Sprayer operating pressure is one of the major factors that affect sprayer performance. Pressure directly affects your application rate as well as the droplet size produced by your sprayer nozzles. This leads to the natural question: What pressure should you spray at?
You should operate your sprayer at a pressure that is in the middle of the operating pressure range of the spray nozzle(s) that you are using. For most flat fan nozzles this is going to be about 40 PSI, but it can vary depending on the specific nozzle. Keeping operating pressure in this range offers flexibility during operation to speed up or slow down and maintain a constant rate by increasing or decreasing the pressure accordingly.
If the operating pressure is too low the spray nozzle will not produce a consistent spray pattern, and if the pressure is too high it may result in droplets that are too fine or a mist. In either case, the effectiveness of your sprayer will be greatly reduced. Let’s take a closer look at how pressure affects spraying.
How Does Pressure Affect Application Rate?
The operating pressure of your sprayer directly relates to your application rate. Application rate is generally measured in gallons per acre or GPA. The actual amount you apply with your sprayer per acre is determined by the speed you travel, the nozzle size you use, and the pressure at which you operate.
If you maintain a constant speed and do not change your nozzle size, then your rate will go up when you increase pressure and down when you decrease pressure. If you speed up you need to increase your operating pressure to maintain the same application rate. Likewise, if you decrease your speed your per acre rate will increase unless you lower your pressure.
Gallon Per Acre Formula = [5940 x GPM flow each nozzle] / [speed(mph) x nozzle spacing (inches)] Example: Each of your sprayer nozzles produce 0.2 GPM, you spray at 4 mph, and your nozzles are 20 inches apart. (5940 x .2) / (4 x 20) = 14.85 GPA.
You will find the gallon per minute (GPM) flow of your nozzles by identifying what size they are. Most flat fan sprayer nozzles follow a universal color code that indicates the size. For example, a yellow tip will flow 0.20 GPM at 40 psi. You can refer to the spray nozzle chart to see what the flow is at different pressures.
Your speed is determined by your preference and conditions. A lower speed will generally allow for better canopy penetration and is easier on your boom. Higher speeds will help you cover more ground but can be hard on equipment and may require higher operating pressures which can lead to more drift. We will talk more about how pressure affects drift below.
How Do I Adjust My Sprayer’s Pressure When I speed Up or Slow Down?
If you can maintain a constant speed while spraying then adjusting the operating pressure is not necessary. In this case, you would only need a simple sprayer setup with a bypass or regulating valve that allows you to set your operating pressure and then begin to spray. If you need to make adjustments you will likely need to stop and adjust the control dial.
However, if your speed will vary then you need to be able to adjust the pressure on the go. This can be accomplished in two main ways: pressure or flow meter-based control.
Pressure-Based Sprayer Control
Pressure-based or manual control is when you calculate ahead of time what operating pressure will produce your desired application rate at the different speeds you will potentially travel. Then, as you increase or decrease your speed, you can manually adjust the pressure to match the speed and maintain a constant rate.
This is achieved by either controlling the pump speed or using a regulating valve. In either case, you use a simple control console that allows you to increase the flow rate and therefore the pressure by speeding up your pump or opening up the regulating valve more.
Regulating Valve Pressure Control Components:
- Regulating Valve
- Pressure sensor/Gauge
- Control Console
Rheostat Pump Speed Controller Components:
- Rheostat motor speed controller (Used with 12-volt pumps)
- Pressure Sensor/Gauge
Flowmeter Based Sprayer Control
Flowmeter-based control is more elaborate. This setup gives you automatic rate control. Your rate control is set to your desired GPA rate and it will automatically adjust the sprayer output accordingly as you speed up and slow down.
Components Needed For Flowmeter Based Automatic Rate Control on Sprayer:
- Regulating Valve
- Ground Speed Sensor or GPS Speed Input
- Automatic Rate Controller
Read this post if you want to see a more detailed look at sprayer control setups and plumbing.
How Does Pressure Affect Spray Pattern?
Another factor that you should consider in determining your optimum spray pressure is the spray pattern of your nozzles. Each different nozzle type will have a range of PSI to stay within. This range is usually 20-80 psi but varies depending on the nozzle.
Pressure too low and the nozzle will not maintain an even spray pattern. Pressure too high will produce a fine mist and will hurt your coverage.
How Does Pressure Affect Spray Drift?
Droplet size is also affected by pressure. At lower operating pressures your nozzles will create larger droplet sizes than at higher pressures. If drift is a concern then you will want to stay at a pressure that will produce larger droplets. This may mean you need to go up to the next size nozzle so you can operate at a lower pressure and still achieve your desired rate.
Different nozzle types will indicate in their individual nozzle charts what droplet size the nozzle produces a certain PSI. Droplet sizes are measured in microns and you can view this article to see how droplet size is shown in a nozzle chart.
Pressure Range of Different Nozzle Types:
|Nozzle||Pressure Range (PSI)|
|Turbo TeeJet (TT)||15-90|
|Xtended Range TeeJet (XR)||15-60|
|Air Inducted TeeJet (AI)||40-100|
|Hypro Ultra Lo-Drift Nozzle (FCULD)||30-100|
|Wilger ComboJet (ER)||20-80|
|Wilger ComboJet (DR)||30-80|
|Green Leaf Turbo Drop (TADF)||30-120|
|Green Leaf Turbo Drop (TDXL)||30-120|
Before You Go
The ideal operating pressure will vary depending on many factors. Your sprayer nozzles, your drift tolerance, and your speed will affect what pressure is correct for you. Having the right pieces of information will help you select the ideal operating pressure. The most important information you need is your spray nozzle operating pressure range and the droplet size recommended by the chemical you are using.