JP2024077642A - Water retention measuring device and water retention measuring method - Google Patents

Abstract

To provide a device used to measure a water holding property in a sample and has a simpler configuration than a conventional device.SOLUTION: A water holding property measurement device is characterized by setting a cylindrical container for storing a test sample and a pressure gauge for measuring the suction of the sample on a balance, and measuring the suction and weight at the same time. Furthermore, the device includes a heat source to dry the sample as means of removing moisture from the sample.SELECTED DRAWING: Figure 1

Description

The present invention relates to a sample water retention measurement device and a sample water retention measurement method.

Knowing the water retention capacity of a sample is important in various technical and industrial fields. For example, the water retention capacity of soil is important for knowing the strength of the ground before building a house. In soil contamination surveys, water retention capacity can be a valuable parameter for understanding the mechanism of contamination.
In the agricultural field, soil water retention has a significant effect on crop growth when cultivating crops, so it is important to understand the soil water retention in advance.

To date, methods for measuring soil water retention have been established, depending on the measurement range, including the sand column method, pressure method, and centrifugal method (see, for example, Non-Patent Document 1). These measurement methods involve increasing or decreasing the pressure or gravity on the sample to gradually remove or add water from the sample, and measuring the water retention amount at each stage by weighing the mass of the sample at each stage.

However, conventional devices tend to be large, require space, and are expensive, making them unsuitable for quickly and easily measuring the water retention of samples.

Journal of the Japan Society of Erosion Control Engineering, Vol. 73, No. 3, pp. 91-94, 2020

In view of the above, the present invention aims to provide an apparatus used to measure the water retention of a sample, which has a simpler configuration than conventional apparatuses.

As a result of intensive research to solve the above problems, the inventors have succeeded in developing an apparatus that allows the water retention of a sample to be measured more easily than conventional apparatus by placing the entire apparatus on a balance and using a drying method to remove moisture from the sample.
More specifically, the suction of the sample at each stage of the dehydration process (suction is the force with which unsaturated soil draws up water through capillary action) is measured with a small tensiometer. Because the entire measurement device is placed on a balance, the amount of dehydration can be obtained from the loss in weight, making it possible to simultaneously obtain the suction and amount of dehydration, which are parameters necessary for water retention tests. As the entire device is small enough to be placed on a balance and has a simple configuration, it has advantages over conventional testing methods in terms of the freedom of where to perform the water retention test and the cost of introduction.
The present invention was completed based on the above findings.

The present invention relates to the following (1) to (7).
(1) A water retention measuring device characterized in that a cylindrical container for storing a test sample and a pressure gauge for measuring the suction of the sample are set on a balance, and the suction and weight are measured simultaneously. (2) The device described in (1) above, in which the sensor for measuring the suction of the sample in the cylindrical container is made of ceramics. (3) The device described in (1) above, which is equipped with a heat source for drying the sample in the cylindrical container. (4) The device described in (1) above, which is equipped with a water-adding device using a Mariotte siphon for saturating the sample in the cylindrical container. (5) The device described in (1) above, which is connected to a data recording device for recording the weight and pressure of the sample in the process from drying to saturation of the sample in the cylindrical container. (6) The device described in (1) above, which is connected to a device for controlling the on/off of a heat source for drying the sample in the cylindrical container and the on/off of a water-adding device for saturating the sample. (7) A method for measuring the water retention of a sample, characterized in that it includes a step of drying a sample saturated with water, and a step of creating a moisture characteristic curve. In this specification, the symbols "to" indicate a numerical range including the values on either side of the symbol.

The present invention makes it possible to easily carry out the water retention test. The introduction cost of the test device is cheaper than that of the conventional test method.
Furthermore, by creating a moisture characteristic curve from a water retention test using the water retention measuring device, it is possible to convert suction data into volumetric moisture content. Here, the "moisture characteristic curve" refers to the relationship between soil matric potential and moisture content. Matric potential indicates the magnitude of the force that attracts soil moisture into the gaps between soil particles due to capillary action and adsorption. In other words, when a tensiometer is installed in a farm field, etc., by conducting a water retention test on the soil at the installation location, it is possible to convert suction at that location into volumetric moisture content, and the tensiometer functions as a moisture content measuring device.

An example of the outline of a water retention tester is shown below. 1 to 5 are the main body of the water retention tester, 6 is a balance that measures changes in weight, 7 to 9 are water adding equipment, 10 is a heat source, and 11 to 13 are devices that record changes in weight and pressure. The data recorder 12 is connected to the tensiometer 5 and records changes in pressure. The heat source 10 and solenoid valve 8 are controlled by the program relay 13 with the value measured by the tensiometer 5. The changes in weight from drying to water absorption are recorded by the PC (personal computer) 11. The results of measuring the pressure head and volumetric water content using the water retention measuring device of the present invention (moisture characteristic curve) are shown below. The vertical axis is the volumetric water content (%), and the horizontal axis is the pressure head ( _cmH2O ). The hysteresis and scan curve of the moisture characteristic curve are shown here, excerpted from "Methods and Explanations of Testing Soil Materials (First Revised Edition) - Volume 1 of 2 - Chapter 7 Soil Water Retention Test (p184)". The vertical axis is the common logarithm of the matric potential (kPa), and the horizontal axis is the moisture content (%) (volume moisture content %). 1 shows a moisture characteristic curve created using a water retention measuring device according to the present invention, in which the vertical axis represents the common logarithm of matric potential (kPa) and the horizontal axis represents volumetric moisture content (%).

Hereinafter, an embodiment of the present invention will be described.
The first embodiment is a water retention measuring device (hereinafter also referred to as the "water retention measuring device according to this embodiment") that is characterized by having a cylindrical container for storing a test sample and a pressure gauge for measuring the suction of the sample set on a balance, and measuring the suction and weight simultaneously.
Here, the sample refers to a material that forms granules or aggregates, such as soil, sand, and adsorbents. The material of the cylindrical container for storing the sample is not particularly limited and may be any material, but examples include metals such as stainless steel and resins such as acrylic. When collecting undisturbed soil (undisturbed sample), the cylindrical container is directly driven into the soil with a hammer, so a sturdy stainless steel container is preferable. The measurement device is designed to allow the setting of commercially available stainless steel sample cylinders for soil testing.

The pressure gauge that measures the suction of the sample (the force that pulls soil water held by adsorption, capillary force, osmotic pressure, etc. into the atmosphere) is a measuring instrument called a tensiometer, and since it needs to be installed on a balance, it is preferable that it is small in size, and a commercially available product may be used. Here, the tensiometer is a measuring instrument for measuring the suction of soil water in the unsaturated zone, Ψ m {\displaystyle \Psi _{m}}. This measuring instrument has a porous ceramic (porous cup) as the sensing part, and is connected to a pressure measuring device by a pipe, etc., and the area from the porous ceramic (porous cup) to the pressure gauge is filled with water.
Tensiometers are primarily buried in soil, and the soil water in contact with the porous ceramics (porous cup) is balanced with the water potential inside the tensiometer. When soil water decreases due to evaporation, etc., the matric potential of the soil water decreases, and the pressure inside the tensiometer also decreases. When moisture is added to the soil, the matric potential increases, and the pressure inside the tensiometer also increases. In this way, the pressure of the tensiometer represents the matric potential of the soil water, and it is a measuring instrument that can grasp the behavior of soil moisture.

The sensing part of the tensiometer is a porous ceramic (porous cup), but the material is not limited to ceramics. Any porous material can be selected taking into account the air infiltration value. Examples include glass filters and sintered metal filters. However, ceramics are easy to obtain and process. The air infiltration value here is a value that indicates how difficult it is for air to pass through the filter, and is expressed as 50kPa, 100kPa, etc.

In this embodiment, a drying means is used as a means for removing moisture from the sample, instead of the conventional means of gradually dehydrating the water in the sample by increasing or decreasing the pressure or gravity on the sample. Air drying (natural drying) can be selected as a drying means, but dehydration can be more efficient if a heat source is provided. Therefore, the water retention measuring device according to this embodiment may further be equipped with a means for drying the sample. Considering that many resins are used in the material of the measuring device, it is preferable that the heat source be a device that does not become too hot. The measuring device may use a heating device such as a ceramic heater that is screwed into a light bulb socket. These heating devices are widely available commercially for use in raising reptiles.

The water adding device uses the principle of the Mariotte siphon, and although its structure is simple, it is capable of dripping at a constant flow rate. The Mariotte siphon is a water adding device that achieves a constant flow rate based on Torricelli's theorem. This water adding device is designed so that a tube is passed through the plunger of a plastic syringe and the position at which it is open to the atmosphere can be adjusted. The bottom end of the tube inserted into the syringe is the position open to the atmosphere, so there is a pressure difference between the height of the nozzle outlet and the bottom end of the tube. This makes it possible to drip at an almost constant flow rate regardless of the amount of liquid in the syringe. Lowering the bottom end of the tube reduces the pressure difference and slows the flow rate, and raising the position of the bottom end of the tube speeds up the flow rate.

The balance on which the water retention measuring device is installed measures the increase or decrease in moisture as weight, and does not necessarily need to be one that can automatically record measured values. The same goes for the pressure value of the tensiometer. It is sufficient that something that can display the tensiometer pressure value, either directly or indirectly, is connected. The tensiometer pressure and the weight at that time are recorded at the recording time planned in advance by the person measuring. Here, "indirectly" refers to cases where the indicated value is output as a current value or voltage value rather than a pressure value. These are converted to pressure values using a calibration formula.
However, in measurements that take time, it is tedious for the person making the measurements to record each and every measurement. Therefore, a PC (personal computer) or data collection device can be connected that can record the weight data from the balance and the pressure data from the tensiometer at any set interval. The data recording time for both is set to the same time. Many of the electronic balances available on the market in recent years have a function that allows weight data to be output externally. Similarly, many recording devices that can easily record the output data from a tensiometer are also available on the market.

The water retention measuring device according to this embodiment dries the target sample by air drying (natural drying) or with a heat source, and uses a hydration device for wetting. The increase or decrease in weight is measured with a balance, and the increase or decrease in pressure is measured with a pressure gauge, but this series of operations does not necessarily need to be performed automatically. However, in measurements that take time, it is tiring for the person performing the measurements to operate the heat source and hydration device one by one. Therefore, a device can be connected to perform this series of operations automatically. The dehydration process is performed by drying, but when the set pressure is reached, the heat source is stopped, and water addition from the hydration device begins at the set timing. When saturation is reached, water addition from the hydration device is stopped. The power supply for the heat source and hydration device required from this dehydration to saturation are connected to a programmable relay and controlled.

The water retention measuring device according to this embodiment is characterized by its extremely simple configuration, and its applicable range is within the range that can be measured by a tensiometer, including suction (negative pressure: pF3 or less) and pressure head of approximately 0 to -800cm (up to pF2.9). With regard to pressure head, 0cm indicates saturation, and negative values indicate unsaturation. The more negative the value, the drier the condition. The common logarithm of the absolute value of this pressure head is expressed in pF.
Here, as an example of a method for testing water retention, there are several methods that are explained in "Methods and Explanations of Soil Material Testing, by the Geotechnical Society Indoor Testing Standards and Criteria Committee," in which soil is dehydrated in stages using the sand column method, suction method, pressure method, and centrifugal method, and the mass at each stage is measured to determine the amount of water retained at each stage. Each dehydration method is explained below.

Sand pillar method
The sample is placed on a sand column with a constant free water surface of 0 cm to -100 cm (~pF2) and drained until equilibrium is reached.
Suction method
Water with a constant negative pressure (0cm to -200cm (up to pF2.3)) is brought into contact with the water in the sample and allowed to drain until equilibrium is reached. Depending on how the negative pressure is applied, there are two methods: the "decompression method" which uses a vacuum pump, and the "head method" which adjusts the height of the water surface in the drainage burette.
Pressurization method
0cm to -15,000cm (up to pF4.2) Positive air pressure is applied to the water in the sample and the water is allowed to drain until equilibrium is reached. There are two methods: the "pressure plate method" which uses a ceramic plate, and the "pressure membrane method" which uses a cellulose membrane.
Centrifugation
0 cm to -15,000 cm (up to pF4.2) The sample is placed in a rotating drum and rotated at a constant speed to apply a constant gravity and allow the water to drain until equilibrium is reached.

From the above explanation, the water retention measuring device according to this embodiment can be used for a test method that is intermediate between the suction method and the pressure method in terms of its range of application, but it is characterized in that drainage is achieved by drying and water absorption is achieved by adding water, and a cylindrical container for storing the test sample and a pressure gauge for measuring the suction of the sample are set on a balance, and the suction and weight of the sample are measured simultaneously. This is a test method that can be realized because of the simplicity of this invention.

A second embodiment is a method for measuring the water retention of a sample, the method comprising the steps of drying a sample saturated with water and generating a moisture characteristic curve.
Conventionally, as a method for removing moisture from a sample saturated with water, the pressure or gravity applied to the sample is increased or decreased to gradually remove the water from the sample, as described above. In order to remove moisture from the sample using this method, a large-scale device is required. In contrast, according to the method of the present embodiment, the moisture can be removed from the sample by natural drying or drying using a simple heat source. The method of the present embodiment also includes a step of creating a moisture characteristic curve, which makes it possible to easily convert pressure into volumetric moisture content.

A more detailed explanation is given below. In the process of removing moisture contained in a sample such as soil, suction and weight are measured simultaneously. Next, water is added to the sample, and in the process of saturating the moisture in the sample, suction and weight are measured simultaneously. After measuring the suction and weight of the sample, the sample is dried and its dry weight is measured. The volumetric moisture content at each stage is calculated based on the measured sample weight and the dry weight value at each stage. A moisture characteristic curve can be created by graphing the relationship between the measured pressure (plotted as pressure head) and the volumetric moisture content. Using this moisture characteristic curve, it is possible to convert the pressure into the volumetric moisture content.

Where this specification is translated into English and contains the singular words "a,""an," and "the," these are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.
The present invention will be further described below with reference to examples. However, these examples are merely illustrative of the embodiments of the present invention and are not intended to limit the scope of the present invention.

1. Water retention test 1-1. Apparatus settings The procedure for the water retention test is described below with reference to Figure 1. Ceramic 2, pedestal 4 and tensiometer 5 were degassed in advance, ceramic 2 was saturated, and the space from the ceramic to the sensor part of the tensiometer was filled with water.
The weights of the No. 1 sample cylinder and the No. 3 bottom plate were measured, and the tare weights were recorded. The No. 1 sample cylinder filled with the test sample was set on the No. 3 bottom plate, and the sample where the ceramics would be inserted was removed using a φ6mm pipe, and the holes were filled with cotton whose weight had been measured to prevent collapse. The weights of these were measured and recorded as the preliminary weights. The sample used in this example was black soil. Black soil is a blackish volcanic ash soil that can be collected from the surface layer of the Kanto Loam layer.
The test sample-filled sample cylinder 1 and bottom plate 3 were immersed in a container of water to saturate the test sample, and then the weights were measured and recorded. In order to attach bottom plate 3 to base 4, vacuum grease was applied to base 4, and ceramics were inserted into the holes from which the cotton to prevent collapse had been removed, and sample cylinder 1 and bottom plate 3 were attached to base 4.
These devices (1-5) were placed on the balance, and the tensiometer 5 was connected to the data recorder 12 and the program relay 13, and the balance 6 was connected to the PC (personal computer) 11. Next, the Marriott syringe 7, the solenoid valve 8, the nozzle 9, and the heat source 10 were set, and the solenoid valve 8 and the heat source 10 were connected to the program relay 13. The program relay 13 was set so that the heat source was stopped when the pressure reached -60 kPa (approximately -610 cmH ₂ O), and the solenoid valve 8 opened and started adding water when the pressure reached -80 kPa (approximately -816 cmH ₂ O). Furthermore, the solenoid valve 8 was programmed to open for 6 seconds and close for 5 minutes, and to repeat this process slowly until the test sample was saturated. The series of operations by the program relay was used for automatic measurement.

1-2. Measurement To determine the relationship between soil suction and water content, soil samples to be measured are taken and "soil water retention tests" are carried out using various methods.
The purpose of the water retention test performed here is to determine the volumetric water content for a range of suction (negative pressure: pF3 or less) that can be measured with a tensiometer.
The results obtained by automatic measurement at regular intervals using the configuration shown in Figure 1 are referred to as continuous measurement results. In this example, the suction and sample weight of the test sample were measured continuously at regular intervals from the dehydration process when drying a saturated soil sample to the water absorption process when the dried soil sample is allowed to absorb water. The measurement results for the dehydration and water absorption processes were obtained as pressure head ( _cmH2O ) and volumetric water content (%).

The relationship between pressure head and volumetric moisture content obtained by continuous measurements from the dehydration process to the water absorption process (moisture characteristic curve) is shown in Figure 2. As is clear from Figure 2, the dehydration process and the water absorption process follow different paths, and this phenomenon in which the dehydration process and the water absorption process follow different paths is called hysteresis. In general, the average value of the volumetric moisture content during the dehydration process and the volumetric moisture content during the water absorption process for the same pressure head on the moisture characteristic curve is calculated, and this is applied to the calculation of the volumetric moisture content from the pressure head obtained by on-site measurements such as in a farm field, based on the relationship between the pressure head and the average volumetric moisture content.
In Figure 2, the average value of the volumetric water content for the same pressure head ( _cmH2O ) during the dehydration and absorption processes obtained by continuous measurements is shown by the dashed line. In this measurement example, the relationship between the pressure head and the average value of the volumetric water content can be expressed by the following approximation formula:
Here, y is the volumetric water content (%) and x is the pressure head ( _cmH2O ).
By inputting the pressure head obtained by on-site measurement in a field or the like into this approximation formula, the volumetric water content at the measurement depth can be calculated.

Figure 3 is an excerpt from "Methods and Explanations of Soil Material Testing (First Revised Edition) - Volume 1 of 2 - Chapter 7 Soil Water Retention Test (p184)" entitled "Hysteresis and Scanning Curve of Moisture Characteristic Curve". It shows that there is hysteresis during the drainage process and the water absorption process. Figure 4 shows the moisture characteristic curve (relationship between matric potential and volumetric water content) obtained by the water retention measuring device of this embodiment (device with the configuration shown in Figure 1). For comparison with the excerpt from the literature shown in Figure 3, the measurement results shown in Figure 2 (results measured by the water retention measuring device of this embodiment) are converted to the common logarithm of the matric potential (kPa) on the vertical axis and the volumetric water content (%) on the horizontal axis and plotted. From the comparison of the shapes of the graphs, it can be seen that hysteresis curves are shown during the drainage process and the water absorption process, even when measured by the water retention measuring device of this embodiment, as in Figure 3.

This invention is expected to be of great use in fields such as agriculture, disaster prevention, and research.

1 Sample cylinder 2 Ceramics 3 Bottom plate 4 Pedestal 5 Tensiometer 6 Balance 7 Marriott syringe 8 Solenoid valve 9 Nozzle 10 Heat source 11 PC (personal computer)
12 Data recorder 13 Program relay

Claims (7)

A water retention measuring device that is characterized by simultaneously measuring suction and weight by setting a cylindrical container for holding the test sample and a pressure gauge for measuring the suction of the sample on a balance. The device according to claim 1, in which the sensing part for measuring the suction of the sample in the cylindrical container is made of ceramics. The apparatus of claim 1, further comprising a heat source for drying the sample in the cylindrical container. The apparatus of claim 1, which is provided with a water supply device using a Mariotte siphon to saturate the sample in the cylindrical container. The apparatus according to claim 1, which is connected to a data recording device that records the weight and pressure of the sample in the cylindrical container during the process from drying to saturation. The apparatus according to claim 1, which is connected to a device for controlling the on/off of a heat source for drying the sample in the cylindrical container and the on/off of a water adding device for saturating the sample. A method for measuring the water retention capacity of a sample, comprising the steps of drying a sample saturated with water and constructing a moisture characteristic curve.