Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-01-24
2001-10-23
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000, C204S427000, C205S781000
Reexamination Certificate
active
06306271
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a sensor for measuring nitrogen oxide. Particularly, the present invention provides a method for measuring a very small amount of nitrogen oxide in a gas containing a great amount of interfering gas with a high preciseness.
With regard to a sensor for obtaining NOx amount in a gas to be measured by measuring an amount of oxygen generated by decomposing NOx in the gas to be measured, there has conventionally been, for example, a sensor disclosed in European Patent Laid Open No. 0678740A1 as shown in FIG.
13
.
In
FIG. 13
,
80
is a NOx sensor having a main pump means
60
and an electric signal conversion means
70
. The main pump means
60
includes an electrochemical pump cell
6
comprising a substrate
3
comprising an oxygen ion conductive solid electrolyte and an inner pump electrode
5
and an outer pump electrode
4
formed on the inner surface and outer surface of the substrate, respectively. The mail pump means
60
treats oxygen contained in the measuring gas introduced from an external space by pumping processing on the basis of a control voltage applied between the inner pump electrode
5
and the outer pump electrode
4
by a power source
7
. The electric signal conversion means
70
includes a pair of detection electrodes
11
and
14
formed on the side where the measuring gas that was treated by pumping processing by the main pump means is introduced. The electric signal conversion means
70
generates an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the measuring gas after treated by the pumping processing by the main pump means
60
.
There has conventionally been used the NOx sensor
80
for measuring an amount of nitrogen oxide in a gas by introducing the gas to a first inner space
2
connected with the external space by means of a first diffusion resistance
1
, controlling an oxygen partial pressure in an atmosphere in the first inner space
2
to be a predetermined low value at which NO is not decomposed substantially by the main pump means
60
, introducing a controlled atmosphere of the first inner space
2
into a second inner space
9
connected with the first inner space
2
by means of a second diffusion resistance
8
, and converting the atmosphere into an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the atmosphere by the electric signal conversion means
70
.
Incidentally, in
FIG. 13
, a potential difference detection means
40
measures an electromotive force V
1
corresponding to a difference in oxygen partial pressure between an electrode
10
exposed in the first inner space
2
and an electrode
11
exposed in a space
15
where a standard gas is present. A voltage applied to the main pump means
60
is adjusted so that the electromotive force V
1
is controlled to have a predetermined value.
However, it is difficult by this sensor to completely remove an influence of an interfering gas, for example, oxygen gas, carbonic acid gas steam, or the like, in an exhaust gas of an internal engine. Because of a difference between a gas concentration in the side facing the first inner space
2
of an inner pump electrode
5
of the electrochemical pump cell
6
and that in the side of the outer pump electrode
4
, a disorder of an atmosphere in the first inner space
2
by pulsation of a gas to be measured, an error caused by decomposition of water, a change of electrode activity or diffusivity of each gas component with a change of a temperature of a sensor element, a change of standing of electrodes and electrolyte, a discordance between an average value of an electrode potential of the electrochemical pump cell
6
and an average of a gas concentration, etc., it was difficult to maintain an oxygen partial pressure of a gas introduced into the second inner space
9
to be a certain value. When NOx in a gas having a high oxygen partial pressure is measured, an oxygen partial pressure of a gas introduced into the second inner space
9
is changed to be higher. According to the change, a base line of an electric signal (a pump current value Ip in
FIG. 13
) by an electric signal conversion means
70
corresponding to NOx amount is also changed. Particularly, it causes a large error when a very small amount of NOx is measured. On the contrary, when a steam amount of a gas to be measured increases, a hydrogen amount in the first inner space
2
in an equilibrium condition, a balance between hydrogen and oxygen in the second inner space
9
is lost by the difference in diffusion coefficients, a part of oxygen generated by decomposition of NOx is consumed by combining with hydrogen thereby causing an error in measuring NOx.
SUMMARY OF THE INVENTION
The present invention solves the problems of a conventional measuring method and aims to provide a highly precise sensor for measuring an nitrogen oxide in a gas, the sensor has little influence of a concentration of oxygen or steam in the gas and little fluctuation by a temperature change.
According to the present invention, there is provided a sensor for measuring nitrogen oxide (NOx) in a gas by decomposing NOx in the gas and measuring an amount of oxygen generated by the decomposition, using a sensor comprising:
a main pump means including an electrochemical pump cell comprising a substrate comprising an oxygen ion conductive solid electrolyte and an inner pump electrode and an outer pump electrode formed on the inner surface and outer surface of the substrate, respectively, and the main pump means treating oxygen contained in the measuring gas introduced from an external space by pumping processing on the basis of a control voltage applied between the inner pump electrode and the outer pump electrode, and
an electric signal conversion means for generating an electric signal corresponding to the quantity of. oxygen generated by decomposition or reduction of NOx contained in the measuring gas after treated by the pumping processing by the main pump means in which one side thereof has a pair of detection electrodes formed on the side where the measuring gas that was treated by pumping processing by the main pump means is introduced;
the method for measuring nitrogen oxide (NOx) comprising the steps of:
introducing the gas to a first inner space under a first diffusion resistance and connected with the external space,
controlling an oxygen partial pressure in an atmosphere in the first inner space to be a predetermined low value at which NO is not decomposed substantially by the main pump means,
introducing a controlled atmosphere of the first inner space into a second inner space which is connected with the first inner space and which is under a second diffusion resistance, and
converting the atmosphere into an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the atmosphere by the electric signal conversion means;
wherein a first oxygen concentration detecting means is disposed for detecting an oxygen partial pressure in the second inner space, and a control voltage to be applied to the main pump means is adjusted on the basis of an output of the first oxygen concentration detecting means.
According to the present invention, there is further provided a sensor for measuring nitrogen oxide (NOx) in a gas by decomposing NOx in the gas and measuring an amount of oxygen generated by the decomposition, using a sensor comprising:
a main pump means including an electrochemical pump cell comprising a substrate comprising an oxygen ion conductive solid electrolyte and an inner pump electrode and an outer pump electrode formed on the inner surface and outer surface of the substrate, respectively, and the main pump means treating oxygen contained in the measuring gas introduced from an external space by pumping processing on the basis of a control voltage applied between the inner pump electrode and the outer pump electrode, an
Ina Noriyuki
Kato Nobuhide
NGK Insulators Ltd.
Parkhurst & Wendel L.L.P.
Tung T.
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