Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For oxygen or oxygen containing compound
Patent
1995-01-19
1997-01-21
Tung, T.
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For oxygen or oxygen containing compound
204424, 204425, 204426, 204427, 422 98, G01N 2712, G01N 2741
Patent
active
055956475
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is based on a method of determining gas components and/or gas concentrations in gas mixtures, particularly in exhaust gases of internal combustion engines, in which the adsorption and/or desorption speeds of the gas mixture caused by a change in gas concentration at a gas sensor are evaluated.
2. Description of the Related Art
A generic method of determining different gas concentrations of a gas mixture in which the concentration of the gas mixture is modulated in a defined manner in a reaction chamber disposed upstream of a gas sensor is known from Sensors and Actuators, 20 (1989), 277-285. The sensor signal following the modulation is analyzed, and subsequently a conclusion is drawn regarding the corresponding gas components. This method is refined in Sensors and Actuators B, 4 (1991), 337-343 in that a sudden switch between a reference gas and the measured gas has a surging effect on the gas concentration at the gas sensor. A conclusion regarding the gas species is then drawn from the reply function of the sensor signal. In both known methods, the change in the gas mixture concentration takes place in the gas phase, because of which the influenced gas mixture only reaches the sensitive region of the gas sensor by way of a diffusion step. The system thus has a long reaction time. Moreover, the reply function is distorted by the occurring gas diffusion. Finally, the gas diffusion limits the possible frequency spectrum of modulation.
A sensor arrangement for determining CO concentrations, in which an electrochemical oxygen pump cell pumps oxygen to a gas sensor, is also known from Sensors and Actuators B, 9 (1992), 183-189 and 233-239. The gas sensor in this instance is disposed in a measuring chamber without a defined reference to the pump cell and the gas mixture. It is only required that a sufficient oxygen concentration be present at the measuring element. It was determined that the resistance value of an SnO.sub.2 gas sensor is three times greater in CO in air with 21% oxygen than in CO in nitrogen.
SUMMARY OF THE INVENTION
The invention makes use of the fact that the different gas species can be characterized by typical adsorption and/or desorption speeds at a gas sensor. The method according to the invention, which has the characterizing features of the main claim, has the advantage that the use of gas sensors with an oxygen cross-sensitivity is possible in selective determination of gas components, for example CO, NO.sub.x, HC, with simple means. The gas concentration is directly influenced at the gas sensor by the supply of oxygen, which causes a rapid reaction of the gas sensor to changes in the concentration.
Advantageous refinements and improvements of the method disclosed in the main claim are possible due to the measures outlined in the dependent claims. A particularly realistic time response of the reply function is achieved when the oxygen is supplied to the gas sensor in ion form. Consequently, a surface migration of the oxygen at the sensitive region of the gas sensor can take place significantly faster than by diffusion of the oxygen in the gas phase. A first advantageous embodiment of the method consists of analyzing the transient response of the sensor signal due to an externally-generated, surging change in the gas concentration at the gas sensor. A second advantageous embodiment further consists of evaluating the concentration balance necessary to maintain a constant oxygen concentration at the gas sensor effected by corresponding oxygen supply, using control technology.
BRIEF DESCRIPTION OF THE DRAWINGS
Two embodiments of the invention are described in detail in the following description. The drawing shows, in FIG. 1, a cross-section of a fundamental representation of an embodiment of a sensor system for performing the method of the invention and in FIG. 2, the course of the conductivity .sigma. of an SnO.sub.2 semiconductor gas sensor over time.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The sensor system i
REFERENCES:
patent: 4272331 (1981-06-01), Hetrick
patent: 4416763 (1983-11-01), Fujishiro
S. Vaihinger et al: "Detection of halogenated and other hydrocarbons in air: Response Functions of Catalyst/Electrochemical Sensor Systems". In: Sensors and Actuators B, vol. 4, 1991 month unavailable, Lausanne CH, pp. 337-343.
J.H. Visser et al: "Sensors for measuring combustibles in the absence of oxygen", In: Sensors and Actuators B, vol. 9, 1992 month unavailable, Lausanne CH, pp. 233-239.
G.J. Maclay: "Use of Time-dependent Chemical Sensor Signals for Selective Identification". In: Sensors and Actuators, 20, 1989 month unavailable, pp. 277-285.
E.M. Logothetis: "Chemical and physical sensors based on oxygen pumping with solid-state electrochemical cells". In: Sensors and Actuators B, 9,1992 month unavailable, pp. 183-189.
Hoetzel Gerhard
Neumann Harald
Riegel Johann
Robert & Bosch GmbH
Tung T.
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