When a rain gage is out of calibration, it is usually because of the buildup of dirt and grime on the internal surfaces of the tipping bucket mechanism. Cleaning the internal surfaces usually brings the rain gage back into calibration. It is also possible that a rain gage is out of calibration because it is no longer level.
To minimize the possible occurrence of calibration errors, perform routine cleaning and maintenance of the rain gage at least once every three months. The environmental conditions at a particular site may require a facility to perform cleaning, leveling, and maintenance on a much more frequent schedule.