Interface measurement:
Guided wave radar can measure the Interface, such as the oil-water interface, the interface between liquid and slurry, etc. This function is very important in petrochemical, chemical and other industries, especially in multiphase liquid systems to measure the height of the boundary between different media. The following details explain its principle, implementation mode and working condition requirements.
1. Basic principle of interface measurement
Guided wave radar measurement interface is based on dielectric constant difference and electromagnetic wave reflection principle.
1. Electromagnetic wave reflection mechanism:
• The electromagnetic wave emitted by the guided wave radar will partially reflect when it encounters different media. The strength of this reflection depends on the difference in permittivity between adjacent media.
• A medium with a high dielectric constant reflects a stronger signal. For example, the dielectric constant of water (≈80) is much higher than that of oil (≈2~4), so the reflected signal is very obvious at the oil-water interface.
2. Signal distribution:
• Electromagnetic waves first encounter the liquid surface (for example, the top of the oil layer), where the first reflection occurs.
• The remaining electromagnetic wave continues to propagate until it reaches the oil-water interface, producing a second reflection.
• After receiving the two reflected signals, the instrument calculates the liquid level height and the interface height respectively through the time difference and signal strength.
3. Dual interface measurement:
• For oil-water mixtures, the guided wave radar can simultaneously measure the oil level position at the top and the oil-water interface height at the bottom.
2. Method of interface measurement
2.1 Signal Processing
Guided wave radar uses a special signal analysis algorithm to achieve interface measurement:
• Signal strength analysis:
• Distinguish the top liquid level from the bottom interface by analyzing the strength of the reflected signal.
A medium with a high dielectric constant (such as water) reflects a stronger signal, while a medium with a low dielectric constant (such as oil) has a weaker signal.
• Time difference calculation:
• The instrument records the time of each reflected signal and, combined with the known wave velocity, calculates the position of the top liquid level and interface respectively.
2.2 Multiple Calibration
In actual conditions, the interface measurement requires factory calibration or field calibration of the guided wave radar:
• Factory calibration: Manufacturers pre-set parameters according to the permittivity of common media.
• On-site calibration: The user sets and optimizes the instrument according to the specific medium, such as entering the dielectric constant value of different media.
3. Working condition requirements of interface measurement
3.1 Medium Requirements
1. Dielectric constant difference:
• The accuracy of the interface measurement is directly related to the dielectric constant difference. The greater the dielectric constant difference, the stronger the interface reflected signal and the more reliable the measurement.
• Examples of typical media differences:
• Water and oil: large differences, easy to measure.
• Alcohol vs. oil: The difference is smaller and may require a more sensitive instrument.
2. Uniformity:
• The measured medium should be as uniform as possible, for example, the oil-water interface should be clear. If the medium has a large fluctuation or mixing zone (emulsion layer), it may lead to measurement errors.
3.2 Environment Requirements
1. Stirring and fluctuation:
• If the interface fluctuates violently (such as violently stirring or tossing), the reflected signal may be unstable.
• It is recommended to measure under static or more stable conditions.
2. Temperature and pressure:
• Guided wave radar can generally adapt to high temperature and pressure, but it is necessary to ensure that the rod material can withstand the actual working conditions.
• Large temperature gradients may have a slight effect on signal propagation speed, but the instrument can be corrected by compensation.
3. Container shape and obstacles:
• The probe rod should avoid stirrers, escalators or other structural obstacles to avoid interfering with signal propagation.
3.3 Dielectric constant input
• Interface measurement requires the permittivity of both media to be entered in advance.
• If the permittivity of the two media is too close (e.g., the difference is less than 5), the guided wave radar may have difficulty accurately distinguishing the interface.
4. Advantages and limitations of interface measurement
advantage
1. Non-contact measurement (through the probe rod) : no direct contact with the interface, strong durability.
2. Accurately distinguish the interface: it can measure the top liquid level and the interface position at the same time, providing comprehensive information of multi-layer liquid.
3.Resistant to complex conditions: suitable for high temperature, high pressure, corrosive media environment.
4. Easy integration: Compatible with industrial automation systems, remote data monitoring can be achieved.
limitation
1. Strong dependence on dielectric constant difference: the interface with small dielectric constant difference is difficult to measure.
2. Impact of emulsion layer:
• If there is an emulsifying layer between the two media (such as an oil-water mixture), the reflected signal may be dispersed and the height of the interface can be measured inaccurately.
3. Interference signals: stirrers or other devices may cause pseudo-reflected signals.
4. Calibration complexity: It is necessary to accurately understand the characteristics of the measured medium in order to carry out effective calibration.
5. Typical application scenarios
1. Oil-water separator: used to measure the height of the oil level and the position of the oil-water interface to ensure the purity of the oil.
2. Chemical reaction tank: monitoring the stratification state of different liquids during the reaction process.
3. Sewage treatment: Measure the height of the clean water layer and the sludge interface to optimize the process operation.
4. Tank level management: Accurate measurement of each liquid layer in the mixed liquid tank.
Sum up
Guided wave radar can accurately measure the interface height of liquid by detecting the reflected signals of different media. The key lies in the difference of dielectric constant and signal processing technology. Although it has certain requirements for working conditions and medium characteristics, its high accuracy and wide applicability make it the preferred tool for multiphase liquid interface measurement.
Thank you
