Measuring principle and advantages and disadvantages of capacitance liquid level gauge

Capacitive level gauges use the capacitive liquid level sensor principle. It is suitable for liquid level measurement in electric power, metallurgy, chemical industry, food, brewing, pharmaceutical, sewage treatment, boiler drum, etc. Because capacitive liquid level detection is basically not affected by changes in process conditions. The buoyancy type, differential pressure type, and capacitance fuel level sensor are all related to the weight of the medium in the detection, and the change of the weight of the gas-liquid phase medium will cause additional errors in the detection results. 

The reason why capacitive level gauges are more universally applicable than other types of level gauges. 

First. The principle of capacitive liquid level meter The capacitive liquid level measurement principle is that the probe wire and the conductive liquid form a capacitor, in which the metal core of the probe wire is one pole of the capacitor, the conductive liquid is the other pole of the capacitor, and the middle is high stability The PPR or polyvinyl fluoride, that is, the insulator outer layer of the probe wire is used as the medium between the two stages. With the change of the liquid level, the area of the liquid surrounding the probe wire changes, so that the relative area of the two poles of the capacitor changes, The capacitance changes. According to the formula of concentric cylindrical capacitance, the relationship between the liquid height and capacitance can be written. The capacitance of the capacitor is affected by three factors: 1. The distance between each electrode surface 2. The size of each electrode surface 3. The distance between the electrode surfaces It is often said that the dielectric constant of the intermediate medium is fixed because Co, ε and D/d are fixed, so: C=KH, that is, the capacitance is only proportional to the height of the liquid immersion probe (the relative area of the capacitor plate). 

Second. Capacitance level meter parameter measurement range: 0.2-20 meters Accuracy: 0.5 class; 1.0 class Measurement medium: acid, alkali, salt, water and other amorphous conductive liquids and organic solvents with electrical conductivity ≥ 10-3s/m Ambient temperature: - 40℃-60℃ Medium temperature: -50℃-200℃ Power supply: 12.5-30VDC Output signal: 4-20mA range and zero adjustment range: ≥30%FS Installation size: M20×1.5, M27×2, flange type , Suspended container pressure: -0.05MPa ~ 32 MPa 

Features of Capacitance Level Gauge:

1. Simple structure: There are no movable elastic parts, so the reliability is relatively high, and the maintenance is very little. 2. Easy installation: The built-in structure especially shows this feature, one person and one wrench can be installed in ten minutes. 3. Easy to adjust: the two potentiometers of zero position and span can arbitrarily carry out zero point migration or span compression or expansion within the effective range of liquid level detection. 4. Liquid level detection is basically not affected by changes in process conditions. The buoyancy type, differential pressure type, and isotope type liquid level gauges are all related to the weight of the medium in the detection, and the change of the weight of the gas-liquid phase medium will cause additional errors in the detection results. 5. Wide range of application: It is specially customized according to the specific characteristics of the detection object, which can be adapted to liquid level detection under various harsh conditions. 6. Low operating cost, no additional impact, no wearing and consumable parts, no additional problems such as depreciation of radiation source, waste source treatment fee and radiation protection compared with isotope type liquid level gauge. 7. Lightweight: The weight of a high-pressure liquid level gauge working under 32MPa is less than two kilograms, which is several tenths of the weight of the high-pressure float level gauge. 8. The liquid level is not suitable for the detection of highly viscous media, such as molten asphalt IV.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        Disadvantages of capacitive level measurement: The dielectric constant of the measured medium and the upper part of the liquid surface must be kept constant for accurate measurement; the measurement range is limited by the length of the metal rod; there are high requirements on the material of the container; the measured medium must be conductive.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                The transmitter adopts the principle of capacitance measurement, and the outer insulation layer of the probe can be based on the corresponding insulation materials of different media (such as high temperature resistance, corrosion resistance, anti-aging and high chemical stability). Teflon material), it is insulated from the liquid under test during normal operation. When installing, be sure to protect the outer insulating layer of the probe wire. Once damaged, the service life will be shortened or the installation will fail. After the installation of the probe wire, when it is completely immersed in the liquid, the insulation resistance between the probe wire and the liquid (or the outer wall of the metal container) should be > 20M (measured with a digital multimeter 20M). The connection to the transmitter is temporarily disconnected. 2. When the transmitter is installed in the open air, the probe wire should not be exposed outside the container, so as to avoid measurement errors when the probe wire hits the water in rainy days. 3. The casing of the transmitter or the stainless steel process connection parts at the lower part of the junction box must be reliably connected to the outer wall of the container (grounded), and its contact resistance should not be greater than 2. 4. In normal operation, the probe wire should not have a relatively large diameter in the container. Large swing amplitude, otherwise there will be signal instability. 5. When installing the probe wire, it should be as far away from the inner wall of the container as possible, and the minimum distance should not be less than 100mm. When limited by conditions, when the distance is less than 100mm, the distance between the probe line and the container must be relatively fixed. 6. For the single-wire soft probe, the excess part can be pulled out through the upper end of the process connection and then cut off, and then the deposit bolt is tightened. For the twisted-pair probe, the excess part can be coiled above the liquid level to be measured.It is absolutely not allowed to coil excess parts around the bottom of the vessel or the effective measuring section. 7. When there is agitation in the container or the liquid may produce a lot of bubbles, in order to protect the probe line and avoid the false liquid level caused by liquid fluctuations and bubbles, a metal or non-metal tube with an inner diameter > 80mm can be placed in the container. The lower end should be opened to receive the liquid inlet hole, and the vent hole should be left below the liquid level. When using a metal tube, the position of the probe wire in the tube should be guaranteed to be quite stable, and the probe wire should be supported and straightened if necessary.

Many of us have heard about level gauge and level transmitter and seen some of these units in operation in customized level indicator, customized level indicator and customized level indicator spaces.
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