Category: Rates and Calibration

For basics category

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
Validate Airblast Output – Nozzle Calibration
Sprayer math is important. It ensures the operator applies the correct product rate and has enough to complete the job. But, it assumes the airblast sprayer is behaving as expected… and it often doesn’t. After confirming the airblast travel speed, use one of the following methods to assess sprayer output. There are pros and cons to each.
The area method
Operators that claim the sprayer empties in the same place every time assume everything’s alright. They are performing a variation on the area method.
Essentially, you fill the sprayer with enough water to spray one hectare (or acre) and then spray that area. If the tank empties where expected, you know your output rate (i.e. volume / area). But, there are a few problems with this method:
- Most operators don’t have an accurate test area marked off, and even when they think they know the area, measurements prove otherwise. They’re always amazed when this happens.
- The area method has poor resolution. It reveals the total output but does not assess individual nozzles. For example, partially-blocked nozzles and worn nozzles average out (we’ve seen it). Rate controllers provide whatever pressure is required to match the desired output, masking individual nozzle problems.
The dip stick method
Another method is to fill the sprayer to a known volume using a flow meter, while observing a sight level or a graduated dip stick. Then, while parked, the operator sprays for a given amount of time and determines the difference in the volume remaining in the tank.
This method can be defeated if volume is misread. It’s an easy error to make if the sprayer is parked on a grade, or the dipstick shifts in a tank with a rounded bottom. And, of course, it also masks individual nozzle problems.
Sight levels can be misleading when the sprayer is parked on a grade. They are often opaque and hard to read.
The timed output method
The preferred method is to measure the output of each nozzle individually. We performed a review on several timed output methods here. It can be messy and time consuming, but it’s accurate. Appropriate personal protective equipment is required to perform the timed output method – expect to get wet.
1. Fill the rinsed sprayer half-full with clean water and park it on a level surface.
2. With the fan(s) off, bring the sprayer up to operating pressure. Start spraying with all nozzles open (closing any will change the pressure).
3. You will need 1 meter (3 feet) of 2.5 cm (1″) diameter braided hose (have a second, longer hose to reach the top of a tower sprayer). It should be stiff enough that you can slip it over a nozzle body while holding the other end. Use it to guide flow into a collection vessel, held with your other hand. The hose not only reaches the top nozzle on towers, but it lets foam dissipate before it gets to the vessel.
4. When the flow from the hose is steady, direct it into the collection vessel for 30 seconds (a partner with a stopwatch is very helpful). It is preferable to collect for a minute because it improves the accuracy.
5. Determine and record the nozzle output per minute. Graduations on plastic collection vessels are unreliable. It’s preferable to weigh the output on a cheap, digital kitchen scale. One milliliter of clean water weighs one gram. Don’t forget to subtract the weight of the vessel (this is called taring) and double the output if you only collected for 30 seconds.
Interpreting the results
Once you have recorded all the outputs, you will have to convert the output to U.S. gallons or liters per minute, depending on units in the nozzle manufacturer’s catalogue (see common conversions below).
Replace any nozzles that are 10% (or preferably 5%) more or less than the rated output. This not only indicates a rate problem, but likely a problem with droplet size as well. If enough nozzles are worn, consider replacing all of them. Nozzles should go on as a set, and come off as a set (unless replacing a broken tip, of course). This can be an expensive proposition for large airblast sprayers, but it is part of operational costs.
Don’t assume new nozzles are accurate. We’ve found +/- 5% flow variation right off the shelf. Keep your receipts.
Testing and replacing nozzles is an important part of sprayer operation, no matter how many there are. This Air-O-Fan is nozzled for Australian almonds.
Helpful conversions
Anyone that has tried the timed output method in Canada knows the pain of our Metric-esque (Mocktric?) units. We’re an odd hybrid because our label rates are in metric, but our nozzles and many of our sprayers are US Imperial. You can find a complete collection of conversion tables here, but the most common calculations are reproduced below:
If collecting in ounces, converting to U.S. Gallons per minute:
If collecting in millilitres or grams converting to U.S. Gallons per minute:
If collecting in ounces, converting to litres per minute:
If collecting in millilitres or grams converting to litres per minute:
If collecting in ounces, converting to Imperial gallons per minute:
If collecting in millilitres or grams converting to Imperial gallons per minute:
A more sophisticated option
The timed output method is slow and requires math. You can avoid both problems by using electronic calibration vessels like the Innoquest SpotOn SC-4. We’ve tested both, and they are as accurate as weighing the output – but much faster.
They can, however, be fooled by foam. We’ve had good results using a length of braided hose to direct the flow and dissipate most of the foam. Typically, foaming means the sprayer wasn’t rinsed enough.
The SpotOn calibration vessel is easier, faster and more accurate than the classic pitcher-and-stopwatch approach to timed output tests. The SC-4 (pictured) is for airblast and SC-1 is for field sprayers.
Another approach is to hose-clamp multiple hoses over nozzle bodies and spray all at once. This is tricky and takes time. Plus, if you suffocate the nozzle’s exit orifice (creating back pressure) or block the air inlets on AI nozzles, you will get a false reading.
Be careful not to plug air inlets on air induction nozzles – you may get a false reading.
We prefer nozzle clamps over hose clamps (see the AAMS-Salvarani nozzle clamp pictured below). There are pincers designed to latch behind the nut of the nozzle body, but compatibility can sometimes be an issue (e.g. with Turbomist sprayers).
Passive flow meters (also pictured below) remove the need for a collection vessel, but they’re a better fit for field sprayers since they have to be held in place manually. They are difficult to source in North America because their accuracy is questionable, but they are fine for comparing relative flow from tip to tip.
Nozzle clamp or flow meter, avoid suffocating the nozzle exit orifice or AI nozzle air inlets.
Left: Nozzle body hose clamp. Right: Passive flow meter.
Some grower groups, or professional consultants, spring for very sophisticated and accurate units, such as AAMS-Salvarani flow measurement system pictured below.
AAMS-Salvarani flow measurement system. We used these on a pumpkin sprayer in New Hampshire, but they work with airblast too.
No matter your preferred method, take the time to confirm your sprayer output at the beginning of the season and whenever you make repairs or significant changes to your sprayer.
February 1, 2020
How Low Can You Go?
Listen to an audio recording of this article by clicking here
There’s a lot of talk about lowering the boom to reduce drift and make twin fan nozzles more effective. But how low can we actually go with a boom before striping becomes a problem?
We’ve done some calculating and have come up with answers.
First, a few guidelines. Tapered flat fan nozzles require overlap to generate a uniform volume distribution under the boom. Traditionally, we’ve recommended 30 to 50% overlap with fine flat fan sprays. The small droplets tended to redistribute to fill in any gaps that might occur.
Overlap from fine sprays is less critical than from coarser sprays because the small droplets redistribute readily.
The advent of low-drift nozzles changed that advice. This nozzle type produces fewer droplets overall, and, like all fan-style nozzles, puts the coarser ones towards the outside edges of the fan. These don’t redistribute.
A typical flat fan spray places the coarser droplets at its periphery, and the smaller ones in the middle. When only the outed edges overlap, that can creates a band of poor coverage.
When we had 30% overlap and these two edges met, a region of relatively few, coarse droplets was formed, and this region contained almost no small droplets. On a patternator, the volume distribution was still good. But when we measured the droplet density, we saw a deficit in coverage at the overlap.
With low-drift nozzles, we need 100% overlap to distribute both small and large droplets uniformly under the spray swath. Too little overlap and we create bands of relatively few but large droplets that can cause striping.
Since then, we’ve been recommending 100% overlap for low-drift sprays. This means that the pattern width at the target will be twice the nozzle spacing, and all regions under the boom receive droplets from two adjacent nozzles.
With this adjustment, small droplets appeared throughout the spray swath, and striping was eliminated.
That leaves the question, just how low can a boom be set without creating this problem? The following tables provide some theoretical numbers.
Minimum boom heights for achieving 50% and 100% overlap of flat fan spray nozzles (US units)

Minimum boom heights for achieving 50% and 100% overlap of flat fan spray nozzles (metric units)
A word of caution: The advertised fan angle on a sprayer nozzle often differs in practice. Not only will it be slightly different by design, it also depends on spray pressure and tank mix. As a result, it’s best to do a visual check. Set the spray pressure to the minimum you expect to use. Inspect the spray patterns and set the boom height so that the edge of each nozzle pattern reaches to the middle of the next nozzle. That means your pattern width is twice the spacing and will give 100% overlap. No tape measure required.
The tables were generated from a spreadsheet which can be downloaded here:
Boom-heights-at-fan-angles-March-2022 Download
- The values are theoretical and assume the fan angles are accurate. Some nozzles don’t produce the advertised fan angle. Enter your actual angle in the spreadsheet if you know it.
- The theory assumes that the droplets at the edge of the fan always move in their projected direction. In fact, after some distance, say 50 to 75 cm, gravity pulls the droplets down and the pattern no longer widens at the same rate. The rate of pattern collapse depends on the droplet sizes.
- Use the 0% overlap column to help with banding nozzle pattern width. Simply use the nozzle spacing column to enter your desired band width.
- Note that angling the nozzles forward or backward decreases your minimum boom height, but depending on the deflection of the spray in the wind, this too has limits.
- Too high a boom obviously increases drift. But patternation from overlap isn’t affected that much, largely because the pattern is now subject to aerodynamics and that becomes more important.
Pro Tip: Attach a length of plastic hose or a large zip tie to the boom, cut to your minimum boom height. This makes it easier to see what your boom height is, from the cab or the ground.
The bottom line is that a boom can be quite low and still allow excellent overlap and pattern uniformity from the nozzles. Yet we all know that most sprayer booms can’t reliably operate that low because they don’t control sway well enough. The ball’s in your court, sprayer manufacturers!
February 4, 2019
Boom Heights at Fan Angles Worksheet
Use this spreadsheet to calculate the minimum boom heights needed for various applications.
Some caution:
- The values are theoretical and assume the fan angles are accurate. Some nozzles don’t produce the advertised fan angle. Enter your actual angle in the spreadsheet
- The theory assumes that the droplets at the edge of the fan always move in their projected direction. In fact, after some distance (say 50 to 75 cm, gravity pulls the droplets down and the pattern no longer widens at the same rate. The rate of pattern collapse depends on the droplet sizes.
- Use the 0% overlap column to help with banding nozzle pattern width. Simply use the nozzle spacing column to enter your desired band width.
- Note that angling the nozzles forward or backward decreases your minimum boom height, but depending on the deflection of the spray in the wind, this too has limits.
- Too high a boom obviously increases drift. But patternation from overlap isn’t affected that much, largely because the pattern is now subject to aerodynamics.
Boom-heights-at-fan-angles-March-2022 Download
February 4, 2019
Diagnosing Airblast Coverage
Assuming there are no mechanical or maintenance problems, water-sensitive paper can be used to diagnose sprayer performance. Go here to read more about water-sensitive paper. Interpreting the results and knowing what changes to make is the critical part of the process. Observing no coverage, or a sodden paper, make for obvious conclusions… but what about everything in between? Here are the ground rules:
First: Only ever test coverage in environmental conditions you would normally spray in. Temperature, humidity and wind speed can make or break an airblast calibration.
Second: When altering sprayer settings, only make one change at a time for each test pass so you can isolate what’s wrong.
Third: Each pass requires a new set of papers located in the same place, oriented the same way, distributed throughout the canopy. Mark their locations with bright flagging tape and write the pass number and canopy position on the back of paper prior to placement. This helps you to compare the passes later on. Don’t collect papers until they’ve had an opportunity to dry a little, or they will smear and stick together.
Fourth: Pass down one alley first. Have a look at the papers without removing them. Then, spray the target canopy from the other side. Now the papers can be removed for analysis. This order is important because it reveals the impact of wind direction and the cumulative effect of spraying from both sides. In some cases, the sprayer operator may wish to travel an additional upwind alley to reflect the cumulative coverage on a typical spray day. Alternate row applications are not recommended.
This Turbomist has been outfitted with sensors that detect the presence of a canopy. Each eye corresponds to a boom section, turning the section on and off as required and improving efficiency. If it’s not there, why spray it?
Once the papers are retrieved, it’s time to diagnose the coverage. The following situations are typical in calibrations, and possible fixes are suggested. Remember, this is a process that takes time. Several passes may be required before satisfactory coverage is obtained. Once the correct settings are determined for the block, continue to use them until there is a significant change in the crop staging or weather. At that point, repeat the process.
Seven Situations
Situation One:
<15% coverage and <85 Fine/Medium droplets/cm² at top of target (e.g. tall targets such as hops or trees). Suggested Fixes:
- Wind might be stealing fine droplets. Try Coarser droplets (e.g. using air induction nozzles). Be aware that you may have to increase volume to compensate for reduced droplet counts and that they may fall out of the airstream before reaching distant targets.
- Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.
- Fan may have to be set to higher gear, or if using GUTD, return to 540 rpm to increase fan speed. If still insufficient, you may need a sprayer with higher air capacity.
Situation Two:
<15% coverage and <85 Fine/Medium droplets/cm² deep in canopy – sometimes papers on outside of canopy are visibly wet. Suggested Fixes:
- Ground speed may be too high. Use flagging tape indicator on far side of target and see if air is getting through.
- Canopy maintenance may be required (e.g. pruning, hedging, leaf stripping, etc.). No sprayer can consistently penetrate really dense canopies.
- Fan may have to be set to higher gear, or if using GUTD, return to 540 rpm to increase fan speed. If still insufficient, you may need a sprayer with higher air capacity.
- Increase carrier volume.
Situation Three:
Papers are drenched, dripping or show channels of running liquid. Suggested Fixes:
- Reduce spray volume, either overall or in key locations on the boom corresponding to the drenched papers.
- Ground speed may be too low. Use flagging tape indicator on far side of target and see if too much air is getting through. If so, increase ground speed.
Situation Four:
Considerable overspray beyond target row. Suggested Fixes:
- Turn off upper nozzles until spray JUST clears target.
- Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.
Situation Four:
Considerable blow-through beyond target row. Suggested Fixes:
- Slow the fan speed by shifting to low gear, or using GUTD method
- Ground speed may be increased as long as coverage is not compromised. Use flagging tape indicator on far side of target and see if air is getting through.
Situation Five:
Ground under target row is drenched. Suggested Fixes:
- Rotate lower nozzles slightly upward, but do not shut them off. If ground remains drenched, turn them off entirely. Each hollow cone produces up to an 80º spray angle, so the next higher nozzle often compensates by spraying lower than expected.
- Deflectors may not be channelling air and spray correctly – extrapolate air direction using ribbons on deflectors.
Situation Six:
<15% coverage and <85 Fine/Medium droplets/cm². Remember that this coverage threshold is only a point of reference, not a hard fact. It does not apply when using Coarser droplets. Suggested Fixes:
- Increase spray volume, either overall or in key locations on the boom corresponding to the under-sprayed papers.
- Wind might be stealing fine droplets. Try coarser droplets (e.g. using air induction nozzles). Be aware that you may have to increase volume to compensate for reduced droplet counts.
- Ground speed may be too high. Use flagging tape indicator on far side of target and see if enough air is getting through. If not, decrease ground speed.
- Canopy maintenance may be required (e.g. pruning, hedging, leaf stripping, etc.). No sprayer can consistently penetrate really dense canopies.
Situation Seven:
Inconsistent coverage on outer edge of canopy (e.g. one spot never seems to get spray.) Suggested Fixes:
- Nozzle spray angle may be too acute (e.g. full cones), and spray is not overlapping before reaching target. Try wider spray angles.
- Some tower sprayers have ‘dead spots’ in their air. Check for limp or flagging ribbons tied to nozzle bodies and/or deflectors. Deflectors may need to be adjusted, or adjacent nozzle body angles repositioned to compensate. Try an air induction nozzle in the dead zone.
- Canopy may be brushing against nozzles as the sprayer passes, temporarily blocking them. Canopy management required.
Some sprayers, such as Rears, Turbomist, FMC or this Durand Wayland have an option for electronic ‘eyes’ that detect spray targets. The boom will shut off completely if there is a gap in the planting. This can save a great deal of wasted spray. It is less applicable in trellised plantings where it has been known to be “fooled” by wires and posts.
If you still are unable to achieve satisfactory coverage, you may have to consider more extreme solutions. You may have an under- or over-powered sprayer. You may have to perform significant canopy management. Or, you may be trying to spray in poor weather conditions.
August 29, 2018