1. Field of the Invention
The present invention relates in general to an electrochemical device, and more particularly to an oxygen sensor for detecting the concentration of oxygen in a measurement fluid, in particular, exhaust gases emitted by an internal combustion engine.
2. Discussion of the Prior Art
There has been known a sensing device including an electrochemical element having a solid electrolyte body, for example, an oxygen sensor which detects or determines the oxygen concentration of exhaust gases emitted from an internal combustion engine, for the purpose of controlling the combustion or fuel burning conditions of the engine according to signals produced by the oxygen sensor, and thereby purifying the exhaust gases and reducing the fuel consumption of the engine. An example of such oxygen sensors uses a sensing element which comprises a body of oxygen-ion conductive solid electrolyte such as zirconium oxide doped with calcium oxide or yttrium oxide, and which further comprises siutable electrodes formed on opposite surfaces of the solid electrolyte body. In this oxygen sensor, one of the electrodes is exposed to a reference gas such as the ambient air, while the other electrode is exposed to a desired measurement fluid such as exhaust gases. In operation, the oxygen sensor produces an output signal representative of an electromotive force induced between the two electrodes according to the principle of an oxygen concentration cell.
In recent years, there has been an increasing tendency to use an elongate planar sensing element, rather than a conventionally used tubular sensing element, in view of ease of fabrication and structural simplicity of the sensor. Such an elongate planar sensing element has, at its one longitudinal end, an oxygen detecting portion which is exposed to exhaust gases or other measurement fluids. Such planar oxygen sensing elements or oxygen sensors incorporating a planar oxygen sensing element are disclosed, for example, in Laid-Open Publications No. 58-153155 and 58-172542 (published in 1983) of Japanese Patent Applications.
In the electrochemical device exemplified by the oxygen sensors described above, one of the electrodes provided at the detecting portion of the sensing element functions as a measuring electrode to be exposed to the measurement fluid, while the other electrode functions as a reference electrode to be exposed to a reference gas which is present in a reference-gas space or passage formed in the sensing element so as to extend toward the detecting portion. The reference-gas space is open at the longitudinal end of the sensing element remote from the detecting portion, so that the reference-gas space communicates with the ambient air or atmosphere, through air-inlet openings formed through a housing in which the sensor is accommodated. In this arrangement, the ambient air is introduced as a reference gas into the reference-gas space within the sensing element, so that the reference electrode is exposed to the introduced reference gas.
However, the oxygen sensors constructed as described above tend to suffer from various inconveniences due to entry of undesirable external foreign substances such as water, salt water and muds, through the air-inlet openings provided in the sensor housing. These foreign substances may have adverse effects on the sensing element. For example, these substances may cause electrical insulation failure or similar trouble, which leads to inaccurate output signals of the sensor, or which results in damaging the sensing element due to rupture or breakage of the ceramic components.
Another drawback experienced in the oxygen sensors indicated above arises from the presence of the reference-gas space or passage which extends through the sensing element over its substantially entire length, which therefore reduces the structural strength of the sensing element.
In another type of oxygen sensor, the entirety of the sensing element is located within the measurement fluid, such that the measuring electrode is exposed to the measurement fluid, while the reference electrode communicates with the measurement fluid, either directly, or indirectly via a porous layer or diffusion-resistance layer having a given diffusion resistance to the measurement fluid. In this arrangement, the reference oxygen is continuously supplied to the reference electrode by a pumping cell of the sensing element, while at the same time the supplied reference oxygen is discharged into the external measurement fluid through the porous layer indicated above or other means. In this case, the reference gas to which the reference electrode is exposed is easily influenced by the ambient atmosphere, and the sensor output is accordingly influenced. If a large amount of pumping current is applied to the pumping cell to avoid such influences, the ceramic components of the sensing element are likely to be deteriorated, or the detecting accuracy of the sensor is lowered due to an excessive voltage drop caused by the adjustment of the reference gas. Further, the application of an excessive current to the pumping cell may cause separation or flake-off of the sensing element at the reference electrode.