How often do I need to calibrate my pH meter?

Calibration frequency depends upon the importance of what you are measuring. If you are measuring pharmaceuticals or foods for human consumption then follow federal guidelines such as from the FDA and GLP, Good Laboratory Practices. These require recalibrating for each lot or batch or shift as your protocol requires. If your process could result in large civil fines for water pollution then you should recalibrate often and keep good records. If on the other hand your process is not too important it is wise to recalibrate at least on a set schedule of once a week. In the end the user must make this determination themselves. Calibration only takes a few minutes so the more often the better.