The calibration of the neutron soil moisture meter is the basis of the neutron measurement technique. The neutron method for determining soil moisture, although using the hydrogen nuclei of fast neutrons and the water in the upper soil, has a direct effect, but the measured result is the number of neutrons generated by fast neutrons after the slow completion of hydrogen nuclei. In order to obtain the upper soil moisture content, the relationship between thermal neutron count and water content must be clearly defined. This is a prerequisite for the determination of soil moisture using a neutron moisture meter. Circular Loom,Pp Woven Bag Circular Loom,Leno Mesh Bag Circular Loom,Woven Sack Loom Zhejiang Sanlong Universal Machinery Co.,Ltd. , https://www.sanlongmachinery.com
In general, the generated thermal neutron density has a certain linear relationship with the soil moisture content. Practice has also proved this fact. However, in practical applications, different instruments (mainly the structure of the probe and the location of the source are different), the physical and chemical nature of Shigeo (such as chemical composition, texture, weight, etc.), Affects the shape, intercept, and slope of the calibration curve. Therefore, strictly speaking, for each instrument and each type of soil, there is a need for a specific calibration curve, so as to obtain more accurate measurement results.
There are two commonly used calibration methods: indoor calibration method and field calibration method. Indoor calibration method is to collect a representative soil, according to experimental requirements, formulated into a certain volume, different volume moisture content of the bucket, and then use the instrument to determine each The number of thermal neutrons in the bucket and the relationship between the known volumetric water content and the corresponding thermal neutron count. The field calibration method is to select the measuring points of different soil moisture content in the field, measure the thermal neutron count with the instrument, and at the same time, collect the soil at the side points, determine the soil bulk density and water content by the drying method, and use the obtained data to make thermal neutrons. Count the calibration curve with the volumetric moisture content. The structure and physicochemical characteristics of soil vary widely. Indoor calibration curves are used in different soils, and soil moisture in the field is often more different. Therefore, to accurately measure field soil moisture, field calibration must be performed. In this study, the LZS plow neutron moisture meter was used to perform indoor and field calibration tests.
1. Instruments used for instrument and material tests The LZS-30, LZS-50, LZS-100 type neutron soil moisture analyzer (241Am-Be neutron source) developed by the Institute was used to measure the top moisture content of soil moisture. The used catheter was a commercial hard aluminum tube with an inner diameter of 5 cm and a wall thickness of 0.25 cm.
The test soil was yellow-brown soil and collected in dry farmland of this farm. It can represent the main dry soil in the southwestern hilly region of Jiangsu Province. The soil texture is silty clay loam soil. The weight density of the section is in the 2m soil layer except the tillage layer. About 1.6g/cm3, the texture is more consistent. Organic matter content is less than 2%.
II. Indoor calibration 1. Calibration of the specifications of soil hoppers The size of the soil buckets has a great relationship with the calibration. Generally, the concept of “Sphere of importance†is used. It is defined as: If the media and water outside the sphere are removed, in this range The resulting neutron flux density is equivalent to 99% of the neutron flux density obtained in infinity media. Also useful for the concept of the Sphere of influence of God, is a sphere that can achieve 95% of the neutron flux density. According to experiments, the radius of thermal neutron cloud spheres in soil media can be calculated by the following formula:
R=100/(1.4+loQv)cm
In the formula, R is the radius of “important ball restâ€, Qv is volumetric moisture content, and 1.4 is a constant. With reference to relevant data and theoretical calculations, cylindrical drums 100 cm in diameter and 100 cm in height are required. An aluminum duct is placed in the center of the bucket, sealed at the bottom, capped at the top, and suspended in a silicone bag to keep it dry. A tick mark is made every 10cm on the inner wall of the barrel and on the outer wall of the duct as an indication of earth loading.
2. Preparation of Soil Samples with Different Moisture Content The soil samples were collected from the field and were first crushed in the field and collected through an upper sieve of 5 mm pore size so as to obtain a more uniform water distribution when the water was added later. The soil samples were transported to the interior and thoroughly air dried to measure soil moisture using a soil moisture recorder.
It was originally planned to prepare soil samples with different levels of water content ranging from air-dried soil to near-field water holding capacity. The soil enthalpy weight should be the same as that of the field soil as much as possible. However, in the actual preparation process, it was found that the silty clay loam soil for testing can only be compacted to 1.39/cm3 in the case of moisture content in the dry soil. Taking into account the production conditions, the test weight of 1.39/cm3 was selected.
When formulating moisture soil samples at all levels, the following formula is applied:
(l) Volume of cast sub-soil
V=Ï€(R2-r2)h
Where: R is the inner radius of the bucket, r is the outer radius of the pipe, and h is the height of each soil loading.
(2) Each time the dry soil weight Ws
Ws=Vp(1+Q)
In the formula: p is the volume of soil per unit weight, and Q is the moisture content (weight) of the dry soil.
(3) The weight of water added to make the volumetric water content Qv
W=V(Qv-PQ)
In order to make the bulk density of the soil samples in the buckets uniform, 10 cm is used as a layer when loading soil, and each bucket is divided into 10 layers. According to the above formula, the quantitative dried soil is calculated, placed on a plastic film, and the soil is laid into a thin layer. The quantitative water is added in stages by spraying, and the mixture is fully mixed in batches, and then the barrel is packed. The soil in the 10 cm layer is all After the addition, compaction was done with a pad, so that the soil surface was exactly flush with the 10 cm mark. After that, it is stratified by this method till it is filled with 100cm. In this process, the soil samples after the mixing of the water are divided and the moisture content is measured by the drying method as the actual soil weight water content of the soil drum. After loading the soil, add a layer of gravel to the soil cover plastic film and a layer of gravel of about 3cm. Finally, add a lid to reduce the evaporation of water. The soil barrel is allowed to stand for 3-5 days until the moisture in the soil is fully balanced and the instrument is calibrated. .
3. Calibration of the instrument Before calibration, the probe was used to measure the depth of each 10cm depth from the soil surface to the bottom of each bucket to check the uniformity of moisture distribution. The results of the measurement showed that counts within the range of 20-7 oc are consistent with each other except that the counts within 1 scm above and below each level are affected by the interface. Therefore, determine the depth of 50cm for each bucket as a calibration point.
In the experiment, the LZS-30, LZS-50, and LZS-100 instruments were calibrated and measured. Before and after each calibration, the instrument was placed on a polyethylene plastic block and counted 5 times a minute for a total of 10 times. The average is used as the standard count for the probe. At the same time, the probe is lowered to the position of the measuring point of 50cm depth in each bucket and counted for 10 minutes. The average value is used as the corresponding moisture count. The ratio of the count rate of each measurement point was calculated from the average count value of each measurement point and the average value of the standard count, and was used as the calibrated neutron count to eliminate the influence of environmental and instrument drift. The counting ratio and the corresponding volumetric moisture content of each instrument were used for regression statistics, and a soil moisture calibration curve was made. The experimental results (Table 1) show that all three types of probes have a good linear relationship. With the exception of the LZS-100 model with a slightly larger error at high water content, it is 1.5% (volume water content), and the rest are all less than l%. The sensitivity of the LZS-30 model is 906 cpm per 1% volumetric moisture, 1480 cpm for the LZS-100 model per 1% volumetric moisture content, and 886 cpm for the LZS-50 model. 
It should be pointed out here that the LZS-30 type is a protective container for vinyl chloride and its silicone is a polyethylene protection device. The bucket used for calibration was prepared a year ago. In the two buckets of high water content mice, the soil bulk density and volumetric water content could not be kept constant. The reason is that the water content in the preparation of the soil is too high, and the mixture becomes agglomerate and it is difficult to load the barrels; Water, drums, and evenly spray the rest of the water should be added, compaction and volume. When calibrating the instrument, it was found that after a long period of time in the soil with high water content, the soil sample had a sink to achieve home and the volume had changed. Therefore, after the calibration measurement, samples were taken with a bulk density earth picker to determine the actual bulk density and the water-containing mice. From the measurement results (Table 2), the soil bulk density and water content of the two low water content upper barrels (No. 0 and No. 1) basically changed, while the high water content 1 was two soil barrels (No. 2 and No. 3). ) The change is greater. Therefore, the preparation method of soil buckets with water content needs further improvement. 
From the good linear correlation of the results obtained, it can be concluded that the sub-counts are closely related to the volumetric water content under conditions of uniform soil composition and texture. When the bulk density changes within the range of 0.130-0.143 g/cm3, the influence of the heavy change on the calibration is not significant.
Third, the field calibration Field calibration is in a representative field, the choice of profile is more consistent, natural 
Or a few points with a certain amount of water content difference established by artificial plus nuisance interception method, the stone simultaneously determines the neutron moisture meter count and field actual water content (drying method), in order to obtain the neutron meter count and The relationship between the amount of water, making a calibration curve. Two trials have been conducted in this experiment: One time at the beginning of the water test, two inspection sections were dug, mainly to observe the soil texture and structure of the experimental plots. At the same time, according to the natural development level of the section, the soil sample was taken with a cylinder picker and the bulk density and moisture content were determined within the treasure. When the buried water pipe was drilled, the sample was taken in layers and the soil moisture was measured by a drying method. Insert the catheter in the same well, and after two days, the catheter and the surrounding soil will be stabilized. Use the neutron moisture meter Han Qizhi and the sample water to measure the same number of hot towels at two different points. This will save time and workload. The obtained results show that although there is a certain correlation between the counting rate and the volumetric water content after statistical analysis, the error is larger. This may be due to errors in the process of soil sampling and buried pipe. Another time the Taoyuan irrigation experiment was conducted, because the catheter was stable with the surrounding soil after a certain period of time. According to the count distribution measured by the neutron instrument in each catheter, the points with different count rates were selected. To less disturb the soil around the catheter, Three depths of soil samples were taken from the same depth of the pipe 50 cm around the pipe. Soil moisture content of the soil was measured with a drying rake. The volumetric moisture content was calibrated based on the results of the cross-sectional volumetric and volumetric loading tests, and the thermal neutron count and volumetric content were calculated. The results obtained are also relevant, but the points are scattered and the errors are large. The reason may be the spatial variability of soil moisture content and the impact of plant root drift. The differences among the three repetitions of each point are larger. 
The instrument used for the third calibration was the LZS-30 type neutron soil moisture meter, which was measured in a high-yield cultivated wheat field test area. At the end of the test, according to the results of the last water measurement, the measuring points of different water content were selected at each conduit, and the soil was taken at the same depth and placed on the catheter at the same depth at l0cm above and below the measuring point, and the soil was measured by the drying method. Weight and weight moisture content. Repeat 3 times for each point. In soils with known bulk density, it is also possible to measure only the amount of water. From the resulting different count ratios and volumetric water content, a standard curve was obtained by regression analysis (see figure). The linear equation is:
Re=5.173Qv+0.924
The correlation coefficient is 0.994 and there is a good linear relationship.
Comparing the results of indoor calibration and field calibration (Figure 3), we can see that the two calibration curves are parallel lines with the same slope, indicating that the performance of the instrument is accurate and reliable. The intercept of the field calibration curve is higher than the indoor calibration curve, indicating that the soil conditions in the field are different from the soil conditions in the interior soil bucket. The artificial soil in the room is basically a uniform medium. The soil between the soil is more complex and heterogeneous, with a bulk density of about 1.6 g/cm3. One is reflected in the water standard error. The average standard error of the indoor calibration is plus or minus 0.008, the relative standard error is plus or minus 0.004, the maximum error of the soil analyzer is plus or minus 0.018, and the corresponding standard error of the field calibration is plus or minus 0.009. Positive and negative 0.011 and positive and negative 0.029. Although the latter's error is slightly larger, but from the point of practical application, there is also considerable accuracy, and even more representative of the actual situation in the field, so it has practical value.
IV. Concluding remarks From the process of exploring the field calibration method, it can be seen that the key to technology lies in the setting of the catheter. When setting up, the surrounding soil should be less disturbed, and the catheter and the surrounding soil should be stabilized after being set for a long time. When soil sampling is used to dry and measure water, the soil sampling point should be as close as possible to the surrounding area of ​​the pipe to avoid errors due to the distance and the spatial variability of the soil moisture. The field calibration method is generally easier than the method of making the soil bucket in space. Easy to do. The indoor calibration is more accurate, such as measuring only the soil moisture variation, can also be applied. At the same time, Bao internal calibration is a method to test the performance of the instrument. Therefore, both have their own characteristics and cannot replace each other.