Common Level and Level-Sensing Instruments and Their Principles and Characteristics.doc

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Common liquid level and level measurement instruments, along with their principles and characteristics. Liquid level gauges commonly used for measuring liquid levels include the communicating vessel type, bubble type, differential pressure type, and capacitive type; whereas instruments for measuring level include ultrasonic level gauges and radioactive level gauges. Its principles and characteristics are as follows: a. The communicating-tank type is the most commonly used glass level gauge; it features a simple structure, low cost, and ease of use, making it suitable for on-site application. However, it is prone to damage, and contamination of its inner surface can make reading difficult, as well as hindering remote transmission and adjustment. b. Buoyant level gauges are divided into two categories: constant buoyancy type and variable buoyancy type. (1) Constant buoyancy level gauge: A constant buoyancy level gauge relies on a float or buoy that floats in the liquid and rises or falls as the liquid level changes. It features a simple structure and low cost, making it suitable for measuring levels in various storage tanks ； (2) Variable buoyancy level gauge: The variable buoyancy type is also known as the float tube level gauge. When the liquid level changes, the volume of the float tube submerged in the liquid varies, which results in different buoyant forces and thus displacement. This displacement is converted into an angular displacement through mechanical transmission to measure the liquid level. Such instruments enable remote transmission and automatic adjustment. 　 c. The bubble level gauge uses the principle of hydrostatic pressure to measure the liquid level in open containers. After passing through the filter and pressure reducing valve, the compressed air is delivered to a certain pressure by a regulator; then, after passing through a throttling element, it splits into two paths: (1) One path goes into the conduit installed inside the container, from where it is blown out from the bottom of the container ； (2) The other path enters the pressure gauge for indication. When the liquid level is at its lowest, there is no resistance to bubble formation; the back pressure is zero, and the pressure gauge reads zero ； As the liquid level rises, the bubbles must overcome the static pressure of the liquid column; as a result, the backpressure increases and the pressure reading goes up. Therefore, the back pressure, which is the pressure indicated by the gauge, reflects the level of the liquid surface. The bubble level gauge has a simple structure and is inexpensive; it is suitable for measuring the level in open containers that are corrosive, have high viscosity, or contain suspended particles, but its accuracy is low. d. The differential pressure level gauge has pressure tapping points for both the gas phase and the liquid phase. The pressure at the gas-phase pressure tapping point is the pressure of the gas phase inside the equipment ； At the pressure measurement point in the liquid phase, the pressure is influenced not only by the gas-phase pressure but also by the hydrostatic pressure of the liquid column; the difference between the pressures in the liquid and gas phases is the hydrostatic pressure generated by the liquid column. Therefore, the liquid level can be measured using the following formula: Such instruments include pneumatic and electric differential pressure transmitters as well as flanged liquid level transmitters; they are easy to install, facilitate remote transmission and automatic control, and are widely used in industry. e. A capacitive level gauge measures the height of the liquid level by detecting changes in capacitance. It is a metal rod inserted into a container filled with liquid; the metal rod serves as one pole of the capacitor, while the wall of the container acts as the other pole. The dielectric between the two electrodes is the liquid and the gas above it. Since the dielectric constant ε1 of the liquid is different from the dielectric constant ε2 at the liquid surface – for example, ε1 > ε2 – as the liquid level rises, the overall dielectric constant between the two electrodes increases, thereby increasing the capacitance. Conversely, as the liquid level drops, the ε value decreases, and the capacitance also decreases. Therefore, the level of the liquid can be measured by the change in capacitance between the two electrodes. The sensitivity of a capacitive level gauge depends primarily on the difference between the two dielectric constants. Moreover, only when ε1 and ε2 remain constant can accurate level measurement be ensured. Since the medium being measured is conductive, the metal rod electrodes are covered with insulating layers. Capacitive level gauges are compact, easy to enable remote transmission and adjustment, and suitable for measuring the level of corrosive and high-pressure media. f. An ultrasonic level gauge utilizes the properties of ultrasonic waves, such as their attenuation, penetration ability, and differences in acoustic impedance in gases, liquids, or solids, to measure the interface between two media. Such instruments have high precision and fast response times, but they are expensive and difficult to maintain; they are used in applications where high measurement accuracy is required. g. A radioactive level gauge measures the level of a fluid by taking advantage of the differences in the absorption of radiation by radioactive isotopes as a function of the level height. It has a wide measurement range, and can be used to measure the level of highly viscous, highly corrosive, flammable, or explosive fluids in containers operating under low, high temperatures or high pressures. However, such instruments are expensive, inconvenient to use and maintain, and the radiation poses a serious risk to human health.