by means of an alternating power supply further referring to fig3 s is a double thrown switch for toggling the polarity of the electrode r is a resistor an mcu is the control unit and a is a voltage sample point two sets of double thrown switches are used to toggle the polarities of the electrodes to thereby achieve an anti polarization of electrode function where analog switches can be used as the toggling switches referring to fig3 again the control unit 1 samples a voltage at the point a of the electrodes actually a voltage between the electrodes and calculates the conductivity of liquid through analog to digital conversion thereby deriving the concentration of urine it shall be noted that fig3 illustrates only a preferred embodiment of the invention and the voltage of the electrodes can be sampled differently a relationship between the voltage sampled at the point a and the conductivity shall be determined dependent upon a specific circuit design and there will be more than one possible calculation equation in brief the lower the voltage sampled at the point a is the larger the conductivity will be on the contrary the higher the voltage sampled at the point a is the smaller the conductivity will be the conductivity sensor 2 is installed at the location where a siphon.