Pearl Water Manager calibrated by default at the factory. Then you should calibrate your sensors at the start of every season, starting from the 2nd one, before getting your pool back up and running to ensure that you start the season in the best possible condition. Furthermore, if at any moment during the season you have a doubt about the accuracy of your pH and Redox readings do not hesitate to re-calibrate the sensors in order to ensure that the readings are as accurate as possible.