JP2001264279A - Gas sensor - Google Patents

Abstract

(57) [Summary] [PROBLEMS] For a gas sensor unit suitable for a field requiring a simple and compact size such as a monitor application, even if the condition of the ambient environment or the wind is changed, the temperature of the signal baseline value is changed. It is an object of the present invention to provide a gas sensor whose correction method is simple. SOLUTION: In a gas sensor having a sensor cover part a made of an airtight porous material, a gas detection part, a means for heating the gas detection part, and a means for detecting the temperature, a hollow is used as the porous material. Using a solid containing glass or a foamed resin, the thickness of the sensor cover a is 0.8 to 3 mm, and the porosity of the sensor cover a is 30% to 70%. I do.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor, such as a gas sensor, a solid electrolyte gas sensor, or an infrared absorption gas sensor, which needs to be measured by heating, using a resistance value, a capacitance, or the like of a metal oxide semiconductor. . The output of these gas sensors often depends on the ambient temperature, and furthermore, the sensor output needs to be corrected because it contains various drifts.

[0002]

2. Description of the Related Art Various types of gas sensors for detecting a gas concentration have been proposed in the past. As a gas detection method, a gas sensor using the resistance value or capacitance of a metal oxide semiconductor,
There is a concentration cell type using a solid electrolyte. These gas sensors are used by heating the sensing element to 300 ° C to 600 ° C.

[0003]

However, many of the conventional gas sensors change their characteristics due to long-term use or environmental changes. When used as a device, the baseline of the output of the detection means drifts if there is no correction means, resulting in inaccurate measurement values, and a malfunction may occur when used as a control. To correct and calibrate the change in the baseline of this output, for example, automatic calibration performed by introducing a reference gas is conceivable. However, since the equipment and circuits are complicated and the size is increased, the cost is increased. It is difficult to apply to fields such as a simple monitor that requires high performance.

[0004] For example, as described in Japanese Patent Application Laid-Open No. 7-270315, a method of correcting and calibrating assuming that two detection means are provided and that the base lines of the outputs of these two detection means have the same drift in the baseline. Has been proposed. However, it is unpredictable that the baseline drifts are completely equal among a plurality of detecting means, and therefore, the reliability is low. Further, since a plurality of detecting means are provided, it is difficult to reduce the size of the apparatus, resulting in an increase in cost. That is a problem.

On the other hand, as a simple method of correcting the baseline value, there is known a method of correcting the value with little change, for example, keeping the concentration of carbon dioxide in the atmosphere constant. That is, since the concentration of carbon dioxide present in normal air is considered to be about 400 ppm, this method is a method of correcting this value to match the baseline. However, the output corresponding to the baseline of the sensor in the air changes depending on the daily atmospheric environment, and is greatly affected by changes in the outside air temperature and the wind.

As described above, the conventional gas sensor is used by heating the gas detecting element to 300 ° C. to 600 ° C. Therefore, when the environmental temperature changes, the temperature of the gas detecting element changes and the baseline output shifts. Further, even when the ambient temperature is constant, when the wind hits the gas sensor, heat is deprived and the baseline output shifts. In the conventional gas sensor, the temperature of the baseline is corrected using the temperature detecting element. However, there is a problem that the correlation between the gas detecting element and the temperature detecting element is not sufficiently obtained, and the accuracy of the output concentration is poor.

SUMMARY OF THE INVENTION The present invention relates to a method for correcting the temperature of a signal baseline value for a gas sensor unit suitable for a field requiring simplicity and compactness, such as a monitor application, even when the ambient temperature or wind changes. It is an object of the present invention to provide a simple gas sensor.

In order to simplify the temperature correction method, there are the following two conditions. One is that the temperature correction coefficient is linear in a practical temperature range and has no temperature dependence. Second, the difference in temperature correction coefficient between the case where the ambient temperature changes and the case where the wind hits is small.

[0009]

SUMMARY OF THE INVENTION The present invention provides a gas sensor having a sensor cover made of an airtight porous material, a gas detector, a means for heating the gas detector, and a means for detecting temperature. In the above, a solid containing hollow glass or a foamed resin is used as the porous material, the thickness of the sensor cover is 0.8 to 3 mm, and the porosity of the sensor cover is 30% to 7
0%.

Thus, even when the temperature of the atmospheric environment changes, the gas correction coefficient is linear in a practical temperature range, there is no temperature dependency, and a simple method of correcting the signal baseline value is provided. be able to.

In the above arrangement, the temperature detecting element may be arranged outside the sensor cover.

With such a configuration, it is possible to provide a gas sensor in which the difference between the temperature correction coefficients is small between when the ambient temperature changes and when the wind hits, and the method for correcting the signal baseline value is simple.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to a first aspect of the present invention provides a sensor cover made of a porous material having airtightness, a gas detector, a means for heating the gas detector, and a means for detecting temperature. In a gas sensor having a solid material containing hollow glass or a foamed resin as a porous material, the thickness of the sensor cover is 0.8 to 3 mm, and the porosity of the sensor cover is It is a gas sensor characterized by being 30% to 70%, and has an effect of improving heat insulation and heat retention against external temperature changes.

When the thickness of the sensor cover portion is 0.8 mm or less, the effect of improving accuracy is small, and when the thickness is 3 mm or more, it takes time until the sensor element temperature stabilizes. If the porosity of the sensor cover part is 30% or less, the heat insulation and the heat retention are not sufficient, and the correction accuracy deteriorates, which is not desirable. If the porosity of the sensor cover is 70% or more, the heat insulating effect is not sufficient and the heat capacity is small.
The correction accuracy is deteriorated, which is not desirable.

According to a second aspect of the present invention, in the gas sensor according to the first aspect, the temperature detecting element is disposed outside the sensor cover. It has the effect of being able to sense with high accuracy without interference by the heat of the heater.

Hereinafter, embodiments of the present invention will be described.

A carbon dioxide gas sensor was used as the gas sensor of the embodiment. The concentration of carbon dioxide present in the atmosphere was 330 ppm in the Chemical Handbook (Revised 3rd Edition), and the science chronology (Heisei 8)
In the normal state, the change is small, so that it is suitable as an embodiment. However, the present invention does not depend on the type of the sensor, and other temperature-dependent gas sensors. But the same effect can be expected.

FIG. 1 is a schematic view showing the structure of the sensor of the present invention. a is a sensor cover part. A gas passage hole having a diameter of 1 mm is formed in the upper part. If the size of the hole is too small, the flow of gas is slow, and if the size is too large, the hole is easily affected by the outside world.

Reference numeral b-1 denotes a heater section, which is formed on the surface of the alumina substrate b-3. b-2 is a carbon dioxide gas detection unit, which is CeO ₂ , BaC
Electrodes are attached to an element made by sintering O ₃ and CuO powders, and are fixed to the surface of the alumina substrate b-3. The element was heated to about 500 ° C. by a heater and measured.

Reference numeral c denotes a temperature detecting component, and a thermistor is used in the embodiment. The temperature detecting component c is not limited to the thermistor, but may be any component whose characteristics change with temperature. The correction can be performed using not only a thermocouple having a wide measurement range but also a characteristic change at a specific temperature. Table 4 shows the results of measurements made by mounting this thermistor on an alumina substrate b-3 so as not to be affected by heat from the heater section b-1.

D-1 to d-6 are terminals, and terminals d-
Terminals d-3 and d-4 are for extracting the characteristic value of the gas detection unit, and terminals d-3 and d-4 are for the heater unit b-1. The terminals d-5 and d-6 are connected to thermistors, that is, temperature detecting components c.
Terminal.

The sensor having the shape shown in FIG. 1 prepared by changing the sensor cover material is mounted on a substrate, and the outside air temperature is set to “−10 ° C.” “0 ° C.” “10 ° C.” “20 ° C.” ”
A change in characteristics when the temperature was changed to 40 ° C. and 50 ° C. was measured, and a sensor having a sensor cover a made of an epoxy resin was prepared as a reference product for comparison, and measured at the same time.

An oscillation circuit using an IC was used as the characteristic and temperature detection circuit. This is a circuit that reads a change in frequency due to a change in impedance value, and is a circuit that is generally used. Based on the measured frequency, the coefficient of the temperature change was calculated from the measured value of “−10 ° C.” according to the calculation formula 1. Many of the sensor characteristics are proportional to the log value, and the thermistor value is also proportional to the log value. That is,
The temperature coefficient is represented by the following equation (1).

Α = (log (Sa) −log (S0)) / (log (Ta) −log (T0)) (1) α: Temperature coefficient Sa: Measurement frequency S0 of the gas detector at temperature a : Sensor element measurement frequency at −10 ° C. Ta: Thermistor measurement frequency at a temperature T0: Thermistor measurement frequency at −10 ° C. The closer the temperature coefficient (α) is to “0”, the smaller the temperature change of element characteristics. Represent. The fact that the numerical values are the same at each temperature indicates that the element characteristic change amounts at each temperature are the same, indicating that the correction accuracy is high. The closer the temperature coefficient is to "0" and the smaller the coefficient difference at each temperature is, the better the correction accuracy for the outside air temperature is. However, if the temperature coefficient is "0", it is not necessary to perform temperature correction, and the element characteristics at each temperature are not required. Examining the variation of the variation is more suitable for the actual situation. If the correction method is used for each temperature, the variation of the correction coefficient for each temperature can be corrected, but the cost is increased due to the complexity and size of the circuit, and it is required to be small, simple, and simple monitors that require practicality. Is difficult to apply.

Therefore, the calculated coefficient (α) of each sensor for each temperature is converted from the maximum value (MAX (α)) to the minimum value (MIN).
The value (β) obtained by subtracting (α)) is evaluated as the correction accuracy (the following equation (2)).

Correction accuracy (β) = MAX (α) −MIN (α) (2) (Table 1) shows the correction accuracy (β) and temperature coefficient (α) due to the difference in the material of the sensor cover part a. ) Indicates the average value.

A. The epoxy resin (standard) was obtained by curing a commercially available uncured epoxy resin into a shape shown in FIG.

B. The epoxy resin (containing glass beads) is obtained by mixing a commercially available epoxy resin before curing and a commercially available hollow glass bead at a weight ratio of 1: 1 and curing the resin. Processed into the shape of

C. The fluororesin (containing glass beads) is obtained by mixing commercially available fluororesin before curing and commercially available hollow glass beads at a weight ratio of 1: 2, and curing the resin. Processed into the shape of

D. As the urethane foam resin, a commercially available urethane foam resin was used after being processed into the shape of the sensor cover part a in FIG.

The thickness of each sensor cover a was processed to 2 mm.

[0032]

[Table 1]

As is clear from Table 1, the reference a.
A sensor using an epoxy resin sensor cover and b.
c. d. Comparing the values of β of the sensors of b. c. d.
The value of the sensor is smaller than 1/3,
In addition, the value of the temperature coefficient (α) compared with the average value is small, and it can be said that the correction accuracy is considerably improved. c. Fluororesin (with glass beads) shows the best results, but this is due to the difference in the amount of glass beads that can be mixed due to the difference in viscosity of the resin raw materials, and the fact that the amount of fluorine resin material mixed is larger. Seems to be.

To improve the correction accuracy, hollow glass beads or urethane foam are used, so that small air bubbles (small holes) are dispersed in the sensor cover material, so that the heat insulating property is increased and the influence of the outside air temperature is reduced. At the same time, it is determined that the heat of the heater also became difficult to diffuse from inside.

Table 2 shows the value of the correction accuracy (β) due to the difference in thickness between the sensor using the epoxy resin as the material of the sensor cover part a and the sensor using the epoxy resin mixed with glass beads. Numerical values were determined by the same method and calculation formula as in (Table 1).

[0036]

[Table 2]

As can be seen from Table 2, the reference a. A sensor using an epoxy resin sensor cover a and glass beads b. Comparing the values of β of the sensors of
At a thickness of 0.8 mm, the values of both do not change and the effect of containing hollow glass beads is not obtained. This is considered to be due to the fact that the thickness of the material of the sensor cover part a is thin, and therefore, there is no heat insulating effect. When the thickness is 3 mm or more, the change in the value is small, and the effect of increasing the thickness is not obtained. There is a limit to the effect of the thickness, and increasing the thickness of the sensor cover a increases the heat capacity, stabilizes the temperature of the entire sensor, increases the time until the characteristics are stabilized, and is not suitable for practical use. .

Therefore, the thickness of the material of the sensor cover part a is suitably 0.8 mm to 3 mm, more preferably 1 m
A value of m to 2 mm is desirable. Sensors using hollow glass bead-mixed materials or foamed materials as the material of the sensor cover part a do not have a large difference in the value of the correction accuracy (β). This is probably because the effect is greater.

Table 3 shows the values of the correction accuracy (β) when urethane foam prepared by changing the porosity was used as the sensor cover material. The numerical values were obtained by the same method and calculation formula as in (Table 1). The thickness of the material of the sensor cover part a was 2 mm. The porosity is the weight of the urethane resin that does not foam (W0)
Was determined from the weight (Ws) of the same volume of urethane foam using the following equation (3).

Porosity (%) = 100 × {(W0) − (Ws)} / (W0) (3)

[0041]

[Table 3]

When the value of β is compared based on the reference value of 0% porosity, the effect is found between 30% and 70%.
This is because up to 30% of the insulation effect is not achieved by the pores,
It is conceivable that at 70% or more, the pores are large and the pores are connected, so that the heat insulating effect is small.

Therefore, the porosity of the material of the sensor cover part a is suitably 30% to 70%, more preferably 40% to 70%.
A value of 60% is desirable.

Table 4 shows the results obtained by changing the position of the thermistor, that is, the temperature detecting component c, and measuring the values when the power of the thermostat is turned on (on) and turned off (off). Show. The temperature of the thermostat is 25 ° C. When the thermostat is turned off, the fan stops and the difference in characteristics between when there is wind and when there is no wind can be seen. Sensors are often used in windy environments, and changes in characteristics due to differences in wind speed have a significant effect on accuracy. FIG. 1 shows the position of the temperature detecting component c.
The position in the sensor shown in Fig. 7 was compared with the position removed from the sensor and transferred onto the alumina substrate b-3. The thickness of the sensor cover part a of the used sensor is 2 mm.

The values in Table 4 are the correction coefficient (αon) when the power of the thermostatic chamber is turned on and the correction coefficient (αon) when the power of the thermostatic chamber is turned off, based on the value of −10 ° C. obtained by the above equation (1). αof
f), and the value obtained by subtracting these two values (βon-off)
It is. If this value (βon-off) is close to “0”, the effect of turning on and off the power of the thermostat is small, that is, the effect of the wind is small. That is, the influence of the wind speed and the air volume is small, and the sensor accuracy is good.

[0046]

[Table 4]

The thermistor, that is, a sensor in which the position of the temperature detecting component c is inside the sensor, has a large difference in the correction coefficient.
Unless this value is corrected, the carbon dioxide concentration is 1500p
It is a numerical value that changes by more than pm, for example, a value that causes a malfunction when used as a sensor for monitoring the indoor environment. On the other hand, the position of the temperature detecting component c is changed to the alumina substrate b-
The sensor set at 3 above has a small difference in correction coefficient depending on whether the thermostat power supply is turned on or off, and does not require a correction means and a circuit based on wind speed. Although the sensor changes the heat balance and the temperature by the wind speed, the difference in the correction coefficient is small because the thermal effect of the heater part b-1 used in the sensor on the thermistor becomes small, and the influence of the environment is reduced. It is considered to be more accurate.

The wind speed and air volume when the power of the thermostat is turned on are equal to or larger than the wind speed and air volume of the atmosphere in which the sensor is normally used, and when the power is off, there is no wind. The conditions for turning on and off the power of the thermostatic bath are conditions under which a characteristic difference due to the wind speed and air volume of the sensor can be sufficiently examined.

[0049]

According to the present invention, it is possible to correct the temperature of the signal baseline value even when the conditions of the ambient environment such as the temperature and wind change, which are suitable for the field where the simplicity and compactness are required such as the monitor use. A gas sensor whose method is simple can be provided.

Further, a simple correction circuit can be provided by setting the mounting position of a temperature detecting component such as a thermistor outside the sensor.

[Brief description of the drawings]

FIG. 1 is a schematic view showing the structure of a gas sensor of the present invention.

[Explanation of symbols]

 a Sensor cover part b-1 Heater part b-2 Carbon dioxide gas detecting part b-3 Alumina substrate c Temperature detecting parts d-1, d-2 Gas detecting part characteristic value extracting terminal d-3, d-4 For heater Terminal d-5, d-6 Terminal for thermistor

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Shogo Matsubara 1006 Kazuma Kadoma, Kazuma, Osaka Prefecture F-term in Matsushita Electric Industrial Co., Ltd. (Reference) 2G004 BB04 BC03 BF14 BJ03 BK01 BL06 BL08 BL19 2G046 AA01 AA12 BA01 BB02 BH02 CA09 DB05 DC02 DC15 DC18 DD02 EA12 EA14 EB01 FB02 2G060 AA01 AB09 AE19 BD02 HB06 HC02 HC03 HC13 HC20 HE03 KA01

Claims (2)

[Claims] 1. A gas sensor having a sensor cover made of an airtight porous material, a gas detector, means for heating the gas detector, and means for detecting temperature, wherein the porous material is hollow. Solids, including the glass of
Alternatively, a gas sensor using a foamed resin, wherein the thickness of the sensor cover portion is 0.8 to 3 mm, and the porosity of the sensor cover portion is 30% to 70%. 2. The gas sensor according to claim 1, wherein a temperature detecting element is disposed outside the sensor cover.