Working principles of 20 commonly used level gauges

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By explaining the working principles of 20 commonly used level gauges and analyzing their installation, usage, and precautions, this article helps to identify potential faults in these level gauges and determine how to address them. It provides a comprehensive understanding of level gauges, thereby offering a basis for making accurate decisions when selecting one suitable for a particular application. Common types of level gauges: 1. Magnetic flip board level gauge 2. Float level gauge 3. Steel belt level gauge 4. Radar level gauge 5. Magnetostrictive level gauge 6. Radio frequency admittance level gauge 7. Fork-type level gauge 8. Glass plate/glass tube level gauge 9. Hydrostatic level gauge 10. Pressure level transmitter 11. Capacitive level gauge 12. Intelligent electric float level gauge 13. Buoy level gauge 14. Float level transmitter 15. Electrical contact level gauge 16. Magnetosensitive two-color electronic level gauge 17. External measurement level gauge 18. Hydrostatic level gauge 19. Ultrasonic level gauge 20. Differential pressure level gauge (double flange level gauge). Working principles of commonly used level gauges: 1. Magnetic flip board level gauge: Also known as magnetic float level gauge or magnetic column level gauge. Principle: It operates based on the principle of a communicating vessel, and was developed using the principles of buoyancy and magnetic coupling. When the liquid level in the container being measured changes, the permanent magnet inside the float transmits a magnetic signal to the magnetic flip-column display panel, causing the red and white flip columns to rotate 180°. When the liquid level rises, the flip columns change from white to red; when the liquid level drops, they change from red to white. The point where red and white meet on the panel indicates the actual height of the liquid level in the container, thereby enabling the display of the liquid level. 2. Float level gauge: The structure of a float level gauge is designed and manufactured based on the principles of buoyancy and static magnetic fields. The position of the float with a magnet (referred to simply as the float) in the medium being measured is influenced by buoyancy; changes in liquid level cause changes in the position of the magnetic float. The magnet and sensor (reed switch) inside the float ball cause a change in the number of components connected in series in the circuit (such as fixed resistors), which in turn alters the electrical parameters of the instrument’s circuit system. In other words, it is the change in the position of the magnetic float that causes a change in electrical quantities. The level of liquid inside the container is indicated by detecting changes in electrical quantities. 3. Steel strip level gauge: It is designed and manufactured based on the principle of mechanical equilibrium. When the liquid level changes, the original mechanical equilibrium is disrupted by the buoyant force acting on the float, and a new equilibrium is achieved through the movement of the steel strip. The level detection device (float) moves the steel belt in accordance with the liquid level; the displacement transmission system causes the transmission pin to rotate through the movement of the steel belt, which in turn acts on the counter to display the liquid level. 4. Radar level gauge: A radar level gauge is a measuring instrument based on the time-of-travel principle. Radar waves travel at the speed of light, and the travel time can be converted into a level signal using electronic components. The probe emits high-frequency pulses that travel along the cable-like probe; when these pulses encounter the surface of the material, they are reflected back and captured by a receiver inside the instrument, which then converts the distance signal into a level signal. 5. Magnetostrictive level gauge: When the sensor of a magnetostrictive level gauge is in operation, its circuitry generates a pulsed current in the waveguide wire; as this current propagates along the waveguide wire, it creates a magnetic field of pulsed current around the wire. A float is attached to the sensor rod of the magnetostrictive level gauge; this float can move up and down along the rod as the liquid level changes. Inside the float, there is a set of permanent magnetic rings. When the magnetic field generated by the pulsed current meets the magnetic field of the magnetic ring produced by the float, the magnetic field around the float changes, which causes a torsional wave pulse to be generated in the waveguide wire made of magnetostrictive material at the location of the float. This pulse travels back along the waveguide wire at a constant speed and is detected by the detection mechanism. By measuring the time difference between the pulsed current and the torsional wave, the position of the float, that is, the level of the liquid surface, can be determined accurately. 6. Radio frequency admittance level gauge: A radio frequency admittance level gauge consists of a sensor and control instruments; the sensor can be in the form of a rod-type, coaxial, or cable-type probe, and it is installed on the top of the tank. The pulse converter in the sensor can convert changes in liquid level into pulse signals, which are sent to the control instrument. After processing these signals, the control instrument converts them into numerical values for display, thereby enabling continuous measurement of the liquid level. 7. Tuning fork level gauge: The operating principle of the tuning fork level controller is that a pair of piezoelectric crystals mounted on the tuning fork base cause the tuning fork to vibrate at a certain resonant frequency. When the tuning fork comes into contact with the medium being tested, its frequency and amplitude change. These changes are detected and processed by an intelligent circuit, which then converts them into a switch signal. 8. Glass plate level gauge (glass tube level gauge): The glass plate type level gauge is connected to the container via flanges to form a communicating vessel, allowing the liquid level inside the container to be read directly through the glass plate. 9. Pressure level transmitter: Pressure-type level gauges operate on the principle of hydrostatic pressure measurement. When the level transmitter is placed at a certain depth in the liquid to be measured, the pressure acting on the sensor’s surface in contact with the liquid is transmitted to the sensor’s positive pressure chamber through stainless steel tubes. Meanwhile, the atmospheric pressure Po at the liquid surface is connected to the sensor’s negative pressure chamber, thereby canceling out Po on the back side of the sensor. As a result, the pressure measured by the sensor is ρ.g.H. By measuring this pressure P, it is possible to determine the liquid level depth. 10. Capacitive level gauge: A capacitive level gauge measures the height of a 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 container wall serves as the other pole. The medium between the two electrodes is the liquid and the gas above it. Since the dielectric constant ε1 of the liquid is different from that ε2 at the liquid surface – for example, ε1 > ε2 – as the liquid level rises, the overall dielectric constant between the two electrodes of the capacitive level gauge increases, thereby increasing the capacitance. Conversely, as the liquid level drops, the ε value decreases, and the capacitance also decreases. Therefore, a capacitive level gauge can measure the level of a liquid by detecting changes in the capacitance between two electrodes. 11. Intelligent float level gauge: The intelligent float level gauge is a level measurement instrument designed based on Archimedes’ principle and magnetic coupling. It can be used to measure liquid levels, interface levels, and density, and it is responsible for generating alarm signals for upper and lower limit conditions. 12. Float level gauge: It is designed and manufactured based on the principle of mechanical equilibrium. When the liquid level changes, the original mechanical equilibrium, disturbed by the buoyancy force acting on the float, will reach a new equilibrium through the movement of the steel strip (cable). The liquid level detection device (float) moves the steel belt (rope) in accordance with the liquid level, and the displacement transmission system drives the on-site indication device through the movement of the steel belt (rope), thereby displaying the liquid level on the display device. 13. Float level transmitter: The float is submerged in the liquid within the float chamber and is rigidly connected to a torsion tube system. The force acting on the torsion tube system is the net value of the float’s own weight minus the buoyant force acting on it; under this combined force, the torsion tube twists by a certain angle. Changes in the position, density, or level of the liquid inside the float chamber cause variations in the buoyant force acting on the float submerged in the liquid, thereby causing changes in the rotation angle of the torsion tube. This change is transmitted to the sensor that is rigidly connected to the torsion tube, causing the sensor’s output voltage to change; this voltage change is then amplified by electronic components and converted into a 4–20mA current output. The float level transmitter uses a microcontroller and associated electronic circuits to measure process variables, provides a current output to drive the LCD display, and offers HART communication capabilities. 14. Electric contact level gauge: The electric contact level gauge is designed based on the difference in resistivity between water and steam. The electrodes of the measuring cylinder have a low impedance to the cylinder body in water. The impedance of the cylinder in steam is high. As the water level changes, the number of electrodes in the water changes as well. Converted into changes in resistance value. It is transmitted to the secondary instrument, thereby enabling functions such as water level display, alarm, and protection interlocking. 15. Magnetic-sensitive dual-color electronic level gauge: The magnetic-sensitive electronic dual-color level gauge is manufactured using high-quality stainless steel and imported electronic components. Its display area features high-brightness LED dual-color lights that form a columnar display; through the red and green changes in these LED lights, it is possible to achieve alarms and control for the upper and lower limits of the liquid level. 16. External level gauge: An external level gauge is a device that uses the principle of sonar ranging and \"micro-vibration analysis\" technology to measure the liquid level from outside the container. Two compact external level gauge ultrasonic sensors are installed, one at the bottom of the tank and the other on its side wall, to compensate for changes in density. The signal from the external level gauge sensor is processed by a microprocessor and sent to either a local display or the user control system. It is possible to calculate the height of the liquid in the tank and its volume. 17. Hydrostatic level gauge: A hydrostatic level transmitter in which a diffused silicon oil-filled core is enclosed within a stainless steel housing. The front protective cap serves to protect the sensor diaphragm while also allowing liquid to come into smooth contact with it. Waterproof wires are sealed to the housing, and an air vent connects to the outside world inside the cable; the internal structure is designed to prevent condensation. 18. Ultrasonic level gauge: An ultrasonic level gauge/level sensor consists of a complete ultrasonic sensor and control circuit. The ultrasonic waves emitted by the ultrasonic sensor are reflected off the liquid surface; the time it takes for them to return is used for calculation. The temperature sensor is used to account for any effects of temperature on the propagation of these ultrasonic waves, and this information is then used to determine the distance between the liquid surface and the ultrasonic sensor. The result is displayed on an LCD screen, and an analog signal of 4mA–20mA DC is generated, enabling remote reading of the instrument at the site. 19. Differential pressure level gauge (double-flange level gauge): A differential pressure level transmitter measures the pressure difference between high and low pressures, and then uses a conversion unit to transform this value into an electrical signal that is sent to the electrical components in the control room. Differential pressure level gauges are primarily used for measuring the level in pressurized, sealed containers. The magnitude of the differential pressure also represents the level height of the liquid. The differential pressure gauge is used to measure the differential pressure between the gas and liquid phases, thereby determining the liquid level. Installation, use, and precautions for level gauges – Precautions for the installation and use of glass tube level gauges: When transporting and installing these level gauges, care must be taken to avoid mechanical impacts to prevent the glass from breaking; this is something that requires special attention. Level gauges come in various specifications and models; it’s essential to select a suitable model. For instance, certain media require colorless, light-transmitting level gauges, while others may necessitate level gauges equipped with steam jackets for thermal insulation purposes. Humidity sensor probes, stainless steel electric heating tubes with PT100 sensors, cast aluminum heaters, fluid solenoid valves for heating coils. After the level gauge is installed, when the temperature of the liquid in the container is very high, the valve should not be opened immediately; it needs to be preheated for a while. The valve should be opened only after the glass tube of the level gauge has reached a certain temperature, in order to prevent the glass from cracking due to thermal expansion and contraction. During the use of the level gauge, it is necessary to regularly clean the inner and outer walls of the glass tube to prevent blurred vision. When cleaning, be sure to close both the upper and lower valves tightly to avoid any leakage of liquid from the container. After that, rinse the removed glass tube with water or soak it in alcohol. Care should also be taken during installation, and there must be a proper method. If the glass tube is broken and needs to be replaced, care must be taken during disassembly and installation. After installation, it is also necessary to check for any leaks; it can only be put into use if no leaks occur. It should be regularly inspected and maintained during use to prevent leakage caused by rust and corrosion, and records of usage and maintenance should be kept. Precautions for the installation and use of glass plate level gauges: During installation, to ensure proper automatic sealing, the liquid inside the container must be under a certain pressure, at least 0.2 megapascals. When opening the upper and lower valves, the valve stem should rotate at least four full turns; this is necessary so that the steel ball used for sealing does not come into contact with the top of the valve stem, thereby preventing damage to it. During transportation and handling after unboxing, be sure to avoid touching hard objects to prevent the glass panels from breaking. It is also important to pay attention to the specifications and models; liquid level gauges of inappropriate specifications must not be installed. There are restrictions on certain types of media when it comes to liquid level gauges – for example, media that are corrosive to glass or steel plates cannot be used. For other maintenance and cleaning tasks, the methods for disassembly, cleaning, installation are similar to those for glass tube level gauges. Precautions for the installation and use of magnetic float level gauges: Consider the measurement object, such as the physical and chemical properties of the medium to be measured, as well as the operating pressure, temperature, installation conditions, and the rate of level change ； Another set of considerations are the measurement and control requirements, such as the measurement range, accuracy, display method, on-site indication, remote indication, interface with computers, safety and corrosion resistance, reliability, and ease of installation. All these factors need to be taken into account; only by considering them thoroughly can one select a suitable option for use. During installation, ensure that the level gauge is vertical, and that the float can move freely. It must not be installed upside down. Otherwise, the installation procedure is similar to that for glass-plate or glass-tube level gauges. Therefore, we can only properly install various level gauges if we know their installation and usage methods. Precautions for the installation and use of magnetic flap level gauges: Before use, use calibration magnets to set the ball below the zero point of the level gauge to red, while setting the balls in other areas to white. When users install tracing pipes on their own, they should choose non-magnetic materials, such as copper pipes. The temperature of the tracing pipes must be determined based on the specific medium being transported. The installation location of the magnetic flap level gauge should also avoid or be kept away from the inlet and outlet of the medium, otherwise rapid changes in the medium density in those areas can affect the accuracy of the measurement data. It is also important to note that the medium should not contain any solid impurities or magnetic materials, as these can interfere with the operation of the float. Around the magnetic flap level gauge, it is also not allowed for any magnetically conductive materials to be in proximity, and iron wires should not be used to secure the gauge, as this can affect its proper operation. During installation, the first thing to note is to open the bottom of the level gauge and insert the float; make sure that the magnetic end faces upward, as it must not be installed in the reverse direction. After installation, during debugging, the valve on the pipe leading to the level gauge should be opened first, followed by the slow opening of the valve below, so that the fluid can enter the detection tube gradually and smoothly. It is also necessary to check whether the red and white balls on the level gauge move properly; if they do, the valve on the pipe should be closed, and then the drain valve should be opened to allow the liquid level in the main pipe to drop gradually. This process should be repeated three times. Once it is confirmed that the level gauge is working properly, it can be put into normal operation. Precautions for the installation and use of capacitive level gauges: When installed outdoors, the probe wire must not be exposed outside the container, to prevent measurement errors caused by water contacting the probe wire on rainy days. The stainless-steel process connection components on the lower part of the enclosure or junction box must be reliably connected (grounded) to the outer wall of the vessel; their contact resistance must not exceed 2. During normal operation, the probe wire should not swing excessively within the container; otherwise, signal instability will occur. When installing the probe wire, it should be kept as far away as possible from the inner wall of the container; the minimum distance must not be less than 100 mm. When constrained by conditions and the distance is < 100 mm, the distance between the probe wire and the container must be kept relatively constant. For single-wire soft probes, the excess portion can be pulled out through the upper end of the process connector and then cut off; afterwards, the set screw should be tightened. For twisted-pair probes, the excess portion can be coiled above the liquid surface being measured – it is absolutely not allowed to be coiled at the bottom of the container or within the effective measurement section. When there is stirring inside the container or when the liquid may produce a large number of bubbles, in order to protect the probe wire and avoid false liquid levels caused by liquid fluctuations and bubbles, a metal or non-metallic tube with an inner diameter greater than 80 mm can be placed inside the container. The lower end of the tube should be open to serve as a liquid inlet, while an exhaust hole should be provided below the liquid surface. When using a metal tube, it is necessary to ensure that the probe wire remains stable within the tube; if needed, the probe wire should be supported to keep it straight. Precautions for the installation and use of float level gauges: Welding or riveting of the lower support for the guide wire – Determine the appropriate position at the bottom of the container according to the direction in which the float of the level gauge moves, and then weld or rivet in place the lower support for the guide wire. If the equipment does not meet the welding requirements, the lower support of the guide wire can be fixed using a heavy anchor. If the liquid level in the container fluctuates little, it is not necessary to install guide wires. Installation of the guide wire: (1) Pull the wire straight with force and secure it to the lower support, being careful not to cause any bending or kinking in the wire, as this could affect the up and down movement of the float. (2) Pass the guide wire through the guide ear hook of the float in the float level gauge, then thread it through the hook screw. Secure the end of the wire and clamp it using a wire clamp. Tighten the hook nut to put the wire under tension; subsequently, tighten the upper nut to prevent any loosening. Finally, put the cover back on. (3) The two guide wires must be perpendicular to the ground and parallel to each other, with a distance of 300 mm between them. Ruler installation. The length of the scale on a float level gauge is determined based on the measurement range specified by the user at the time of ordering. During installation, it is required that (1) the connections between sections of the scale be straight and smooth, with no irregularities, in order to prevent any interference with the proper functioning of the weight pointer or to avoid measurement errors. (2) The scale should be perpendicular to the liquid level inside the storage tank; there should be no tilt. The verticality of the scale’s installation must not exceed 5°, to prevent the weight pointer from getting stuck and causing measurement failures. (3) When welding the scale tripod (piece 11), efforts should be made to ensure that the mounting surfaces are on the same plane and the mounting holes are on the same straight line; in other words, it is necessary to keep the scale’s scale surface as well as the guides on both sides of the plumb bob pointer straight, so that the plumb bob pointer can move up and down freely along the scale. Note: The distance between the two scale tripods is 1 meter. Installation of the buoy attachment wire and weight pointer: One end of the attachment wire is secured to the buoy using a wire clamp. The other end of the wire is then fed through two pre-installed guide pulley boxes and connected to the weight pointer; this connection is also secured with a wire clamp. Finally, the guide pulley brackets are adjusted so that the attachment wire is perpendicular to the horizontal plane. This ensures that the buoy moves freely along the wire, gliding smoothly without any jamming, twisting, knotting, coiling, or damage. Note: Determine the required length of the wire. Installation of the buoy connection wire and the weight pointer: The buoy of the float level gauge is fixed in place using a wire clamp at one end of the connection wire; then, the other end of the connection wire is inserted into the two guide pulley housings that have been installed in advance, and it is connected to the weight pointer, which is also secured with a wire clamp. Finally, the guides for the pulleys are adjusted so that the connection wire remains perpendicular to the horizontal plane, allowing the buoy to move freely along the wire without any issues such as jamming, twisting, knotting, curling, or damage. Precautions for installing and using ultrasonic level gauges: Selection of measurement range for ultrasonic level gauges: When measuring liquids, the gauge can be selected based on its rated measurement range. If measuring a solid (please consult the manufacturer’s sales staff in advance), it is necessary to increase the measurement range; it cannot be used for soft materials such as flour, cotton, or sponge. If there are many bubbles covering the liquid level during measurement, it is also necessary to increase the range; a bubble thickness of over 5 cm is not recommended. If there is dust or vapor in the air, please consult the manufacturer’s technical staff in advance to use a higher range. When selecting the probe material for an ultrasonic level gauge, the main factor to consider is whether the environment is corrosive: in general, weak acid or weak alkali environments can use ordinary probes, while in highly corrosive environments, corrosion-resistant probes are required. In the presence of strong acids and bases, we also need to consider whether fog will be formed; in cases where fog does form, it is necessary to use a higher measurement range. The installation position of the probe depends on the emission angle and the potential for false reflected echoes: the ultrasonic beam is focused through the probe, with the emission of the pulse beam resembling the beam from a flashlight; probes of different types and ranges have varying emission angles. Any object within the firing angle, such as pipes, container supports, and other devices, will generate strong spurious echoes, especially in the few meters closest to the probe. When the transducer emits ultrasonic pulses, it does so at a certain emission angle. There must be no obstacles in the area between the transmitting surface of the transducer and the surface of the medium being tested, as well as within the range radiated by the ultrasonic wave beam. Therefore, when selecting the installation location for the transducer, obstacles such as ladders, brackets, pump and valve equipment should be avoided as much as possible. When avoidance is not possible, false echoes can be filtered out through program adjustments. The transducer should be perpendicular to the surface of the medium being tested to ensure that the reflected echo signals can be received. Furthermore, the transmitting surface of the transducer must be at a sufficient distance from the highest liquid level, so that the highest liquid level does not enter the measurement blind zone. When the transducer emits ultrasonic pulses. They all have a certain emission angle. There must be no obstacles in the area between the transmitting surface of the transducer and the surface of the medium being tested, as well as within the range radiated by the ultrasonic wave beam. Therefore, when selecting the installation location for the transducer, obstacles such as ladders, brackets, pump and valve equipment should be avoided as much as possible. When avoidance is not possible, false echoes can be filtered out through program adjustments. The transducer should be perpendicular to the surface of the medium being tested to ensure that the reflected echo signals can be received. Furthermore, the transmitting surface of the transducer must be at a sufficient distance from the highest liquid level, so that the highest liquid level does not enter the measurement blind zone. Precautions for the installation and use of radar level gauges: The measurement range starts from the point where the beam reaches the bottom of the tank; however, in special cases where the bottom of the tank is concave or conical, measurements cannot be taken when the liquid level is below this point. When the dielectric constant of the medium is low and the liquid level is low, the bottom of the tank becomes visible; to ensure measurement accuracy, it is recommended to set the zero point at a height of C. Theoretically, it is possible to measure the position at the tip of the antenna, but considering the effects of corrosion and adhesion, the end of the measurement range should be at least 100 mm away from the antenna tip. For overflow protection, a safety distance can be defined and added to the blind area. The minimum measurement range is related to the antenna. Depending on the concentration, foam can either absorb microwaves or reflect them, but it is possible to make measurements under certain conditions. Points to note when using a radar level gauge: 1. The measurement range of a radar level gauge starts from the point where it comes into contact with the beam; however, if the bottom of the tank in which the gauge is used is concave, the measurement should start from its lowest point. 2. When in use, pay attention to the dielectric constant of the medium. If the medium has a low dielectric constant, it is best to set the zero point at a position at a low height of C when the liquid level is low; this will enable better measurement accuracy. 3. When making measurements, the effects of corrosion and adhesion must be taken into account; the final value within the measurement range should be at least 100 mm away from the tip of the antenna. 4. The antenna can affect the minimum measurement range. 5. Set a safety distance in addition to the blind area, which can serve as an overflow protection mechanism. Fault Phenomena and Solutions for Magnetic Flip-Plate Level Gauges: No display on the panel: Check whether the float is damaged or demagnetized; check whether the flip column of the panel is demagnetized. Unstable remote transmission output: Check the line voltage; look for intermittent short circuits, open circuits, or multiple ground connections. Faulty module circuit board.

For Steel-Belt Level Gauges: The instrument does not function properly: Check whether the counter is stuck; check the float; check the belt. Unstable remote transmission output: Check the connection between the remote transmission unit and the sprocket. Faulty module circuit board.

For Float Level Gauges: The display does not change with changes in liquid level: Check whether the rotating shaft is in good contact with the transmitter; check the power supply voltage; check the zero point and range settings. Sensor failure, circuit board failure. Actual liquid level changes, but the display does not change: The external balance rod has separated from the rotating shaft; the weight has not been adjusted properly; internal connectors are loose or detached; the ball rod is deformed; the float has fallen off; the float is broken; the medium has vaporized.

For Differential Pressure Level Gauges: Large changes in liquid level: Large fluctuations in the medium or severe vaporization; the upper or lower pressure lead wires are blocked; crystals are present in the medium; the pressure-transmitting fluid in the capillary tube is lost; the diaphragm box is damaged; the heating temperature is too high. No change in display: The control valve is not open; the pressure lead wires are blocked; the range and zero point settings are incorrect; debris has accumulated around the diaphragm box; the capillary tube is compressed and blocked. Circuit board failure.

For Guided Wave Radar Level Gauges: Fluctuations in liquid level, output percentage, and circuit values: Reconfigure the probe length and deviation settings; use other devices to determine the accurate liquid level; adjust the damping coefficient; reconfigure the circuit values. The output remains the same regardless of the liquid level: Verify the probe length; adjust the bias value to achieve an accurate reading. No liquid level signal: Check the dielectric constant of the medium. The liquid level is in the top transition zone, and this was not taken into account during configuration; the circuit board or 16-pin connector is not functioning properly. Probe length configuration issues; there may be medium buildup on the probe; incorrect dielectric constant selection. The output is either at its maximum or minimum value, resulting in inaccurate readings: The medium is impure, such as oil mixed with water; medium or debris has built up on the probe; the guided wave rod is blocked; there is foam or viscous substances present; debris is present at the seal at the top of the probe.