Undoubtedly, the number one question we get from operators is: “Which nozzle should I get”? Luckily there’s no simple answer, or we wouldn’t have jobs! The reason it’s not simple is because selecting the “right” nozzle for a sprayer is a process. It can be broken down into two steps:
identifying the right flow rate (aka nozzle size)
choosing a specific nozzle model (i.e. brand, spray pattern type, spray quality, etc.)
It’s a big question, so let’s tackle just the first bullet: identifying the right flow rate.
All sprayer nozzles come in standardized (ISO) sizes, and these sizes are usually identified by numbers stamped on the nozzle as well as the colour of the nozzle itself. The nozzle’s key characteristics (i.e. the fan angle and nominal flow rate), are identified in a format that looks like some version of this (Fig. 1):
Fig. 1: Typical information printed on modern nozzles.
The 110 refers to the fan angle (110°) and the 04 refers to the flow rate. 04 means 0.4 US gallons of water per minute (gpm) at 40 psi. Each nozzle brand has a slightly different convention, but no matter how the information is presented it ought to be on the nozzle somewhere.
Nozzle colour has an ISO standard across fan-style nozzles, and we have this table to match the nozzle colour to the flow rate:
Fig 3: ISO nozzle colours and flow rates
You’ll note that the nozzle we pictured earlier was “flame red”, matching the 0.4 gpm on the table. So how do we use the table to pick the right size nozzle?
Application rate (i.e. gallons per acre or L/ha) is a function of travel speed, nozzle spacing along the boom, and nozzle flow rate. Traditionally, this has been expressed as the following formula in US units:
This formula is famously represented in nozzle charts found in all sprayer catalogues (Fig 4). Along the left side are nozzle sizes and pressures. Along the top is sprayer speed. The body of the table contains application volume. Pick your speed, and look for your application volume in the columns. If you want to apply five gpa, you need to look for the number 5 (or as close as you can get to it), among these numbers.
Fig 4: Typical nozzle flow rate chart, with speed at top and volumes in body. Ugh.
The format of the chart can be confusing because it doesn’t follow a modern sprayer operator’s priorities. Usually, an operator decides on an application volume first, and this decision is not very flexible. Travel speed, decided second, has more flexibility.
We’ve therefore re-worked the table to make more sense (Fig. 5). Along the top are common water volumes. The body of the table are travel speeds. Pick a water volume at the top and follow the column underneath this value to find a speed range you’re comfortable with. To the left, the nozzle size and corresponding operating pressures are now visible.
Fig. 5: Nozzle flow rate chart with volumes at top makes it user friendly.
Try to operate at a spray pressure that’s in the middle of the nozzle’s operating range. For an air-induced nozzle, the range is usually from 30 to 90 psi, so the middle is 60 to 70 psi. That should be the target pressure. Look for a nozzle size that delivers this pressure at your expected travel speed.
These columns can be used to work out a nozzle’s travel speed range. If a nozzle can be operated between 30 and 90 psi, for example, the corresponding speeds are listed in the same rows in the volume column.
For example, say you want to apply seven gpa and think that 13 mph would be a good average travel speed.
Fig 6: Five solutions for the question, “which nozzle to apply 7 gpa at 13 mph?”
Move down the seven gpa column, and you’ll encounter a value close to 13 mph five times – the yellow nozzle at 90 psi, the lilac nozzle at 60 psi, the blue nozzle at 40 psi, the dark red at 30 psi, and the red at about 25 psi. Now use the columns to see which of these three best matches your expected travel speed range.
The yellow nozzle would allow between seven and 12.5 mph from 30 and 90 psi, the lilac nozzle nine to 16 mph, the blue nozzle 11 to 19 mph, the dark red 13 to 22 mph, and the red 15 to 26 mph.
The best choice for a typical air-induced tip would be the lilac 025 size, since it would meet the target speed of 13 mph at a perfect 60 psi, about right for nozzles of that size, and allowing some travel speed flex on the slower side.
Some operators try to extend that range, but dropping below 30 psi will likely result in too narrow a pattern, or too coarse a spray quality, so it’s not advised.
Note that the three-fold change in pressure from 30 to 90 psi translates to only a 1.73-fold change in travel speed. That’s due to the square-root nature of the relationship, as illustrated by this formula:
This exercise applies to sprayers with rate controllers that adjust pressure to regulate flow rates. However, if you use pulse-width modulation (e.g. Case AIM Command, Capstan Sharpshooter, Raven Hawkeye, or TeeJet DynaJet) check out this article describing these systems.
There are a number of apps and websites, usually developed by nozzle manufacturers, which provide similar answers. These are also very useful, and all of them rely on the same formulas used in our new, simplified table. You can go here to download a high resolution version, suitable for framing, in both US and metric units.
Low water volumes can mean less effort to apply pesticides. But there is a limit to how low water volumes can go before problems appear. To understand the reasons why, and help applicators use the right volume for a given situation, we briefly outline what happens to a spray cloud as it reaches the crop canopy.
Basic Principles
To choose the right water volume, we have to remember three criteria for sprays to be effective.
First, the spray must reach the target.
Second, there must be enough droplets to sufficiently cover the target.
Third, the droplets have to be in a form (size and pesticide concentration) that allows the pesticide to be efficiently taken up by the target.
Reaching the target
Let’s start with the first criteria, reaching the target. Droplet size is important for minimizing both spray drift and droplet evaporation. Small droplets move off-target easily, they also evaporate to dryness very quickly and may not have the expected performance as a result. Larger droplets clearly reduce drift, but may bounce off the target and offer less coverage per water volume.
Droplets of various sizes are actually important to cover all parts of a target, so we shouldn’t eliminate all the small ones. For example, penetration of dense broadleaf canopies, or coverage of small targets like stems is best achieved with smaller droplets, while larger droplets are useful for penetrating grassy canopies or targeting the top of a broadleaf canopy.
Target coverage
We need to get the right number of droplets to the target. The more leaf area to be covered (i.e., the taller or denser the crop canopy), the more droplets will be required. Leaf Area Index (LAI), defined as the total leaf area per unit ground area, is a good indicator of canopy density.
To put this in perspective, consider a pre-seed burnoff or an early post-emergent herbicide spray vs. a late season fungicide. In the first case, the canopy can be described as being in a single plane near ground level, with leaf areas of target plants fully exposed and with an LAI of <1. High droplet density on the leaves will be achievable with relatively low volumes.
In the second case, the canopy will have more depth, and will contain large leaf areas in each of the lower, mid, and upper canopy regions, with LAI >>1. Providing the same droplet number to each of the regions in the second case will require more droplets, and therefore more volume.
Taken as a whole, the exclusive use of finer droplets can be counterproductive due to evaporation and drift. Higher water volumes have the advantage of allowing larger average droplet sizes to be used, minimizing evaporation, drift, and enhancing deposition.
Deposit efficacy
The third criteria, maximizing the performance of specific pesticides with droplet size, is more complicated. Typically, contact modes of action and grassy or difficult-to-wet targets require somewhat finer sprays and higher water volumes (Table 1). With tank mixes, such as glyphosate and Heat or AIM, the higher water volume and finer spray criteria should be used. For any specific herbicide, use the higher volume with coarser sprays.
Table 1. Herbicide modes of action, minimum water volumes with low-drift nozzles, and maximum spray quality
In practice, an applicator rarely encounters just one type of targeting situation. Most herbicides are either broad-spectrum, or are tank mixed to target both grass and broadleaf weeds. As a result, the same spray operation has to be effective on grass weeds and broadleaf weeds, some of which may be near the top of the canopy, or be more mature, whereas others may be just emerging. In these cases, a number of different droplet sizes will be required.
Low-drift nozzles
A low-drift nozzle can be used for most applications, as long as small adjustments are made for specific conditions. Increases in pressure above 60 psi (for finer droplets, Medium to Coarse spray quality) and volume to at least 7 to 10 US gpa (for better penetration) with this nozzle optimizes performance for grassy weeds. Lower pressures (down to 40 psi, Coarse to Very Coarse spray quality) are sufficient for systemic broadleaf products or when additional drift control is necessary. Higher volumes (12 – 15 US gpa) may be needed to obtain coverage in dense canopies. Always check with nozzle manufacturer information to learn what spray quality is produced by the nozzle you’re using – this will vary with nozzle type, flow rate, and spray pressure.
Droplet sizes in sprays
All nozzles produce a wide variety of droplet sizes ranging from 5 µm to 1000 µm in diameter. The main difference between sprays is the proportion of their volume in any given size fraction, with low-drift sprays having less of their volume in the drift-prone sizes.
Size distribution (by volume) of two spray qualities. Not that both of these sprays contain small and large droplets. The difference is the volume (=dosage) in each of these size fractions. Shaded areas highlight drift-prone droplets (left) and bounce-prone droplets (right).
But even low-drift nozzles produce small droplets, and these provide sufficient coverage in most cases. Low-drift sprays do create more larger droplets, and these do not contribute to coverage due to their relatively low number and poor retention.
Our main tools for droplet size selection are spray pressure (higher pressure reduces droplet size) or nozzle choice.
Spray Pressure
Higher pressures are sometimes thought to increase canopy penetration because they force the spray into the canopy. This is not true. While higher pressures create faster moving droplets, this speed quickly diminishes. By the time the spray enters the canopy, the faster velocity is lost, especially for the smaller droplets, and the only effect that remains is the finer spray. Finer droplets will penetrate many canopies further, but only if they are protected from wind. On a windy day, the finer sprays are more likely to blow downstream, or perhaps evaporate. The main benefit of higher pressure is better operation of the nozzle, especially air-induced nozzles, leading to more uniform patterns and better overall results.
Large Droplet Advantages
Although coarser sprays are often thought to work less well, they offer certain advantages.
One advantage is that a coarser spray tends to provide the air assist mentioned above (dragging air into the canopy, and giving smaller droplets a greater chance of moving where they’re needed).
Larger droplets also take longer to evaporate, increasing opportunities for uptake and translocation within the plant.
Larger droplets are more efficient at targeting the exposed, large leaves of plants requiring disease protection, leading to greater deposition and fungicide performance.
Most importantly, coarser sprays produce less drift, enabling application under windier conditions and thus ensuring that the timing of the application with respect to the crop or disease stage can be optimized.
Water Volume
Higher water volumes are the single most effective way of increasing dense canopy penetration. Higher volumes will deliver a greater number of droplets to the lower canopy, leading to greater performance when lower canopy coverage is of importance. When used in combination with lower travel speeds, the downward air flow created by sprays can provide significant benefits in forcing the smaller droplets further down. Larger volumes also decrease sensitivity to droplet size, permitting coarser sprays that reduce spray drift.
Nozzle Angling
Research has shown that exposed (upper canopy) vertical targets such as heads or stems will benefit from an angled spray. Forward-pointed sprays offer a slight advantage over backward-pointed sprays. Since angled sprays must maintain this trajectory to be useful, it is recommended that coarser spray qualities be used to minimize fine droplet production. Angled fine droplets will quickly deflect from their initial angled path and move with prevailing winds. Low booms heights also help in maximizing the benefit of angled sprays. Canopy penetration has not been shown to be improved with forward angled sprays, but backward angled sprays can help place some spray deeper into grassy canopies.
Broadleaf vs Grassy Canopies
How can an applicator decide the most appropriate water volume and spray quality for a specific application scenario? The following guides should help.
First determine the canopy density and form (broadleaf or grassy), and the target site within it (upper, mid, or lower). If the canopy is dense, but fairly vertical (i.e., a cereal), and a significant portion of it needs to be protected, the best strategy is to apply a higher water volume using a reasonably slow ground speed to allow the spray’s built-in air assist to work. If, on the other hand, only the upper layer of leaves, or the heads, are to be targeted, slightly less water can be used. If the water volume is appropriately high for the canopy, larger droplet sizes do not significantly diminish coverage or pesticide performance.
If the canopy is dense but more horizontally oriented (broadleaf crops), similar rules apply for water volume and travel speed, but now the use of a somewhat finer spray may be of benefit. The smaller droplets will be better able to move around and through the leaves to reach deeper into the canopy. Ensuring a downward trajectory of the spray through travel speed and water volume selections will be important.
Nozzle suggestions
A very good starting point for a conventional rate-controlled sprayer is any one of the low-pressure air-induced tips that now form the majority of the market. These tips are similar enough in terms of pressure range (30 – 100 psi), spray quality (Medium-Coarse-Very Coarse, depending on pressure), and spray pattern fan angle (about 100 degrees) to have comparable performance with most pesticides. Such tips are best operated in the middle of their pressure range, which is about 50 – 70 psi, offering some room to move as travel speeds change.
For those with Pulse-Width Modulation (PWM), where most air-induced tips cannot be used, nozzle choice is more limited but growing
Nozzle choice can be overwhelming due to the large selection available from many suppliers. But nozzles are the most important part of the sprayer, being responsible for metering the liquid, atomizing it into droplets, and distributing it across the boom. Review this list for what’s available, then follow these general recommendations.
1. Choose a Coarse spray quality for a multi-purpose spray
All sprayer manufacturers voluntarily publish the spray quality of their nozzles at various flow rates and operating pressures. This information is available here from their websites, catalogues, or apps. The most popular tips for field sprayers are the air-induced and pre-orifice low-drift style, and these are typically operated between 30 and 90 psi. As a starting point, look for tips that produce a Coarse spray quality at an intermediate pressure of 60 to 70 psi. This seemingly high pressure is normal for air-induction style tips and provides some necessary travel speed range for self-propelled sprayers.
The name, symbol and colour code used to describe the spray quality produced by nozzles, according to the ASABE standard S572.2.
2. Match water volume to spray quality and crop canopy
Spray quality is a useful way to manage spray drift and coverage. The coarser your spray, the higher your water volume must be because you must have enough droplets per unit area to hit your target and provide enough droplet density on the leaves. This is most critical for pre-seed burnoff, where weeds are smallest, and where low-volume, very coarse sprays will likely miss weeds entirely. It is also important for contact herbicides (that require high droplet densities) and for grassy weeds, most of which have a hard time retaining very large droplets. Use at least 7 to 10 gpa for in-crop herbicides, 10 to 15 gpa for fungicides. The taller your crop canopy, the more leaves there are to cover and the more water is required.
Relationship between spray quality and water volume. The consequence of coarser sprays is fewer droplets per unit area. The real question is what the density threshold is for any given application.
3. Know and use the right pressure for your nozzle
Even a good nozzle won’t work well at the wrong pressure. Air-induced nozzles and some pre-orifice nozzles require higher pressures to operate properly. The most common reason for performance complaints is when the spray pressure of a low-drift nozzle is too low, resulting in poor spray distribution between nozzles (see next point). If your sprayer cannot produce sufficiently high pressures, you should not be using these nozzles. Use your spray pressure gauge as your speedometer, and aim for pressures in the middle of the nozzle’s recommended operating range. Higher pressures increase drift potential, but less so for pre-orifice and air-induced nozzles. These tables help size your nozzles for your travel speeds.
Your spray pressure is the most important measurement while spraying. It determines droplet size (and therefore drift potential and coverage) as well as fan angle, which affects overlap and pattern uniformity.
4. Ensure good patterns
Whereas finer sprays from conventional nozzles can re-distribute themselves with wind or turbulence, covering up poor patterns, the coarser droplets produced by low-drift sprays will go where they’re pointed. Therefore, there is only one chance to get uniform coverage across the boom. Before spraying, set your sprayer to your lowest expected spray pressure (say 30 psi) and your lowest expected boom height and inspect the spray pattern overlap. For low-drift sprays, try to achieve a nozzle pattern width that is twice your nozzle spacing at the target height. If necessary, adjust your boom height, increase pressure, or select wider angle nozzles. This will ensure that the coarsest droplets at the pattern edge are mixed in with the more abundant, finer droplets found in the middle of a pattern.
