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The working principle of soil temperature and humidity sensor monitoring soil moisture

by:Rika Sensors     2021-11-19
The working principle of soil temperature and humidity sensor monitoring soil moisture

Soil is one of the natural resources that humans rely on for survival, and it is the basis for plant growth and development. The state of the soil environment is closely related to the survival and reproduction of human society. In addition to hydroponic plants, soil moisture is the main source for plants to absorb water. The dissolution and absorption of various fertilizers in the soil, the decomposition and transformation of organic matter are also closely related to the content of soil moisture.

The measurement standard of soil moisture is soil moisture content, that is, the percentage of soil weight by moisture weight. Measuring soil water content and understanding the needs of crops for water are of great significance to agricultural production. The current methods for measuring soil moisture include drying weighing method, tensiometer method, resistance method, neutron method, r-ray method, standing wave ratio method, optical method, time domain reflectometry (TDR) and high frequency oscillation method ( FDR) and so on. Among them, the more common and simple methods are the drying method and the resistance method.

Drying method:

Drying method is a very common method for measuring soil moisture, and it is also a standard method. Specifically: Obtain a certain amount of soil from the field, then put it in an oven at 105°C and wait for it to dry. The standard of drying is that the weighing is constant before and after twice. The moisture lost after drying is the moisture content of the soil. The calculation formula is soil moisture contentu003dW/M*100%, M is the weight of soil before drying, and W is the weight of soil moisture, that is, the difference between M and the weight of soil after drying M’.

Resistance method:

The resistance method uses the resistance of gypsum, nylon, glass fiber, etc., which is related to their water content. When these intermediates plus electrodes are placed in moist soil, and then after a period of time, the water content of these things reaches equilibrium. Due to the relationship between resistance and moisture content, we have previously calibrated a certain corresponding relationship between resistance and percentage, and then we can use these components to obtain moisture readings within the suction range of 1 to 15 atmospheres.

The value measured by the drying method is accurate, but it has the disadvantage of poor real-time performance. If we want to measure quickly, we can consider the soil temperature and humidity sensor. The soil temperature and humidity sensor time-domain reflectometry (TDR) or high-frequency oscillation method (FDR) two monitoring principles can provide more accurate soil moisture content in real time.

TDR method:

The Chinese name of TDR is Time Domain Reflectometry, which is a soil moisture measurement method developed in the 1980s. This method is very common in foreign countries. In China, it has only been introduced in recent years, but it has been paid attention to by various departments immediately after its introduction. TDR time-domain transmission is a common way to quickly monitor soil moisture. The principle is that the waveform on a mismatched transmission line will be transmitted, and the waveform at any point on the transmission line is the superposition of the original waveform and the transmitted waveform. The response time of TDR principle equipment is about 10-20 seconds, which is suitable for mobile measurement and fixed-point monitoring.

TDR has strong independence, and its results are basically not affected by environmental factors such as soil type, density, and temperature. In addition, there is another important point: TDR can measure soil moisture under freezing conditions, which is unmatched by other methods. In addition, TDR can monitor the soil water and salt content at the same time, and there is almost no difference between the results of the two measurements. The accuracy of this measurement method is evident.

FDR method:

The Chinese name of FDR method is high frequency oscillation method, and the birth time of FDR is slightly later than TDR. At first, because the TDR equipment was expensive, foreign companies such as AquaSPY and Sentek developed FDR in order to find a simpler method than TDR to measure the dielectric constant of the soil. FDR is not only cheaper than TDR, but also has a shorter measurement time. After a specific soil calibration, the measurement accuracy is high, and the shape of the probe is not limited, and multiple depths can be measured at the same time, and data acquisition is easier to achieve.

The FDR method adopts the working principle of electromagnetic pulse, and measures the apparent permittivity of the soil according to the frequency of electromagnetic wave propagation in the medium, so as to obtain the volumetric water content of the soil.

Soil moisture monitoring is now not only applied to agricultural soil moisture detection, many engineering buildings such as railways, highways, hydropower stations, channels, houses, etc., soil moisture status is also an indispensable information. When we need accurate data and sufficient time, we can choose the traditional drying method. If you want to know the soil moisture in real time, you can choose the soil moisture sensor (TDR/FDR).


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