Everyone has heard plenty of preaching on the importance of calibrating before spraying. There are many good methods for calibration, but they all have the same purpose - to determine the volume or gallons per acre a sprayer is applying. Without knowing the volume applied, we cannot know how much chemical to add to the tank to control the target weed or insect and stay within legal rate limits. Additionally, if there are control failures or injury problems, you need to prove that you made the application at the proper rate to substantiate your claim.
The Noble Research Institute Web site has tools to do many common calculations on the farm or ranch. These are located at www.noble.org/Ag/Tools. One of these is a calculator introduced in 2007 for calibrating boomless sprayers. To use this calculator, determine the effective spray width, operating pressure, output per 30 seconds and actual driving speed. Type this information into the blue spaces, and it will calculate the gallons per acre being applied. Print this information for your files so you can reference it for additional applications the same year. Sprayers should be calibrated, at a minimum, before starting each spray season.
For any application to be accurate, all parameters must stay the same as during the calibration. Following are a few tips for sprayer set up and calibration to help you do this.
Determine your operating speed first. The ideal operating speed is as fast as you can safely and accurately drive over the roughest part of the field. The speed is more accurate when measured on as long a run as practical and similar terrain to what will be sprayed. Once the speed is set, refer to the nozzle manufacturer's chart to select the right nozzle size for your target volume and effective spray width.
Set the pressure within the acceptable range for your nozzles, and measure actual output from each nozzle. The pressure should be as low as possible, but still maintain a good spray pattern. As spray pressure increases, droplet size becomes smaller, and the spray is more susceptible to drift. Do not use spray pressure adjustment to make large changes in volume, make those with nozzle selection and speed. It requires a quadrupling in spray pressure to double output. Use adjustments in spray pressure only for fine-tuning output.
Use foam markers, a GPS light bar or some other method to maintain the effective swath width. Too wide swaths will result in streaking or lack of control between the swaths. Too narrow swaths will result in higher rates between the swaths, resulting in wasted money and potential crop injury or illegal chemical residues.
Check the calibration accuracy by measuring the actual volume required to spray a field of known acreage. If the application volume is off by more than 10 percent, adjust your mix calculations accordingly or re-calibrate.
The Noble Research Institute Web site has another calculator for calibrating orchard sprayers, and a calculator for boom sprayers is under development. There are also calibration instructions printed on stickers available from the Noble Research Institute Ag Division at 580-224-6500 or at www.noble.org/Ag. The key is to select the calibration method you are most comfortable with, use it and then keep those parameters constant during the application.
If you have questions about calibrating a sprayer, contact your soils and crops specialist.