Thread Content
This post was last edited by zhwsyb on 2011-9-13 11:02. I. Coriolis mass flow meter. A Coriolis mass flow meter is a new type of instrument for directly measuring the mass flow rate of fluids in closed pipelines, based on the principle of the Coriolis force. Its structure generally consists of a signal measurement sensor and a signal converter. Its working principle is as follows: the measuring tube vibrates continuously at a certain resonance frequency, and this vibration frequency changes depending on the density of the fluid; therefore, the resonance frequency is a function of the fluid density. By measuring the resonance frequency, it is possible to determine the density of the fluid. A mass flow meter can simultaneously measure mass flow rate as well as density and temperature. It also features high measurement accuracy and stable operation; the downside is that it tends to wear out and get clogged when used to measure particulate media. II. Tuning fork densitometer – Working principle: The sensor is designed based on the principle of vibration; this vibrating element resembles a tuning fork with two prongs. Vibration is generated in the prong due to a piezoelectric crystal located at the base of the prong, and the frequency of this vibration is detected by another piezoelectric crystal. Through phase-shifting and amplification circuits, the prong is stabilized at its natural resonant frequency. As the liquid flows through the fork, the vibration changes, causing a shift in the resonant frequency; this allows an accurate density value to be calculated using the electronic processing unit. Practice has shown that this densitometer does not perform well when used to measure media that are prone to crystallization, scaling, or containing particles. III. γ-ray type density meter: The principle of operation of a radioactive density meter is that radiation loses intensity as it passes through a material, and the extent of this attenuation depends on the density of the material in the measurement channel. When the measurement channel remains constant, the degree of attenuation is a function of the material’s density. A radioactive densitometer can measure parameters such as the density of materials inside a container without coming into contact with the object being tested, especially in conditions of high temperature, high pressure, high corrosivity, or toxicity. The disadvantages are that scaling and wear on the inner wall of the pipes cause measurement errors, complicated approval procedures, and strict management and inspection. IV. The measurement principle of the differential pressure density meter is based on the difference in gravitational force generated by a liquid column, ΔP = ρgh. When h remains constant, the differential pressure value measured by the differential pressure transmitter, divided by the acceleration due to gravity and the length, yields the density value. The differential pressure densitometer is a simple, practical, and cost-effective product. This product may have drawbacks such as large errors and unstable measurements. However, all these drawbacks can be improved in different ways. V. Buoy-type density meter: The principle of operation of a buoy-type density meter is to determine the density of a liquid by measuring pressure changes. Its structure is similar to that of a buoy-type level gauge; by measuring the buoyant force acting on the floating ball, it is possible to calculate the density of the medium in question. Buoyant ball densitometers are susceptible to disturbances in the medium, resulting in relatively large errors. VI. The working principle of the ultrasonic density meter is based on the propagation speed of ultrasound in liquids. Theory and experiments have shown that, at specific temperatures, the speed at which ultrasound travels in a liquid of certain concentration or density is fixed; changes in the liquid’s concentration result in corresponding changes in the speed of ultrasound propagation. The propagation speed of ultrasound in a liquid is a function of the liquid’s elastic modulus and density; therefore, differences in the ultrasound propagation speed within a liquid at a given temperature reflect corresponding changes in the liquid’s concentration or density. In this way, when the ultrasonic sensing unit of the instrument emits an ultrasonic signal and measures its propagation speed in the process fluid as well as the current temperature of the fluid, the instrument can use these data related to concentration, temperature, and speed to accurately determine the current concentration or density value. The price of this product is also relatively high, and its measurements are greatly affected by bubbles. In addition, there are limitations in its own circuitry as well as environmental interference in industrial settings; therefore, the accuracy of this product needs to be improved.