Principle and fault analysis of the hottest RF adm

Principle and fault analysis of RF admittance level meter

Abstract: This paper analyzes the working principle of RF admittance continuous level meter, puts forward the use of RF admittance technology to solve the problem of material hanging in actual production, and puts forward the corresponding solutions to the common faults of RF admittance level meter

key words: RF admittance; Hanging materials; Fault analysis

introduction

level refers to the interface position between the liquid, powdery or granular solid and gas in the storage container or industrial production equipment. It can also be the interface position formed by the different density between the two immiscible liquids. Due to the different properties of various materials, the working conditions in production are very different, and there are many methods of level measurement. At present, commonly used level meters include: weight load level meter, ultrasonic detection level meter, liquid float level meter, RF admittance capacitance level meter. Among them, the capacitive level measurement method has the characteristics of wide application range (applicable to all kinds of media, including liquid, powdery solid, liquid-solid slurry and media interface), the measurement results have nothing to do with media density, chemical composition and other factors, and the measuring instrument has simple structure, reliable performance, low price, and can work under all kinds of harsh working conditions. It is the most widely used method for level measurement. However, the capacitive level measurement method has the problem of hanging material. When measuring the adhesive conductive material, the material will adhere to the outer insulating cover of the sensing electrode (hanging material), forming a false level, resulting in a large measurement error, which hinders the wider application of the capacitive level instrument. In recent years, capacitive material testing technology has made new development. Foreign scholars have used RF admittance technology to solve the material hanging problem in capacitive level measurement method, and relevant products have been produced abroad. The RF admittance continuous level meter produced by AMETEK drexelbrook company in the United States has been widely used in China

1 working principle of capacitive level meter

working principle of capacitive level meter

capacitive level meter is to establish a capacitor in the container, one pole is a rod-shaped probe immersed in the container, and the other pole is a grounded metal plate (usually the container wall), as shown in Figure 1a. If the capacitance with air as the medium is C1 and the capacitance with the measured material as the medium is C2, the capacitance between electrodes is:

c=c1+c2

if the measured medium is a conductor, an insulating layer must be added to the probe, as shown in figure 1b. Suppose that the capacitance of the upper part of the tested material with the insulating layer as the dielectric is C1a, the capacitance of the air as the dielectric is C1b, and the capacitance of the part of the tested material with the insulating layer as the dielectric is C2A, then the capacitance between the electrodes is:

C = C1a · c1b/(C1a + C1b) + C2A

through derivation, the relationship between the capacitance C between the two electrodes and the height h of the tested material level is

c=co+k · h

where C0 and K are constants related to dielectric constant and container structure

when the level changes, the immersion height h of the measured medium to the probe changes, so that the capacitance C changes. The level height h is obtained by measuring the capacitance C

2 working principle of RF admittance level meter

the traditional capacitive level sensor cannot eliminate the influence of electrode hanging on the measurement, especially when measuring viscous conductive materials, the error is extremely serious, which greatly limits the use and development of capacitive level meter

as shown in Figure 2a, in a container filled with viscous conductive materials to protect their investment and realize all the functions of projet MJP 5600, a measuring electrode with an insulating layer is installed on the measuring electrode. At this time, there is a material capacitance in the container. Due to the large cross-section of the conductive material, it can be considered that the resistance of the tested material in the detection circuit is zero after being confirmed by the evaluation team on site. Since the two poles of this capacitor are electrode core and conductive material respectively, it can be seen from the working principle of the capacitive level meter that the measured capacitance is directly proportional to the height of the material at this time. However, this capacitive level measurement principle has a serious disadvantage, as shown in Figure 2B. When the level is reduced from high h to low H0, an adhesive layer (i.e. hanging material) may be left on the probe, resulting in false level and measurement error

level detection principle of viscous conductive materials

in Figure 2a, because the cross-sectional area of the hanging material is small, the equivalent resistance of the hanging material is large (the equivalent circuit of the hanging material is shown in Figure 2b), the hanging material can be regarded as composed of many tiny resistors and capacitors. It can be mathematically proved that as long as the adhesive layer is long enough, the resistance and capacitance of the adhesive layer have the same impedance, which is the RF admittance theorem. The outer wall of the container with adhesive layer on the measuring electrode is grounded, and a high-frequency excitation signal is added between the measuring electrode and the ground. There is no DC path between the measuring electrode and the ground. Therefore, the actual material level of the storage tank can be obtained by measuring the current

3 realization of RF admittance measurement system

the standard sinusoidal signal generation circuit sends out a 100 kHz RF signal, and the interference components are filtered out by the filter circuit to obtain a relatively pure 100 kHz RF signal. The capacitance drive circuit is used to improve the level measurement range. The measuring circuit of transformer bridge converts the level signal into voltage signal. The measured voltage signal is divided into two channels: the measurement signal containing material level information and hanging information; Provide synchronization signal at sampling time. The synchronous signal is processed and the sampling time is given. At this time, the measured signal is sampled to obtain a pure level signal, so as to eliminate the influence of electrode hanging. The detected material level signal passes through the low-pass filter circuit to eliminate some interference, and then is sent to the microcomputer system to keep the instrument clean, complete parts and accessories, conduct a/d conversion and data processing safely, and display the actual material level through the display circuit. The actual level is compared with the set upper and lower limit value of the level, and an out of limit alarm signal is given. Users can input command signals through the keyboard to self calibrate the system and improve the measurement accuracy. The block diagram of RF admittance measurement system is shown in Figure 3

4 analysis of common faults of RF admittance level meter

the RF admittance continuous level meter produced by AMETEK drexelbrook company in the United States has stable performance and high measurement accuracy, but improper process parameters such as use conditions and application conditions may cause inaccurate measurement or non operation of the instrument. If it is caused by the instrument, please check the following aspects in turn:

(1) check the electronic unit. Move the sensor power supply and signal cable, plus 15 ~ 28vdc power supply, check and calibrate the system, and use the signal generator to check whether the output signal of the system is within the range of 4 ~ 20mA

(2) check the sensor. Use an analog ohmmeter to measure the resistance between the sensor terminals, including the resistance between the center line and the shield, the resistance between the shield and the ground, and the resistance between the center line and the ground for 10 minutes. When the sensor element has no hanging material (aggregate) in the air, the above three resistance values should be infinite. If there is hanging material on the sensor element, the resistance value is low. When the hanging material is removed, the resistance returns to infinity. Under normal conditions, the minimum resistance value when there is material hanging on the sensor is: 1000 Ω between the center line and the ground; 600 Ω between the center line and the shield; 300 Ω between shield and ground

(3) check the transmission cable. Water or conductive objects in the conduit may cause instrument failure

conclusion

this paper analyzes the working principle of the RF admittance continuous level meter and the realization of the measurement system in detail, analyzes the common faults of the RF admittance level meter, and puts forward the corresponding solutions, so that the RF admittance level meter can better serve industrial production

references

[1] Chen Xiaozhu, Chen le Research on material hanging in capacitive level meter [j] Journal of Metrology

[2] zhangbaofen Automatic detection technology and instrument control system [m] Beijing: Chemical Industry Press, 2002

[3] Wang Yonghong Process instrumentation [m] Beijing: Chemical Industry Press, 2005

[4] Shi Ren, Liu Wenjiang, et al Automatic instrumentation and process control (Third Edition) [m] Beijing: Electronic Industry Press, 2003