Humidity chart-microwave measurement method to obtain two-dimensional and three-dimensional humidity distribution map

summary
Using existing microwave remote sensing heads, it is easy to make the non-destructive side moisture distribution and approximate the Z-level of the humidity value. Since these measurements are a lot more than 2D but not yet true 3D images of moisture distribution data. The true depth level moisture profile is recommended using a new endoscopic microwave nozzle.
Key words
Humidity chart, humidity distribution, humidity profile
1 Introduction
In many cases of industrial and civil engineering, the investigation of water content is very important. In fact, the moisture distribution is not uniform because of the non-uniformity of the material and the adhesion between pores and water molecules. In terms of civil engineering applications, humidity distribution is an important information for architects and civil engineers. In order to eliminate water damage and leakage, a new microwave method of establishing a two-dimensional and preliminary three-dimensional humidity distribution map by a humidity diagram. The core of this new technology is a quick and easy method to measure moisture in the microwave using several types of reflected microwave remote sensing heads. Although conventional methods for moisture determination are affected by other mechanical, chemical and physical properties of materials, microwave moisture measurement based on it is possible to overcome these shortcomings. Microwave glue can be strong, and the interaction between the volume of electromagnetic waves and wet materials can be large enough to ignore the unevenness of the material or small enough to obtain good spatial resolution.
2 basic microwave principles
We must think that the dielectric constant of water is compounded at a higher frequency. It has a real part and an imaginary part
The maximum medium loss is about 20 MHz. This corresponds to a wavelength of about 15 mm in air, and about 5 to 10 mm in many materials. Therefore, a good measurement effect can be achieved at this frequency, but only if it penetrates the material slightly. This is why humidity measurement is not widely used at ultra-high frequencies. A large number of experiments show that the well-known ISM frequency of 2.45 GHz is a pretty good choice, because even if the wavelength is greater than about 10 times of 20 GHz, the dielectric loss is still high enough to be measured.
At 2.45 GHz, it is feasible to construct microwave antennas. They are small, easy to operate, and focus microwave radiation well. Focusing needs to increase the transmission depth in the material until the omnidirectional radiator up to several times the penetration depth at the same frequency and effective wavelength.
If the microwave head has no antenna, but the antenna interference area is like an open resonance, the influence range is about 20 cubic centimeters³ and the penetration depth can reach about 3 centimeters. Using these microwave basic principles to compare the results of surface and volume measurements, it is feasible to obtain a moisture distribution on the sample surface of the material and its core. To this end, two types of microwave sensor probes were developed: MOIST R for surface measurement and MOIST P for volume measurement.
In many cases, the reflection range should be the best choice based on the humidity measurement problem being solved, because it is related to the side of the sample to be measured. The reflection coefficient G is the ratio of the echo and the forward wave Figure 1 Reflectance method for measuring microwave humidity The design principle of the sensor probe is very simple. They contain a microwave source, a ring or reflector, separate echo and forward wave and some microwave detectors [5]. The use of radiating antenna structures, such as patch antennas, has a maximum recording depth of 30 cm. It can also detect the internal humidity value of thicker samples. Coupled with the influence of the nearby area, the antenna emits electromagnetic waves. For the patch antenna, it has a certain amount of air on the surface of the sample and has an influential impedance, so the input reflection coefficient G depends appropriately on the dielectric constant of the sample. Open linear resonance is suitable as a humidity sensing device because of its inherent planar shape. The extension of the antenna interference is the distance between the centers of oscillations of the opposite charge (maximum distance 1/2).
Due to the lower installation position of the triggering microwave structure in the probe, this part interacts with the sample through the air band. In the range of millimeters, the balance of small unevenness can be achieved. In a larger range, the electrical coupling of several such oscillating line segments can achieve a further average effect. For a frequency of 2.45 GHz, the effective average range is 30 to 60 mm in diameter. Figure 2 MOIST P, a far-field sensing probe for volumetric humidity measurement
Fig. 3 Impedance field sensing probe MOIST R for surface humidity measurement The highly symmetrical resonant structure prevents the independent oscillation of the sensor thickness of the sample thickness due to reflected microwave radiation. The penetration depth of the resonant probe without radiation is up to 3 cm.
3 Humidity chart
Using the sensor probe described above to measure humidity is not only a single point, but also a matrix measurement. In principle, images can be obtained from the measurement. In these images, the single-point humidity value is like a pixel in a computer drawing or photograph. The measurement must be as shown below, in the form of an array. The organization of MOIST series hygrometer is that the humidity value can be stored easily. In the figure, MP is an abbreviation of measurement point. Figure 4 Typical humidity measurement array combination
This new humidity test method is called the humidity graph method. In this method, the change value measured by a single point is not very important, because behind a large amount of data obtained in a short time, it is a statistical data. Figure 5 shows the humidity distribution in the xy plane made with a surface sensing probe.
Figure 5 The lateral distribution of volume-surface measurement (MOIST R) To obtain the humidity distribution in the Z direction, in addition to the damage method, sensor probes with different penetration depths can be used, such as MOIST R for surface measurement and MOIST P for volume measurement. Figure 6 The lateral distribution of volume-surface + volume measurement with existing types of sensor probes can be measured with two main depth ranges, which are interesting in many applications: Surface layer-depth 2 ... 4 cm Media layer-depth 10 ... 30 cm. A very important parameter is the selection of grid size in the humidity test. On the one hand, a very fine measurement grid will cause over-measurement. On the other hand, a rough grid will cause a significant statistical gap between the inhomogeneity and the incompatibility, especially if it is a reinforced concrete structure with a metal structure inside. In civil engineering applications, the mesh sizes of the non-reinforced materials shown below can be useful: In a large range, a summary test of 0,5 ... 1,0 meters
High-resolution measurement of leak detection 0,1 ... 0,25 meters An interesting application of the combination of surface measurement and volume measurement is the humidity survey of houses. It is often necessary to determine whether the humidity is in the center of the wall, the humidity comes from damage, or just find the humidity (such as condensed water) on the wall surface. Figures 7 and 8 are such tests. In the test, the calibration curve of the brick wall needs to be used. The parts where the surface humidity value is higher than the volume of the material will be marked. In addition, the surface humidity distribution shows a higher humidity area than the volume measurement, and more single-point humidity peaks can be found. This indicates that there is condensation on the surface of the wall. Figure 7 Humidity measurement of the house-surface Figure 8 Humidity measurement of house-volume In reinforced (concrete) buildings, each large piece of metal can be used as a reflector, causing interference in humidity measurement. The degree of interference depends on the location of the interference and the size of the reinforcement mesh. In general, if the diameter of the coupling area between the sensing probe and the sample and the wavelength of the material are smaller than the size of the steel grid, measurements can also be made in such materials. The valuations are as follows: At a measurement frequency of 2.45 GHz, the minimum grid size is 0,08 ... 0,1 meters. Figure 9 Humidity distribution of concrete wall
For concrete, there is no graded curve during the test. Therefore, the humidity distribution is obtained by relative measurement using the humidity index. Fig. 10 Leakage detection of an underground parking lot. The left half of Fig. 9 shows the added value area, which is equivalent to a reasonably higher humidity value than the normal area in the picture. The structure on the right is a picture of the combination of humidity and reinforcement. Figure 10 is a similar test conducted in an underground parking lot.
Three-dimensional humidity profile In the near future, when the MOIST system is introduced to the market, the problem of true depth humidity measurement will be mentioned. This requirement is also the reason for advancing an endoscope called MOIST ENDO, which is similar to a microwave nozzle. This nozzle is an interference field nozzle similar to MOIST R and is based on four λ / 2 linear resonances. The active area is about 3 cm long. The highest activity of the nozzle is located in the center of the resonant structure, at least the depth affects about 1.5 cm. MOIST ENDO is used with microwave-optimized guided plastic tubes to better contact materials and avoid air bands. Figure 11 Considering the depth level of the water profile using a probe probe

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