Gas analyzer and method of calibrating the same

It is an object of the present invention to provide an improved gas analyzer capable of correctly monitoring the concentration of a discharged NOx even if an air ratio lambda is less than 1. It is another object of the present invention to provide a gas analyzer calibrating method involving the use of the improved gas analyzer. The gas analyzer of the present invention comprises: a gas sensor (2,100) which includes a first diffusion rate controlling passage (12), a first internal space (6) communicated with the first diffusion rate controlling passage (12), a second diffusion rate controlling passage (13), a second internal space (7) communicated with the second diffusion rate controlling passage (13), a third diffusion rate controlling passage (14), a third internal space (8) communicated with the third diffusion rate controlling passage (14), and an air introducing duct (31); wherein the first internal space (6) is provided for effecting the combustion of some combustible gases in the object gas, and is provided with a first electro-chemical pump (4b,16,18) for adjusting an oxygen partial pressure within the first internal space (6), so that the internal space constantly contains a sufficient amount of oxygen capable of effecting the combustion of the combustible gases contained in the object gas which has been introduced into the internal space (6) through the first diffusion rate controlling passage (12); wherein the second internal space (7) is provided with a second electro-chemical pump (4b,22,24) which is so constructed that when an amount of oxygen is drawn from the second internal space atmosphere consisting of the object gas introduced into the second internal space (7) through the second diffusion rate controlling passage (13), the oxygen partial pressure is decreased to a sufficiently low value which does not reduce or decompose the object gas and which is low enough for controlling the oxygen partial pressure in the third internal space (8) wherein the third internal space (8) is provided with a third electro-chemical pump (4d,28,30) next to the third diffusion rate controlling passage (14) and a fourth electro-chemical pump (4d,36,38) next to the third electro-chemical pump (4d,28,30), the third electro-chemical pump (4d,28,30) is so constructed that when the oxygen partial pressure of the atmosphere within the second internal space (7) has a value which will not substantially reduce or decompose the object gas, the oxygen partial pressure i