To ensure the long term accuracy of your Bluelab EC meters, we recommend cleaning and testing them regularly. The Bluelab EC pens and meters are factory calibrated, but to check cleaning we recommend using the Bluelab 2.77 EC Standard Solution - 250ml. It is recommended that you clean and test your EC meter every 30 days, with step by step instructions on cleaning and care provided by Bluelab. The Bluelab 2.77 EC Standard Solution - 250ml is essential for use with all Bluelab EC products.