Abstract: In a treatment system for feeding a liquid from a tank having an outlet by a controllable pump operated cyclically, an automatic calibration/measuring system. The system is arranged for determining the volume of liquid pumped each cycle of operation by the pump and comprises a hollow sight glass having first and second sensors coupled thereto. The sight glass is in fluid communication with the tank. The first sensor is arranged to provide a first signal when the level of liquid in the container reaches it while the second sensor, which is located below the first sensor, provides a second signal when the level of liquid in the container reaches it. The container defines a predetermined volume between the first and second sensors. The system also includes a controller to determine the actual number of cycles of operation of the pump required to pump the predetermined volume and if that number has deviated from a preestablished number. If so, the pump is controlled in response to the detected deviation.