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Application of WET portable soil moisture, temperature and conductivity measuring instrument in space planting
"Small instrument, large application" series
WET portable soil moisture, temperature, conductivity meter
Application in space planting
The so-called "the terracotta warriors are not moving, the grain and grass are the first", the future of China's space industry must vigorously develop the space station, and must consider the issue of feeding in space. In the Shenzhou 11 spacecraft launched by our astronauts on 10.17 last year, we brought the seeds of lettuce and the specially-designed vegetable incubator, and for the first time in the docking of Tiangong No. 1 as a “space vegetable farmerâ€.
In order to better understand the "soil" moisture and nutrient status during the growth of vegetables, the Aerospace Research Center has selected a cost-effective WET portable soil moisture, temperature and conductivity meter provided by our company. In the incubator unit, the measurement data is recorded in real time, labeled as "space-based"; at the same time, the recorded data is simultaneously measured at the Earth's astronaut work center, labeled "Foundation."
The water in the soil is mainly affected by three kinds of forces, gravity, adsorption and capillary force (surface tension). There are two main forces for maintaining soil moisture, namely adsorption force and capillary force. Gravity mainly affects the infiltration rate of water.
On the earth, when the soil is watered, the water is subjected to gravity, and at the same time, it is subjected to adsorption force and capillary force, which will gradually decrease the soil water content. Therefore, a curve of the water content of the foundation in the figure will appear.
When the same experiment was carried out in space, the main influence became the adsorption force and surface tension due to the loss of gravity due to soil moisture. These two forces will gradually homogenize the water in the soil, so the moisture content around the probe will gradually increase from the beginning of watering, and to a certain extent, a water balance will be reached. Shown in the figure is a process of 8-9 days.
Since the instrument itself is metal, when the probe emits electromagnetic waves, a certain polarity will appear around the probe, and then the adsorbed moisture will gather around the probe to form a water film, so that the measured water value of the probe is similar to the soil saturated water. Content (about 50% - 60% or so). This balance will remain for a certain period of time unless it is re-irrigated or if the amount of evaporation is large. The figure shows a 9-18 day curve process.
When water is re-watered, the water increases sharply, breaking the original water balance, and there will be a high peak and begin to gradually decrease. This is consistent with the curve measured by the foundation. The figure shows the curve process of 19-27 days.
At present, there are mainly three methods for measuring soil moisture content: time domain reflection or frequency domain reflection (high frequency electromagnetic wave method), capacitance method and drying method.
The first type is to emit high-frequency electromagnetic waves, and the moisture content is determined by receiving different reflected electromagnetic waves or electromagnetic waves reflected back for different times.
The second is the emission current, which determines the change in capacitance in the soil moisture to define the moisture content.
The third type is the drying method, which is the absolute water content value. The absolute moisture content of the soil is determined by weighing the soil before drying and after drying, which is a method of calibrating the first two instruments.
For experiments in space, the third method requires measurement of soil weight, so it cannot be achieved. When the first two are compared, the first one is less affected, so it is recommended to use an electromagnetic wave method to measure.
The determination of soil moisture content is more often observed in the change curve, rather than the absolute moisture content value, so it is recommended to consider the effect of weight loss on moisture when experimenting in space. At the same time, with reference to the measured conductivity values, this has great reference significance for space planting.