Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-10-30
2003-05-20
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S658000, C204S432000, C436S151000
Reexamination Certificate
active
06566894
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for detection of oxidizable and/or reducible gases in the air for the purpose of controlling devices for ventilation in buildings or vehicles and for the purpose of surveillance of combustion processes or of waste gas catalyst plants, by employing at least one heated and current-passing sensor, furnished with contact electrodes and made of a metallic sensor material, as well as by employing an electrical evaluation circuit, wherein the sensor is passed through by an alternating current, which alternating current either comprises at least two alternating currents of different frequencies or is switched between at least two frequencies, and wherein, on the one hand, the change of the capacitances between the sensor material and the contact electrodes is evaluated by the evaluation circuit as an indicator of a presence of reduceable gases and, on the other hand, the change of the capacitances within the mass of the sensor material (sensitive material) is evaluated as an indicator of the presence of oxidizable gases, as well as to a device for the performance of the method.
2. Brief Description of the Background of the Invention Including Prior Art
It is necessary in numerous cases to have knowledge about the kind and concentration of gases present in the breathable air in order to be able to perform technical control processes and technical automatic control processes or in order to be able to perform steps for the protection of the human being relative to dangerous or unpleasant, smelling gases. The knowledge of the contents of damaging materials inside or outside of the living space of the human being is important in particular for the purpose of the necessary and situation-required ventilation, in order to either initiate the ventilation or in order to interrupt the feeding of outside air when concentrations of damaging gas are present outside of the region to be ventilated.
During the surveillance or automatic control of combustion processes it is important to know the concentration of non-combusted hydrocarbons or of carbon monoxide or, respectively, the concentration of nitrous oxides in the waste gas. It is necessary in the course of monitoring the function of catalysts, for example, in motor vehicles, to know the concentration of gases before and after the catalyst, in order to be able to determine the degree of effectiveness of the catalyst.
It is a property of all examples that both, oxidizable gases, hydrocarbons, carbon monoxide, etc., as well as of reducible gases, in particular nitrous oxides, are to be detected for a determination of the individual situation and for the control of processes or, respectively, for the initiation of proper steps.
Based on the massive requirement of such sensors, technologies are sought which are not associated with large financial costs. Sensors based on polycrystalline metal oxides are associated with favorable costs and are sturdy and have been proven in practical situations. However, at this time experiences exist in fact relative to the detection of oxidizable materials, but hardly any experiences or practical solutions exist relative to the detection of both oxidizable gases as well as reducible gases, which occur simultaneously and which are to be detected at the same time, i.e. simultaneously, without making the costs and the equipment requirements too expensive relative to the application.
In principle, metal oxide sensors change their electrical resistance upon presence of a gas. The generally known sensors consist of a heated and contacted layer made for example of tin dioxide or of another metal oxide such as, for example, zinc oxide, gallium oxide, tungsten trioxide, aluminum vanadate, and other sensitive materials, wherein the sensor material is applied in a thin film technique or a thick film technique to a substrate made of ceramics or silicon and exhibits contact electrodes. If an oxidizable gaseous material impinges onto the sensor, then the metal oxide releases oxygen and becomes therefore reduced, whereby the conductivity value is increased. This process is reversible because the heated metal oxide combines later again with the oxygen of the air. The conductivity value of the sensor material is a function of the concentration of the oxidizable gases present, since a continuous exchange process is present between the offered gas, the metal oxide and the oxygen of the air and because the concentration of the oxygen of the air is a quasi constant value.
The electrical effect is opposite in case of reducible gases such as ozone, nitrous oxides and the like, because the sensitive material of the sensor is in no case a pure metal oxide, but instead this metal oxide is always in part reduced [SnO (2−x)]. Thus, the conductivity value is therefore lowered in case of an application of gas to the sensor. Complex chemical reactions between the gases with the sensor material as well as of the gases amongst each other occur in the simultaneous presence of oxidizable gases and of reducible gases in the neighborhood of the hot surface of the sensor, furnished with catalytic materials such as for example platinum or palladium. An unequivocal statement relating to the concentration of the gases present cannot be made up to now in this case.
It has been observed that the reactivity of metal oxide sensors relative to oxidizable or reducible gases is dependent on temperature. In case of low temperatures of for example below 150° C., the tin dioxide sensors are only slightly sensitive relative to oxidizable gases and hardly at all react with a conductivity-value increase upon application of a gas. If however reducible gases are present, the sensor reacts immediately with a significant decrease in the value of the electrical conductivity. For this reason it has been proposed to switch either sensor groups of in each case constant temperature based on the different reactions of the sensor in case of high (above 300° C.) temperatures and of low (below 150° C.) temperatures, which sensor group is in each case more sensitive for the group of gases involved.
It is known from the German printed patent document DE-A-38 27 426 to vary the temperature of the sensor and thus to obtain the desired information with a single sensor. This method has not proven to be of practical interest, because it takes a long time for the sensor to recover a standard value after applying the gas to the sensor at a low temperature. It has further being observed that the metal oxide sensors have under no circumstances an Ohmic resistance changeable depending on the gas present, as is frequently reported in a simplified fashion.
A method is known from the German printed patent document DE-A-3917853, where the impedance course of the sensor element is determined through the frequency of an alternating current and is employed for the identification of the gas present. J. Gutierrez describes the electrical equivalent circuit diagram of a tin dioxide sensor (
FIG. 1
) as a combination of resistors and capacitors and determines that all parameters are changed upon presence of a gas, in the literature citation “Sensors and Actuators, Volume 4, 1991, pp. 359 to 363”.
An electrically reactive compound device has in addition become known from the U.S. Pat. No. 5,387,462, which compound device exhibits random and regular fields of microstructures, which are in part disposed within an enveloping layer, wherein each microstructure exhibits a structure similar to a hair crystal and possibly exhibits a cover layer, wherein the cover layer envelopes the hair-crystal-like structure. The compound device is electrically conducting and serves as a component of an electrical switching circuit, of an antenna, of a micro-electrode, as a reactive heating element, or as a multimode sensor, in order tQ prove a presence of vapors, gases or liquids. The change of the orientation of the hair-like crystals is employed for measuring a presence of the material to be detected.
Technical Object:
It
Deb Anjan K.
Kasper Horst M.
Rosemarie Brand-Gerhart
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