Patent ID: 6889536

Claim:
An apparatus for detecting an air/fuel ratio, comprising: a heater which generates heat through outside energization; a gas diffusion layer provided to the heater, the gas diffusion layer allowing a gas to be measured to diffuse therein; a solid electrolyte layer provided to the heater outside the gas diffusion layer, the solid electrolyte layer being activated by heat of the heater, the solid electrolyte layer being oxygen-ion conductive; a first electrode arranged between the gas diffusion layer and the solid electrolyte layer, the first electrode being exposed to the gas passing through the gas diffusion layer; a second electrode arranged on an outer surface of the solid electrolyte layer, the second electrode facing the first electrode across the solid electrolyte layer, the second electrode and the first electrode allowing a pumping current to flow therebetween when externally applying voltage thereto; a third electrode arranged on the outer surface of the solid electrolyte layer away from the second electrode, the third electrode and the first electrode allowing an electromotive force corresponding to an oxygen concentration in the gas to be generated therebetween; a first compact layer arranged on the outer surface of the solid electrolyte layer, the first compact layer concealing externally the gas diffusion layer, the solid electrolyte layer, the second electrode, and the third electrode to restrain entry of the gas; and an opening formed in the first compact layer, the opening having a predetermined width, the opening allowing entry of the gas toward the gas diffusion layer and the second electrode in a range of the width.