Method and apparatus for measuring concentration of a component in a gas

Abstract

A method and apparatus which enable accurate measurement of the concentration of NOx in a gas under measurement, even when the NOx concentration is low. A gas under measurement is introduced into a first chamber through a first diffusion passage A pump voltage V is applied from a direct-current power source E to a first oxygen ion pump cell such that the electric potential of an oxygen-concentration-measuring cell is held constant. As a result, oxygen is pumped out from the first chamber such that the oxygen concentration measured at an inlet to a second chamber becomes constant. A first pump current I flowing to the first oxygen ion pump cell is measured. Next, the gas under measurement contained in the first chamber is introduced into the second chamber through a second diffusion passage A pump voltage V is applied to a second oxygen ion pump cell of the second chamber by means of a direct-current power source E thereby decomposing nitrogen monoxide contained in the gas under measurement and pumping out oxygen from the second chamber A second pump current I flowing to the second oxygen ion pump cell is measured. In a measurement region in which NOx concentration is low, the second pump current I is proportional to the square of the NOx concentration. Thus, the NOx concentration is obtained from the second pump current I by use of an expression of second order.