The invention relates to a device for sensing and indicating the state of a medium in a conduit. The device comprises a sensor means (10) for sensing the value of a parameter of the medium in the conduit (8) and an indicator means (13) which is coupled to the sensor means and is intended to provide an indication which is dependent on the sensed value. The sensor means (10) is coupled into the conduit (8) and is provided with a channel which, when the sensor means is coupled in place, constitutes a portion of the conduit. The sensor means (10) has two electrodes which are electrically isolated from each other and extend into the channel. The indicator means is provided with or is coupled to a current source for applying a voltage between the electrodes of the sensor means (10) and is arranged to provide an optical and/or acoustic signal when the resistance of the medium exceeds a predetermined threshold value. According to the invention, a second sensor means in the form of a measuring probe (23) is coupled to an instrument (12) comprising the indicator means (13). The measuring probe (23), when immersed in a medium, senses a calibration value for calibrating the indicator means (13).