Sensor for measuring nitrogen oxide

A NOx sensor for measuring nitrogen oxide (NOx) in a gas which comprises:

• a first inner space and a first diffusion resistance for introducing a gas to be measured from outside the sensor to said first inner space,
• a main electrochemical pump cell for controlling the oxygen partial pressure in said first inner space,
• a means for applying a control voltage to said main electrochemical pump cell in order to effect oxygen pumping thereby,
• a second inner space connected to said first inner space via a second diffusion resistance,
• an electrical signal conversion means for effecting decomposition or reduction of nitrogen oxide on an internal electrode in said second inner space and measuring the amount of oxygen produced by the decomposition or reduction by producing an electrical output corresponding to the amount of oxygen produced, and by controlling at least the oxygen partial pressure of the atmosphere in said first and second internal spaces such that the concentration of the NOx component to be determined of the atmosphere in said second internal space by means of said electrical signal conversion means corresponds to a concentration of the NOx component in said measurement gas,
• a second electrochemical pump cell arranged for adjusting oxygen concentration in the gas which has passed through said first inner space into one of either
• (a) said second inner space, or
• (b) a third inner space of said sensor arranged in series connection between said first and second inner space and connected to said first inner space by a third diffusion resistance, said second electrochemical pump cell having an internal electrode in said second or third inner space,
• a means for applying electrical power to effect pumping of oxygen by said second electrochemical pump cell and for obtaining thereby an electrical measurement output dependent on the oxygen concentration at said internal electrode thereof,
• a means for adjusting said control voltage applied in said main electrochemical pump cell in dependence on said measurement output of said second electrochemical pump cell so as to maintain said measurement output of said second electrochemical pump cell at a constant level.