Author: Jason Deveau

Coverage is King
We’ve often heard the adage “Coverage is King” but what does that mean, exactly? It means that in order for your spray application to yield acceptable results, a threshold amount of the active ingredient in your tank must end up on the target. But at what point have we achieved sufficient spray coverage without wastefully over-applying to the target? What does good coverage look like?
Let’s manage expectations right here at the beginning of the article: There is no single, definitive answer because it depends on the nature of the application. In other words, you have to understand which factors are relevant to your specific situation before you can understand what success looks like.
Let’s highlight some of those factors:
Transfer Efficiency, Catch Efficiency and Retention
This relates to the spray’s ability to span the distance from nozzle to target (transfer efficiency) get intercepted by that target (catch efficiency) and then deposit a biologically-active residue on the target surface (retention).
- First, the spray must reach the the target location. This may be the soil, or it might be the underside of a leaf deep in a plant canopy. The degree of success will depend on the droplet size(s), distance to the target and the environmental conditions.
- Then the droplets have to be retained by the target surface and not bounce or slide off. Difficult-to-wet surfaces such as fruit, stems and waxy vertical leaves may be more easily covered with finer droplets and/or formulations that include activator adjuvants (e.g. surfactants).
- Then the deposit must stay wet long enough to be absorbed by the tissue, or leave a hardy residue on the surface that can withstand weathering (e.g. precipitation, sun, and even bacteria) long enough to encounter the pest. More on this below.
Mode of Action
This relates to where spray must deposit (or relocate to) in order for it accomplish it’s objective. Here are a few examples of how products might work. Read your pesticide label to determine your situation.
- Some products require contact. Insects must touch them, either via a droplet landing on them or as they move through a deposit. Similarly, certain fungicides must contact fungal hyphae on the plant surface. A few products are designed to drench the target, as is the case with oil-based miticides.
- Some insecticides must be ingested. That may be in the form of a surface deposit or in plant material that has absorbed the chemistry. Similarly, some fungicides are absorbed by plant tissue.
- Many herbicides are mobile (i.e. systemic). They may be drawn up through the roots, or enter the cytoplasm via leaves and travel to the growing points on the plants, or move through the xylem. Others are contact, staying relatively close to the original deposit.
The sprayer operator should consider these factors when planning the application and when evaluating the resulting coverage. So how do we visualize coverage? Some operators look for the shine on leaves, or a cloudy residue once the spray has dried. That’s better than nothing, but we recommend water sensitive paper (WSP), which is still the most versatile and economical way to visualize coverage.
WSP can be purchased from most retailers that carry spray equipment. It is available in three sizes, of which the 1” x 3” size is the most common. It can be folded and clipped to a plant surface, or placed on the ground. We’ve written several articles on how to use it (such as here and here and in pretty much a third of the articles on Sprayers101).
There are two metrics that must be evaluated when assessing coverage on water sensitive paper:
- the area of the target that has spray on it, and
- the distribution of the droplets over that area.
Let’s use a metaphor to explain:
The Battleship® / Coverage Metaphor
Imagine the boats in this Battleship® game are the insect pests, and the board they’re on is a leaf. The white pegs represent the spray deposits. In this first image, we see 100% coverage and a very high deposit density. Sure, we got every boat, but this is literal and figurative overkill. There’s no need to completely drench the target in order to control most pests. When you spray a target past the point of run-off, you are not adding more pesticide to the target – you are displacing what was already there. The surface will not exceed the concentration of product you sprayed (with the possible exception of mixes that include certain adjuvants). While additional volume can improve coverage to a point, there is a diminishing return.
Unless the label specifically asks for a drench, this is too much coverage.
In this second image, we’ve covered about 15% of the target area, which is reasonable. However, note the lack of distribution. You can see that we’ve missed quite a bit of the leaf. If our pretend pests are sedentary and if this was a contact product, then we’ve missed. If this was WSP we would advise the sprayer operator to note how much space there is between the deposits. Could a pest such as an insect or small weed easily fit between the deposits?
15% coverage is good, but the distribution is bad.
In this third image, we are still covering about 15% of the target, but now the spray is distributed more evenly. Some of you are likely noticing that we missed a pest. That observation reminds me of one of my favourite exchanges from the movie “Christmas Vacation” where Clark finally got his house illuminated, but his father-in-law only sees the problems: “The little lights aren’t twinkling.” “I see that and thanks for noticing, Ed.”
15% coverage, distributed evenly. Deposits may have some pest activity beyond the edge of the residue (light red circles).
Yes, we still missed a pest, but spraying is playing a game of odds. You want enough spray to increase the odds of controlling a pest, but not so much to waste spray (and money and time). This image represents an ideal coverage situation. If this pest moves, or this pesticide redistributes even a little, it will affect the pest.
Plus, we should not discount the threshold of influence that lies around pesticide residue. Imagine a small circle around each droplet (illustrated here as light red haloes) where active ingredient may redistribute beyond the initial deposit to affect an adjacent pest. Perhaps even more importantly, deposits do not spread on WSP the way they do on actual plant tissue, so WSP always gives an underestimate of the potential coverage.
In this last image, we see that red deposits have been introduced. This represents a disease control program where an earlier (white) application retains some residual activity when next application (red) is applied. The second spray application almost never lands on top of the first, giving much more protection on the target. For those keeners out there, note that we got that last pest!
In the case of many disease management programs, subsequent sprays tend to fill in gaps left by previous sprays. If timing is prompt, residual activity will see you through.
If you Absolutely Need a Number…
So, what if you’ve read all this but still insist on a firm number to define adequate coverage? We’ll reiterate that there’s no universally-accepted threshold of deposit density or area covered. It would be nice if pesticide labels included this information, but they don’t.
We’ll stick out necks out and say that in general practice we see excellent results when we achieve 85 discrete deposits per cm² as well as 10-15% surface coverage on at least 80% of the water sensitive papers in a spray application. If you can manage this, it should give satisfactory results in most situations.
Ontario Agriculture Conference – 2022
For a really in-depth conversation on the topic of coverage, check out our presentation from the 2022 Ontario Ag Conference. We tried to deliver a fun and memorable demo at the end of this presentation to show how different droplet sizes might contribute to coverage. Enjoy.
January 18, 2022
‘Twas the Night Before Christmas – Parody
Here’s our rendition of the Christmas classic: ‘Twas the Night Before Christmas… but with a bit of a sprayer-winterizing twist. You can hear us read it in the audio bar below, read it yourself, or watch as the talented Bridgette Readel (@bmreadel) reads it to you (end of article). Happy holidays from Sprayers101!
‘Twas the night before Christmas and all through the farm
No sprayers were stirring, not even a Dramm
The nozzles were stored in a bucket with care
In the hopes that the herbicides were no longer there
The impeller spun freely in its little bed
We can’t spray next year if the controller is dead
I took off my gloves and my PPE cap
As the sprayer settled down for a long winter’s nap
When from under the hood there arose such a clatter
I sprang back to the cab to see what was the matter
Away to the ladder I flew with a flash
Tripped over clean filters and swore as I crashed
The light from my smartphone held up from below
Gave a luster of mid-day so I could behold
Some movement above as my vision did clear
Eight mice moving ‘round in the engine and gears
With a flash of their tails, so lively and quick
I knew that our cat wasn’t doing the trick
More rapid than eagles my curses they came
I remembered my checklist with each point by name:
- I’d drained all the places that water could hide
- I’d used RV antifreeze throughout the inside
- I looked closely for leaks from tank lid to sump
- I checked the spray tender and both transfer pumps
- I cleaned the outside and cab wall to wall
But I’d forgotten to mouse-proof… dash it all
And then came a scratching from up near the roof
The cat was responding, no longer aloof
Down from the rafters she came with a bound
She coiled as she listened to each squeaking sound
Her claws sprang to life from the tip of each foot
Her fur was all matted with ashes and soot
Her eyes, how they twinkled, her visage so scary
She grinned like the cat that had ate the canary
She leaped over me and into the fray
The mice scampered past, running every which way
One ran up my leg and jumped onto to a shelf
And I screamed when I saw them in spite of myself
Then the cat reappeared strutting all ‘round my head
And I lay there, and knew there was nothing to dread
As I climbed to my knees, I felt I had shirked
Steel wool and some traps would have probably worked
Then the cat rubbed against my legs with her nose
I reached out to pet her, as I slowly rose
And that’s when she bit me and hissed like a kettle
I jumped up too fast and my head hit the metal
As I swayed back and forth the cat ran out of sight
I’m getting a dog by the end of the night
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December 1, 2021
Adventures in Lecturing – Turn Off PowerPoint
Harvest is mostly done and growers want to hear what we’ve learned and what’s coming next. Lecture season is upon us once again.
In 2021 we’re still finding our way through virtual conferences and hybrid models, but I like to think we’re slowly returning to the in-person format. Just last week I gave my first in-person talk in 20 months. It felt wonderful after having spoken into a dead-eyed camera for so long. Half-way through my lecture I remembered a lesson I learned a few years back and spontaneously decided to go off-script.
Let me explain.
In 2016 I was invited to present at the 40th annual Tomato Days conference in Southern Ontario. I knew what I wanted to say, but didn’t have a decent slide deck for that particular topic. I’d have to pull one together.
I work hard on my presentations. I employ lots of imagery (I create all my own illustrations). I get persnickety about fonts, white space and slide transitions. I try to tell a story that educates and hopefully, entertains. Prideful? Perhaps. But if you’re willing to sit on a hard chair for an hour, I’m going to do my best to make it worth your while.
I finished the slide deck, drove three hours to the conference, handed my USB data key to the organizers and sat down to wait my turn. It was a clear, bright winter morning and I saw that the pavilion we were in was more-or-less windows and a roof. It was so bright, in fact, that none of the 150 attendees could see the projector screen!
I watched sympathetically as the first speaker spent 30 minutes trying (and failing) to verbally describe his graphs. I cringed as the second speaker pantomimed her illustrations in some kind of brave, interpretive dance. Then it was my turn.
I decided I wasn’t going down that road.
When the moderator brought up my talk, I turned the useless projector off. I asked the squirming and disinterested audience:
Q. “What’s the most terrifying thing you can do to an academician?”
A. “Take their Power Point away.”

For the next 30 minutes we had a discussion about spray coverage. No props. No slides. The audience slowly warmed up to the new format. They shared experiences. They debated. They asked questions. I became more facilitator than speaker.
When our time was up I think everyone was pleased. Sure, I missed a lot of my key points and never really addressed the subjects I thought I would, but who cares? Everyone learned something.
For me, I learned that speakers should abandon the script every now and again. It’s not always ideal since we’re there to teach and structured visuals are often required. But, the next time you’re asked to speak, consider the possibility of using your time to engage your audience and establish a dialogue… not just talk at them until the moderator gives you the 5-minute warning.
I have a colleague who does this masterfully. Whenever he is the last speaker on the agenda, and the previous speakers have discourteously gone over-time and whittled his time in half, he jumps straight to his take-home slide. He leads a quick discussion with the audience and becomes a hero. The moderators are now back on schedule and no one is late for lunch.
Since “Tomato Days”, I now try to do this once a year. I never know when the mood will take me, but when it does I give the audience a choice: They can hear my canned presentation or I can shut it down and we can have a conversation. To date, given the option, every audience has opted to go off script. It’s scary, it’s fun and like I said earlier, everyone learns something.
I challenge you to try it the next time you’re lucky enough to be in front of an audience in person.
November 29, 2021
Air-Assisted Spraying in Greenhouse Ornamentals
The aesthetic value of ornamental plants requires a near-zero tolerance for insect pests, which cause up to 10% of crop losses per season. Controlling them with insecticides is a difficult proposition:
- Key pests such as thrips, aphids and whiteflies tend to feed on the underside of leaves – a notoriously difficult surface to target because of it’s orientation relative to the spray nozzle (see image below).
- Other pests, such as mealybugs, are found on stems. Stems are hard-to-wet plant surfaces because spray tends to run off. Further, as the plant canopy grows and densifies, these surfaces are buried deep inside, out of line-of-sight.
- The insecticides available for closed environment spraying must be compatible with biological controls and are therefore “softer” chemistries. Examples include soaps, oils and entomopathogenic fungi. These products require contact with the pest and are at best translaminar, so coverage becomes critical for performance.
Whitefly on the abaxial laminar (under-leaf) surfaces of Poinsettia.
Spraying for Insects
The planting architectures and canopy morphologies are highly variable in ornamental greenhouses. Perhaps they are young plants with sparse canopies, densely packed in pots on raised tables. Perhaps they are mature, hanging plants with dense canopies. Perhaps they are something in between.
Crop canopy morphology and planting architecture are highly variable from operation to operation.
Ideally, each combination of canopy morphology, planting architecture, pest and chemistry would have a specific sprayer designed to optimize coverage and efficiency. This is economically unrealistic. Instead, many producers utilize technologies that rely on high water volumes and hydraulic pressures to “drench” targets indiscriminately. Others employ highly manual methods that allow the operator to aim the nozzle in relation to the canopy on a case-by-case basis, but still rely solely on water to distribute the insecticide.
Typical application technologies in ornamental greenhouses. The backpack sprayer (left) with its manual pump is inexpensive and the operator can aim the nozzle more accurately. The trailed tank-and-handgun (right) utilizes higher hydraulic pressure and water volume in an attempt to improve the work rate. Both rely solely on water and hydraulic pressure to distribute spray.
These technologies have their place, but the reliance on hydraulic pressure and carrier volume has drawbacks:
- High water volumes lead to higher humidity in closed environments which may favour disease.
- The inevitable run-off creates waste water that may require treatment before leaving closed environments.
- High carrier volumes dilute an already “soft” chemistry and hydraulic pressure doesn’t always improve canopy penetration or coverage uniformity.
Air-assisted spraying can be a viable alternative (and an improvement) over these approaches. Stationary or mobile, many ultra-low volume sprayers already employ air to capitalize on the mechanical advantage offered by smaller and more numerous droplets. Finer droplets have very little mass, so they must be directed and carried by air currents to get them to the target. Sufficient air energy will also displace the air within the target canopy and physically expose otherwise hidden plant surfaces to the spray.
The upshot is that air can partially replace water as a carrier and it has the potential to improve coverage uniformity throughout the target canopy.
Testing Air-Assisted Spraying
We chose to test this assertion in a chrysanthemum nursery. Our objective was to compare the coverage from the grower’s conventional hydraulic gun to that of a customized backpack mist blower.
Crop Canopy and Architecture
The crop canopy wasn’t fully mature but still represented a very dense target. In order to compare canopy penetration the canopy was divided into three depths: The Top exterior, the Middle (8″ from ground) and the Bottom (just above the pot soil). Each treatment area contained 8×2 plants and a buffer of three plants was maintained between treatments. We made three sprays (reps) for each condition.
Sprayers
Several attempts were made to redirect and redistribute air from a commercial backpack mist blower. The goal was to create an air outlet that would distribute the same air speed over a long and narrow swath. Air is highly compressible and early attempts using baffles, straightening vanes and variable outlet sizes were unsuccessful. A compromise was reached by reducing the swath to about plant-width (40 cm). This was confirmed by spraying water on dry pavement and measuring the width of the swath. While not ideal, the operator could span the full 75 cm plot width by shifting the outlet back-and-forth laterally while spraying. There are videos below that show examples of both applications.
Several iterations of the air outlet design.
Through trial and error, the outlet was held above the canopy at a height and angle that optimized air penetration. If the outlet was held too far away, there was insufficient air energy to penetrate the canopy. If held too close, too much spray-laden air would escape the canopy. These attempts were performed at a comfortable walking pace to account for dwell time (E.g., the longer the outlet remained stationary over a canopy, the deeper it penetrates).
With the gravity flow set to “1” and moved as it would be used during spraying, we measured walking pace and timed how long it took to spray a known volume. The application volume was 1,250 L/ha (~133 US gal./ac).
The grower’s conventional sprayer was used according to their typical practices. Walking pace and flow rate were measured to establish application volume for both sprayers.
By timing walking pace and performing a timed output test, the application volume was 2,400 L/ha (~256 US gal./ac) for the conventional sprayer.
Coverage Indicator
Coverage was quantified using dye recovery and fluorimetry. The process is described in detail in this article and this article. Basically, a known concentration of Rhodamine WT dye is applied to the plant. Sprayed leaves are collected from key locations in the canopy and placed in labelled containers with a known volume of water. Later, that water is analyzed in a fluorimeter and the data is normalized by leaf weight (or in this case, leaf surface area) to account for the volume used and the size of the leaf sampled.
Dye pooling on leaf surfaces following an application using conventional methods.
Relative size and number of leaves sampled from each canopy depth.
In addition to dye recovery, we also used water sensitive paper as a qualitative indicator. Papers were placed at the Middle depth facing into and away from the direction of travel and sprayed with both methods. This was used as a visual check to ensure spray went where it was intended, but it also provided insight into how spray might deposit on the leaf surface. As an artificial collector, water sensitive paper does not behave like a leaf surface, but it is helpful for relative comparisons.
There were obvious visual differences in how spray deposited on water sensitive papers located in the middle of the canopy. The mist blower had far less drenching and an even distribution of finer deposits compared the the conventional method. From left to right: Mistblower, facing sprayer travel direction. Mistblower, facing away from sprayer travel direction. Conventional sprayer, facing away from sprayer travel direction. Conventional sprayer, facing sprayer travel direction. When comparing these papers, remember that the mist blower was using approximately half the volume of the conventional method.
Results
As mentioned previously, dye recovery was normalized by spray volume and leaf area for each condition. The results align with inferences made in the above image. Spray coverage can be highly variable which often leads to statistically insignificant results, but the mean-dye-recovered does demonstrate clear trends. The top of each canopy received a similar dose of dye for each condition. This comes as no surprise and is typical of any overhead application into a canopy. However, the air-assisted condition resulted in more than 2x the dye in the middle of the canopy and more than 10x the dye at the bottom compared to the conventional method.
Bars represent standard error.
When considered as a percentage of overall dye recovered, we see that the dye deposited was more uniform in the air-assisted condition. 16% of total dye recovered in mid-canopy in the air-assisted condition canopy versus 7% in the conventional condition. 13% at the bottom on the air-assisted condition versus 2% at the bottom of the conventional condition.
Conclusions
Based on this study, there is compelling reason to consider air-assisted applications in closed environments. Canopy penetration and coverage uniformity was improved in the air-assisted condition. In addition, there is potential for reduced water volumes, which mean less contaminated run-off and lower humidity levels in closed environments.
Future work would require a better-engineered sprayer than the prototype used here. Further, while improved coverage often improves spray efficacy, it is not always a direct correlation. An efficacy study comparing crop damage and pest counts should be performed to confirm that this method of application represents a positive return on investment.
This research was performed with Dr. Sarah Jandricic, OMAFRA Greenhouse Floriculture IPM Specialist. Thanks to Schenk Farms and Greenhouses Co. for collaborating in the study.
November 19, 2021
Centrifugal and Diaphragm Pumps
Press Play to hear the audio version of this article
Adjusting Sprayer Settings
Operators are encouraged to adjust airblast sprayer settings to conform to the variability in canopy size, density, spacing, and weather conditions. The efficiency and accuracy of the application is improved through the regular and independent adjustment of travel speed, nozzle output, and air settings.
Airblast design is highly variable.
Inflexible sprayer design results in a suboptimal match between equipment and crop. For example, sprayers intended to blow across multiple rows in a single pass are promoted for their high productivity, but typically compromise either coverage uniformity or drift control. In another example, low volume mist blowers utilize high speed air to atomize spray and are promoted as a means for saving water and/or pesticide. But, for many such sprayers, moderating air speed to reduce drift potential causes undesired changes to spray quality.
Even with geared fans, many of Ontario’s airblast sprayers are overpowered for vines, canes, bushberries and high-density orchards. I am uncomfortable with manually obstructing the air intake or adjusting fan blade pitch for safety reasons. Fan gears and travel speed are excellent means for adjusting air energy. Alternately, we have sometimes had success reducing air energy by gearing the tractor up and throttling down (GUTD), but it’s only for very specific situations.
It has been my experience that centrifugal pumps on axial airblast sprayers can undermine adjustment efforts when spraying small to medium sized canopies (i.e. not tree nut or citrus). In the case of GUTD, slowing the fan reduces pressure at the nozzle. Modest pressure regulation may be possible, but typically the operator must swap to larger nozzles to maintain flow. Hollow cone nozzles are only available in large flow increments (average 0.5 gpm), and stepping-up often results in excessive flow. The operator may be able to increase travel speed to compensate, but this frustrates the original intention by affecting dwell time: air settings must now be reconsidered.
Within this context, why do some Ontario airblast operators still choose airblast sprayers with centrifugal pumps? Let’s consider Ontario’s Georgian Bay area, which many manufacturers, distributors and mechanics refer to as “the last bastion of the centrifugal pump in Canada”.
Remember as you read on, Ontario’s airblast crops are predominantly small to moderate sized canopies. Centrifugal pumps are a common and appropriate pump for large canopies like tree nut and citrus.
Airblast Pumps (in Ontario)
The Georgian Bay region of Ontario.
Airblast sprayer design is highly variable, featuring a diversity of pump styles. Piston (or plunger), peristaltic, tractor-hydraulic driven centrifugal pumps are but a few. Historically, piston pumps and centrifugal pumps on John Bean and FMC sprayers were the airblast norm in Canada.
In the 1950s, Georgian Bay was home to Swanson Sprayers (now part of DW), who manufactured airblast sprayers featuring the Myers centrifugal pump. The sprayer was a good fit for the standard apple orchards found in the region. Huge canopies required high volume applications, and the rough and craggy bark harboured mites that drove the need for drenching sprays. To achieve this, sprayers traveled at 5 km/h (3.1 mph) on 7 m (24 foot) spacing, operating at 10 bar (150 psi) to emit as much as 3,750 L/ha (400 US gal./ac). At the time, a diaphragm pump could not manage this, even traveling at 0.8 km/h (0.5 mph).
A Swanson Sprayer (This one likely from Georgia, USA).
By the 1970s Holland’s Kinkelder air-shear sprayer (centrifugal pump) was introduced to Ontario and promoted as a way to use less pesticide. Perhaps ahead of their time, they never really took off because orchards were still too large for their concentrated (i.e. low-volume) applications. By the 1980s a wave of Italian-made sprayers (e.g. the Good-Boy or GB) featuring diaphragm pumps were imported into the Niagara region by distributors such as Rittenhouse.
Similar to the Kinkelder, this was one of Ontario’s last KWH air shear sprayers. RIP 2018.
The Italian-made Good-Boy (or GB).
There were many cases of misuse as unfamiliar operators failed to grease direct-drive diaphragm shafts, ran the throttle beyond 540 rpm or diverted flow intended for agitation to increase flow to the booms. Decreased agitation in relatively large tanks left concentrated spray mix to clog suction filters and destroy the diaphragm pumps. It was an inauspicious start, but the diaphragm pump rallied and today we estimate that 90% of Ontario’s airblast sprayers have diaphragm pumps, while the rest are mostly centrifugal. One Ontario airblast dealer claims to sell 50 diaphragms for every centrifugal, but not in Georgian Bay.
Is it regional history or a long memory of diaphragm “growing pains” that propagate the demand for centrifugal pumps? Perhaps considerations of maintenance, expense or ease of use play a role. Dealers claim that the centrifugal pump is cheaper, but these savings are offset by custom installation costs. Perhaps weather conditions or the crop morphology make centrifugal a better fit? Let’s consider the relative benefits and limitations of diaphragm and centrifugal pumps.
Design
Centrifugal Pumps
Centrifugal Pump – Exploded View.
Most centrifugal pumps prime by gravity feed which is why they are located at the bottom of the sprayer. While less common in Ontario, there are self-priming versions that reserve fluid in the case, or employ clever plumbing, permitting a more accessible location on the sprayer.
Engine-driven centrifugal sprayers are artefacts in Ontario. The more common PTO-driven impeller operates at high speeds, requiring a >1:4 speed step-up mechanism (e.g. gearbox, pulley or hydraulic motor), and unlike diaphragms, they create smooth flow that does not require pulse suppression. While not technically required, most have a relief valve between the pump outlet and nozzle shut off valve to handle changes in pressure.
Diaphragm Pumps
Diaphragm Pump – Exploded View.
Diaphragm pumps are self-priming and readily accessible because the shaft runs through the pump to power the fan at 540 RPM, with no need to step-up. Flow is directly proportional to pump speed which in turn depends on the tractor PTO speed. A pressure regulator is used to control bypass flow, which is convenient for making adjustments in nozzle output.
Pump Flow and GUTD
Centrifugal pumps are capable of higher flow at lower nozzle pressure and require more horsepower than diaphragm pumps. Note the large relative difference in flow for a centrifugal pump between the operating pressures of 90 and 100 psi (red curve shaded red) versus that of a diaphragm pump (blue curve shaded blue).
Relative difference in flow versus PSI at constant RPM for a common Centrifugal (red) and Diaphragm (blue) pump. Shaded pressure represents 90 to 100 psi.
Centrifugal Pumps
The flow curve of a centrifugal pump drops off dramatically; pressure (not RPM) dictates flow. If you were to throttle back on a PTO-driven centrifugal pump, reduced flow would reduce the ability to build nozzle pressure. This means fan speed cannot be separated from nozzle pressure, and reducing air speed means re-nozzling.
Centrifugal flow at different RPM. Shaded pressure represents 90-100 psi.
While (unfortunately) still rare in Ontario, rate control monitors can be used (regardless of pump type) to calibrate output based on a target rate, speed and material flow using travel speed and flow sensors. Nevertheless, they cannot compensate for the aforementioned pressure loss at the nozzle if a centrifugal pump is throttled down to reduce air speed.
In any case, throttling back on a centrifugal pump can cause a condition called suction or recirculation cavitation (aka pinging). Tiny high-pressure air bubbles form on the suction side of the impellor, explosively pitting the impellor. The damage is similar to corrosion and it causes vibration that will wear the pump prematurely.
Any restriction on the inlet side (e.g. clogged suction strainer, collapsed/undersized line) can cause a loss of volume that can damage a centrifugal pump. “Dead-heading” (i.e. closing the outlet) is possible for a short period of time, but it quickly results in heat build-up which can cause damage.
Diaphragm Pumps
The flow curve of a diaphragm pump is flatter and more efficient; RPM (not pressure) dictates flow. If you slow the airblast fan by throttling the PTO below 540 rpm, flow decreases moderately, but surplus capacity allows sufficient flow to the nozzles without pressure drop. As long as the tractor does not lug, there is less noise, lower fuel consumption and therefore operator can typically adjust the air without having to change nozzles. Even if the flow changes the pressure regulator on the diaphragm pump can be used to adjust nozzle operating pressure, precluding a change in nozzle size. Convenient.
Diaphragm flow at different RPM. Shaded pressure represents 90-100 psi.
Diaphragm pumps are capable of high pressure, but are rarely operated above 150 psi in Ontario. Molded hollow cones (eg. TeeJet’s TXR or Albuz’s ATI) operate well in the lower psi range compared to pressure-loving disc-cores. Therefore, while regulators and springs are sized according to the pump’s maximum settings, they do not reflect the usage pattern. The relatively heavy spring is too stiff to compensate for changes in pressure (e.g. driving on hills or closing one boom) behaving more like a fixed bypass and undermining a calibration. The phenomenon is discussed more detail in this article.
Maintenance
Centrifugal Pumps
A centrifugal pump with self-lubricating bearings and quality seals (e.g. carbide) that is maintained seasonally and operated in the best efficiency point of the curve will run reliably for many years.
Proponents of the centrifugal pump claim they are low maintenance (compared to the diaphragm pump). This may be anecdotal, because of the pump’s out-of-sight position on the sprayer and their tolerance for neglect. A mistreated centrifugal pump fails by degrees, often forgotten until a seal leaks or a pressure drop is noticed. In the later situation, increased flow from nozzle wear can mask the problem as the sprayer continues to cover the same number of hectares. Often overlooked, worn or misaligned sheaves/belts on a centrifugal sprayer can also cause a loss of flow. Operators might notice a tail breeze that blows spray onto the belts can cause slippage and lower the nozzle pressure.
Diaphragm Pumps
Opinion is divided on the longevity and maintenance of diaphragm pumps. Some claim they are reliable and low maintenance as long as regular oil changes occur. Others suggest the complication of connecting rods, o-rings and valves require more upkeep than the simpler centrifugal. Unlike the centrifugal pump which merely loses pressure, failure on a positive displacement pump is complete and requires immediate repair
Much depends on the diaphragm material and the products being sprayed. For example, corrosive materials (e.g. copper sulfate, urea, etc.) require polymer manifolds to minimize contact with metal. Metal manifolds do not weather well.
The diaphragm pump can run dry for extended periods. This creates heat but does not often lead to failure. Failures occur from exposure to vacuum, which can happen with dirty suction filters or long and/or improperly sized suction lines, or even lack of oil support on the compression stroke (caused by over-revving).
While three-cylinder designs may not require pulsation dampening, most require an accumulator to suppress the pulsing created by each stroke. Improper adjustment can lead to “hammering” that cracks mounts and valves, and can exacerbate rub-points on hoses. Diaphragm pumps that use direct drive shafts (i.e. carry the PTO to the fan) are subjected to the thrusting of the drive shaft during turns. It is important to keep them greased.
Summarily, the longevity and maintenance requirements for either pump design seem about equal. They depend on the products being sprayed, the quality of pump materials, and adherence to the manufacturer’s instructions on correct usage and preventative maintenance.
Conclusion
Ontario’s airblast-specific crops have become smaller, closer and denser. High liquid volumes and air speeds are typically not required. Operators are encouraged to use Crop-Adapted Spraying to adjust fan speed and nozzle output to the crop and the weather. In my opinion, the diaphragm pump facilitates this, resulting in lowered input costs, reduced drift and improved coverage uniformity. I recognize that this requires skill and effort on the part of the operator, and setting-and-forgetting a centrifugal pump can be attractive, but it’s unacceptable if it leads to unnecessary environmental impact.
In the end, the sprayer manufacturer chooses the pump, atomization and air-handling system while considering safety, effectiveness, reliability and price point. The operator must acknowledge the capabilities and limitations of the sprayer design when choosing the best fit for their operation.
I still don’t know why regions like Georgian Bay seem to prefer one pump over another. Perhaps it’s simply herd mentality. Perhaps they know something I don’t. But consider: an airblast sprayer’s average lifespan is 30 years. That’s a long time to live with a decision.
Choose wisely.
Special thanks to the many dealers, manufacturers, engineers, mechanics and end-users that helped to inform this article.
November 8, 2021