RF admittance is a new level control technology developed from capacitive type, which is more reliable, more accurate and more applicable. It is an upgrade of capacitive level technology. The so-called radio frequency admittance means the reciprocal of impedance in electricity. It is composed of resistive components, capacitive components and inductive components. Radio frequency is the spectrum of high-frequency radio waves. Therefore, radio frequency admittance can be understood as measuring admittance with high-frequency radio waves. When the instrument works, the sensor of the instrument forms admittance value with the irrigation wall and the measured medium. When the material level changes, the admittance value changes accordingly. The circuit unit converts the measured admittance value into material level signal output to realize material level measurement.
For continuous measurement, the difference between RF admittance technology and traditional capacitance technology, in addition to the above, two very important circuits are added, which is improved according to a very important discovery in the practice of conductive hanging. At this time, the above technology also solves the problem of connecting cable and the problem of hanging material at the root of vertically installed sensor. The two circuits added to the lock are oscillator buffer and AC converter chopper driver.
For a container with strong conductivity of the measured medium, because the measured medium is conductive, the grounding point can be considered as the surface of the probe insulation layer, which is only a pure capacitance for the transmitter. With the discharge of the container, the hanging material is generated on the probe rod, and the hanging material has impedance. In this way, the former pure capacitance has now become a complex impedance composed of capacitance and resistance, which leads to two problems.
The first problem is that the liquid level itself is equivalent to a capacitance to the probe, which does not consume the energy of the transmitter (pure capacitance does not consume energy). However, if the equivalent circuit of hanging material to probe contains resistance, the impedance of hanging material will consume energy, which will pull down the oscillator voltage, resulting in the change of bridge output and measurement error. A buffer amplifier is added between the oscillator and the bridge to supplement the consumed energy, so the oscillation voltage applied to the probe will not be reduced.
The second problem is that for the conductive measured medium, the grounding point on the surface of the probe insulating layer covers the whole measured medium and the hanging area, so that the effective measuring capacitance is extended to the top of the hanging material. This will produce hanging error, and the stronger the conductivity, the greater the error. However, any measured medium is not completely conductive. From the electrical point of view, the hanging material layer is equivalent to a resistance, and the part covered by the hanging material of the sensing element is equivalent to a transmission line composed of countless infinitesimal capacitors and resistance elements. According to the mathematical theory, if the hanging material is long enough, the capacitance and resistance of the hanging material are equal. Therefore, according to the error research of hanging impedance, an AC driver circuit is added. The circuit can measure capacitance and resistance together with AC converter or synchronous detector. Since the impedance and capacitive reactance of hanging material are equal, the measured total capacitance is equivalent to C + c hanging material, and then subtract the resistance r equal to c hanging material, the real value can be measured, so as to eliminate the influence of hanging material.
I.e. C measurement = C + c hanging
C = C measurement – c hanging
=C measurement – R
These, multi parameter measurement, is the foundation, and AC phase discriminating sampler is the means to realize. Due to the use of the above three technologies, RF admittance technology shows extraordinary vitality in field application.
Post time: 24-01-22