As spring approaches, vegetation managers will begin readying for another spray season. This includes ordering herbicides, setting schedules and checking equipment for proper functionality. An important step in this process, that is often overlooked, is calibration. Properly setting up your herbicide sprayer is crucial for effective weed control and helps to prevent application errors. This article will walk you through the necessary preparatory steps to take in order to ensure you are getting the best results every time.
Why Calibrate?
Equipment calibration is necessary to ensure you are applying the correct amount of herbicide to the identified target area. Over-application of herbicides can contribute to off-target damage, result in over-label applications and ultimately become very costly. Under-applying herbicides will typically produce inferior results, requiring retreatment of the area. This is a waste of product and applicator time. For commercial applicators, lack of weed control due to under-application can lead to lost profits and jeopardize contracts.
System Preparation
Before you can effectively calibrate your herbicide sprayer, it's essential to ready your equipment for the spray season. Even with properly winterized equipment, there are steps that should be taken to prepare the spray system prior to application.
1. Visually inspect the spray system for damage, paying close attention to the tank, valves and hoses for cracks and dry-rot. Make repairs as necessary, and replace any hoses that are showing excessive wear to mitigate potential future problems. Tighten all clamps and fittings.
2. If a winterizing solution was used, completely empty the tank in a proper disposal location and refill with clean water.
3. Start the pump and allow for the clean water to circulate through the spray system, flushing all hoses and nozzles. Look for noticeable leaks and check that your pressure gauge is operational at this time.
4. Remove all nozzles and filters, inspect each component for damage, and remove any dirt or residue with a soft brush and soapy water prior to replacing.
5. Get ready to calibrate.
Area - Time - Volume
Calibration techniques may vary depending on the equipment being used and the number and type of spray nozzles. However, regardless of approach, we are only concerned with answering three questions - How big is the area? How long did it take to spray it? How much did I spray? Answering these questions will allow you calculate your gallons per acre (GPA,) and ultimately lead to how much product to put in your tank. To get started, you'll need a timer, a calculator, a measuring wheel and/or tape, marking stakes, a bucket, a measuring cup (in ounces,) pen and paper.
Handguns, Backpacks and Other Handheld Sprayers
Calibrating handheld sprayers can be accomplished in a few easy steps by following the 1/128th method for calibration. There is very little math required and no conversion equations to follow. The procedure below will allow you to calibrate your spray volume simply, and accurately.
1. Fill your rinsed sprayer with clean water.
2. Measure off an area 18.5 feet by 18.5 feet, which is equal to 1/128th of an acre.*
3. Apply water to the marked area at your normal working pace, ensuring a uniform spray pattern. Time this in seconds.
4. Spray into a bucket for the same number of seconds it took to spray the marked area.
5. Measure the amount of water you have collected in ounces.
6. Because there are 128 ounces in a gallon, there is no additional conversion necessary. The number of ounces you have collected is equal to your gallons per acre.
For increased accuracy, steps 3 - 5 can be repeated to get an average time and volume. If spray pressure or nozzle type/size changes, recalibration will be required. Finally, due to variations in working speeds, each applicator should complete the above exercise, even if they are using the same equipment.
*margin of error is .56933%
Calibrating Boom Sprayers
Due to the number of nozzles and the complexity of some spray systems, boom sprayers can appear more intimidating when attempting to calibrate for application accuracy. However, we are still addressing the same area, time and volume questions that were answered with handheld sprayers and, with a few extra steps, the 1/128th method can be used to simplify the calibration process.
1. Fill your rinsed sprayer with clean water.
2. Ensure that the nozzles are of the same type/size and are evenly spaced across the spray bar. In this case our nozzle spacing is 20 inches.
3. Measure the output of each nozzle for 30 seconds and take the average of these numbers by dividing the total output by the number of nozzles on the boom. In the example above you would have (12+12.5+13.0+12.5+11.5)÷5 = 12.3. Rounding to 12 is acceptable. Pump pressure should also be recorded at this time.
4. Clean or replace any nozzles that have greater than a 10% variance from your determined average. In this instance, any nozzles producing less than 11 oz or greater than 13 oz would be out of range.
5. Now that your nozzle output is confirmed to be accurate, measure off a course of travel, in feet, based on your nozzle spacing according to the table below. These tables can easily be found online, or travel distance can be determined by using the formula beneath the table. In this instance we would be traveling 204 feet. The terrain should be similar to what would be traversed during spray operations.
Table courtesy Alabama A&M and Auburn University Extension
6. Setting your equipment to the gear of normal spray operations, record the time, in seconds, that it takes to traverse the predetermined course. Speed and/or RPMs should also be noted to maintain consistency. Repeat this step 2 or 3 times to determine an average. For this exercise, let's say that it took an average of 23 seconds to traverse the 204 feet. We now have the information needed to calculate gallons per acre.
7. With the equipment in park, engage the spray system and capture the water output from one spray nozzle for the amount of time it took to complete the calibration course. (23 seconds). Pressure and engine RPMs should be regulated to what was recorded in steps 3 and 6. The number of ounces collected is equivalent to your gallons per acre for a single nozzle.
8. In this example, we collected 9 ounces in 23 seconds. Because we already determined that all 5 nozzles have effectively the same output, multiplying the number of ounces (9) by the number of nozzles (5) results in 45 gallons per acre for the entire boom.
Boomless Sprayers
Boomless sprayers offer advantages to applicators who may be spraying uneven terrain and/or around obstacles. Due to their compact profile and lack of spray bar, boomless systems allow for greater maneuverability and reduce the risk of equipment damage. Boomless sprayers, however, can be less accurate than traditional spray booms, due to nozzle configuration and wind impact. In the end, calibration of these sprayers is equally important and can be accomplished in a similar fashion. The process below will outline the 1/8th method of calibration for boomless spray systems.
1. Fill your rinsed sprayer with clean water.
2. Engage your spray system on dry pavement and an measure your total spray swath in feet. We will use 24 feet for this example. Pressure should also be noted at this time.
3. Based on the length of your spray swath, measure off a course of travel in feet according to the table below. Again, these tables can be found online, or travel distance can be determined by using the formula beneath the table. In this instance we would be traveling 227 feet. The terrain should be similar to what will be sprayed during operations.
Table courtesy Alabama A&M and Auburn University Extension
4. Setting your equipment to the gear of normal spray operations, record the time, in seconds, that it takes to traverse the length of the course, noting speed and/or RPMs. Repeat this step 2 or 3 times to determine an average time of completion. For this exercise, let's say that it took an average of 31 seconds to traverse the 227 feet.
5. With the equipment in park, engage the spray system at the same pressure and engine RPMs that were recorded in steps 2 and 4. Capture the total water output for the amount of time it took to complete the calibration course. (31 seconds.) Due to nozzle design and water volume, a garbage bag or funneling device may be needed to accomplish this.
6. Measure the water collected in pints. Because the area traversed was equal to 1/8 of an acre and 1 pint is 1/8 of a gallon, the number of pints collected equals your gallons per acre.
Making Adjustments
The scenarios above use area, time and volume to calculate gallons per acre for different types of sprayers. In order to increase or decrease the GPA being applied, we must either adjust the speed of the sprayer or the amount of water being dispersed by the nozzles.
Sprayer speed and GPA have an inverse relationship. Simply put, if the sprayer nozzles and pressure remain unchanged, the faster you go, the lower the gallons per acre. If the sprayer speed slows down, GPA will increase. Oftentimes only slight adjustments can be made to the speed of the application. Uneven terrain can limit how fast a piece of spray equipment can safely operate, and going too fast on level ground can increase the potential for drift.
The other way to adjust gallons per acre is by changing the amount of water coming out of the spray system. Nozzle size and GPA have a direct relationship. Therefore, increasing nozzle size will allow for more water to be disbursed by the sprayer while keeping the pressure and sprayer speed constant, thus increasing the gallons per acre being applied. Smaller nozzles will have the opposite effect by reducing the output of the sprayer. Another way to adjust the sprayer output is by increasing or decreasing pressure, but this is generally not recommended except for minor adjustments. Increasing spray pressure by 4X will only double the sprayer output, and reducing the pressure 4X will decrease it by half. Equipment should always be operated according to manufacturer recommendations, including optimal operating pressure for the spray nozzles.
Answering One Final Question
Now that we've determined how many gallons per acre are being applied, let's answer a final, important question. How much herbicide should I put in my tank? We'll use the the boom sprayer example to answer this. We have a spray rig with a 200 gallon tank. Knowing that we are spraying 45 gallons per acre, we can determine that 200 ÷ 45 = 4.44 acres per tank. We'll round to 4.5 to simplify the math. From our herbicide label, we want to apply at a rate of 16 oz per acre. Therefore: 4.5 x 16 = 72 ounces per tank.
We're ready to spray.
