JPH0638073B2 - Soil moisture measuring device - Google Patents

Description

The present invention relates to a device for measuring the amount of water in soil, and more specifically to measuring the amount of water in the soil by measuring the thermal conductivity of the soil by the twin unsteady heating probe method. It relates to a new device for measuring.

Currently, there is an extremely widespread demand for a device for measuring the amount of water in soil over time. Conventionally, for this purpose, a soil water potential measuring device using a tensiometer, an electric resistance measuring device, a neutron moisture meter, a gamma ray moisture meter, etc. have been devised and tested.

However, among these methods, the tensiometer method is inexpensive, but has a narrow moisture measurement range, a slow response, requires time for maintenance, and is not necessarily high in accuracy. The electric resistance method is inexpensive and easy to maintain, but its accuracy is extremely low. A neutron moisture meter and a gamma ray moisture meter are relatively accurate but expensive, and cannot be easily used in a field such as a field because they are limited in handling due to the use of radiation.

In this way, not only in Japan but also in foreign countries,
There has been no moisture meter that is highly accurate and easy to use, and there has been a long-felt demand for a device that has quick response, is easy to maintain, and is not too expensive.

As a result of repeated research by the present inventors in order to meet such a demand, a novel soil moisture meter that overcomes the shortcomings of the existing measurement methods and is highly accurate and practical can be developed by using a completely different principle from the conventional one. did. As a result, it can be widely used for automatic measurement of soil moisture content in fields, automation of irrigation, monitoring soil moisture for environmental conservation, disaster prevention-for example, prediction of sediment collapse.

The soil moisture meter of the present invention focuses on the fact that the thermal conductivity of soil has a high correlation with the soil moisture content, and uses a twin-type unsteady probe for soil thermal conductivity measurement buried in the soil for a short time. The thermal conductivity was measured and the soil water content was
It is characterized in that it is possible to obtain soil moisture by performing calibration conversion from a relational expression or curve of thermal conductivity.

First, when the principle of the measuring apparatus of the present invention is briefly described,
When a constant heat generation q is generated in a unit time from a linear heat source in an infinite individual having a temperature T ₀ at the time t = 0, the temperature difference between the temperature T of the linear heat source after t hours and the previous T ₀ is expressed by the following equation. It is known that it is expressed by an equation including a logarithm.

However, in the above equation, λ is the thermal conductivity of the individual, d is a constant, and ε is a correction term for simplifying the equation.

The temperature difference between the temperature T and T ₀ of the linear heat source after continuing the heat generation by the linear heat source until t _x time and stopping the heat generation at that time is also expressed by the following equation.

Next, there are two types of infinite individuals A and B, which have different thermal conductivities such as λ _a and λ _b , respectively. In each of these individuals, if heat generation is started simultaneously from completely similar linear heat sources, then in both individuals, (1) and (2) is holds together the same, yet constant d preliminary correction term ε is because it sees the same, the temperature of each line heat source in the a and B individuals T _a, T When _b, t _x time from the heat generation line heat source until the heat generation is stopped, and even heat generation stop after subsequent temperature change T _a -T _0a and T _b during both the individual
It can be seen that the ratio with −T _0b is equal to λ _b / λ _{a as} in the following equation.

(T _a -T _0a ) / (T _b -T _0b ) = λ _b / λ _a · (3) However, if the temperature difference [(T _a -T _0a ), (T _b -T _0b )] is increased Since the convection of moisture in the soil occurs and the above equations (1), (2) and (3) cannot be applied, it is necessary to set q so that the temperature difference is around 1 ° C. Also, t _x is set in a short time according to the temperature change.

From the equation (3), if the thermal conductivity of either one of the A and B individuals is known, the thermal conductivity of the other one can be easily measured. For example, if b is known, the unknown quantity a is given by the following equation.

The ratio of the temperature difference due to such temperature rise or fall is X-
It can be recorded by a Y recorder. That is, (T _a-
When T _0a ) and (T _b -T _0b ) are recorded on the Y-axis and the X-axis of the recorder, respectively, these trajectories are displayed as straight lines, and the inclination gives the ratio immediately. Alternatively, a microcomputer may be used to automatically perform on / off operation of the heater power supply, measurement of temperature difference, calculation of thermal conductivity, and the like.

In actual measurement, there are cases where the measured values obtained by the equations (1) and (2) do not match. This is because one or both of the substances A and B undergo a slight temperature change due to causes other than the heater during measurement. In this case, a correct value can be obtained by performing measurements by both (1) and (2) and averaging the values obtained by equation (4) for each.

Compared with the single non-stationary probe method, the twin type non-stationary probe method can eliminate the logarithmic term in each equation (1) and (2), and the principle and operation are extremely simple. In addition to being quick to perform, the thermal conductivity can be measured with an accuracy of ± 1%, which is clearly superior.

The apparatus for measuring water content in soil according to the present invention, which is used to measure water content in soil based on the above-described principle, includes (i) a thin metal pipe, an electric wire for heating and an electric resistance for temperature measurement, each of which is insulated and coated. A heating and temperature measuring probe in which a wire is encapsulated and fixed, and a tubular member surrounding a wire connecting portion for connecting an electric wire for heating and an electric resistance wire for temperature measurement of the probe to the upper end of the probe is attached, A standard resistance device for setting a comparative temperature, in which an electric resistance wire made of the same material as the temperature measurement electric resistance wire of the probe and having the same resistance value is wound around a wire core in the tubular member around a metal core having good thermal conductivity. And (ii) a reference sensor in which a sensor substantially the same as the soil moisture measurement sensor is stored and fixed in the center of the cylinder, and the reference substance is filled in the cylinder. age The DC variable stabilized power supply for energizing and heating both of these sensors at the same time, the electrical resistance measuring device for measuring the temperature change of the sensor after the start and stop of the heat generation, and the temperature change of both sensors are recorded. An apparatus for measuring soil moisture by measuring heat conduction, characterized by being equipped with a device that

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

FIG. 1 and FIG. 2 are both explanatory views schematically showing the moisture measuring device in soil of the present invention, which is a device for reading the temperature change of the soil moisture measuring sensor and the reference sensor and performing the necessary conversion work. In the device shown in FIG. 1, X-Y
The recorder, the apparatus shown in FIG. 2, uses a computer incorporating a predetermined program. In the figure, 1 is a soil moisture measuring sensor, 2 is a reference sensor, 3 is a DC variable stabilizing power supply, 4 is a resistance measuring device, 5 is an XY recorder, 6 is an A / D converter, and 7 is a relay switch. , 8 is a microcomputer. Appropriate known devices can be used for these devices except for the sensors for soil moisture measurement and the reference sensor 1 and 2, respectively.

FIG. 3 is a central longitudinal sectional view of a conventional soil moisture measuring sensor. In this sensor, a stainless steel tubular pipe 9 with an inner diameter of 0.5 mm, a wall pressure of 0.25 mm, and a length of 20 cm is closed, and a platinum wire with insulation resistance of 40 Ω is used as a resistance wire for temperature measurement and a heating wire, respectively. A manganin resistance wire 11 that is insulation-coated as in 10 is fixed by filling it with paraffin 12. Both ends of these wires are taken out from the open ends of the metal pipes and connected to electric wires for connection with other devices. The resistance value of the resistance wire for temperature measurement does not necessarily have to be 40Ω. Further, the material thereof may be platinum wire, for example, nickel wire. As the heating wire, for example, a constantan wire may be used in addition to the manganin resistance wire.

It is preferable that the metal pipe used for the sensor for measuring soil moisture has a large thermal conductivity in view of function and a pipe excellent in corrosion resistance because it is embedded in soil and used. A typical example is stainless steel.

The filler for fixing the resistance wire for temperature measurement and the electric wire for heating is not particularly limited as long as it is an electrically insulating material such as paraffin.

The resistance wire for temperature measurement and the electric wire for heating are connected to the shielded 6-core cable 13 and connected to external equipment.
The wire connection portion is housed in a synthetic resin tubular member 14 attached to the open end of the metal pipe 9, and the tubular member 14 is sealed to the outside with a filling adhesive 15 or the like.

The sensor for soil moisture measurement is as thin as possible because it replaces the linear heat source, and preferably has an outer diameter of about 1 mm, but it may be somewhat thicker than that due to its strength. However, if it is too thick, the measurement accuracy will be low, so even if it is thick, the outer diameter should be about 2 mm or less.

FIG. 4 shows a soil moisture measuring sensor used in the soil moisture measuring device according to the present invention, and the same material as the temperature measuring electric resistance wire is used in the connection portion of the soil moisture measuring sensor shown in FIG. A standard resistance device (16) for setting a comparative temperature in which an electric resistance wire having the same resistance value is wound around a metal core having good thermal conductivity is provided. Although the sensor for soil moisture measurement shown in FIG. 3 is functionally sufficient for measuring moisture in soil, the sensor can be used by using a standard resistance device for setting a comparative temperature as shown in FIG. The temperature change is easy to read, and compared with the case of using the soil moisture measurement sensor shown in Fig. 3, other attached equipment is cheaper and therefore measurement values can be obtained with the same accuracy even if a low-grade equipment is used. You can

The reference sensor 17 is shown in FIG. This sensor is an acrylic resin cylinder with an inner diameter of 5 cm, a wall thickness of 5 mm, and a length of 25 cm.
A tubular member 14 made of synthetic resin having a wire connection portion of the probe 2 having the same structure as the soil moisture measuring sensor shown in FIG. A 1% agar gel is enclosed as a reference substance in the cylindrical body.

It is preferable that the cylindrical body of the reference sensor can contain as much of the reference material as possible, and in that sense it is better that the cylindrical body is larger, but if it is larger than necessary, mobility as a device to be buried in soil Will be constrained. Therefore, in order to obtain a measurement value of accuracy required by the moisture measuring apparatus of the present invention, when accommodating a soil moisture measuring sensor having an outer diameter of about 1 mm, an inner diameter of about 5 cm is necessary and sufficient.

Further, the material of the cylindrical body is preferably one having a small thermal conductivity and a certain strength. As long as it is a synthetic resin, not only acrylic resin but also any other synthetic resin can be used, and synthetic resin foam is the most preferable as long as it can satisfy the strength condition. Needless to say, materials other than synthetic resins may be used.

As a reference substance used for this reference sensor, in order to improve the measurement accuracy, the value of thermal conductivity has already been calculated accurately and has a thermal conductivity as close as possible to the thermal conductivity of soil, causing a temperature difference. A substance that does not generate convection when exposed to light is preferable. For example, water is known to have a highly accurate thermal conductivity value, is easily available, and has a value close to that of soil. Therefore, it is expected to be used as a reference substance. However, it is a liquid at room temperature, and if there is a temperature difference, convection will occur,
In this respect, it is unsuitable as a reference substance. In order to eliminate this drawback, if a small amount of substance that does not change the thermal conductivity of water can be dissolved to prevent convection of water, it can be used as a reference substance. Agar gel with a concentration of about 1%
It is one of the most preferable.

The soil moisture measuring sensor and the reference sensor do not necessarily have to have the same length, and when the lengths are different, either one may be longer.

Next, the operation of the apparatus for measuring moisture in soil according to the present invention will be described.

First, the soil moisture sensor is embedded horizontally (or vertically in special cases) at the desired depth in the soil where the moisture is to be measured in the field, while the reference sensor is at the same depth as the soil moisture sensor. And it will be buried at a distance of 30 cm or more. However, when several points are simultaneously measured by computer control, only one reference sensor installed at the deepest position can be used in common. Next, the reference material and each sensor for the soil to be tested are simultaneously energized from the DC variable stabilizing power supply. The supply voltage at this time is about 3 minutes after the start of energization and the temperature rise of the probe is 1
Adjust in advance to keep the temperature inside or outside. Simultaneously with the energization, measurement of the resistance value of the temperature measurement electric resistance wire of both sensors is started, and this continues even after the energization is stopped. Energization time is usually about 3
The resistance measurement time is about twice this energization time. When the XY recorder is used, the linear shape drawn by the XY recorder has a general feature as shown in FIG. That is, it becomes a straight line except at the beginning of heat generation and the initial stage of stop. From the arithmetic mean of the slopes of these two straight lines, the ratio of temperature change of both sensors can be obtained.

By substituting the thermal conductivity of water for b in the equation (4), the thermal conductivity of soil can be immediately calculated as a by using the value of this ratio.
Separately, as shown in FIG. 7, if the relationship between the thermal conductivity of the soil and the water content is previously investigated, the water content in the soil can be measured with time.

Experimental example Stainless steel tube with outer diameter of 1 mm, inner diameter of 0.5 mm, and length of 20 cm is insulation-coated as a heating wire with a constantan wire of 40 cm (electrical resistance value of about 40 Ω) in 0.1 mm and with an insulation coating as an electric resistance wire for temperature measurement. 0.03mmφ nickel wire 4
Seal the probe with 0 cm (electrical resistance of about 40 Ω), fill it with paraffin and fix the wire, and connect it to the 6-core shield wire as shown in Fig. 3, and attach it to the upper end of the probe to protect the connection part. Acrylic tubular member with a diameter of 10 mm is attached, and further, in the tubular member of the connection part, brass with an outer diameter of 3 mm and a length of 25 mm,
Seven sensors for soil moisture measurement were prepared by attaching the same nickel wire as the one enclosed in the probe and wrapping it with the same length (40 cm) as a standard resistance device for setting the comparative temperature. A reference sensor was created using one of these.
That is, one end of a transparent acrylic pipe having an outer diameter of 60 mm, an inner diameter of 50 mm, and a length of 25 mm is sealed with a rubber stopper, 1% agar solution is poured into the transparent acrylic pipe, and the remaining end is also a rubber stopper having a 10 mm hole in the center. The acrylic pipe was sealed so that air could not enter, and the above-mentioned soil moisture measuring sensor was inserted and fixed in the hole of this rubber stopper to prepare a reference sensor. These sensors were connected to other devices as shown in FIG.

After putting the sensor for soil moisture measurement in the same 1% agar gel used as the reference sensor, set the voltage of the DC variable stabilizing power supply to 5.0 V, energize for 3 minutes, and both during energization and after de-energization. The temperature change of the probe was recorded by an XY recorder. It is shown in FIG. The linear changes were clear except at the beginning of energization and immediately after the de-energization, and the slopes of the straight lines during energization and after de-energization were 0.995 and 0.980, respectively, with an average of 0.99. The slope should theoretically be 1.0 because the same substance is measured, but in practice
There is a difference in output between the sensors within the range of about 2%. This reflects the inevitable differences that occur when making the sensor. In the actual calculation of the thermal conductivity, the reciprocal of this value is determined for each sensor, and this is corrected by multiplying the measured gradient as the instrument coefficient. In order to check the water content change in the field using these sensors, it was buried in the actual field and measured. The field is Field D, Ministry of Agriculture, Forestry and Fisheries National Institute for Agro-Environmental Sciences, Yatabe-cho, Tsukuba-gun, Ibaraki Prefecture. Sensors for soil moisture measurement are 5, 10, 20, 3 from the surface
It is buried horizontally at a depth of 0, 40, 50 cm and the reference sensor is 50
It was embedded at a depth of about 50 cm away from the soil sensor to be tested. With these sensors, the second every 30 minutes
Automatic measurement was performed using the microcomputer shown in the figure. Part of the obtained results is shown in FIG. The numbers in FIG. 8 are the numbers of the sensors, and the number 1 indicates the sensor with a depth of 50 cm, which indicates the sensors at shallower positions. Although there was about 30 mm of rainfall from the first day of measurement to the next day, it is clearly observed that water gradually increases from the shallow layer. As is clear from this, it can be seen that the sensor has sufficient accuracy as a soil moisture measurement sensor.

[Brief description of drawings]

1 and 2 are explanatory views schematically showing a soil moisture measuring device of the present invention, FIG. 3 is a central longitudinal section of a conventional soil moisture measuring sensor, and FIG. 4 is a diagram showing the soil of the present invention. A central longitudinal cross-sectional view of a soil moisture measuring sensor used in a moisture measuring device,
FIG. 5 is a central longitudinal sectional view of a reference sensor used in the soil moisture measuring apparatus of the present invention, and FIG. 6 is both sensors before and after energization when the soil moisture measuring sensor and the reference sensor of the apparatus of the present invention are energized. Fig. 7 is a schematic diagram of the record obtained by reading the ratio of the temperature change of the soil with an XY recorder, Fig. 7 is a graph showing the relationship between the thermal conductivity of soil and the moisture content, and Fig. 8 is the measurement result of the moisture content in the field over time. It is a graph which shows.

Claims (8)

[Claims] (I) A heating and temperature-measuring probe in which a heating electric wire and an electric resistance wire for temperature measurement, which are respectively insulated and coated, are enclosed and fixed in an elongated metal pipe, and for heating the probe at the upper end of the probe. A tubular member surrounding a connecting portion for connecting the electric wire and the electric resistance wire for temperature measurement to an externally attached device is mounted, and the connecting material in the tubular member is made of the same material as that of the electric resistance wire for temperature measurement of the probe. A soil moisture measuring sensor provided with a standard resistance device for setting a comparative temperature, in which an electric resistance wire having a resistance value is wound around a metal core having good thermal conductivity, and (ii) the soil moisture measuring sensor and It is mainly composed of a reference sensor in which the same sensor is housed and fixed in the center of the cylinder, and the reference material is filled in the cylinder, and a DC variable stabilized power supply for energizing and heating both of these sensors at the same time. A device for measuring electrical resistance for measuring temperature changes of the sensor after starting and stopping the heat generation, and a device for recording temperature changes of both sensors, the soil moisture measuring device by thermal conductivity measurement. . 2. The soil moisture measuring device according to claim 1, wherein the metal pipe of the soil moisture measuring sensor is made of stainless steel. 3. The soil moisture measuring device according to claim 1, wherein the metal pipe of the soil moisture measuring sensor has an inner diameter of about 0.5 mm or more, an outer diameter of about 1 mm or more, and a length of about 5 cm or more. . 4. The soil moisture measuring device according to claim 1, wherein the cylindrical body of the reference sensor has an inner diameter of 5 cm or more. 5. The soil moisture measuring device according to claim 1, wherein the reference substance is 1% agar gel. 6. The soil moisture measuring device according to claim 1, wherein the cylindrical body of the reference sensor is made of acrylic resin. 7. A device for recording the temperature change of both sensors is X.
A soil moisture measuring device according to claim 1, which is a Y recorder. 8. The soil moisture measuring device according to claim 1, wherein the device for recording the temperature change of both sensors is a data logger equipped with a microcomputer.