Level measurement
With level measurement (also: level measurement ), the level (level) of liquids and bulk solids in a container is recorded using level measuring devices. A distinction is made between continuous measurement using level sensors and level limit switches.
With continuous level measurement, the measured values are output in percent or in units of length, volume or mass as an analog signal or digital value , depending on the height that can be achieved . See also: display (technology)
Level limit switches monitor limit levels, for example to avoid running dry or overfilling a container. The measured values are output via relay outputs (switching contacts). If the limit switches are built into the side wall of a container, their positions must be determined beforehand.
Combination devices, consisting of continuous measurement in connection with relay units, are often used where, for safety reasons ( leakage ), a side opening of the container is not appropriate. The probes are installed from above. The switching points can be freely selected according to the level. There is no planning or assembly effort.
Level measurements have different tasks in industry. The selection of the suitable measurement method essentially depends on whether it is liquids or bulk solids. The physical properties of the product, the operating conditions such as Ex zones, explosion protection , overfill protection according to § 19 WHG and the nature of the container must also be taken into account.
Measuring systems
Mechanical level measurement
swimmer
A body with a low density floating on the liquid. The height of the swimmer is determined. For this purpose, the float can be attached to a lever or a thin rope; thereby a switch or a displacement transducer, such. B. a potentiometer moved. A float can also be guided on a vertical pipe. The position of the float can then be detected without contact with an inductive sensor, or a magnet attached to the float can activate a magnetic switch ( reed contact ) or be detected with a magnetostrictive sensor.
Vibration sensors
A sensor in the form of a tuning fork is excited to vibrate at its resonance frequency. The drive is usually piezoelectric. Immersion in a medium changes the vibration frequency or amplitude. This change is evaluated and converted into a switching signal. Adjustment and maintenance-free operation. For all liquids, even with build-up, turbulence or air bubbles, regardless of the electrical properties of the medium.
Rotary switch
The rotation of a wing around an axis is stopped by covering it with bulk material; then a relay (limit switch) switches. When using a conventional synchronous motor, the required holding torque can only be adapted to the bulk material by changing the wing geometry. When using a modern stepper motor, the required holding torque is generated by individually setting the motor torque. This eliminates the need to adjust the wing geometry. The measuring method is independent of dust and caking.
Electromechanical soldering systems
A measuring tape weighted down with a sensing weight is lowered into a silo. When the weight hits the surface of the product, the tensile force on the measuring tape decreases, causing the motor to switch over. The weight returns to the starting position. The fill level can be calculated from the length of the unwound tape. Application: For continuous measurement of bulk solids in high silos, unaffected by heavy dust formation.
Hydrostatic level measurement
The hydrostatic level measurement is based on the measurement of the difference between the pressures on the surface of the liquid and the bottom of the container, which is generated by the height of the column of liquid. The measured pressure difference is therefore a direct measure of the fill level and the height of the liquid above the sensor and is independent of, for example, foam, turbulence and boiler installations.
See also: Pneumatic level measurement
Displacement body principle
The weight of a displacement body immersed over the entire fill level range of a liquid is measured. The displacement body is dimensioned in such a way that at the maximum fill level it is still heavier than the resulting buoyancy. There is no change in position as with a swimmer. The suspension of the displacement body transfers the force to a corresponding system, which uses it to generate an electrical or pneumatic signal that is to be processed.
Differential pressure
In closed, pressurized containers, the hydrostatic pressure of the liquid column leads to a pressure difference. This pressure difference leads to a deflection of the measuring element, which is proportional to the hydrostatic pressure. One sensor is installed above the highest liquid level and one sensor is installed at the bottom of the container and fed to the corresponding evaluation unit.
Level probes
Level probes are special pressure transmitters for measuring the level of liquids in open containers and tanks. Level probes are lowered directly into the liquid to be measured and remain permanently suspended above the bottom of the tank. The measurement is based on the hydrostatic principle. The gravitational pressure of the liquid column causes an expansion of the pressure-sensitive sensor element, which converts the detected pressure into an electrical signal. The connection cable of level probes has to fulfill several tasks. In addition to the power supply and signal transmission, the level probe is held in place by the cable. The cable also contains a thin air hose with which the ambient air pressure is conducted to the level probe. Accordingly, level probes are usually designed as relative pressure sensors, which use the current ambient pressure as the zero point of their measuring range.
Conductivity measurement
Measurement of electrical conductivity
This method is also known as conductive measurement. When a certain level is reached, the electric current is passed through the liquid between two electrodes. The resistance between two measuring electrodes changes due to the presence or absence of the medium.
By using alternating current in the measuring circuit, corrosion of the probe rod and electrochemical reactions of the product are avoided. A time delay is often built into the output signal for reliable measurement when the surface of the liquid is moving. Conductive probes with one electrode can be used in metal containers as full or empty alarms, with two electrodes in metal containers as full and empty alarms and with three electrodes as full and empty alarms also in non-metallic containers. In the case of single rod probes, the electrically conductive container wall serves as the counter electrode.
Conductivity measurement is a simple and inexpensive measurement method for conductive liquids such as water, waste water and liquid food for point level detection.
Conduction
Compared to air, liquids have a much better thermal conductivity . A heating element with constant power supply reaches a lower temperature when immersed in liquid. A temperature sensor combined with the heating element registers the temperature change. Combined with a second temperature sensor, the influence of the outside temperature can be eliminated. Elimination of the outside temperature is slower if only one temperature sensor is used and the heating element is switched off cyclically.
The thermal conductivity measurement is only suitable to a limited extent for solids, as the heat transfer is often poor due to poor contact between the sensor and the medium.
Superconductivity
This measuring principle is mainly used for liquid helium . Current is conducted through a vertically stretched wire. In the area in which it is immersed in liquid helium, the wire is superconducting ; above it it is warmed up by the current ( Joule heat ); at this higher temperature the wire is normally conductive. As a result, only that part of the wire that is not immersed in liquid helium contributes to the electrical resistance . By measuring the resistance, it is possible to determine the height up to which liquid helium is in the container. The wire usually consists of a niobium - titanium alloy.
Capacitive measurement
Continuous level measurement
With capacitive level measurement , the change in the electrical capacitance between the electrodes is detected when they are surrounded by a medium. This change depends on the dielectric constant (also abbreviated as DK in measurement technology) of the medium. If this is constant, the measured capacitance can be used to determine how far the electrodes are immersed in the medium. In this case, not only can a limit switch be built, but the continuous level can also be determined.
Level switch
When measuring with capacitive switches, a change in the dielectric constant is detected and this change is converted into a switching signal. The advantage of this technology is that the medium can partly be detected behind container walls. No drilling is therefore necessary in the container. The function depends on the ratio of the dielectric constant of the medium to the dielectric constant of the container wall. A typical pairing is a water-based liquid behind a plastic or glass wall.
Switch with frequency swing technology
A sensor determines the dielectric constant of the medium. The signal can then be converted into a switching signal. In this way, unwanted substances that adhere to the sensor can be hidden. This applies to buildup on the sensor tip, foams or a medium flowing past during filling.
Optical measurement
The absorption of light or the disappearance of total reflection is detected when the sensor is immersed in the medium. The optoelectronic level sensor consists of an infrared LED and a light receiver. The light from the LED is directed into a prism at the tip of the sensor. As long as the tip is not immersed in liquid, the light is reflected inside the prism to the receiver. If the liquid rises in the container and surrounds the tip, the light is refracted by the liquid and no longer reaches the receiver or is weakened. The evaluation electronics convert this change into a switching process. The problem with this measuring method is the sensitivity to contamination.
Ultrasonic
The measurement with ultrasound is based on a transit time measurement . The ultrasonic impulses sent by a sensor are reflected from the surface of the medium and recorded again by the sensor. The required running time is a measure of the distance covered in the empty container part. This value is subtracted from the total height and the level is obtained from it. Due to the decay behavior of the sensor, there is an area immediately below the sensor called the blocking distance, in which area no pulses can be received. The so-called blocking distance determines the minimum distance between the measuring device and the maximum level.
This echo sounding method is a contactless and maintenance-free measurement without being influenced by product properties such as dielectric constant, conductivity, density or humidity.
In addition to this widespread ultrasonic measurement from above through the gas space, there are also sensors that measure the fill level through the container wall from below or serve as limit switches from the side. The transit time in the medium itself is measured here. The most important functional requirement of this method is the soundness of the wall and medium. The application is therefore limited to single-walled containers with liquids.
Microwaves
The measurement with microwaves is based on a transmitter-receiver principle. Microwave pulses sent by a transmitter are registered by the receiver through an empty silo, but attenuated by the medium through a full silo. In the case of metallic container walls, viewing windows must be installed. Applications: Since the microwave barrier uses a contactless detection method, it can be installed in containers, pipelines, shafts or on free-fall shafts. In the case of non-metallic container materials, a measurement through the container from the outside is possible. Microwave barriers report blockages, signal limit levels, solve positioning and counting tasks, measure contact-free from the outside and are therefore wear-free and maintenance-free. Typical areas of application are e.g. B. wood chips, paper and cardboard chips, lime, gravel, sand or even whole bags and boxes. Process temperatures and pressures are arbitrary, however, with direct installation, the limit values of the respective manufacturer must be observed.
radar
The radar sensor works with high-frequency radar signals that are emitted by an antenna and reflected from the surface of the product. The transit time of the reflected radar signal is directly proportional to the distance covered. If the container geometry is known, the fill level can be calculated from this. The determined level is converted into an output signal and output as a measured value. The radar technology on the device is always the frequency-modulated continuous wave radar method . False echoes from possible installations such as cooling or heating or even agitators are masked out by the radar software.
With a well-filled container, the echo from the bottom of the container appears farther away than with an empty container, because the propagation speed of the electromagnetic waves within the product is lower than in air. The measured apparent distance from the ground is a second measurement result with the level. Only when both results (direct reflection on the product surface and apparent distance from the bottom) agree in the result for the fill level, then the echo or the measured value is displayed. With this "multi-echo tracking" procedure, all false echoes are masked out by built-in components. Thanks to the non-contact measuring principle, this type of level measurement is particularly suitable for applications with aggressive media or dynamic process conditions, such as density fluctuations.
Guided radar
The radar sensor works with high-frequency radar signals that are guided along a probe. When the signals hit the media surface, the wave resistance changes and part of the transmission energy is reflected. The length of time measured and evaluated by the device between sending and receiving the reflected signals is a direct measure of the distance between the process coupling and the media surface. The maintenance-free measurement can be used in liquids, even with turbulence and foam. The measurement is independent of density, temperature, conductivity and humidity and is not influenced by vapors above the liquid. Interfering signals from built-in components, such as with radar measurements in free tanks, cannot occur.
Radiometry
With the radiometric level measurement , the intensity of gamma rays is recorded, which penetrate the medium on the way from the radioactive source to the detector. A gamma source, mostly cesium or cobalt isotope, emits radiation that is attenuated when it penetrates materials. The measuring effect results from the absorption of the radiation by the medium to be measured. It is not necessary to open the container or fixtures in the container; the measurement is carried out contactless from the outside and is therefore suitable for extreme applications such as highly corrosive, aggressive and abrasive media.
literature
- Ellen Amberger: Level measurement technology: Basics and application examples. Verlag Moderne Industrie, 1988, ISBN 978-3-478-93014-7 .
- Rüdiger Settelmeyer: From field device to automation solution. Christiani, Konstanz 2007, ISBN 978-3-86522-305-0 .
Web links
Individual evidence
- ↑ Level measurement of bulk material
- ↑ Michael J. Heim and Andreas Mayr, Selection criteria for radar systems for level measurement in the chemical industry , February 2001, p. 3 ( PDF )
- ↑ Filling level control / Glossary / Support / Service & Support / Bürkert Fluid Control Systems. Retrieved August 20, 2018 .