With the development of RF level detection technology, capacitive products have gradually developed into conductive products, which can be used to measure the level of media in containers under different working conditions. However, from the communication with users, many users still know little about the principle of RF Admittance and RF Admittance level switch, and they do not know how to select the correct type. For this reason, this article combines the concept of RF conductivity and the principle of RF conductivity level switch to talk about how to correctly select RF conductivity level switch. 

First. Concept and principle of RF conductivity

　　RF conductivity technology is mainly used to detect the level by detecting the dielectric constant and conductivity of the material. When the conductivity of the measured medium is large, capacitive products will produce errors due to the adhesion of the measured medium to the sensor. However, conductivity products can eliminate this error by detecting both capacitance and resistance.

　　Conductance as a physical concept is the reciprocal of impedance, and since there is very little inductance in the actual process, that conductance is actually capacitance and resistance. For accurate level measurement, an appropriate radio frequency (RF) signal in the frequency range of 15 to 400 kHz is also required. People often refer to this technique of measuring level as RF conductance.

　　After understanding the concept of RF Admittance, the principle of RF Admittance Level Switch is also easier to understand. The RF conductivity level switch is used to measure and control the level by sensing the change in reactance (capacitive and impedance) between the probe and the tank body. The internal electronic unit, which consists of a probe measuring pole and the reactance between the unloaded tank together form a balanced bridge circuit and generate a stable oscillation signal. When the measured medium covers the probe measurement pole, it causes the reactance between the probe measurement pole and the tank to change, causing the bridge circuit to be unbalanced and stop generating the oscillation signal, and the back-end circuit detects this change and outputs an alarm signal. The oscillation signal as RF signal applied to the probe measuring pole, but also after 1:1 voltage follower sent to the probe's protection pole, measuring pole and protection pole RF signal with equal potential, the same phase, the same frequency and isolated from each other. When the probe has hanging material, there is no potential difference between the measuring pole and the protection pole to form electrical isolation to ensure that the signal change of the protection pole does not affect the detection, so that the change of reactance on the probe measuring pole can only be determined by the material between the probe measuring pole and the tank, so that the hanging material on the probe will not affect the normal detection.

Second, the correct selection of RF Admittance level switch

　　Since the actual working conditions vary, the correct selection of RF Admittance level switch directly affects the matching degree and accuracy of measurement between the expiration and the site working conditions. Generally, when purchasing RF Admittance level switches, users need to pay attention to the following aspects.

　　1、Understanding the properties of the measuring medium

　　According to the different properties of the medium, choose the appropriate RF Admittance level switch.

　　(1) RF Admittance level switch can only be used for level measurement with a dielectric constant ≥ 1.6, so the measurement medium should have a certain dielectric constant.

　　(2) RF Admittance level switch is contact measurement, so the measured material cannot be too large, so that the RF Admittance probe will not be smashed and bent and damaged, so it is not suitable for the measurement of density and particles of larger media.

　　(3) When measuring non-conductive media, the probe with insulating layer should be used as much as possible to prevent corrosive substances from being mixed into the media and corrode the measuring probe. For insulated probe, the temperature and pressure of the medium should be considered. But the temperature and pressure has a corresponding curve range, not working in the curve range will damage the insulation layer, so that the medium through the insulation layer penetrate the probe rod at the crystallization, affecting the measurement results. In addition, in the larger diameter of the non-conductive medium container, in order to improve the sensitivity and accuracy of the measurement, the probe with auxiliary electrode can be used, or the rf level switch as far as possible installed in the edge of the container.

　　(4) If the viscosity of the material is high, the flow speed of the material between the probe rod and the reference electrode of the grounded tube will be restricted, making the level in the tube not really represent the actual level in the tank, at this time, the RF conductivity level switch should be selected with caution.

2、Insertion depth and length of protection pole

　　RF conductivity level switch is a point contact measurement, in the case of installation position is determined, it is necessary to choose the appropriate insertion depth and protection pole length according to the tank situation. The insertion depth is the key to accurate level measurement, generally the probe can completely contact the material as appropriate, the length of the insertion depth also depends on whether the instrument will be affected by material impact, damage, etc..

　　If the application site is more severe working conditions, such as in the application of similar power plant ash conveying system of the ash hopper, in order to reduce the probe damage caused by material impact, in the selection, should choose to buy the radio frequency conductivity level switch with stainless steel protection casing. The length of the protection pole of this kind of meter must generally be deeper than 50mm inside the silo. When the customer has special requirements, it can also be customized according to the user's needs.

　　3、Process temperature and pressure

　　Different working conditions and rf admittance level switch working principle have different requirements on temperature, so when choosing RF Admittance level switch, it is quite necessary to choose according to the specific requirements of the site temperature. If the ambient temperature of the instrument is high, the RF Admittance level switch should be selected to match that temperature.

　　1）Limited by the probe material, when the temperature is greater than 260℃, the ultra-high temperature type RF Admittance level switch should be selected. Kaidi ultra-high temperature type RF Admittance level switch, withstanding temperature up to 400℃, can be applied in most industrial environments.

　　2）Limited by the manufacturing process of the probe, the pressure resistance of the probe is generally within 2MPa, so the RF Admittance level switch is generally applied in atmospheric pressure or micro-pressure environment.

　　4、Process connection

　　The process connection is generally divided into three types: thread, flange and clamp, which are the key components to install the instrument on the tank, pipeline and other equipment. In the selection of process connection, generally should be from the site needs to select and customize the appropriate size, thickness and material process connection.

　　5、Protection function

　　In the application of the site environment for dust or humidity conditions, should choose a higher level of protection of the RF Admittance level switch, so as to avoid the performance and service life of the instrument due to poor sealing. The protection level of Kaidi RF conductor level switch is IP66, which has high sealing and can be used in dusty and humid conditions.