Patent ID: 8248087

Claim:
A liquid concentration measuring device comprising: a detection electrode pair including a pair of electrodes facing each other; a switching section connected to switch charging and discharging of the detection electrode pair; an operation signal outputting section configured to produce operation signals of a predetermined frequency to control switching operation of the switching section at a predetermined period; and a measured value outputting section configured to produce a voltage for charging the detection electrode pair through the switching section and to produce a detection voltage corresponding to a static electric capacity of the detection electrode pair as a measured value, wherein the operation signal outputting section is configured to produce alternately a first operation signal of a first frequency and a second operation signal of a second frequency different from the first frequency for operating the switching section during a first period and a second period, respectively, the first and the second period being determined such that the detection voltage is converged in each period; wherein the measured value outputting section is configured to produce a first detection voltage and a second detection voltage when the switching section is operated at the first frequency in the first period and the second frequency in the second period, respectively; and wherein the device further comprises a difference calculating section configured to produce a difference between the first detection voltage and the second detection voltage, which are produced when the frequency for operating the switching section is switched over between the first frequency and the second frequency, and the difference is produced as the measured value.