For agricultural cultivation, it is necessary to understand the moisture content of the soil in the cultivated land. When the technology was underdeveloped in the past, it was generally observed with the naked eye, and the hand touched these simple sensory methods. With the development of society, some precision instruments were successively developed and soil analysis was conducted. Instruments, soil moisture testers, etc., are applied to agricultural production and make great contributions to the modernization of agricultural production. When these instruments were researched, they encountered a lot of problems, but they were solved by our scientists one by one, especially in the solution of instrument calibration. Now we will describe in detail how the soil moisture tester performs calibration work.
The principle and composition of the soil moisture tester: The soil moisture tester uses the FDR principle, uses a stable and reliable high-precision sensor, according to the reflection of the electromagnetic wave emitted by the detector in different dielectric constant substances, to calculate the water content of the measured object. . The measurement range of the device on the market is generally 0 to 100%, and the accuracy is generally ±2%.
The soil moisture tester consists of four parts: a probe (sensor), a detection system, a hand-held signal processing and display system, a communication cable, and an accessory software system, as shown in Fig. 1.

Fig.1 Block diagram of soil moisture tester

The working principle of the soil moisture probe (sensor) consists of a waterproof chamber containing electronic components and four stainless steel needles connected to one end. The probes are inserted directly into the soil. The cable at the end of the probe is connected to a suitable voltage source and an analog output signal. A moisture probe is a device that uses analog signal processing to continuously produce electrical signals and soil properties expressed as voltage ratios. Specially designed transmission lines generate high frequency signals and measure soil parameters. The impedance of the transmission line varies with the impedance of the soil. Impedance includes apparent dielectric constant and ion conductivity. The frequency of the selected electrical signal minimizes the effect of ion conductivity, so that the transmission line impedance changes almost exclusively depending on the change in the dielectric constant of the soil. These changes produce a voltage standing wave that increases or decreases the voltage produced by the crystal oscillator as the medium around the probe changes. The output voltage is 0 to 1V (DC) indicating that the dielectric constant of the soil is between 1 and 32. This range represents the volumetric moisture content of general mineral soils of 0.5m3/m-3.
Signal processing and display systems use microprocessors for control, calculation, and storage. The 16-bit precision AD converter improves the measurement display accuracy and displays the measurement results on a 32-bit LCD display. When measuring soil moisture, the meter can directly display the volumetric moisture content. The humidity probe uses high frequency technology to measure the relative permittivity, Ka, of the measured medium. According to the different moisture content of the soil corresponding probe detection of different analog signals (millivol voltage signal), the inorganic soil was rigorously calibrated, and then the standard curve (moisture content - millivolt voltage signal) as a standard curve, after data processing After that, the moisture content is displayed directly.
The principle of calibration of the soil moisture tester is conceived from the composition and working principle of the device, and the accuracy of the device can be calibrated by calibrating the accuracy of changes in the power of the device. However, the accuracy of the device power signal can only explain the accuracy of the internal signal processing method of the device, and cannot fully explain whether the power signal truly reflects the accuracy of the actual change of the actual soil moisture, so the calibration power is used to calibrate the device. The method is not exactly correct. Then, the soil moisture content measured by the device has to be compared with the moisture content measured by the dry method. That is, the device is calibrated using a comparative method to determine the accuracy of the measurement result of the device.
The calibration data of the soil moisture tester is processed using the comparison method to calibrate the equipment. The first consideration is whether the data is linearly related. If it is a linear correlation, a correction curve and correction formula can be calculated.
The correlation is a non-deterministic relationship, and the correlation coefficient r is the amount of linear correlation between research variables. The property of the correlation coefficient is |r|≤1, that is, the correlation coefficient r takes a value between -1 and 1. When r=0, the two variables x and y are said to be irrelevant; when |r|=1, x and y are completely related. At this time, x and y have a linear function; |r|<1, x Changes cause partial changes in y, the greater the absolute value of r, the greater the change in x caused by the change in Y; |r|>0.8 is called highly correlated, when |r|<0.3, it is called low correlation, Others are moderately related. The formula for calculating the correlation coefficient is:

The formula for calculating the correlation coefficient

In this paper, take 10 soil standards, first use the soil moisture tester to measure the results (x), and then use these dry samples to measure the results, measure the results (indicated by y), calculate the error. The measurement results are shown in Table 1.

Table 1 Test Results of Soil Moisture Tester and Drying Results

Substituting the data in Table 1 into equations (2), (3), and (4) yields

The linear relationship between x and y two variables is moderately related

The linear relationship between x and y variables is moderately correlated, but it is close to low correlation. That is to say, soil moisture is measured by a soil moisture tester and a drying method. The correlation between the results of the two methods is moderate, so no correction formula or correction line can be given for the measurement data of the equipment. When calibrating, only a range of data calibration can be performed by selecting the range that the user most needs to use, and data processing correction is performed according to the error.
Through the description of the principle and structure of the soil moisture tester, the calibration method was studied from the theoretical and actual measurement results. The method of calibrating the soil moisture tester is to compare the measurement result with the drying method measurement result, calculate the error, and then make correction and traceability according to the error result.

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