The products that measure the height of liquid interface in the market compete with each other, and their moves have their own advantages. Xiaobian will count them one by one to see which measurement principle can be the top!
1、 Radio frequency admittance level gauge (capacitance level gauge)
The sensor of RF admittance liquid level gauge is a metal rod or flexible metal cable with Teflon coating, which needs to be inserted into the measured liquid body. In this way, a capacitance will be generated between the metal rod (cable) and the metal tank wall containing the measured liquid. This capacitance will be amplified to a sufficient measured admittance value under the RF circuit (frequency up to 300GHZ). If the measured liquid is stratified to form the stratification of upper oil and water, three capacitance values of C1, C2 and C3 will be formed. Because the conductivity of water maximizes the value of C3, the capacitance measured by RF admittance level gauge mainly depends on the value of C3. Therefore, the height of water level is almost directly proportional to the value of C3.
According to the above measurement principles, the necessary conditions and limitations of RF admittance measurement interface:
1. The upper liquid cannot conduct electricity, such as benzene, hydrocarbons, etc; The liquid under test must be conductive liquid, such as ordinary water or aqueous solution.
2. The measurement accuracy of the split interface is not high. Because the height of the upper oil and the shape of the measured tank will be affected by the measurement accuracy.
2、 Guided wave radar level gauge
The sensor of guided wave radar liquid level gauge is also a metal rod or metal cable inserted into the measured liquid body. The electronic hair of the liquid level gauge emits radar wave pulses. The radar wave pulse will propagate towards the measured liquid along the metal rod or metal cable, and will be reflected when it reaches the position where the dielectric constant changes. The electronic head of the liquid level gauge calculates the time difference between transmitting radar wave pulse and receiving radar wave pulse to determine the height of the measured liquid level. As shown in the figure below, if the oil on the upper layer of the measured liquid is referred to as water, the radar wave is reflected respectively when entering the oil from air and when entering the water from oil.
According to the measurement principle of guided wave radar, the necessary conditions of its measurement interface are as follows:
1. The necessary condition for radar wave reflection is the change of dielectric constant. If the change of dielectric constant is very small, the reflected signal of radar wave is too small to be measured. Therefore, the dielectric constant of the lower liquid is much larger than that of the upper liquid. The dielectric constant can not be measured by conventional means, and the dielectric constant is easily affected by ambient temperature, electromagnetic frequency, chemical reaction in the tank and so on. This determines that it is not easy to judge whether the guided wave radar can adapt to the application environment.
2. In order to keep enough energy and Reflection on the interface after the radar wave passes through the upper liquid, the sensor is generally required to be a coaxial structure. The above figure shows the installation requirements of the measurement interface of the guided wave radar. On the left is the measuring rod and measuring cylinder of the guided wave radar, which is a coaxial structure. In the right figure, the measuring rod of the guided wave radar is installed in a measuring tube full of liquid pouring holes. Sometimes the interface changes inside and outside the coaxial structure can not be truly synchronized, which will lead to measurement errors. Moreover, the viscous liquid will adhere to the coaxial structure and interfere with the measurement.
3. If you want to accurately measure the interface of the measured liquid, you must know the specific dielectric constant of the upper liquid. Because the speed of radar wave passing through air is the speed of light, while the speed passing through the upper liquid is less than the speed of light and inversely proportional to its dielectric constant. Because the dielectric constant is not easy to measure and changeable, it is difficult for guided wave radar to accurately measure the interface height.
3、 Float level gauge
The measuring interface of the float level gauge is generally installed on the side of the tank, and the measured liquid is introduced into the measuring cylinder from the measured tank through two flange ports. At the same time, there is a float in the measuring port, which is connected with the electronic head through some mechanical structures. The average density of the liquid in the measuring cylinder is induced by measuring the change of buoyancy of the float, so as to evaluate the height of the measured interface (assuming that there are only two liquids in the measuring cylinder). The float level gauge does not require the conductivity and dielectric constant of the measured liquid.
According to the principle and internal structure of pontoon level gauge, the necessary conditions and limitations of its measurement interface:
1. It can only be installed on the side of the tank to be measured, and there is no air in the measuring cylinder.
2. In order to ensure the accuracy of measurement, calibration must be carried out regularly, because the force transmission structure from the float to the electronic head will offset the setting over time.
3. Because of the narrow structure of the measuring cylinder, the solid or viscous substances in the measured liquid will make the float stuck in the measuring cylinder and can not really reflect the height of the interface.
4、 Double flange level gauge
The double flange liquid level gauge indirectly measures the height of the interface by measuring the pressure difference between two fixed positions to reflect the average density of the liquid between the two positions. The double flange liquid level gauge is also installed on the side of the tank, but the measuring cylinder is not required. Therefore, the problem that the float in the upper float liquid level gauge is easy to get stuck is solved.
The double flange liquid level gauge transmits the two pressure values in the tank to the electronic head through the oil filled capillary tube. Therefore, the interface measurement of double flange level gauge is affected by the following factors:
1. The change of ambient temperature has a significant impact on the oil pressure in the capillary, so the long measurement range must have a long capillary, resulting in the temperature change, which also has a great impact on the measurement accuracy.
2. It must be ensured that the liquid is filled between the two flanges.
3. Capillary oil filling requires special production technology, so the measurement range has a great impact on the price of the instrument.
4. During the installation process, ensure that the capillary tube is not impacted by external machinery, so as to prevent the capillary tube from being damaged or the oil circuit from being blocked.
5、 Magnetostrictive liquid level gauge
Measure the height of the magnetic float on the interface of the magnetic float. The level plus series liquid level gauge produced by MTS can control the measurement error within 1mm, and the maximum measurement range can reach 22m. The measurement of magnetostrictive liquid level gauge does not depend on the physical parameters such as liquid conductivity and dielectric constant, but is based on the density difference of different liquids. Density difference and mutual insolubility are the necessary conditions for the formation of interface.
Level plus series
As long as the density of a specific float is selected for the magnetostrictive liquid level gauge, the long-term reliability and accuracy of the liquid level gauge can be guaranteed without regular maintenance and calibration after installation.
The application limitation of magnetostrictive liquid level gauge is that it can not be used to operate liquids with high viscosity (viscosity less than 400 centipoise is recommended). High viscosity liquid is easy to stick the float in one position, resulting in the float position measured by the magnetostrictive liquid level gauge can not accurately reflect the height of the interface.
Application example of level plus magnetostrictive liquid level gauge
1. Interface between dinitrotoluene (DNT) and water
Because the specific gravity of dinitrotoluene (DNT) relative to water is 1.52, it is the lower liquid in the tank, so RF admittance and guided wave radar cannot be used. Moreover, because the measurement range on site exceeds 4m, pontoon level gauge and double flange level gauge cannot be used. By using the level plus series magnetostrictive liquid level gauge, the liquid level and interface height in the reaction tank can be measured at the same time, so as to accurately control the chemical reaction process.
2. Interface between liquid white phosphorus and water
White phosphorus is the basic raw material for the production of phosphoric acid. However, because white phosphorus is easy to be natural in the air and toxic, it is generally stored in storage tanks with water seals. The level plus level gauge can monitor the white phosphorus stock under the water seal in real time, and ensure the thickness of the water seal to prevent the white phosphorus from contacting with the air.
3. Large finished oil tank
Refining plants and fuel delivery systems all have some product oil storage tanks and transfer tanks, which are 15 ~ 20 meters high. Because some finished fuel will absorb moisture in the air, resulting in ponding at the bottom of many tanks. The level plus level gauge produced by MTS sensor can simultaneously measure the liquid level of product oil in the tank and the height of oil water interface. At the same time, it can also calculate the capacity in the tank according to the tank capacity table inside the level gauge and output the temperature of product oil in the tank. The level plus level gauge can output the measurement results of liquid level, interface height, tank capacity and temperature at the same time through only one measuring hole on the tank, meeting the handover measurement requirements of the international petroleum organization for finished fuel oil.
Post time: 18-02-22