To correctly measure concentrations of NOx and oxygen by a single NOx sensor.
The apparatus comprises a first measurement chamber 20 communicating with a gas to be measured via a diffusion-controlled layer 4d and a second measurement chamber 26 communicating with the first measurement chamber 20 via diffusion control layers 6d, 22d. A first pump cell 4 and a Vs cell 6 are formed in the first measurement chamber 20. A second pump cell 8 is formed in the second measurement chamber 26. An oxygen concentration and an NOx concentration are measured with the use of an NOx sensor 2 having the first and second measurement chambers. A first pump current IP1 is controlled so that an output of the Vs cell 6 becomes a reference voltage VC0, thereby controlling the interior of the first measurement chamber 20 at a constant low oxygen concentration. Moreover, a constant voltage is impressed to the second pump cell 8 thereby to decompose NOx components in the second measurement chamber 26 and extract oxygen. The NOx concentration and oxygen concentration are measured from a second pump current IP2 and the first pump current IP1 respectively. At the measurement time, a sensor temperature is detected from an internal resistance of the Vs cell, thereby to control the amount of a current supplied to the heaters 12, 14. The sensor temperature is thus maintained constant.
KONDO TOSHIAKI
INAGAKI HIROSHI
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