Patent ID: 7252748

Claim:
An NOx measurement apparatus comprising an NOx sensor element which includes: a flow passage to which an analyte gas is introduced; an oxygen partial pressure detection cell for detecting oxygen concentration of the analyte gas introduced to the flow passage; a first oxygen pump cell for pumping oxygen out of the interior of the flow passage to the exterior of the flow passage, or for pumping oxygen into the interior of the flow passage from the exterior of the flow passage, on the basis of the oxygen concentration detected by the oxygen partial pressure detection cell, so that current corresponding to the oxygen concentration of the gas introduced to the flow passage flows through the first oxygen pump cell; and a second oxygen pump cell to which the analyte gas having an oxygen concentration controlled by the first oxygen pump cell flows and which decomposes nitrogen oxides within the gas and causes oxygen dissociated from the nitrogen oxides to migrate, wherein a predetermined voltage is applied across the second oxygen pump cell, and the nitrogen oxides concentration of the analyte gas is measured based on current flowing through the second oxygen pump cell, the NOx measurement apparatus further comprising: voltage generation means, connected between the second oxygen pump cell and a power source, for generating, from a voltage supplied from the power source, a voltage to be applied across the second oxygen pump cell; and clamp means, connected between the voltage generation means and the second oxygen pump cell, for clamping the voltage applied across the second oxygen pump cell to a predetermined voltage which is higher than a predetermined measurement voltage to be applied across the second oxygen pump cell during measurement and which is at a level that does not damage the second oxygen pump cell but less than the voltage supplied from the power source, wherein said voltage supplied from the power source is at a level which would damage the second oxygen pump cell if applied directly thereto.