JP3570666B2 - Gas sensor, method for correcting measured value of gas sensor, and pressure sensor unit - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a gas concentration measurement technique.
[0002]
[Prior art]
The limiting current type sensor element is mainly used in the field of measuring oxygen concentration in gas. This product has the features that its output (current value) is almost proportional to the oxygen concentration, has excellent responsiveness, and can withstand high temperatures, while eliminating the need for a reference oxygen partial pressure. Has been done.
It is known that in such a limiting current type sensor element, by adjusting the voltage applied to the solid electrolyte that transmits oxygen ions, sensitivity can be given not only to oxygen in gas but also to water vapor. I have.
That is, when a relatively high voltage is applied as a monitoring voltage to a limiting current sensor in an atmosphere containing a large amount of water vapor, such as combustion exhaust gas, the water vapor is decomposed and reduced as shown in the following chemical formula (I) to obtain purified oxygen. The gas increases the output.
[0003]
Embedded image
2H ₂ O → 2H ₂ + O ₂ (I)
[0004]
Since the increased output corresponds to the water vapor concentration, the applied electrode is switched to a relatively low voltage at which the decomposition of water vapor does not occur and a relatively high voltage at which the decomposition of water vapor occurs, or two pairs of electrodes are provided. By applying a high or low voltage, the partial pressure of water vapor can be measured.
[0005]
An example of the limiting current type sensor element used in the present invention will be described with reference to FIG. In this method, a porous substrate is used for controlling the diffusion of gas.
A platinum (porous) cathode and anode having a porosity (air permeability) are provided with a zirconia solid electrolyte interposed therebetween, and a porous alumina substrate is provided so as to cover the anode side. A platinum heater is provided on the other surface of the porous alumina substrate to keep the temperature of the zirconia solid electrolyte at a temperature (600 to 700 ° C.) suitable for oxygen ion conduction. Each electrode and the heater are provided with a lead wire so that electrical connection can be easily made.
[0006]
The value measured by such a limiting current sensor element is affected by a change in the pressure of the atmosphere to be detected. FIG. 2 shows the results of an investigation on the effect of changing the pressure on the measured oxygen concentration. At this time, the oxygen concentration of the detection target is constant.
In FIG. 2, the solid line indicates the measured oxygen concentration, and the broken line indicates the value indicated by the semiconductor pressure gauge.
Thus, it can be seen that the value measured by the limiting current sensor element is greatly affected by the fluctuation in pressure.
Therefore, a study was made to correct the oxygen concentration measurement value based on the pressure value detected by the pressure gauge. The results are shown in FIG. The correction was performed according to the formula (I) described later.
[0007]
It can be understood from FIG. 3 that the oxygen concentration measurement value, which should be a constant value, fluctuates due to the correction. This was considered to be due to the improper correction due to differences in response and sensitivity to pressure fluctuation between the semiconductor pressure gauge used and the limiting current gas sensor element.
In the above example, a semiconductor pressure gauge was used, but other pressure gauges, such as a diaphragm pressure gauge or a strain gauge pressure gauge, could not be properly corrected. Only the same correction could be made.
[0008]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION It is an object of the present invention to improve the conventional problems described above, that is, to provide an excellent method for correcting a measured value of a gas sensor which is less affected by pressure fluctuation of an atmosphere to be measured.
[0009]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, a gas sensor according to the present invention is configured such that a limiting current type sensor element is used to correct a gas concentration measurement result by a limiting current type sensor element for detecting a gas concentration. This is a gas sensor having a pressure sensor unit housed in a closed case maintained at the same pressure as pressure correction means.
According to a second aspect of the present invention, there is provided a method for correcting a measurement value of a gas sensor, wherein the measurement result of the oxygen gas concentration by the limiting current type sensor element for detecting the gas concentration is determined by the limiting current type sensor element having the same pressure as the measurement target atmosphere. The correction is performed by using a pressure sensor unit housed in a sealed case kept in the pressure sensor.
Further, the pressure sensor unit of the present invention has a configuration in which the limiting current type sensor element is provided in a closed case maintained at the same pressure as the atmosphere to be measured.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
In the configuration of the present invention, the gas diffusion in the limiting current type sensor element for detecting the gas concentration is affected by the fluctuation of the pressure of the atmosphere to be measured. Since the same occurs in the pressure sensor unit provided in the kept sealed case, it is possible to know the accurate gas concentration by correcting the gas concentration measurement result using the output value of the pressure sensor unit. .
In the gas sensor of the present invention, it is necessary to keep the limiting current type sensor element of the pressure sensor unit used as the pressure correcting means in a closed case kept at the same pressure as the atmosphere to be measured. With this configuration, the pressure around the internal limiting current type sensor element is maintained at the same pressure as the gas concentration measurement target atmosphere, but the oxygen concentration is constant. Excellent pressure compensation is possible without receiving the pressure.
[0011]
Here, in order to keep the inside of the closed case at the same pressure as the atmosphere to be measured, the closed case itself is made of a flexible film having a gas barrier property, or a part of the closed case is formed of a flexible film having a gas barrier property. This can be easily performed. However, since the internal limiting current type sensor element requires that the zirconia solid electrolyte be kept in a temperature range suitable for ionic conduction, the film having gas barrier properties must have relatively high heat resistance. is there. Examples of such a film include films made of silicone, PTFE (polytetrafluoroethylene), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), and the like.
In the present invention, the shape of the case itself does not matter. That is, it may be box-shaped or bag-shaped.
Further, it is desirable that the atmosphere in the closed vessel is as close as possible to the atmosphere for measuring the gas concentration. This enables more accurate correction.
[0012]
It is preferable that the limiting current type sensor element installed in the closed case as described above has the same specifications as the limiting current type sensor element for detecting the gas concentration, since it is possible to more appropriately correct the measured gas concentration value.
The pressure sensor unit having the above-described limiting current type sensor element in a closed case kept at the same pressure as the atmosphere to be measured enables extremely excellent correction as compared with the correction using a conventional pressure gauge.
[0013]
Here, the pressure sensor unit of the present invention will be described with an example.
FIG. 4 shows a model sectional view of the pressure sensor unit A according to the present invention.
In the figure, reference numeral 1 denotes a limiting current type sensor element, and its electrode and heater are connected to a lead wire 2 made of platinum. This lead wire 2 is electrically connected to the outside of the pressure sensor unit A by a sensor mounting pin 3.
Note that the periphery of the limiting current type sensor element 1 of the pressure sensor unit A is a closed container, and the closed container includes a lower portion 4a of the sensor housing, a heat-resistant resin thin film 5 made of PTFE, and a sealing material 6. In addition, the heat-resistant resin thin film 5 is protected by the sensor housing upper part 4b. A pressure detection hole 4b1 is provided in the upper part 4b of the sensor housing. By the action of the pressure detection hole 4b1 and the heat-resistant resin thin film 5, the inside of the sealed container around the limiting current type sensor element 1 becomes equal to the atmospheric pressure around the pressure sensor unit A. It enables accurate gas measurement together with the limiting current type sensor element for gas concentration measurement installed in the system.
[0014]
Here, the state of the output of the pressure sensor unit A when the pressure of the surrounding atmosphere of the pressure sensor unit A is changed is shown in FIG. In the figure, the broken line represents the change in pressure, and the solid line represents the sensor output. All the conditions such as measurement and pressure change in FIG. 5 are the same as those in FIGS. 2 and 3.
FIG. 5 shows that the output of the pressure sensor unit A changes according to the change in the atmospheric pressure.
The correction of the gas concentration measurement value based on the output value of the pressure sensor unit A can be performed using, for example, a gas concentration measurement limit current sensor element, a pressure sensor unit, an OP amplifier, an A / D converter, a microprocessor, or the like. .
That is, when the concentration of the gas to be measured in the atmosphere to be measured is constant and the output to the initial atmosphere is set to 0, the subsequent fluctuation is considered to be the fluctuation due to pressure, and the output fluctuation value of the pressure sensor unit A is α, Assuming that the output (standard output) of the pressure sensor unit A in the initial atmosphere (standard output) is β, and the detection output of the limiting current type sensor element for measuring gas concentration is γ, the pressure correction output can be obtained by the equation (I).
[0015]
(Equation 1)
γ × (1-α / β) (I)
[0016]
FIG. 6 shows the result of obtaining the pressure correction output according to the equation (I) and correcting the output of the limiting current type sensor element at that time. In FIG. 6, a broken line indicates a change in pressure, and a solid line indicates a corrected oxygen gas concentration measurement value.
Here, when only the pressure of the atmosphere is changed while the oxygen gas concentration is constant, the measurement result of the oxygen gas concentration corrected using the conventional semiconductor pressure sensor, and the pressure sensor unit according to the present invention as the pressure correction unit FIG. 7 also shows the results of measuring the oxygen gas concentration using the gas sensor.
[0017]
From FIG. 7, it can be seen that the gas sensor having the pressure sensor unit according to the present invention as the pressure correcting means enables accurate measurement with little error.
Although the above example shows an example in which the pressure sensor unit and the limiting current type sensor element for measuring gas concentration are separately used, an example of a structure in which these two are integrated will be described with reference to FIG.
FIG. 8 is a model cross-sectional view of an integrated gas sensor B having a pressure sensor unit having a limiting current sensor element in a sealed case kept at the same pressure as the atmosphere to be measured as a pressure correcting means.
Reference numeral 1 denotes a limiting current type sensor element, and its electrode and heater are connected to a lead wire 2 made of platinum. The lead wire 2 is electrically connected to the outside of the integrated gas sensor B by a sensor mounting pin 3.
Note that the periphery of the limiting current type sensor element 1 of the integrated gas sensor B is a sealed container, and the sealed container includes a lower portion 4a of the sensor housing, a heat-resistant resin thin film 5 made of PTFE, and a sealing material 6. The portion below the heat-resistant resin thin film 5 in the drawing has the same structure as the pressure sensor unit in FIG.
[0018]
The heat-resistant resin thin film 5 is protected by the upper part 4b 'of the sensor housing. A porous plate 7a is fixed to the upper part 4b 'of the sensor housing via a sealing material 6'. By the action of the porous plate 7a and the heat-resistant resin thin film 5, the inside of the sealed container around the limiting current type sensor element 1 is made equal to the atmospheric pressure around the integrated gas sensor NB.
On the other hand, a limiting current sensor element 7 for measuring gas concentration is provided on the porous plate 7a, and the gas concentration of the atmosphere around the integrated gas sensor B is controlled by a lead wire 8 connected to a heater and electrodes of the sensor element 7. It can be measured.
The integrated gas sensor B having such a structure is extremely compact and does not require an installation place. Further, since the position of the gas concentration measuring limit current type sensor element is very close to the position of the pressure sensor unit, more accurate correction of the measured value can be performed.
[0019]
【The invention's effect】
INDUSTRIAL APPLICABILITY The gas sensor of the present invention can eliminate the influence of pressure almost completely, and is therefore an excellent gas sensor capable of performing accurate measurement even in an atmosphere having large pressure fluctuations.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a limiting current type sensor element used in the present invention.
FIG. 2 is a diagram showing that a value measured by a conventional limiting current sensor element is affected by a change in pressure of an atmosphere to be detected.
FIG. 3 is a diagram showing a result of performing pressure correction according to a conventional technique on a measured value.
FIG. 4 is a model sectional view of a pressure sensor unit A according to the present invention.
FIG. 5 is a diagram illustrating an output state of the pressure sensor unit A when the pressure of the surrounding atmosphere of the pressure sensor unit A is changed.
FIG. 6 is a diagram showing a result of performing pressure correction according to the formula (I) using an output value of the pressure sensor unit A.
FIG. 7 is a diagram showing a comparison between a case where pressure correction according to the related art is performed and a case where pressure correction according to the present invention is performed.
FIG. 8 is a model cross-sectional view of an integrated gas sensor B having a pressure sensor unit having a limiting current type sensor element in a closed case kept at the same pressure as the atmosphere to be measured as a pressure correcting means.
[Explanation of symbols]
A Pressure sensor unit B according to the present invention B Integrated gas sensor 1 according to the present invention 1 Limit current type sensor element 2 Lead wire 3 Sensor mounting pin 4a Sensor housing lower part 4b Sensor housing upper part 4b1 Pressure detecting hole 5 Heat resistant resin thin film 6, 6 ' Sealing material 7 Limit current sensor element 7a for gas concentration measurement Porous plate 8 Lead wire

Claims (3)

In order to correct the gas concentration measurement result by the limiting current type sensor element that detects the gas concentration, the pressure sensor unit that is housed in a closed case where the limiting current type sensor element is kept at the same pressure as the atmosphere to be measured is pressured A gas sensor having correction means. The measurement result of the oxygen gas concentration by the limiting current sensor element for detecting the gas concentration is corrected using a pressure sensor unit that is housed in a closed case in which the limiting current sensor element is kept at the same pressure as the atmosphere to be measured. A method for correcting a measured value of a gas sensor. A pressure sensor unit comprising a limiting current type sensor element in a sealed case maintained at the same pressure as the atmosphere to be measured.

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