NOx measurement apparatus

An NOx measurement apparatus which comprises an NOx sensor element which includes:

• an oxygen-ion-conductive solid electrolyte,
• a flow passage which is disposed within or adjacent to said solid electrolyte and to which an analyte gas is introduced;
• an oxygen partial pressure detection cell for detecting oxygen partial pressure of the analyte gas introduced to the flow passage;
• a first oxygen pump cell for controlling the oxygen partial pressure of said analyte gas introduced to the flow passage by pumping oxygen out of the interior of the flow passage to the exterior of the flow passage, or by pumping oxygen into the interior of the flow passage from the exterior of the flow passage based on the oxygen partial pressure detected by the oxygen partial pressure detection cell;
• and
• a nitrogen oxides detection cell which causes dissociated oxygen resulting from decomposition of nitrogen oxides in the gas controlled for the oxygen partial pressure to migrate within said solid electrolyte,
• wherein
• a predetermined voltage is applied across said nitrogen oxides detection cell, and the nitrogen oxides concentration of the analyte gas is measured based on a current flowing through said solid electrolyte,
• said NOx measurement apparatus being characterized by comprising:
• voltage generation means, connected between said nitrogen oxides detection cell and a power source, for generating, from a voltage supplied from the power source, a voltage to be applied across said nitrogen oxides detection cell, wherein said voltage supplied from the power source is at a level which would damage the nitrogen oxides detection cell if applied directly thereto;
• and
• clamp means, connected between the voltage generation means, for generating a voltage to be applied across said nitrogen oxides detection cell, and said nitrogen oxides detection cell, for always preventing an overvoltage applied across said nitrogen oxides detection cell and allowing a voltage which is sufficiently higher than a measurement voltage but less than the voltage supplied from the power source to be applied across said nitrogen oxides detection cell during activation of said NOx sensor element.

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 but not higher than a voltage supplied from the power source, wherein
• the voltage generation means includes an amplification circuit having a power source terminal for receiving drive voltage from the power source, a non-inverted input terminal for receiving a variably set input voltage, and an inverted input terminal and an output terminal which are connected to each other;
• and
• the clamp means includes a diode which is connected to a node between the inverted input terminal and the output terminal and which enters a conductive state when a voltage higher than the predetermined voltage is applied across the second oxygen pump cell, to thereby limit the voltage applied across the second oxygen pump cell to the predetermined voltage or below.