you engaged and to help ensure accuracy. Consider printing and laminating it for repeated use with a dry-erase marker.
You never know what you’ll find during an inspection. I found a robin’s nest hidden on this vineyard sprayer’s pump.
Anyone that operates heavy machinery should perform a preventative maintenance inspection before using the equipment. It’s no different for airblast sprayer operators; embrace the daily walkaround.
February 10, 2020
Airblast Calibration – Clearing up Confusion
“Sprayer calibration is an important part of any crop protection program.” Everyone says so, so it must be important. But what exactly are they asking you to do, and why?
When delivering presentations I often take the opportunity to ask audiences to define airblast sprayer calibration. Their responses cover a wide range of activities that can be rolled up into three related, but quite different, definitions:
1. Sprayer maintenance inspection
2. Adjusting sprayer configuration
3. Validating sprayer output
Ask a group of managers, sprayer operators, agrichemical reps, gov’t regulators and equipment manufacturers to define “calibration”. Be prepared for very different answers.
Traditionally, calibration refers to Number 3: Validating sprayer output, but all three are required to ensure a safe, effective and efficient application. Don’t panic – your workload didn’t just triple.
There is a time and a place for each of these activities. Some should be performed more often than others, but none of them are difficult. This is easier to accept when you realize that only a portion of the spray-day is actually spent spraying. Filling, travel time, cleaning and calibration-related activities are all essential components.
Let’s consider each activity.
Sprayer maintenance inspection
This is more maintenance than calibration (e.g. is it properly connected, is it worn out, is it plugged, is it leaking?). It should not be confused with spring start-up or winterization. For those lucky readers in temperate regions, “winterization” is preparing the sprayer for long-term storage post season… we just use antifreeze.
The maintenance inspection is the morning walk-around, no different from what any operator of heavy machinery must do before starting their work day. Learn more about sprayer inspection and download a helpful checklist in this article.
Here are some nasty disc & cores revealed during a calibration workshop. It certainly explained the poor performance the operator was complaining about. Is it time to replace yours? Photo credit – Dr. H. Zhu, Ohio.
Adjusting sprayer configuration
This is an ongoing process whereby an operator makes minor sprayer adjustments (e.g. pressure, travel speed, air settings) to reflect environmental conditions, the product’s mode of action and the nature of the target. Would you apply an insecticide to semi-dwarf pears in high wind using the same sprayer settings to apply a fungicide to nursery whips in high humidity? I hope not.
The process is more intensive at the beginning of the spray season and again around mid-season (e.g. petal fall or whenever the crop changes sufficiently to require a reassessment). It’s described step-by-step in many articles on this website as well as in Airblast101.
Yes, it requires an investment of time and effort, but the feedback makes subsequent adjustments faster, easier and more intuitive. There are strategies to reduce the number of adjustments required. Large operations can assign sprayers to blocks with similar crop architecture (e.g. one sprayer works large orchards, another sprayer works young or high-density orchards). Smaller operations can change the order in which crops are sprayed.
Validating sprayer output
This accounting activity ensures the sprayer is applying the intended rate at the intended speed. “Sprayer math” is really only theoretical; It helps the operator plan for how much pesticide and water must go in the tank and how long the job will require. How the sprayer actually performs may be a different story.
According to 1992’s “Tools for Agriculture” a horse can deliver 500 watts of power over 10 hours, but the camel can deliver 650 watts over six. And you thought establishing tractor speed was difficult. Photo Credit – R. Derksen, Ohio. Date and location of photograph is unknown.
Validating output, or calibrating, confirms that each nozzle delivers the desired rate and that the sprayer travels at the desired speed, so the crop receives the correct dose with no unexpected left-overs or shortages.
The operator should perform these activities at the beginning of the season and after any significant change to the sprayer set-up. Examples include new nozzles, new tractor tires, using a different tractor or after replacing a pump or any lines/hoses.
The validation (i.e. calibration) process is explained in our articles on testing airblast sprayer sprayer output and travel speed.
Conclusion
Be sure to perform all three calibration-related activities as required. This will keep records up-to-date, improve your spray coverage, and save you from unexpected sprayer malfunctions – almost all of which are preventable.
February 10, 2020