A mobile system for calibrating, verifying and/or adjusting a sensor for determining a process variable of a process medium that is dependent on a salt concentration, comprising a conveying circuit configured to convey a calibration liquid introduced therein, having: - a fluid line; - a bypass fluid line, which is connected to the fluid line via a first connection point and a second connection point, in a bypass to a first section of the fluid line, with a filter unit configured for the continuous desalination of a portion of the calibration liquid conveyed through the bypass fluid line, and; - a pump disposed in the fluid line and configured to convey the calibration liquid in a conveying direction through the conveying circuit; - a reference sensor which serves to measure the process variable; - a calibration site at which the reference sensor is included into the conveying circuit and at which the sensor to be calibrated, verified and/or adjusted can be included, wherein -- the calibration site is disposed in a second section of the fluid line, which differs from the first section of the fluid line, or -- an interchangeable fitting which serves as the calibration site and which is integrated into the conveying circuit by means of connections (7a, 7b) disposed in a second section of the fluid line, which differs from the first section, and - an adjustable valve unit by means of which a ratio of the portion of the calibration liquid conveyed through the bypass fluid line to a portion of the calibration liquid conveyed through the first section of the fluid line can be set; wherein, by setting, in particular controlling, the valve unit, the process variable present at the calibration site can be set, in particular controlled, to at least one specifiable reference value (RW1;RW2,...) for the process variable. - a fluid line; - a bypass fluid line, which is connected to the fluid line via a first connection point and a second connection point, in a bypass to a first section of the fluid line, with a filter unit configured for the continuous desalination of a portion of the calibration liquid conveyed through the bypass fluid line, and; - a pump disposed in the fluid line and configured to convey the calibration liquid in a conveying direction through the conveying circuit; - a reference sensor which serves to measure the process variable; - a calibration site at which the reference sensor is included into the conveying circuit and at which the sensor to be calibrated, verified and/or adjusted can be included, wherein -- the calibration site is disposed in a second section of the fluid line, which differs from the first section of the fluid line, or -- an interchangeable fitting which serves as the calibration site and which is integrated into the conveying circuit by means of connections (7a, 7b) disposed in a second section of the fluid line, which differs from the first section, and -- the calibration site is disposed in a second section of the fluid line, which differs from the first section of the fluid line, or -- an interchangeable fitting which serves as the calibration site and which is integrated into the conveying circuit by means of connections (7a, 7b) disposed in a second section of the fluid line, which differs from the first section, - an adjustable valve unit by means of which a ratio of the portion of the calibration liquid conveyed through the bypass fluid line to a portion of the calibration liquid conveyed through the first section of the fluid line can be set;