Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For oxygen or oxygen containing compound
Patent
1995-11-13
1997-05-27
Tung, T.
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For oxygen or oxygen containing compound
204424, 204425, 204426, 204427, G01N 27407
Patent
active
056328834
DESCRIPTION:
BRIEF SUMMARY
STATE OF THE ART
1. Field of the Invention
The invention relates to a device for detecting the oxygen content in gases, for example, in the exhaust gas of an internal combustion engine.
2. Background of the Invention
It is known to measure the oxygen content of exhaust gases with the aid of a concentration cell, for example made of ZrO.sub.2 ceramic, against the reference oxygen content in a reference atmosphere. The reference atmosphere can be defined, for example, by the ambient air or alternatively by a reference gas in a space which is sealed off or virtually sealed off with respect to the ambient. In the case of different oxygen partial pressures in the exhaust gas and the reference gas, the cell generates an electromotive force Un which, in accordance with the Nernst equation, depends on the logarithm of the quotient of the oxygen partial pressures. If the reference gas composition is known, Un can be tapped off from the cell via measuring electrodes and can be used as a measure of the oxygen content of the exhaust gas. In conjunction with a limited reference gas volume, the problem arises that the composition of the reference gas can change. Diffusion processes result, for example, in equalization of the oxygen concentrations thereby causing Un to vanish. To overcome this problem, it is known, for example, from U.S. Pat. No. 4,207,159, to impress a pump current Ip onto the concentration cell with the aid of a constant current source via the measuring electrode. The pump current Ip is so directed that the reference gas volume is continuously supplied with oxygen from the exhaust gas. Thus it is possible, on the one hand, to maintain a difference in the oxygen partial pressures. On the other hand, this pump current Ip leads, in conjunction with the electrical resistance Ri of the electrolyte and of the electrodes, to a voltage drop Ri*Ip which superimposes itself additively on the electromotive force Un of the cell to provide a total signal Un+Ri*Ip.
SUMMARY OF THE INVENTION
It is the object of the invention to provide a method and a device by means of which the additive offset Ri*Ip is eliminated. This additive offset Ri*Ip represents a disturbance of the useful signal Un because of the temperature dependence of the resistance Ri.
The invention achieves this object by providing a device for detecting the oxygen content in a gas to be measured. The device has at least one concentration cell which operates according to the Nernst principle and includes a measuring electrode, a solid electrolyte and a reference electrode. The measuring electrode communicates with the gas to be measured and is connected via the solid electrolyte to the reference electrode, which communicates with the reference gas volume. The reference gas volume is separated from the gas to be measured in such a way that a particle exchange between the reference gas volume and the gas to be measured is at least impeded. The output voltage of the concentration cell, which voltage can be tapped off between the measuring electrode and the reference electrode, represents a measure of the difference in the oxygen concentrations in the gas to be measured and in the reference gas volume. The device further comprises a current source which is coupled in between the reference electrode and the measuring electrode in such a way that, in the coupled-in state, a current flow results which is directed such that oxygen is transported in the solid electrolyte from the measuring electrode to the reference electrode. The device further comprises switching means which, in a first switching state, couples the current source to the concentration cell and which, in a second switching state, decouples the current source from the concentration cell so that the voltage between the measuring electrode and the reference electrode in the decoupled state is not affected by the current source.
A corresponding method for detecting the oxygen content in gases to be measured with the aid of the above-mentioned device is characterized in that a variable, which is c
REFERENCES:
patent: 4207159 (1980-06-01), Kimura et al.
patent: 4231733 (1980-11-01), Hickam et al.
patent: 4298573 (1981-11-01), Fujishiro
patent: 4384935 (1983-05-01), De Jong
patent: 4853091 (1989-08-01), Mund et al.
Ottesen Walter
Robert & Bosch GmbH
Tung T.
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