Application of capacitance level gauge in chemical production

Abstract: With the continuous advancement of science and technology, automation systems are increasingly used in the field of chemical production. The application of various intelligent instruments plays a big role in the process of realizing automation in chemical production. Among them, intelligent liquid level meters are like eyes in the production process, escorting the industrial production process and providing timely and accurate information. There are many types of liquid level gauges currently used, mainly including: capacitive liquid level gauge, radar liquid level gauge, ultrasonic liquid level gauge, pressure liquid level gauge, etc. Liquid level detection has become more common in many control fields, and various methods They are all designed according to their needs. Their structure, range and accuracy are suitable for different occasions. Most of them have complex structures and high manufacturing costs. Capacitive liquid level gauges have been widely used in recent years due to their excellent accuracy, anti-interference properties and low requirements on media and environment. This article explains the application of liquid level meters in chemical production by introducing the principle, use and various problems that arise in production of capacitive liquid level meters.

1. Principle of capacitive liquid level meter

The capacitive level transmitter is an instrument that uses changes in measuring capacitance to measure the level of the material. The measurement principle is as shown in the figure above. ：

Among them: 1 is the conductive cylinder with diameter d

2 is the insulating material

3 is the electrode 1 in the measuring cylinder or container wall and the measuring cylinder wall 3 forms a coaxial capacitor C

C=k1 ＊ h2 ＋ k2h1

H ＝ h1 + h2

C ＝ k1 (H-h1) + k2h1 =k1H+(k2-k1)h1

Among them: h1 is the liquid level height

k1h2 is the cylindrical capacitance formed by the gas part

k2h1 is the liquid part formed Cylindrical capacitance

k1 and k2 are constants related to the properties of the medium to be measured and the structure of the container

It can be seen that the capacitance C between 1 and 3 has a linear relationship with the liquid level h1 , detecting the change of probe capacitance C can measure the liquid level.

The circuit block diagram of the capacitive liquid level gauge is as follows:

The adjustable capacitor C0 is a zero-adjusting capacitor set to offset the capacitance k1H formed by the gas part in C. The excitation source generates a specific excitation signal required to drive the differential capacitance detection circuit. The differential capacitance detection circuit converts the difference between C and C0 into a weak small current I1. I1 ∝ C - C0＝ k1H+（k2-k1）h1-C0 Adjust C0＝ k1H, then I1 ∝（k2-k1）h2 Therefore, I1 is in line with the liquid level h2A linearly proportional small current can be obtained by amplifying I1 to obtain a 4-20mA.DC current signal that is linearly proportional to the liquid level height h2.

2. Precautions for the use of capacitive liquid level gauge

In the chemical production process, the environment is relatively harsh. The front and rear covers of the transmitter and the wiring must be completely sealed to prevent rain and corrosive gases. Enter.

(1) During transportation, unpacking and acceptance, care must be taken to prevent damage to the sensor insulation layer during installation.

Unpacking inspection: After unpacking, first check whether the sensor is damaged, and then power on for inspection. After powering on, adjust the zero-adjustment capacitor so that the output current of the meter is 4mA.DC. Hold the sensor with your hand and the meter will indicate should increase, indicating that the instrument is working properly. If there is no current output from the instrument after power is turned on, check whether the power supply is connected reversely and whether the signal processor leads are loose or fallen off.

(2) Model and technical specifications:

The model and main technical specifications of the instrument are printed on the nameplate of the casing. When receiving the goods, please check whether they are consistent with the ones originally ordered. Specifications match. When you send a letter or call, you should inform the model number on the nameplate, the factory number and the sensor probe length.

(3) Precautions during transportation:

To prevent the instrument from being damaged, please do not open the package before the transmitter is shipped to the user's place of use. Care must be taken to prevent damage to the sensor insulation during transportation.

(4) Precautions during storage:

The instrument should be installed in time after arrival to prevent the insulation performance of the signal processor in the transmitter from being reduced and the metal parts to be corroded. If storage is required, please note the following:

1. Do not open the packaging box for storage if possible.

2. The storage location should meet the following conditions: rainproof and moisture-proof. Small mechanical vibration to avoid collision and impact. The temperature is between -40-+80℃, and the ideal temperature is around 25℃.

3. Before storing a used level transmitter, the liquid or attached solids on the transmitter sensor and housing should be carefully removed.

4. If stored outdoors, the performance of the transmitter will be affected. Therefore, once the transmitter is moved to the installation site, it must be installed as soon as possible.

(5) Installation precautions:

Capacitive level gauge is a measuring instrument that uses changes in capacitance to measure the level of the medium in the container. The measurement process mainly relies on two The capacitance change between the two electrodes, that is to say, the sensitivity of the capacitance level meter depends on the difference in the dielectric constant of the two media, gas and liquid. The measurement of the capacitance level gauge must ensure that the dielectric constants of the two media are consistent, otherwise the dielectricChanges in constants will directly lead to errors.

1. When installing in the open air, the probe wire cannot be exposed outside the container to avoid measurement errors caused by the probe wire getting wet in rainy days.

2. The stainless steel process connection parts at the bottom of the shell or junction box must be reliably connected (grounded) to the outer wall of the container, and their contact resistance cannot be greater than 2.

3. During normal operation, the probe wire cannot swing to a large extent in the container, otherwise the signal will be unstable.

4. When installing the probe line, try to keep it as clean as possible.