Introduction to the principle of the RF admittance level switch

The RF admittance level meter is a kind of material level control technology developed from the capacitive level control technology, which is more reliable, more accurate and more applicable. The "admittance" in "radio admittance" The meaning of electric energy is the reciprocal of impedance, which is composed of resistive components, capacitive components and inductive components. The "radio frequency" is high frequency, so RF admittance technology can be understood as measuring admittance with high frequency. The high-frequency sinusoidal oscillator outputs a stable measurement signal source, using the bridge principle to accurately measure the admittance of the sensor mounted in the container to be tested. In the direct action mode, the output of the meter increases with the level. increase.

The difference between RF admittance technology and traditional capacitor technology lies in the diversity of measurement parameters, the driving of three-terminal shielding technology and the addition of two important circuits, which are based on valuable experience in practice. The above technology not only solves the problem of connecting cable shielding and temperature drift, but also solves the problem of vertical mounting of the sensor root. The two circuits added are a high precision oscillator driver and an AC phase detector.



For a container with a strong conductive material, since the material is electrically conductive, the grounding point can be considered as the surface of the probe insulating layer, which is only a pure capacitor for the transmitter probe, and the probe is placed on the probe. The hang material is produced, and the hang material is impedance. In this way, the former pure capacitor has now become a complex impedance composed of a capacitor and a resistor, causing two problems.

The RF admittance technology introduces measurement parameters other than capacitance, especially the resistance parameters, which makes the signal-to-noise ratio of the meter measurement signal increase, greatly improving the resolution, accuracy and reliability of the instrument; the diversity of measurement parameters is also Vigorously expand the reliable application of the instrument.

The first problem is that the material itself is equivalent to a capacitor for the probe. It does not consume the energy of the transmitter. (The pure capacitor does not consume energy.) However, if the material is connected to the equivalent circuit of the probe, the impedance of the material will be consumed. The energy, which pulls down the oscillator voltage, causes the bridge output to change, producing measurement errors. We added a driver between the oscillator and the bridge to supplement the energy consumed, 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 insulation covers the entire material and the hanging area, so that the effective measuring capacitance is extended to the top of the hanging material, thus causing the hanging error, and the stronger the conductivity, the more the error Big. But any material is not completely conductive. From an electrical point of view, the hanging layer is equivalent to a resistor, and the portion of the sensing element covered by the hanging material is equivalent to a transmission line composed of an infinite number of infinitesimal capacitors and resistive elements. According to the mathematical theory, if the material is long enough, the capacitance and the resistance of the material are equal. Therefore, the capacitance and resistance can be measured separately by the AC phase detector. The measured total capacitance is equivalent to the C level + C hanging material, and then subtracting the resistance R equal to the C hanging material, the true value of the level can be obtained, thereby eliminating the influence of the hanging material. That is, C measurement = C level + C hanging material C level = C measurement - C hanging material = C measurement - R

These multi-parameter measurements are the basis of the measurement, and the AC phase detector is the means of implementation. 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 and liquid level measuring instrument.