Mainly divided into non-contact online detection and contact online detection
Non-contact is further divided into 1. Microwave online detection? 2. Infrared online detection? 3. Neutron online detection (according to There are two types of installation: contact and non-contact)
Neutron:
Non-contact measurement (belt conveyor installation)
- Still in the exploratory stage, the technology is not yet Mature
-Complex structure, long response time
-Poor performance and accuracy
-Requires radioactive density meter
-Physical calibration Complex
-It is difficult to eliminate the influence of water vapor
-Changes in the crystal water and hydrocarbon components of the material will affect the measurement results
-If it is close to the mixer Exit installation may bring radioactive hazards to on-site workers
Contact measurement (tank installation)
-The outer wall of the probe contacts the material, causing serious wear (possible radioactive leakage)
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- Materials tend to stick to the probe (not sensitive to moisture changes)
- Materials flow slowly near the probe (not sensitive to moisture changes)
-Measurement lag Very sensitive (not conducive to automatic control of mixture moisture)
- Physical calibration is very complex
- Changes in material crystal water and hydrocarbon components will affect the measurement results
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To sum up in one sentence, neutron online water testing: too expensive and has safety risks! !
In fact, the main non-contact online water measurement methods are microwave and near-infrared. Both have their own characteristics. The introduction is as follows:
Microwave:
Principle : The working principle is that there are two sensors, sending and receiving. The sensor emits microwave signals to the measured medium through the microwave transmitting and receiving system. When the electromagnetic energy passes through the measured medium, the water molecules in the medium will be in the medium due to its polarity. Absorb energy from electromagnetic fields. The energy absorbed by water molecules maintains a linear relationship with the content of water molecules. Different electromagnetic frequency bands, different moisture contents and media have different characteristics. By simultaneously emitting multiple spectrums of different frequencies, combined with data models and special The algorithm can determine the moisture content and density of the measured medium.
Advantages: Can penetrate;
Disadvantages: The thickness of the material layer has an impact. It is usually suitable for situations where the dielectric constant of the material is much smaller than the dielectric constant of water, so it should be used in situations where You must see it clearly and compare it with the dielectric constant table, otherwise you will spend a lot of money.
Near infrared:
Principle: Specific infrared wavelengths can be absorbed by the O-H bonds in water molecules Absorption, when a material is irradiated with infrared light of these specific wavelengths, the water contained in the material will absorb part of the energy of the infrared light. The more water there is, the more it absorbs. Therefore, the reduction in reflected light can be measured to calculate the moisture of the material.
Since the reflectivity of materials to infrared rays varies due to their different absorption characteristics and stray characteristics, if only the absorption wavelength of water is used, the surface state, color, structure and other factors of the material will interfere with the moisture measurement; To this end, the three-wavelength method is used, that is, one wavelength that is strongly absorbed by water (measurement wavelength) and two wavelengths that are not too strongly absorbed by water (reference wavelength). The energy ratio of the reflected light at these three wavelengths is detected and calculated. This can eliminate the interference of other factors on moisture measurement. ?
The working principle of OMM3001 moisture meter is shown in the figure below. The infrared light emitted by the light source passes through the filter on the spectroscopic disc and is directed to the material to be measured through the reflector. Different filters on the spectroscopic disc only allow infrared light of a certain wavelength to pass through. The spectroscopic disc is driven by a motor at high speed. Rotate so that the infrared light of the measurement wavelength and the reference wavelength is alternately emitted to the material to be measured; part of the infrared light is absorbed by the material and part is reflected to the concave condenser, which is received by the photoelectric sensor and converted into an electrical signal, which is processed by the subsequent circuit to calculate Remove the moisture content of the material.
Advantages: safe, harmless to human body; independent of thickness; fast response, high measurement accuracy, easy to realize automatic control of mixture moisture; simple calibration and convenient maintenance; equipped with equipment to eliminate the influence of steam and dust Devices and measures
Disadvantages: Requires a purified air source
OMM3001 Advantages: Brand new structure, second generation sensor (the market generally stays at the first generation, including imported ones),
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High-speed optical modulation, cast aluminum probe, full digital signal processing, suitable for high temperature (air cooling)
Contact? Basically it is the resistance online detection method?:
- The electrode is in direct contact with the material and suffers heavy wear
-The measured resistance is greatly affected by other factors and has poor selectivity for moisture