Patent ID: 7073369

Claim:
A method for determining the concentration of an oxygen-containing compound in a test gas in an external volume containing also additional oxygen, the method comprising the steps of: providing a sensor arrangement including; a sensor having a first cavity connected to said external volume via a first diffusion barrier and a second cavity connected to the first cavity via a second diffusion barrier; means for reducing the oxygen concentration in said first cavity; and, means for detecting the concentration of oxygen in said second cavity; reducing the oxygen concentration in said first cavity in a first phase to a value at which a change of the concentration of the oxygen-containing compound does not occur; in a second phase in said first cavity, further reducing said oxygen concentration to a value at which also a reduction of the concentration of the first oxygen-containing compound takes place and forming an index for the sought concentration as a difference of the oxygen concentrations detected in said second cavity in said first and second phases; and, wherein the means for reducing the oxygen concentration in said first cavity includes a solid electrolyte which is arranged between two electrodes to which a voltage is applied; and, the voltage, which is supplied to said electrodes, is greater in the second phase than in the first phase.