JPH0617897B2 - Gas concentration measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention provides a device for measuring the concentration of a gas by ultrasonic waves, by imparting excellent humidity resistance to an ultrasonic sensor unit to be used for a long period of time. The present invention relates to a gas concentration measuring device capable of continuously and accurately measuring.

[Prior art]

First, the measuring principle of an apparatus for measuring the concentration of a mixed gas or a single composition gas by utilizing the dependence of the ultrasonic wave propagation velocity on the measured gas concentration will be described.

The ultrasonic wave propagation velocity of the mixed gas is determined by various constants, concentration, temperature, etc. of the mixed gas, and is represented by the following equation (1).

V: Ultrasonic propagation velocity of mixed gas Cpi: Specific pressure specific heat of measurement target gas i in mixed gas Cvi: Constant volume specific heat of measurement target gas i in mixed gas Mi: Molecular weight of measurement target gas i in mixed gas Xi: Molar fraction of the gas i to be measured in the mixed gas R: Gas constant T: Absolute temperature of the mixed gas Now, when the formula (1) is rewritten by taking the binary system of the mixed gas as air and CO ₂ as an example, V: ² = (Cpco ₂ Xco ₂ + CpairXair) / (Cvco ₂ Xco ₂ + CvairXair) ・ R ・ T …… (2), each constant is entered, and ultrasonic wave propagation of each CO ₂ concentration at mixed gas absolute temperature 293 ° K The results of calculating the speed are shown in Table 1 and FIG.

Since ΣXi = 1, the equation (2) is expressed by the following equation (3).

From the equation (3), the Xco ₂ concentration is represented by the following equation (4): Xco ₂ = F (V, T) That is, the measurement target gas concentration is a function of the ultrasonic propagation velocity V and the gas temperature T.

An example of a measurement system diagram designed based on the above theory will be described with reference to FIG.

The ultrasonic sensor (1) includes a transmitting ultrasonic element (2) and a receiving ultrasonic element (3) installed at a position facing the transmitting ultrasonic element (2). Install the ultrasonic sensor (1) in the measurement target gas atmosphere (4) by an appropriate method. The ultrasonic wave transmitted from the transmitting ultrasonic element (2) passes through the ultrasonic path section (5) in which the gas to be measured exists, and the receiving ultrasonic element (3)
Will be received at. At this time, the speed of sound passing through the ultrasonic path (5) is inversely proportional to the concentration of the gas to be measured. Ultrasonic element for wave transmission
(2) consists of an electrostrictive oscillator, etc.
(6) and the negative immittance converter (7) are
It amplifies the high frequency signal generated by the signal generator (8) controlled by 0) and improves the response characteristic.
The receiving ultrasonic element (3) is composed of an electrostrictive oscillator or the like, and the preamplifier (9) amplifies the high-frequency signal from the receiving ultrasonic element (3) to provide a return ring oscillation amplifier. This is what you will enter in (10). The resistance (11) and the negative immittance converter (12) improve the response characteristics of the receiving ultrasonic element (3).

On the other hand, the frequency fm of the return oscillation system (13) and the ultrasonic path (5)
Between the ultrasonic wave propagation velocity V in the mixed gas to be measured at fm = k · V / l (where l is the ultrasonic element for wave transmission (2) and the ultrasonic element for wave reception (3)). Since the distance and k represent respective proportional constants, the frequency fm of the return oscillation system (13) and the stabilized reference frequency fo generated by the crystal oscillator (14) are mixed by a mixer ( Input in 15) and take the difference F between fm and fo.
This F is converted into a voltage by the frequency-voltage converter (16) and input to the calculator (17).

The temperature sensor (18) consists of a thermistor, resistance temperature detector, thermocouple, etc. for temperature measurement in the gas atmosphere (4) to be measured, and inputs this temperature information to the calculator (17). Then, temperature compensation is performed to eliminate the temperature dependence of the ultrasonic wave propagation velocity. The temperature-compensated output voltage is displayed on an indicator (19) such as an analog voltmeter, a digital voltmeter, or a recorder.

Next, using this gas concentration measuring device, an example of measuring the CO ₂ gas concentration in a mixed gas composition, that is, a three-component system of air, CO ₂ , and H ₂ O will be described in detail with reference to FIGS. 2 and 3. .

100% CO ₂ gas-filled cylinder (20) and the compressor-type air pump (21) CO ₂ gas and air flow control valve with a flow meter is respectively supplied from (22), set in advance the concentration of CO ₂ gas (23) Then, after the flowmeters (22) and (23), a chamber-type mixing chamber (24) for mixing CO ₂ gas and air is provided, and the mixed gas of CO ₂ / air mixed in the mixing chamber (24) is measured. Chamber (25)
It is introduced into the interior through the introduction pipe (26). Measuring checker (2
A water layer (27) is provided on the bottom surface of 5) so that the introduction pipe (26) is sufficiently submerged, and CO _{2 is} supplied from a gas outlet appropriately provided in the introduction pipe (26).
/ Chamber for measurement of air-mixed gas through the water layer (27) (2
5) Let it blow out inside. By this operation, the relative humidity in the measurement chamber (25) becomes high humidity of 95 to 100%. A stirring blade (29) rotated by a motor (28) is provided above the measurement chamber (25) to make the mixed gas concentration in the measurement chamber (25) uniform. Mixed gas is mixed gas outlet pipe (3
It is discharged from the measurement chamber (25) from the (0). The ultrasonic sensor (1) according to the present invention is installed at an appropriate position in the measurement chamber (25), and the return cable oscillation system (13), the crystal oscillator (14), the mixer ( Operation controller (32) composed of 15), frequency voltage converter (16) and operation unit (17)
Connect to. The measurement resistance temperature sensor (18) for temperature compensation is connected to the computing unit (17) of the computing controller (32) by the cable (33). The output voltage from the calculator (17) is CO ₂ gas concentration 0 to 20V.
It was adjusted to 0 to 20 V with respect to the ol%. Therefore, the output voltage reading is the CO ₂ gas concentration. A digital voltmeter was used as the indicator (19), and the frequency fm of the return ring oscillation system (13) was monitored by the frequency counter (34). The mixed gas derived from the gas outlet pipe (30) is introduced into the infrared gas analyzer (35) through the exhaust pipe (36), and the CO ₂ gas concentration is measured. A sampling port (37) is provided in the middle of the discharge pipe (36) and connected to a gas chromatograph (38) to verify the CO ₂ gas concentration.

A thermistor temperature sensor (39) for measuring the temperature of the mixed gas is installed in the measuring chamber, and a temperature measuring device (40)
Monitor at. The measurement chamber (25) is a completely sealed system except for the mixed gas inlet pipe (41) and the mixed gas outlet pipe (30). Also, the measuring chamber (2
5) is installed in a temperature-controlled air constant temperature chamber (42) controlled at ± 0.1 ° C so that the temperature in the measuring chamber (25) can be arbitrarily changed.

With the above verification equipment, the temperature inside the measuring chamber (25) was changed to 27 ℃, 35 ℃, 42 ℃, and CO was measured at each temperature.
₂ gas concentration meter (22), vary from 0-20% by (23), an infrared gas analyzer (35) and a gas chromatograph (38)
Table 2 and FIG. 4 show the measurement data of the present ultrasonic gas concentration measuring device.

From Table 2 and FIG. 4, the return ring oscillation system frequency fm of the ultrasonic gas concentration measuring device shows linearity with respect to the concentration by the calibration gas chromatograph, and the concentration instruction of the ultrasonic gas concentration measuring device is verified. Conforms to the concentration measured by a gas chromatograph.

As described above, CO _{2 is used} as one example of the ultrasonic gas concentration measuring device.
The description has been given for the air system, but the present invention is not limited to this.

Next, an ultrasonic sensor used in the ultrasonic wave transmitting element and the ultrasonic wave receiving element of the ultrasonic gas concentration measuring apparatus will be described. Both elements have the same structure of ultrasonic sensor as the conventional one. Conventionally, a silver electrode is baked on an ultrasonic vibrator (eg piezoelectric ceramic such as PZT) and this is applied to a plate or holder (eg metal, plastic). It is the same as the ultrasonic sensor used for transmission or reception of ultrasonic instruments for measurement and control, which has a structure of being attached to (such as manufactured). These are electrodes of the surface such as PZT, for example, a conductor such as silver is likely to cause an electrolytic corrosion phenomenon due to water vapor, and it is possible to accurately convert an ultrasonic signal into an electric signal or convert the electric signal into an ultrasonic signal. It often becomes difficult. Therefore, in order to prevent electrolytic corrosion of the electrodes attached to the ultrasonic transducer, a method of preventing water vapor from entering the holder by using a sealing material such as silicone resin, epoxy resin, or polyurethane is adopted. . However, an ultrasonic sensor with a structure in which ultrasonic transducers are bonded and sealed with a sealing material is used for a long period of 1 to 10 years when continuously used in an atmosphere of humidity 80 to 100%, There is a drawback that an electrolytic corrosion phenomenon occurs due to the intrusion of water with time. There is also a method of performing sealing by welding using a metal holder, but there is a drawback that welding is difficult because the adhesive resin used for bonding the ultrasonic transducer and the metal holder deteriorates due to heat.

The prior art will be described in more detail below with reference to the drawings. FIG. 8 shows an example of a conventional ultrasonic sensor,
FIG. 8 (a) shows the cross-sectional shape of the side surface of the ultrasonic sensor (59), and FIG. 8 (b) shows the ultrasonic vibrator (45) of FIG. 8 (a) (for example, a piezoelectric ceramic such as PZT). 2 is an enlarged view of a peripheral portion of a resin having piezoelectric characteristics).

As shown in FIG. 8 (b), the ultrasonic transducer (45) has electrodes.
(60) and (61) are provided, and the ultrasonic transducer (45) is a holder.
It is bonded to (44) (for example, metal and resin) with an adhesive (62) having good ultrasonic propagation characteristics. Electrode (60), (6
As shown in FIG. 8 (b), 1) is connected to terminals (48) and (49) with cables (46) and (47), and the terminals (48) and (49) are connected to a board (50) (for example, , A phenol resin laminated plate, an epoxy resin laminated plate), and a sealing substance (51) is sealed thereon. When used for a long period of time in a steam atmosphere, for example, for 1 year to 10 years or more, moisture will pass through the sealing substance (51) to the holder (44).
There is a drawback that the electrodes (60) and (61) enter the inside to cause electrolytic corrosion, and normal transmission and reception of ultrasonic waves cannot be performed.

[Object of the Invention]

In view of such drawbacks of conventional ultrasonic sensors, the present invention forms a thin film on the surface of an ultrasonic sensor used for an ultrasonic element for wave transmission and an ultrasonic element for wave reception of an ultrasonic gas concentration measuring device. It is an object of the present invention to provide a gas concentration measuring device capable of continuously and accurately measuring for a long time by preventing intrusion of water.

[Structure of Invention]

That is, the present invention basically comprises an ultrasonic transducer, a holder, and a sealing material, and forms a film of conductive and / or non-conductive material having a thickness of 500 to 5000Å on the surface of the sealing material. It is a gas concentration measuring device using a moisture resistant ultrasonic sensor.

Hereinafter, the present invention will be described with reference to the drawings.

FIG. 5, FIG. 6 and FIG. 7 show the sectional shapes of the side surfaces of the ultrasonic sensors (43), (53), (56) of the embodiment according to the present invention. The basic structure is the same as that of the conventional ultrasonic sensor shown in FIG. 8, and the ultrasonic transducer (45) is provided with electrodes (60) and (61) as shown in FIG. 8 (b). The ultrasonic transducer (45) is attached to the holder (44) with an adhesive (62). The electrodes (60) and (61) are connected to the terminals (48) and (49) by cables (46) and (47). The terminals (48) and (49) are fixed to the substrate (50), and the top of them is sealed with a sealing material (51).

FIG. 5 shows an ultrasonic sensor in which a thin film (52) is formed on the surface of the sealing substance (51) or the like of the ultrasonic sensor (43). Thin Film (52)
The material of is a non-conductive substance, for example, a substance that can be formed by a thin film forming method such as vacuum deposition, sputtering, or ion plating, and is particularly preferably a fluorine-based resin such as SiO, SiO ₂ , or polytetrafluoroethylene. . Film thickness is 500
It is preferably Å to 5000 Å, preferably 1500 Å to 3000 Å. Since the thin film (52) is non-conductive, it is a sealing material.
Terminals (48), (49) protruding from the holder (44) from the (51)
To prevent a thin film from being formed on the part, for example, wrap Teflon (trademark) tape or the like and cover it to form a thin film (52).
To form.

FIG. 6 is a thin film of terminals (48) and (49) of the ultrasonic sensor (53).
This is an ultrasonic sensor in which insulating films (54) and (55) are attached to the part in contact with (57), and a thin film (57) is formed on the surface of a sealing substance (51). The material of the thin film (57) is a conductive substance such as vacuum deposition,
Targets are substances that can be formed by thin film formation methods such as sputtering and ion plating, and in particular Al, Au, Pb, C
u, titanium alloy, Ni, Cr, MoS ₂ , MgF _2, etc. are preferable. The film thickness is preferably 500Å to 5000Å, preferably 1500Å to 3000Å. Since the thin film (57) is conductive, the terminals (48), (49)
Insulating coatings (54) and (55) are provided on portions of the substrate (50), the sealing material (51), and the thin film (57) that are in contact with each other. In addition, the terminal (4
To prevent a thin film from being formed on the part of the sealing substance (51) of (8) and (49) which is exposed to the outside of the holder (44), for example, Teflon (trademark) tape is wrapped and covered. Thin film
(57) is formed.

FIG. 7 shows a thin film (58) on the surface of the encapsulating substance (51) of the ultrasonic sensor (56) and a portion of the terminals (48) and (49) in contact with the thin film (57).
The ultrasonic sensor in which the thin film (57) is formed on the thin film (58) is further shown. The thin film (58) is a substance that can be formed by a thin film forming method such as vacuum deposition, sputtering, or ion plating, and the type of the material is not limited, but particularly, the sealing substance (51) and A material having a linear expansion coefficient between the thin film (57) is suitable. The thickness of the thin film (58) is preferably 100Å to 4000Å, preferably 500Å to 1000Å. Further, the total thickness of the thin film (58) and the thin film (57) combined is 500Å to 5000Å, preferably 1500.
Å to 3000 Å is suitable. Also, thin films (58) and (57)
When forming the film, the thin film (58) is formed on the terminals (48) and (49) that are outside the holder (44) from the encapsulating material (51) except the part in contact with the thin film (57). In order to prevent the
8) etc., so that the thin film (57) is not formed on the terminals (48) and (49) protruding above, for example, wrap Teflon (trademark) tape, cover it, and then cover the thin film (58). And (57) are formed respectively.

In addition, the ultrasonic transducer (45) and the holder (4
The material of 4), the substrate (50), and the sealing material (51) is not particularly limited as it is the same as the conventional ultrasonic sensor, and as the ultrasonic vibrator, a piezoelectric ceramic such as PZT or a piezoelectric characteristic is used. The resin and the holder may be made of metal or plastic, the substrate may be a laminated plate of phenol resin, epoxy resin or the like, and the sealing material may be silicone resin, epoxy resin, polyurethane or the like.

Further, FIG. 9 shows that the ultrasonic sensor (68) according to the present invention described in FIGS. 5, 6 and 7 is incorporated in the block (67) to further improve the moisture resistance as the object of the present invention. It is one example which can be improved. In FIG. 9, the block (67) is made of a metal such as aluminum or stainless steel having excellent corrosion resistance or a synthetic resin, and the end face (69) of the ultrasonic sensor (68) on the terminals (48) and (49) side. ) Is the vibrating end face (71) of the ultrasonic sensor (68) at the sensor receiving part (70) of the block (67).
Positioning is performed so that the block end face portion (72) of the block (67) is substantially in the same plane. In the annular space (73) between the ultrasonic sensor (68) and the block (67), the ultrasonic sensor (6
The ultrasonic sensor (68) is fixed to the block (67) by elastic sealing materials (74) and (75) made of an elastic material such as silicon resin and rubber capable of absorbing the ultrasonic vibration of 8). The cable (76) is a two-core electric wire that transmits and receives ultrasonic high frequency signals to the terminals (48) and (49) of the ultrasonic sensor (68), and has a hole drilled in the left end face of the block (67) in the figure. It is connected to terminals (48) and (49) via (77). In the gap between the block (67) and the cable (76), a screw bushing or a highly moisture resistant resin such as polybutadiene or polyvinylidene chloride is sealed.

The surface of the elastic sealing material (75) exposed to the outside air, that is, the vibrating end surface (7
A thin film (78) and / or a thin film (79) is formed on the surface of 1) and the block end face portion (72). The material of the thin film (78) has good adhesion to the elastic encapsulant (75), for example, when the elastic encapsulant (75) is a silicon resin, thin film formation such as vacuum deposition, sputtering and ion plating. The target is a substance that can be formed by the method, and a non-conductive thin film such as SiO or SiO ₂ is preferable.
Further, the material of the thin film (79) has good adhesion to the thin film (78) such as SiO and SiO ₂ , the block (67) and the vibrating end face (71), and vacuum deposition,
Targets are substances that can be formed by thin film formation methods such as sputtering and ion plating, and that generate few pinholes, especially Al, Au, Pb, Cu, titanium alloys, Ni, C
Conductive thin films such as r, MoS ₂ and MgF ₂ are preferable. The thickness of the thin film (78) is 100Å ~ 4000Å, preferably 500Å ~ 1000Å, the thin film (79)
Has a film thickness of 500Å ~ 5000Å, preferably 1500Å ~ 3000Å,
The total thickness of the thin film (78) and the thin film (79) is 1000Å ~ 5000Å,
It is preferably 2000Å to 4000Å.

In this embodiment, the description has been made on the two-layer thin film composed of the thin film (78) and the thin film (79). One of the thin film (78) and the thin film (79) may be provided.

〔The invention's effect〕

According to the present invention, by forming a thin film on the sealing member of a conventionally used ultrasonic sensor, it is possible to significantly improve the humidity resistance of the conventional ultrasonic sensor. The ultrasonic sensor used can be used continuously in a steam atmosphere for a long period of time. FIG. 10 shows the result of the accelerated test of the ultrasonic sensor according to the present invention forming a thin film and the conventional ultrasonic sensor. The test was performed by immersing the ultrasonic sensor in warm water at 60 ° C. and applying a DC voltage. In Fig. 10, the vertical axis (66) is the insulation resistance, the unit is MΩ, and the horizontal axis (65) is
The test time is in hours. Comparing the variation line (63) of the ultrasonic sensor according to the present invention forming a thin film with the variation line (64) of the conventional ultrasonic sensor, the ultrasonic sensor having the structure according to the present invention shows a change. In other words, it can be seen that the insulation resistance is not deteriorated and the moisture resistance is excellent. Further, by incorporating the ultrasonic sensor according to the present invention in the block (67) and forming a thin film on the elastic sealing material (75), the block (67) and the vibrating end surface (71), the moisture resistance of the ultrasonic sensor is improved. Can be further improved.

As described above, the ultrasonic gas concentration measuring device using the ultrasonic sensor according to the present invention having the moisture resistance remarkably improved is
This is an invention that is extremely useful industrially because it has little influence on fluctuations in temperature and humidity and can be accurately measured even during long-term continuous use under high humidity.

[Brief description of drawings]

Fig. 1 is a graph showing the relationship between CO ₂ gas concentration and ultrasonic wave propagation velocity, Fig. 2 is an example of measurement system diagram, and Fig. 3 is an example of measuring CO ₂ gas concentration using a gas concentration measuring device. FIG. 4 is a graph of Table 2, and FIGS. 5, 6, and 7 show examples of the ultrasonic sensor according to the present invention, respectively.
It is sectional drawing of a side surface. FIG. 8 (a) shows a sectional shape of a side surface of a conventional ultrasonic sensor, and FIG. 8 (b) is an enlarged view of a peripheral portion of the ultrasonic transducer (45) of FIG. 8 (a). FIG. 9 is a diagram showing an embodiment of an ultrasonic sensor with a built-in block according to the present invention. Further, FIG. 10 is a diagram showing the results of accelerated tests for comparing moisture resistance performance.

Claims (3)

[Claims] 1. A signal generator controlled by a feedback amplifier, a drive amplifier for amplifying a high frequency signal generated by the signal generator, and an electrostriction for converting a high frequency signal amplified by the drive amplifier into ultrasonic waves and transmitting the ultrasonic waves. Type ultrasonic transducer for transmitting waves and an ultrasonic sensor for receiving waves which receives the ultrasonic waves and converts the ultrasonic waves into an electric signal A feedback transmission composed of a negative immittance converter connected between the element and the driving amplifier, a resistance and negative immittance converter connected in parallel with the receiving ultrasonic element, and a preamplifier for inputting to the feedback amplifier. System, a mixer for calculating the difference between the frequency of the feedback transmission system and the reference frequency generated by a crystal oscillator, a frequency-voltage converter for converting the frequency calculated by the mixer into a voltage, and the frequency-voltage conversion In the gas concentration measuring device including a calculation output system including a calculator that calculates and outputs the gas concentration from the value output by the temperature sensor and the temperature information detected by the temperature sensor, the ultrasonic sensor is The ultrasonic vibrator is basically bonded to the holder, the opening of the holder is sealed with the sealing material, and the surface of the sealing material is electrically conductive or non-conductive. Thickness of conductive material 500-500
A gas concentration measuring device, which is a moisture-resistant ultrasonic sensor having a thin film of 0Å. 2. An ultrasonic sensor forms a thin film of a non-conductive substance having a thickness of 100 to 4000Å on a surface of a sealing substance, and then forms a thin film of a conductive substance on the thin film of the non-conductive substance. The gas concentration measuring device according to claim (1), characterized in that the thickness of the whole thin film is 500 to 5000 Å. 3. A block having an ultrasonic sensor built-in, and a thickness of 1000 to 500 made of a non-conductive material and a conductive material on an elastic sealing material, an ultrasonic sensor vibrating end surface and a block end surface portion.
The gas concentration measuring device according to claim (1) or (2), characterized in that a thin film of 0Å is formed.