Method of measuring a conductivity of a liquid using a conductivity sensor

A conductivity measurement method of measuring a conductivity of a liquid using a conductivity sensor, the conductivity sensor including an insulating layer, a first detection electrode provided on the insulating layer, a second detection electrode provided on the insulating layer away from the first detection electrode, and a protective layer covering the first detection electrode and the second detection electrode and formed of an insulator, the conductivity measurement method comprising:

• a) measuring a first complex impedance between the first detection electrode and the second detection electrode at a first frequency with the liquid being in contact with the protective layer to face each of the first detection electrode and the second detection electrode through the protective layer;
• b) extracting a first extraction value from the first complex impedance according to a predetermined extraction rule;
• c) determining whether the first extraction value is within a predetermined tolerance;
• d) deciding the conductivity of the liquid based on the first extraction value when it is determined that the first extraction value is within the tolerance in c);
• and
• e) deciding the conductivity of the liquid based on a second extraction value when it is determined that the first extraction value is outside the tolerance in c), the second extraction value being extracted according to the extraction rule from a second complex impedance between the first detection electrode and the second detection electrode at a second frequency different from the first frequency with the liquid being in contact with the protective layer to face each of the first detection electrode and the second detection electrode through the protective layer.