Measuring and testing – Gas analysis – Detector detail
Reexamination Certificate
2000-09-18
2002-06-11
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Detector detail
C204S424000, C204S425000, C204S426000, C204S427000
Reexamination Certificate
active
06401522
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas analyzer for measuring a gas (hereinafter referred to as object gas) containing a certain component (hereinafter referred to as object component) such as NO
x
including a combined oxygen, and a method for calibrating the gas analyzer.
2. Description of the Related Art
In the past, various methods and apparatus had been suggested which were used to measure the concentration of an object component contained in an object gas. For example, as a method for measuring NO
x
contained in an object gas such as a combustible gas, there has been known a process which involves the use of a gas sensor manufactured by forming Pt electrode and Rh electrode on an oxygen ion conductive solid electrolytic material such as zirconia, and which achieves the desired measurement by making use of a reducibility which Ph can provide for reducing NO
x
and by generating an electro motive force between two electrodes. However, such a gas sensor has been found to have the following disadvantages. Namely, if an oxygen concentration of an object gas (which might be a combustible gas) varies, such a variation will cause not only a significant change in the electro motive force, but also a trouble that the electromotive force will change only very slightly with respect to a change in NO
x
concentration. As a result, it is difficult to complete the desired measurement without being influenced by the above troubles.
Another disadvantage associated with the above mentioned prior art may be concluded as follows. Namely, in order to reduce the NO
x
component, a reductive gas such as CO is indispensable. However, under a condition where a large amount of NO
x
is generated and hence the fuel amount becomes extremely small, an amount of CO generated will become less than an amount of NO
x
generated. As a result, using a combustible gas formed under such a combustion condition has been found unable to perform the desired measurement.
Further, JP-A-63-38154 and 64-39545 have proposed that a series of electrochemical pump cells and sensor cells formed by Pt electrode and an oxygen ion conductive solid electrolytic material can be combined with another series of electro-chemical pump cells and sensor cells formed by Rh electrode and an oxygen ion conductive solid electrolytic material, so that NO
x
concentration may be measured by making use of differences between the electric current values of various pump cells.
Moreover, JP-A-1-277751 and JP-A-2-1543 have proposed a method comprising the steps of preparing a pair of electro-chemical pump cells and a pair of sensor cells, measuring a critical pump current under an oxygen partial pressure wherein NO
x
is not reduced with a sensor comprising one set out of two sets of pairs of the chemical pump cells and the sensor cells, and measuring a critical pump current under an oxygen partial pressure wherein NO
x
is reduced with another sensor which is the other pair of the pump cell and the sensor cell, then calculating a difference between the two critical pump currents; or employing a sensor comprising a pair of a pump cell and a sensor cell, and measuring a difference in the critical current by measuring a critical current by changing an oxygen partial pressure of an object gas between the cases wherein NO
x
is not reduced and wherein NO
x
is reduced.
However, when using the above described method for measuring NO
x
, most amount of a critical current value is occupied by an electric current generated by virtue of a large amount of oxygen contained in an object gas. Since an electric current value based on an object component NO
x
becomes extremely small, a difference between two large current values can be used to calculate only a small current value corresponding to the concentration of the object component NO
x
. For this reason, there has been a problem that when a change-over is conducted between one pair of sensors, it is impossible to perform a continuous measurement, or the measurement has only a decreased responsibility and a decreased precision. On the other hand, in the case of using two pairs of sensors, if an oxygen concentration of an object gas has a significant change, an error is likely to occur in a measured value. As a result, if an oxygen concentration of an object gas has a significant change, it is not allowed to use the above method disclosed in the above prior arts. This is because a dependency of a pump current on the oxygen concentration on one sensor will be different from a dependency of a pump current on the oxygen concentration on the other. In addition, when a difference occurs between two pairs of sensors with the elapse of time, such a difference will become an error, causing a problem that these sensors can not be used for a long time.
In view of the above, it has been made clear that an amount of oxygen existing in an object gas is responsible for a decreased measurement precision when measuring NO
x
or other object components.
In order to solve the problems described in the above, a further method has been proposed by JP-A-8-271476. According to this method, a first electro-chemical pump cell and a second electro-chemical pump cell are arranged in series so as to measure an object gas component (containing a combined oxygen) such as NO
x
contained in an object gas, and such a measurement can be performed continuously for a long time with a high responsibility, without having to be influenced by an oxygen concentration of the object gas. Further, according to this publication, an object gas containing an object component including a combined oxygen is introduced from an outside space into a first treatment zone under a predetermined diffusion resistance. Then, an amount of oxygen contained in an atmosphere within this zone is controlled in a first electro-chemical pump cell in the first treatment zone to a low partial pressure value which does not bring about any unfavourable influence to a measuring process to be carried out in a second treatment zone. On the other hand, in the second treatment zone, the object component contained in the atmosphere introduced from the first treatment zone is reduced or decomposed, an amount of oxygen generated at this moment is drawn out by virtue of an oxygen pumping action of a second electro-chemical pump cell. Subsequently, a pump current flowing into the second electro-chemical pump cell is detected which is then used to calculate an amount of object component contained in the object gas. However, even when using this improved method, there is still a problem, i.e., if the concentration of an oxygen contained in the object gas is high, such a high oxygen concentration will make it difficult to perform a correct measurement.
In order to solve the above problem, a still further method has been proposed by JP-A-9-113484 and JP-A-10-73563. According. to this method, a first electro-chemical pump cell, a second electro-chemical pump cell, and a third electro-chemical pump cell are arranged in series to measure an object gas component (containing a combined oxygen) such as NO
x
contained in an object gas, and such a measurement can be performed continuously for a long time with a high responsibility, without having to be influenced by an oxygen concentration of the object gas. Further, according to these publications, an object gas containing an object component including a combined oxygen is introduced from an outside space into a first treatment zone and a second treatment zone successively under a predetermined diffusion resistance. Then, an oxygen partial pressure of an atmosphere within this zone is controlled in a first electro-chemical pump cell in the first treatment zone to a low value which is sufficient to control the oxygen partial pressure in a second treatment zone. Subsequently, in the second treatment zone, the oxygen partial pressure of the atmosphere in the second treatment zone is controlled to a predetermined value by virtue of an oxygen pumping action effected by the second electr
Kon Masao
Murase Takao
Cygan Michael
NGK Insulators Ltd.
Parkhurst & Wendel L.L.P.
Williams Hezron
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