The working principle of radio frequency admittance level gauge

The working principle of radio frequency admittance level gauge: radio frequency admittance level gauge is a kind of level control technology developed from capacitive level control technology, which is anti-hanging, more reliable, more accurate and more applicable. The meaning of 'admittance' in 'radio frequency admittance' is the reciprocal of impedance in electricity. It is composed of resistive components, capacitive components, and inductive components, and 'radio frequency' means high frequency, so radio frequency admittance technology can be understood For measuring admittance at high frequencies. The high-frequency sinusoidal oscillator outputs a stable measurement signal source, and uses the bridge principle to accurately measure the admittance of the sensor installed in the container to be tested. In the direct action mode, the output of the instrument increases with the rise of the material level. Increase. The difference between RF admittance technology and traditional capacitive technology lies in the diversity of measurement parameters, driving three-terminal shielding technology and the addition of two important circuits, which are improved based on valuable experience in practice. The above-mentioned technology not only solves the problem of shielding and temperature drift of the connecting cable, but also solves the problem of material hanging at the root of the vertically installed sensor. The two circuits added are a high-precision oscillator driver and an AC phase-detection sampler. For a container of highly conductive material, since the material is conductive, the grounding point can be considered on the surface of the probe insulation layer, which is only a pure capacitance for the transmitter probe. With the discharge of the container, the grounding point on the probe rod A hang material is produced, and the hang material is resistive. In this way, the former pure capacitance has now become a complex impedance composed of capacitance and resistance. Due to the introduction of measurement parameters other than capacitance, especially resistance parameters, the RF admittance technology increases the signal-to-noise ratio of the instrument measurement signal, greatly improving the resolution, accuracy and reliability of the instrument; the diversity of measurement parameters also has It greatly expands the reliable application field of the instrument. The first problem is that the material itself is equivalent to a capacitor to the probe, which does not consume the energy of the transmitter (pure capacitance does not consume energy), but the equivalent circuit of the material to the probe contains resistance, and the impedance of the material will consume Energy, which pulls down the oscillator voltage, causes the bridge output to change, creating measurement errors. We added a driver between the oscillator and the bridge to replenish the consumed energy, thus stabilizing the oscillating voltage applied to the probe. The second problem is that for conductive materials, the grounding point on the surface of the probe’s insulating layer covers the entire material and the material-hanging area, so that the effective measurement capacitance extends to the top of the material-hanging material, which will cause a material-hanging error, and the stronger the conductivity, the greater the error. big. But no material is completely conductive. From an electrical point of view, the coating layer is equivalent to a resistor, and the part of the sensing element covered by the coating is equivalent to a transmission line composed of countless infinitesimal capacitance and resistance elements. According to the mathematical theory, if the hanging material is long enough, the impedance and capacitive reactance of the capacitance and the resistance part of the hanging material are equal, so the capacitance and resistance can be measured respectively with the AC phase detection sampler. The measured total capacitance is equivalent to C material level + C hanging material, and then subtracting the resistance R equal to C hanging material, the real value of the material level can be obtained, thereby eliminating the influence of hanging material. That is, C measurement = C material level + C hanging material C level = C measuring - C hanging material = C measuring - R These multi-parameter measurements are the basis of measurement, and the AC phase detection sampler is the means of realization. Due to the use of the above three technologies, the RF admittance technology has shown extraordinary vitality in field applications. It is currently the world's top material liquid level measuring instrument.

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