JP5726454B2 - Gas sensor unit and gas concentration measuring device for measuring gas concentration in soil - Google Patents

Description

Gas sensor using a micro sensor equipped with a special gas permeable membrane that measures the concentration of carbon dioxide (CO ₂ ) gas in the soil directly by measuring the gas concentration in the soil. It relates to units.

An artificial increase in the concentration of carbon dioxide (CO ₂ ) in the atmosphere accompanying the combustion of fossil fuels has become a cause of global warming. On the other hand, carbon dioxide (CO ₂ ) is primarily involved in the transition of the environment from the birth of the earth, forming a carbon cycle on a global scale, and the natural generation of carbon dioxide (CO ₂ ) on the earth is the root of plants. And oxygen (O ₂ ) consumption due to the respiration of small animals in the soil and microbial decomposition, and the amount generated is sufficiently larger than the amount generated artificially, and the amount is also carbon dioxide (CO ₂ ) in the atmosphere It is said that there are several times the amount.
Among the environment surrounding such carbon dioxide (CO _2), in order to consider the global warming, it is important to evaluate the circulating-balance-abundance of carbon dioxide (CO ₂₎ in a variety of scales.

At present, the measurement of carbon dioxide (CO ₂ ) in soil, which is performed in the agricultural field, etc., is performed by inserting a thin tube 37 shown in FIG. 7 (reference numeral 30) into the ground, and carbon dioxide (CO ₂ : white arrow). ) Is sucked and analyzed (for example, see Patent Documents 1 and 2), or the ground surface shown in FIG. 8 (reference numeral 40) is covered with a chamber 43, and its flux (open arrow) is measured (for example, , Patent Documents 3 and 4).
7 and 8, 31 and 41 are gas sensors, 32 and 42 gas analyzers, 33 and 43 are chambers, 34 and 44 are signal lines, 35 is a flow meter, P is a suction pump, 36 and 46 are exhaust pipes, 37 is a gas sampling tube, 38 is a protective tube, and 47 is an air suction port.
However, none of them directly measures carbon dioxide (CO ₂ ) gas in the soil, but is an indirect method of performing gas analysis after collecting the gas, and does not directly measure the gas concentration in the soil. Absent. Therefore, it was difficult to measure in real time without disturbing the field at the measurement location.

  On the other hand, as a method of directly measuring the gas concentration, volatile organic compounds (for example, Volatile Organic Compounds: VOC such as trichlorethylene and tetrachloroethylene) are detected in the polluted soil investigation and cleaning treatment. An example of a soil gas sensor is described in Patent Document 5.

This soil gas sensor and humidity sensor are those whose sensor part is exposed to the air, and carbon dioxide (CO ₂ ), hydrocarbon (CH), oxygen (O ₂ ), carbon monoxide (CO), Gas sensors such as nitrogen oxide (NOx), ammonia (NH ₃ ), ozone (O ₃ ), and VOC all require direct contact with the gas component to be measured, and an electric circuit is used for the sensor unit. For this reason, there is a problem that failure is easily caused when direct contact with moisture occurs.

JP 2004-184105 A JP 2006-346578 A Japanese Patent No. 3968728 JP 2008-203124 A JP 2009-41996 A

In the suction method proposed in Patent Documents 1 and 2, and the chamber method proposed in Patent Documents 3 and 4, the following problems are involved in measuring the concentration of various gases in soil including carbon dioxide (CO ₂ ) gas. The point is raised.
(1) Since the gas around the inserted point is sucked, the gas concentration at a specific position cannot be measured accurately. [Suction method]
(2) Since it is a method of sucking gas, continuous measurement at a specific position is not possible. [Suction method]
(3) The measurement efficiency is low and automatic measurement is difficult. [Suction method]
(4) It is difficult to estimate the gas concentration in the ground from the measurement of the gas concentration in the surface layer. [Chamber method]
(5) Since the ground surface is covered with a chamber, the environmental conditions (atmospheric pressure, wind, humidity, temperature, etc.) are different from natural environmental conditions. The [Chamber method]

Furthermore, a gas sensor or humidity sensor having a sensor part based on an electric circuit requires direct contact with the gas component to be measured. Therefore, direct contact with moisture (water droplets, condensation, water vapor, etc.) is inevitable. The possibility of causing problems such as short circuit and rust of the electric circuit due to is extremely high. For this reason, protective measures are required for its use, and in the case of using a humidity sensor outdoors, devices such as a 100-leaf box and a multi-stage rain protection umbrella are provided, and measurement protection measures are taken therein.
However, since such protective measures are difficult in the soil, in an environment where the object to be measured (gas component) and moisture (mainly water vapor) are mixed as in the soil, the gas component contained in the environment is reduced. A sensor device to measure was desired.

  Therefore, in the present invention, it is possible to continuously measure a specific position in real time, and by using a direct method capable of obtaining a reliable measurement value, the influence of moisture penetrating the soil on the sensor is suppressed and continued in the soil. A sensor unit for measuring a gas concentration for measuring the concentration of a gas component and a gas concentration measuring device using the sensor unit are provided.

1st invention of this invention is a sensor unit which measures the gas density | concentration in soil, Comprising: An electric sensor, the signal wire | line which transmits the signal from the electric sensor to the exterior, and an electric sensor are covered directly A gas sensor unit comprising a hydrophilic gas permeable membrane.

According to a second aspect of the present invention, there is provided a sensor unit for measuring a gas concentration in soil, the electrical sensor, a unit housing for housing the electrical sensor having a wall surface having air permeability, and the electrical sensor. A signal line for transmitting a signal from the outside, a hydrophilic gas permeable membrane of polymer polyethylene / inorganic powder attached to the outer wall surface of the unit housing, and a membrane protection provided outside the hydrophilic gas permeable membrane A gas sensor unit comprising a body .

According to a fourth aspect of the present invention, there is provided the gas sensor unit according to the second or third aspect, wherein a breathable protective body is provided so as to cover the electric sensor in the unit casing.

According to a fifth invention of the present invention, in the second to fourth inventions, a drainage pipe is provided at the bottom of the unit casing, and a water collection storage part is provided for storing stored water in the unit casing that is discharged by the drainage pipe. This is a gas sensor unit.

A sixth invention of the present invention is a gas sensor unit characterized in that the gas sensor according to the first to fifth inventions is a sensor that directly contacts and measures the concentration of the object to be measured.

The seventh invention of the present invention is a sensor body that is arranged in series with a plurality of gas sensor units of the first to sixth inventions, and a signal from the sensor unit of the sensor body. A gas concentration measuring apparatus comprising: a signal line that transmits to the outside; and a data collection unit connected to the other end of the signal line.

According to the present invention, it is possible to greatly reduce the labor and time required for the measurement, and it is possible to perform a large number of simultaneous measurements and continuous measurements with the atmospheric concentration, thereby enabling a significant cost reduction.
Furthermore, since it is a direct measurement in soil, highly reliable data can be obtained.
In addition, it enables real-time measurement of various gas concentrations in humid environments (such as inside grains, curing facilities, inside concrete, etc.) including in the soil.

It is a figure explaining the gas concentration measuring method in the soil by this invention, (a) is a general view, (b) is an enlarged schematic diagram of the gas sensor unit 1 of the lowest part. It is a figure which shows 1st Embodiment of the sensor unit for gas concentration measurement, (a) A partially cutout external view, (b) is sectional drawing along aa. It is a figure which shows other embodiment of a gas sensor unit, (a) A partially notched external view, (b) is bb sectional drawing. It is the schematic which shows the test apparatus for the carbon dioxide concentration measurement which concerns on this invention used in Example 2. FIG. It is the figure which measured the time-dependent change of the carbon dioxide concentration in soil. It is a figure which shows the carbon dioxide density | concentration in air | atmosphere after 240 hours progress, the ground surface, and underground. It is explanatory drawing of the gas concentration measurement by a suction method. It is explanatory drawing of the gas concentration measurement by a chamber method.

  The unit sensor for gas concentration measurement according to the present invention covers a gas sensor having an electric circuit such as an infrared system at the heart with a special gas permeable membrane that allows only the measurement gas to pass through while preventing the intrusion of pore water in the surrounding soil. Therefore, it is possible to directly and in real time measure the gas concentration in the gap in the soil (however, the target is the unsaturated zone above the groundwater surface).

  In addition, a gas sensor is placed in the unit housing that has air permeability, the outer periphery of the unit housing is covered with a gas permeable membrane, and the membrane surface is covered with a wire mesh or the like to prevent the membrane from being damaged by pebbles or tree roots in the ground. Cover with a membrane protector, and install a mesh-like protective container inside the gas permeable membrane, assuming that moisture will enter due to another cause (for example, water leakage from the joint of the unit housing) . Furthermore, as a precaution, the leaked water is led to the water collecting storage part installed at the lower part of the unit casing through the discharge pipe at the bottom of the unit casing and drained to the ground by the micropump.

As the gas permeable membrane in the present invention, a porous membrane having hydrophilicity having an affinity for moisture (hereinafter referred to as “hydrophilic porous membrane”) is used.
A porous membrane generally known as a gas permeable membrane is a porous membrane made of a hydrophobic resin such as a fluororesin as a base material. In this hydrophobic porous membrane, particles such as raindrops are used. Larger diameters do not permeate, but water vapor like particles with a particle size of 0.04 μm to 0.3 nm (water molecule size) is permeated. Growing to a size larger than tuberculosis (about 1.0 μm), it becomes water droplets, which attach to the electrical circuit of the sensor and cause a short circuit, or cause rust and corrosion, causing the gas sensor to malfunction.

On the other hand, the gas permeable membrane, which is a hydrophilic porous membrane in the present invention, plays a role of preventing the gas sensor from being broken due to moisture adhesion by suppressing the permeation of water vapor.
Below, the property as a gas permeable film of the hydrophilic porous membrane used by this invention is demonstrated.
A microporous membrane that does not allow water to pass through and allows only gas to pass through can be used by adapting any material as long as the average diameter of the pores in the porous membrane is smaller than the size of water and larger than the size of the gas. The size of the gas is about 0.0004 μm for oxygen, 0.00033 μm for carbon dioxide (CO ₂ ), about 0.0003 μm for water molecules, and about 0.04 μm for water vapor.
By the way, of rainwater in the natural world, the so-called drizzle raindrops are about 100 μm, the raindrops are a few hundred μm, and the regular rains are of the order of 1000 μm. If the average diameter of the pores of the porous membrane (hereinafter also referred to as pore diameter) is in the range of 1.0 μm or less, it functions to prevent water (water droplets, raindrops, etc.) from passing through the gas sensor and humidity sensor. It is considered possible.

However, in the gas, water vapor passes through the porous membrane, contacts the internal substrate, parts, and equipment, and becomes water molecules due to condensation and other phenomena, causing rust and corrosion, and causing equipment to operate. May have adverse effects.
For this reason, a porous membrane that blocks the permeation of water vapor is required, but generally known porous membranes having an average pore diameter of about 0.1 to 1.0 μm include polytetrafluoroethylene (PTFE) and polyolefin. These are hydrophobic resin membranes, and these hydrophobic porous membranes cannot prevent the passage of water vapor. This is because, in these hydrophobic porous membranes, the water vapor passes through because the water vapor does not condense even if the pore size is reduced.

  By the way, condensation is a phenomenon in which gas is liquefied when the temperature of the gas is lowered, and is known as generation of fog, clouds, dew, etc. due to cooling of water vapor. Furthermore, water vapor present inside hydrophilic porous pores such as silica gel tends to become liquid compared to water vapor outside, and this phenomenon is called capillary condensation, and water molecules in the pores. This phenomenon appears when the attractive force between the pore wall and water molecules is stronger than the attractive force between them.

  This capillary condensation occurs due to the pressure difference (dP) at the curved interface, and this phenomenon can be expressed by a mathematical formula. When the interface shape is a part of a spherical surface and the contact angle between water and solid is θ, the following formula (1) expressed. Note that the contact angle θ is an angle formed by a solid and a polka dot when a water drop is dropped on the solid. The larger the contact angle θ is, the less wet the PTFE is in the form of a round water droplet, and the contact angle θ reaches 114 degrees.

  Next, when multimolecular layer adsorption occurs inside the capillary, a thin liquid phase is formed inside the capillary and the capillary inner diameter (r) becomes small. In this case, the pressure difference in the capillary tube is expressed by equation (2).

In this formula (2), when the capillary is straight, r _L becomes infinite, and formula (2) is approximated by the following formula (3).

  When the membrane material is PTFE, which is representative of a hydrophobic porous membrane, θ = 114 °, equation (3) becomes the following equation (4), pressure difference (dP) becomes negative, and water vapor does not condense into water. Will pass by.

  On the other hand, the gas permeable membrane used in the present invention is a porous membrane having hydrophilicity, and the contact angle with water is almost close to 0 degree, and cos = 1 is approximately. Therefore, the pressure difference dP is expressed by the following formula (5).

  Furthermore, the average pore diameter of the pores of the porous membrane is 0.1 μm or less, and it is easy to cause strong capillary condensation, and water vapor (the water vapor particle diameter is said to be 0.3 nm to 0.04 μm) is transmitted through this gas. Since the membrane is hydrophilic, water molecules and water vapor condense on the membrane surface and inside the membrane (inside the pores), and the passage of water vapor is prevented by growing from the state of water vapor to the size of water at least larger than the condensation nucleus. To do. Since the size of the water at this time is 1.0 μm or more of the condensation nucleus, water vapor does not pass through the hydrophilic film having an average pore diameter of 0.1 μm. Further, it is clear that the surface tension shown in the equation (5) acts in the complete water droplet state and does not easily pass in the water state.

Table 1 shows the characteristics of the gas permeable membrane and other gas permeable membranes used in the examples of the present invention.
The gas permeable membrane used in the gas sensor unit of the present invention is not limited to that of Example 1 in Table 1, and is a porous membrane having hydrophilicity and an average pore diameter of less than 1 μm, more preferably 0.1 μm or less. It is a porous film having an average pore diameter.

  Hereinafter, based on an accompanying drawing, the gas concentration measuring sensor and measuring device of the present invention are explained in detail.

A state during gas measurement by the gas concentration measuring apparatus of the present invention is shown in the gas concentration measurement diagram of FIG. In FIG. 1, 3 is a gas concentration measuring device, 1 is a gas sensor unit of the present invention, 4 is a sensor body, 5 is a gas sensor, 6 is a signal line, 6a is a unit suspension cable, 18 is a data collection unit, 15 Is a drain pipe, 17 is a micropump for discharging the leaked water 20, and 20 is a leaked water accumulated in the sensor unit.
In FIG. 1, five gas sensor units 1 are vertically installed at predetermined intervals with sensor bodies 4 inserted into holes at measurement points, and the CO ₂ concentration at each position in the ground is measured by the gas sensor 5. The data is sent to the data collecting unit 18 through the signal line 6 and recorded.

2A is an external view showing an example of an embodiment of a gas sensor unit, and FIG. 2B is a cross-sectional view thereof. In FIG. 2, 1 is a gas sensor unit, 5 is an infrared CO ₂ gas sensor, 6 is a signal line, 6a is a sensor suspension cable, 10 is a gas permeable membrane, 11 is a unit casing, and 11a is a unit casing. Pore, 12 is a membrane protector for protecting the permeable membrane, 13 is a unit lid, 14 is a water collection reservoir, 15 is a drain pipe, and 20 is a reservoir for leaking water.

The gas sensor 5 is placed inside the unit housing 11 having air permeability through the air holes 11a and the like. In FIG. 2, the unit suspension cable 6a provided with the signal line 6 is suspended from the upper part in the unit housing 11 and installed.
The gas sensor 5 uses a sensor that matches the object to be measured.

  As shown in the sectional view of FIG. 2B, a gas permeable membrane 10 and a membrane protector 12 for protecting the permeable membrane are provided on the outer periphery of the unit housing 11 in order from the unit housing 11 side. 5 is covered. Furthermore, an internal protective body 16 may be provided in the unit housing 11 so as to cover the gas sensor 5.

  Further, a drainage pipe 15 is provided at the bottom of the unit casing 11 to lead out the accumulated leaking water to a storage section for the leaked water 20 and a water collecting storage section 14 that is detachably fixed to the bottom of the unit casing 11. Yes. Furthermore, although not shown, a micropump for discharging the stored water to the outside may be provided in the water collecting part of the water collecting and storing part 14 (micropump 17 shown in FIG. 1).

FIG. 3 is an external view (a) and a cross-sectional view (b) showing an example of another embodiment of the gas sensor unit.
In this embodiment, the gas sensor 5 is covered with the gas permeable membrane 10 without having the unit housing (reference numeral 11 in FIG. 2) for protecting the gas sensor 5 as in the embodiment of FIG. This is a gas sensor unit 2 in a form equipped with a protective film protector 12.

A gas sensor unit 1 for measuring carbon dioxide shown in FIG. 2 was prepared using a gas permeable membrane made of the material shown in Example 1 of Table 1. This performs a moisture resistance test using a constant temperature bath against the sensor unit 1, after the test and the CO ₂ concentration measured in the humidity resistance test was observed the state of wetness of the gas sensor 5 in the gas sensor unit 1.
The moisture resistance test was performed under the condition of standing for 10 days (240 hours) in an environmental atmosphere at a temperature of 20 ° C. and a humidity of 95%.
During the moisture resistance test, the case where the CO ₂ concentration was measured until completion was indicated as “◯”, and the case where the measured value showed an abnormality was indicated as “X”. The sensor wetness was determined as “◯” when it was dry, and “x” when it was clearly wet.

(Comparative Example 1)
A moisture resistance test was conducted in the same manner as in Example 1 except that the gas permeable membrane was changed to that shown in Comparative Example 1 of Table 1. The results are shown in Table 2.

(Comparative Example 2)
A moisture resistance test was conducted in the same manner as in Example 1 except that the gas permeable membrane was changed to that shown in Comparative Example 2 of Table 1. The results are shown in Table 2.

(Comparative Example 3)
A moisture resistance test was conducted in the same manner as in Example 1 except that the gas permeable membrane was changed to that shown in Comparative Example 3 of Table 1. The results are shown in Table 2.

As is apparent from Table 2, in Example 1 using a hydrophilic gas permeable membrane, CO ₂ measurement was reliably performed during the test period. Furthermore, wetting due to water droplets was not observed in the gas sensor after completion.
On the other hand, in Comparative Examples 1 to 3, as a result of using a hydrophobic gas permeable membrane, CO ₂ measurement was abnormal during the moisture resistance test, and the gas sensor was wet with water droplets.

FIG. 4 is a system diagram showing an outline of a test apparatus that measures the carbon dioxide (CO ₂ ) concentration in the atmosphere, the ground surface, and the ground in real time at a predetermined height and a predetermined depth. The sensor unit of the present invention is used to measure the CO ₂ concentration in the soil.
The concentration of carbon dioxide (CO ₂ ) in the atmosphere is measured with an infrared CO ₂ sensor (indicated by reference symbols A5 to A40) suspended from a balloon at a height of 5 to 40 m at intervals of 5 m, and is measured from the ground surface to a distance of 0. The carbon dioxide (CO ₂ ) concentration at a height of 3 m (indicated by A30), 0.6 m (A60), and 0.9 m (A90) was measured by a CO ₂ sensor provided at each position of the frame box mount.

The carbon dioxide (CO ₂ ) concentration in the ground was measured at a position (denoted by G20 to G100) from a depth of 0.2 m to a depth of 1 m using a gas sensor unit of the present invention. In addition, soil moisture content (denoted by M20 to M100) at the same depth was measured using a soil moisture meter.
Using this test apparatus, the carbon dioxide (CO ₂ ) concentration was measured for 10 days (240 hours). The measurement results are shown in FIGS.

FIG. 5 shows the result of capturing the continuous change over time in the carbon dioxide (CO ₂ ) concentration in the ground.
It can be seen that the carbon dioxide concentration at each depth in the ground can be measured in series. In addition, it has been shown that the carbon dioxide concentration in the ground cannot be measured accurately until a certain amount of time has elapsed since the start of the measurement, and results that are difficult to obtain with conventional suction or chamber methods. Is shown to be obtained in the present invention.
It can be seen that the gas sensor unit is functioning normally during this measurement period. In addition, the volumetric water content in the ground during the measurement period was in the range of 10 to 30%.

FIG. 6 shows the carbon dioxide concentration (CO ₂ ) from the ground 1 m (G100) to the atmospheric altitude 40 m (A40) on the 10th day (240 hours after the start of measurement) in the 10-day measurement using the measurement system of FIG. (Concentration) is plotted.

FIG. 6 reveals that the carbon dioxide concentration in the atmosphere hardly fluctuates, while the carbon dioxide concentration in the ground largely fluctuates up to 1 m in the ground. Moreover, it is clarified that the concentration is 10 to 100 times higher than the carbon dioxide concentration in the atmosphere.
As described above, according to the present invention, it is possible to measure in real time the concentration of various gases such as carbon dioxide in an environment where there is a lot of moisture such as in soil.

DESCRIPTION OF SYMBOLS 1, 2 Gas sensor unit 3 Gas concentration measuring device 4 Sensor body 5 Gas sensor 6 Signal line 6a Unit suspension cable 10 Gas permeable membrane 11 Unit housing 11a Unit housing ventilation hole 12 Membrane protector 13 Unit lid 14 Collection Water storage part 15 Drain pipe 16 Internal protection body 17 Micropump 18 Data collection part 20 Water leakage (reserved water)
30 suction method 40 chamber method 31, 41 gas sensor 32, 42 gas analyzer 33, 43 chamber 34, 44 signal line 35 flow meter 36, 46 gas exhaust pipe 37 gas sampling pipe 38 protective pipe 47 atmospheric suction port P suction pump

Claims (6)

A sensor unit for measuring gas concentration in soil,
A gas sensor unit comprising an electric sensor, a signal line for transmitting a signal from the electric sensor to the outside, and a hydrophilic gas permeable membrane that directly covers the electric sensor. A sensor unit for measuring gas concentration in soil,
An electric sensor, a unit housing that houses the electric sensor whose wall surface is permeable, a signal line that transmits a signal from the electric sensor to the outside, and a height that is attached to the outer wall surface of the unit housing A gas sensor unit comprising a hydrophilic gas permeable membrane of molecular polyethylene / inorganic powder and a membrane protector provided outside the hydrophilic gas permeable membrane . The gas sensor unit according to claim 2 , wherein a breathable protective body is provided in the unit casing so as to cover the electric sensor. Comprising a drainage tube at the bottom of the unit housing, a gas sensor according to claim 2 or 3, characterized in that a water collecting reservoir for collecting the stored water in the unit housing to be discharged by the drain pipe unit. The electric sensor, gas sensor unit according to any one of claims 1, characterized in that a sensor for determining the concentration of the object to be measured in direct contact with the gas of the object 4. A sensor body being dumped in the ground by disposing the gas sensor unit according plurality, in series to claims 1 to 3,
A signal line for transmitting a signal from the gas sensor unit of the sensor body to the outside;
A data collection unit connected to the other end of the signal line;
A gas concentration measuring device for measuring a gas concentration in soil, comprising:

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