Category: General Operation

Articles that discuss general field sprayer operation and productivity factors

How Fast Should I Drive My Sprayer?
It seems simple: The faster you drive the sprayer, the more area you cover. This makes higher travel speeds a seductive method for improving productivity. Sprayer manufacturers knew this 25 years ago when pull-type sprayers first received bigger, suspended outrigger wheels. Since then they’ve delivered more powerful engines, better hydraulic motors, smoother suspension and cruise control.
Each of these innovations still required the operator to consider the relationship between travel speed, pressure, nozzle choice and the desired output per acre. But now we have rate controllers, and we don’t have to think about such mundane things anymore… do we? Do we still do a good job if we go faster? What exactly happens when we speed up?
Before considering the role of the rate controller, you have to decide on an overall target-speed range. Charts, apps, or online tools can help you select nozzles sized to deliver your application volume at a given speed and pressure. This initial travel-speed decision requires an understanding of how spray gets delivered to the target. Let’s start with the spray boom.
As the boom moves through air, the oncoming air does three things to the spray:
- It shears the spray, making it a bit finer.
- It scrubs the smallest droplets from the pattern, leaving them in the wake of the boom.
- Finally, negative pressure behind the pattern sucks even more fine spray into the sprayer wake.
Collectively, these create the dreaded “spray plume” that hangs behind the spray boom… and we’ve lost control over it. The faster we move, the greater the proportion of the spray that ends up in the plume. This can be anywhere from one to 15% of the spray. Once formed, that plume moves with the prevailing winds.
Today’s sprayers have wide booms, and faster speeds often require us to keep these booms higher than we have in the past to prevent impacts. But higher booms reduce our control over the spray’s direction. For example, when spraying vertical targets (e.g. wheat heads) we have begun to employ angled sprays. But droplets lose momentum quickly. The further they are from the target, the more likely they are to slow or even fall vertically before they reach the target. That means that higher booms often negate the benefit of angled sprays.
Still not convinced of the perils of high speeds? Well, think about the aerodynamics of the sprayer itself. As travel speed increases, the sprayer, the boom, and even the spray pattern itself disrupt the air around it. Visualize a sprayer in a wind tunnel with smoke tracer lines. The nice pattern created by the boom gets really messy in a turbulent environment. This can cause a loss of deposit uniformity, resulting in a reduction of overall effectiveness.
So far, we’ve talked about average speeds – choosing to travel eight, 12 or 16 mph overall, and then choosing the nozzle that will suit. Now let’s talk about changes in your travel speed within your target-speed range.
Operators know that even small travel speed changes can result in large pressure changes. That’s because travel speed and pressure enjoy a “square-root relationship”. If you double travel speed, your rate controller needs to quadruple the spray pressure to meet the new flow need!
Even minor changes in speed (to adapt to field conditions) can lead to big fluctuations in pressure, changing average droplet size, and affecting coverage and drift potential. Severe pressure fluctuations are more likely with a faster average travel speed. That’s perhaps why pulse-width modulation, which decouples spray pressure from travel speed and replaces it with a solenoid duty cycle, has a growing role in fast self-propelled sprayers.
To minimize pressure fluctuations, use the pressure gauge as your speedometer. Have the boom pressure displayed prominently in your sprayer cab, and try to operate at speeds that result in a pressure which is optimal for the job you’re trying to do.
So, let’s summarize the effects of fast travel speeds.
Pros:
- More area covered per hour
- Better contact with vertical targets (if the booms are kept low)
Cons:
- More drift
- Less uniform deposition
- Wider pressure fluctuations
So, how fast is too fast? We won’t draw a line in the sand, but we will emphasize how important it is to consider as much information as you can before deciding on a travel speed. Don’t rely on the rate-controller to think for you – it doesn’t have all the information.
May 21, 2015
Pulse Width Modulation
Note: This article was written before significant changes occurred in the marketplace in 2016. While it still explains how the system works, a more current account can be found here.
Pulse-Width-Modulation (PWM) refers to a method for controlling the flow rate of fluids. How does it work? Does it have a fit on your farm? We explain in this article.
Case Aim Command, Capstan PinPoint, Raven Hawkeye, TeeJet DynaJet, John Deere ExactApply, WEEDit Quadro, and Agrifac StrictSprayPlus are Pulse-Width-Modulation (PWM) technologies, where a pulsing solenoid controls flow rate through the nozzles. The solenoid is installed in place of the diaphragm check valve, and shuts off the nozzle flow for a split second exactly 10 times per second.
How it Works
All PWM systems employs the duty cycle of a pulsing solenoid instead of spray pressure to control nozzle output. The pulse width, also known as the duty cycle, is the proportion of time that the solenoid is open, and can range from 10% to 100%, although 20% to 100% is more realistic. Duty cycle is closely related to the nozzle flow. Pressure (and droplet size) stays fairly constant throughout the duty cycle range. This means that a wider range of travel speeds can be used without any change in spray pressure. Pressure can still be changed if necessary, to control droplet size.
The Tip Wizard
If using ComboJet nozzles, use the Tip Wizard to identify the best nozzle. Select “Canada”, and “US gal/acre”. Select “Tip Wizard” on the left side of the screen, and choose “Blended Pulse System, Search for Tips”.
Enter your information in the boxes. For example, 10 (gpa), 350 (µm, VMD), 15 (mph, max speed), 20 (inches nozzle spacing), 110 (degrees, fan angle). Always select 110 for use with Aim Command or Capstan.
Click “Search for Spray Tips”. The pressure that matches your droplet size criteria will be highlighted for each nozzle. In this example, the SR11006 nozzle is highlighted at 52 psi, giving about 349 µm VMD. If you choose the SR11008, the pressure goes up to 73 psi to get the same droplet size, and your minimum and maximum speeds increase as a result. Note that the numbers are calculated and do not always agree exactly with published nozzle charts. Allow for some leeway, and double check with manufacturer flow charts to be sure you’re in the ballpark.
If using TeeJet style bodies, use any wide-angle spray tip that is not air-induced. Good candidates are the TurboTeeJet, the TurboTwinJet, and the Hypro Guardian. Other pre-orifice flat fans can also be used.
Pressure Drop
Also note that the system should not be used at very high pressures – about 60 psi max. Finally, pay attention to the pressure drop across the solenoids. The manufacturer publishes charts that show the drop at various flow rates. For a 11004 tip, the drop is about 3 psi. For a 11008 tip, it’s between 6 at 30 psi and 13 psi at 60 psi. Add these values to your rate controller pressure reading, i.e., if you want to spray at 40 psi, have the rate controller read 40 & pressure drop.
Calculating Duty Cycle
Your expected average speed should be 60 – 80% of the maximum speed that the nozzle is capable of in these charts (100% duty cycle). For example, if you expect to travel 14 mph, select a solution whose maximum speed is 20 mph. This way, the system will be averaging 70% duty cycle at 14 mph (20 mph x 70% = 14 mph), allowing you to increase your application rate (or speed) by 30% where necessary (system moves to 100% duty cycle), or reduce your travel speed to 5 mph (system moves to 25% duty cycle). Slowing down further is an option, but a very coarse spray at low duty cycle may introduce skips under some conditions (low booms, fast speeds). This option also gives you maximum flexibility to change pressure to manage droplet size in both directions. Using a higher average duty cycle (say 80%) increases your flexibility to slow down, but limits your top speed more.
Picking the Right Droplet Size and Pressure
The right nozzle pressure depends on the choice of nozzle. For low-drift tips such as the Wilger SR and MR, higher pressures (>40) are recommended to ensure the spray pattern develops fully. Drift remains acceptably low. The %<200 columns in the Tip Wizard is a drift index. It identifies the proportion of the total spray volume in droplets <200 µm. Use the number to compare drift potential of various nozzles and pressures, making sure you also pay attention to the %<600 µm column. When values in that column are subtracted from 100, the result is volume in droplets >600 µm, an indication of the volume in droplets that are possibly too large to contribute much to coverage or efficacy.
It’s not easy to pick the best droplet size for each application because various pesticides and pests each have their own response. Typically, a Volume Median Diameter (VMD) ranging from 350 to 450 µm is ideal for most pesticides. Choose smaller VMDs for low water volumes, grassy weeds, and contact products, but use these only when drift is manageable. Choose larger VMDs for systemic products, broadleaf weeds, and higher water volumes, or when drift must be avoided. If you aim for 375 µm to start, that will be relatively low-drift and work well for most products.
If using nozzles other than the Wilger ComboJets, the Tip Wizard is still useful for identifying the flow rate and pressure of nozzle that you need. The Tip Wizard’s droplet size feature will not be usable, and instead, reference to the specific nozzle manufacturers’ spray quality data will be necessary. Choose nozzles that have a “Coarse” spray quality on average, and allow some movement into “Medium” and “Very Coarse” to suit a specific application need. Avoid “Fine” (not necessary for any pesticide, and too drift-prone) and “Extremely Coarse” (possibly insufficient coverage) unless specifically instructed by the pesticide manufacturer. Always select 110 degree nozzles, and do not use air-induced nozzles.
Advantages:
- Constant pressure (and droplet size) over a wide range of travel speeds.
- Ability to change droplet size with pressure adjustments on-the-go, without changing travel speed (depends on where you are in the duty-cycle range).
- Ability to change application volume on-the-go, without changing travel speed or pressure (again, depends on where you are in the duty cycle range).
- Instant response to shut-off, turn-on. Sprays at full pressure immediately. Does not drip.
- The new AIM Command Pro or Sharpshooter PinPoint allows for individual nozzle flow control. This enables nozzle-by-nozzle sectional control as well as turn compensation.
Considerations:
- Must keep water clean to avoid malfunctioning of tappet seal.
- Operator needs to understand system to take advantage. For example, at max travel speed (100% duty cycle), one cannot increase application volume or reduce drift by lowering pressure without first slowing down. Most flexibility is available at 70% duty cycle, and nozzles should be selected so that at average travel speed, system is near 70% duty cycle.
- Must use wider fan angle nozzles or higher boom height to get 100% overlap.
- The system’s primary purpose is to increase the consistency and accuracy of spraying by maintaining constant pressure over a wide travel speed range. It does not have a unique ability to reduce drift or water volume over a conventional system.
A conventional nozzle system can still do a very good job. However, using a conventional system with low-drift nozzles often reduces the available pressure range by raising the effective minimum pressure, usually to about 30 psi depending on the tip. Since many sprayers cannot produce pressures over 100 psi, this limits the travel speed range (max speed:min speed) to about 1.8. The Aim Command system removes this limitation by using duty cycle, not pressure, to control flow.
April 22, 2015
Rate Controllers and Spray Pressure
Automatic rate controllers are standard equipment on almost all new sprayers. They ensure consistent application volumes, but they don’t do all the thinking for you. We explore how to make them work properly.
A rate controller needs to know the boom width (entered by the user), the total spray liquid flow rate (from a flow meter), and the sprayer speed (gps, radar). It controls the spray liquid pressure by opening or closing a bypass valve. More pressure equals more flow to the boom.
The rate controller allows the applicator to enter a desired application volume and the controller sets the spray pressure that gives the necessary flow for the application volume and sprayer travel speed being used. In practice, this means that higher travel speeds result in higher spray pressure, and vice versa.
But it’s not that simple. Rate controllers aren’t smart enough to know how pressure affects nozzle performance. Some nozzles don’t work well at low pressures. Others do a poor job at high pressures. Some sprayer pumps may even have a problem generating some of the higher pressures a rate controller calls for. What does that mean for the available travel speed range that’s possible with any given nozzle? To answer that question, we first have to have a closer look at how pressure affects nozzle performance.
Spray Pressure and Nozzle Performance
Nozzle performance depends on a number of factors. Of these, the most critical is spray pressure. Pressure affects the flow rate of the nozzle, the spray pattern (fan angle) and the spray quality (droplet size range). The last two of these affect coverage, overlap, and spray drift, so it’s important to get them right. Each nozzle model has a unique spray pressure range and unique spray qualities within that range, so one must obtain information that is specific to the nozzles on the spray boom from the nozzle manufacturer.
ASABE spray quality for the TeeJet AIXR nozzle.
Catalogues Contain Important Information
Nozzle manufacturer catalogues identify the pressure range over which the nozzle should be operated. At low pressures, engineers look for a uniform pattern that meets the advertised fan angle. The upper pressure limits are kept low enough to prevent the formation of excessively fine sprays. Manufacturers now publish tables containing “Spray Quality”, a broad categorization of droplet size, for their various nozzles and spray pressures in their product line. Common spray qualities for agricultural nozzles are Fine (orange), Medium (yellow), Coarse (blue), Very Coarse (green), and Extremely Coarse (white). An example table from a catalogue is shown in Figure 1. Note that for any given nozzle flow rate (left column), the spray quality changes with spray pressure. For example, the TT110025 nozzle can produce a Very Coarse or a Fine spray, depending on the pressure. Also note that for any given pressure, higher flow rate nozzles produce coarser sprays. At 40 psi, the TT nozzle can produce a Medium, Coarse, or Very Coarse spray, depending on its nominal flow. Both of these relationships depend on the nozzle model and manufacturer.
Speed-Pressure-Spray Quality Relationship
As we increase spray pressure, flow rate increases with a square-root relationship.
The square root relationship between travel speed (or flow rate) and spray pressure for hydraulic nozzles
This means that in order to double the flow rate, we need to increase spray pressure by a factor of four. Figure 2 shows three different flow rate tips, each applying 10 US gpa at a range of travel speeds. Assume the operator uses a AIXR11004 to apply 10 US gpa at 12 mph. The nozzle would operate at about 40 psi, producing an Extremely Coarse spray quality. If the sprayer slows down to 7 mph to initiate a turn, spray pressure will drop to 15 psi, producing an Ultra Coarse spray. The spray pattern would likely become noticeably narrower, and poor pest control performance is likely in this situation due to the coarseness of the spray.
Relationship between travel speed and spray pressure for three nozzles applying 10 US gpa
It would have been better to use the AIXR11003 nozzle. At 12 mph, this nozzle would have operated at about 70 psi, producing a Coarse spray. Slowing down to 7 mph would drop the pressure to 25 psi, producing an Extremely Coarse spray. If the pesticide being used is sensitive to spray quality, then perhaps such slow speeds should be avoided in order to maintain a higher pressure and finer spray.
The lesson from this exercise is three-fold: (a) size the nozzle to operate at a higher pressure at your target speed to avoid dropping the pressure too low when you slow down, (b) avoid going as slow as 7 mph to prevent the pressure from dropping too low (c) compromise by setting a minimum spray pressure on the rate controller, in which case you’d over-apply product somewhat when their speed dropped too low.
Spray Pattern Overlap
Flat fan nozzle patterns need the correct overlap in order to achieve a uniform spray pattern under the boom. Research has shown that the amount of overlap for low-drift nozzles needs to be at least 100% to achieve optimum nozzle performance. In other words, the edge of a fan should reach into the centre of the adjacent fan (Figure 3), with each fan covering twice the nozzle spacing at target height. This amount of overlap assures that not only the spray volume is uniformly distributed, but that the droplet density is equally uniform. Less overlap may result in fewer droplets depositing in the overlap region, resulting in poor coverage and reduced pesticide performance.
100% overlap means that all areas under the boom receive spray from two adjacent nozzles.
Adjust the boom height so that at the lowest expected spray pressure (slowest planned travel speed), the nozzles still achieve 100% overlap. There is no disadvantage with greater than 100% overlap, but higher booms will lead to greater drift. When a choice exists, choose 110º fan angle nozzles. Most air-induced nozzles are produced at one (usually wide) fan angle only, but actual angles often differ from those advertised. It is important to visually check the overlap before spraying.
Recommendations
What does this mean in practice? Spray operators need to know the right spray quality for the job, and should consult with the pesticide product manufacturer. They also need to use nozzle manufacturers’ charts to identify the spray quality their nozzle will likely produce at their expected application volume and travel speed. If it’s a poor match, a different nozzle may need to be found. Here are some rules of thumb:
1. Choose a nozzle that produces a Coarse spray over most of the operating pressures you expect to use. Although Very Coarse sprays can work in most situations, avoid them when using lower water volumes, controlling grassy weeds, or using contact modes of action.
2. Minimize spray drift by avoiding nozzles or pressures that produce Medium or Fine spray qualities.
3. Make your pressure gauge your speedometer. First, choose a pressure that is in the middle of the nozzle’s recommended operating range. If the range is 15 to 90 psi, select 50 psi. If it’s 40 to 100 psi, select 70 psi. This allows you slow down or speed up somewhat without breaching the nozzle’s capabilities.
4. Identify the travel speeds that are possible without creating spray qualities that could compromise your application goals.
5. Visually inspect the spray pattern at the pressure extremes you expect to spray at. At the lowest pressure, your nozzle should still produce 100% overlap (the edge of the spray fan should come to the middle of the next nozzle at target height). If it doesn’t, choose a wider fan angle nozzle, increase spray pressure or elevate the boom.
6. Make sure your pump can produce the higher spray pressures you expect to need. Pressure limitations are greatest at high flow rates (fast travel speeds applying large water volumes).
7. Be prepared to compromise. It’s rarely possible to travel at the exact speed, obtain the perfect pressure, and apply the desired water volume that’s been worked out in the office or using manufacturer’s charts. If in doubt, choose slower speeds or higher water volumes to make things work out.
Nozzle manufacturers are getting much better at producing information that helps applicators produce good spraying outcomes. Learning how to use this information is the first step.
April 22, 2015