A common cause of non-uniform coverage is clogged nozzles. Watch the nozzles periodically while spraying to detect clogging. Always carry extra nozzles in your tool box, and replace bad nozzles with good ones immediately. In most cases, the pressure gauges on sprayers do not represent the actual pressure at the nozzle tip. Therefore, check the pressure at the nozzle tip when calibrating your sprayer.
Safety is extremely important when working with chemicals.
Always wear gloves and protective clothing when handling chemicals and calibrating sprayers. For safety reasons, use water instead of actual chemical mixtures when calibrating. However, some carriers, such as liquid fertilizers, are much denser than water and may cause the nozzle flow rate to vary from the rate obtained with water. In this case, determine the average nozzle output using the actual chemical mix in the field, away from the farmstead. Acknowledgment The author acknowledges the useful suggestions given by T.G. Carpenter and J.A. Gliem of the Department of Agricultural Engineering, Ohio State University.