A mobile system for calibrating, verifying and/or adjusting a sensor (S) 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 (KF) 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 (KF) conveyed through the bypass fluid line , and; - a pump disposed in the fluid line and configured to convey the calibration liquid (KF) in a conveying direction (FR) through the conveying circuit ; - a reference sensor (RS) which serves to measure the process variable; - a calibration site (KS) at which the reference sensor (RS) is included into the conveying circuit and at which the sensor (S) to be calibrated, verified and/or adjusted can be included, wherein -- the calibration site (KS) 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 (KS) and which is integrated into the conveying circuit by means of connections 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 (AB) of the calibration liquid (KF) conveyed through the bypass fluid line to a portion (AF) 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 (KS) 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 (KF) conveyed through the bypass fluid line , and; - a pump disposed in the fluid line and configured to convey the calibration liquid (KF) in a conveying direction (FR) through the conveying circuit ; - a reference sensor (RS) which serves to measure the process variable; - a calibration site (KS) at which the reference sensor (RS) is included into the conveying circuit and at which the sensor (S) to be calibrated, verified and/or adjusted can be included, wherein -- the calibration site (KS) 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 (KS) and which is integrated into the conveying circuit by means of connections disposed in a second section of the fluid line , which differs from the first section , and -- the calibration site (KS) 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 (KS) and which is integrated into the conveying circuit by means of connections 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 (AB) of the calibration liquid (KF) conveyed through the bypass fluid line to a portion (AF) of the calibration liquid conveyed through the first section of the fluid line can be set; <EOS>