Level sensor

For level measurement of liquid level (platform height ) of liquids and solids in a container is detected by level measuring instruments. A distinction is made between the continuous measurement by level sensors and level limit switches.

In the continuous level measurement, the measured values ​​in percentage or in length, volume or mass units are output as analog signal or digital value corresponding to the reachable state level. See also: display (technology)

Level limit switch monitor boundary stands, for example to avoid dry running or overfilling of the container. The measured value is output via relay outputs ( relay contacts ). If the limit switch built into the side wall of a container, their positions must be set in advance.

Combinations, consisting of continuous measurement in conjunction with relay units are often used where, for safety reasons ( leakage ) has a lateral opening of the container is not appropriate. The installation of the probes from the top. The switching points can be chosen freely according to the liquid level. There is no need for planning and installation effort.

Have level measurements in industry different tasks. Selection of the appropriate measurement method depends mainly on whether there are liquids or solids. Here, the physical properties of the filling material, the operating conditions such hazardous areas, explosion protection, overfill protection must be taken into account according to § 19 WHG and the nature of the container.

Measuring systems

Mechanical Level Measurement

Float

A body of low density floating on the liquid. The height of the float is determined. For this, the float may be attached to a lever or thin wire; thereby, a switch or a transducer, such as a potentiometer moves. It may also be a float guided on a vertical tube. The position of the float can then be acquired with an inductive sensor, or fastened to the float magnet to actuate a solenoid switch ( reed switch) or be detected with a magnetostrictive sensor.

Vibration sensors

A sensor in the form of a tuning fork vibrates at its resonant frequency to vibrate. The drive is usually piezoelectric. By immersing in a medium, the oscillation frequency and amplitude changes. This change is evaluated and converted into a switch signal. Adjustment and maintenance free operation. For all liquids, even at build-up, turbulence or bubbles, independent of the electrical properties of the medium.

Rotary vane switch

The rotational movement about an axis of a wing is stopped by covering with bulk material; then a relay switches (limit switches). The required holding torque can be adjusted by changing the wing geometry of the bulk material. The method of measurement is unaffected by dust and caking.

Electromechanical plumb line

A weighted with a sensing weight measurement bandwidth is left down into a silo. Upon impact of the weight on the product surface, the tensile force can after the measuring tape, whereby the motor is switched. The weight goes back to the starting position. From the length of the unwound tape, the level can be calculated. Application: For continuous measurement of bulk solids in silos high, unaffected by strong dust formation.

Pressure measurement

Pressure measurement, both hydrostatic and by means of differential pressure, is independent of dielectric constant, foam, turbulence and installations, such as: struts and agitators.

Hydrostatic

The hydrostatic level measurement in open containers is based on the determination of the hydrostatic pressure generated by the height of the liquid column. The measured pressure is thus a direct measure of the level and the amount of liquid over the sensor. See also: Pneumatic level measurement

Displacer principle

The rate of the weight of a submerged over the entire range of liquid level displacement body. The displacement body is dimensioned so that it is still heavier than the resulting buoyancy force at maximum level. There is no change in position rather than as a float. The suspension of the displacer transmits the power to a corresponding system, which generates an electric or pneumatic signal weiterzuverarbeitendes thereof.

Differential pressure

In a closed and pressurized vessels, the hydrostatic pressure of the fluid column leading to a pressure difference. This pressure difference results in a deflection of the measuring element, which is proportional to the hydrostatic pressure. A sensor above the highest liquid level and a sensor is installed at the bottom of the container and fed to the evaluation unit in each case corresponding.

Conductivity measurement

Measuring the electrical conductivity

This method is also known as Konduktivmessung. Upon reaching a certain level of electric current between two electrodes through the fluid. Here, the resistance between two measuring electrodes by the presence or absence of the medium changes.

Through the use of alternating current in the measuring circuit corrosion of the probe rod and electrochemical reaction of the material to be avoided. For reliable measurement of a moving liquid surface, a time delay of the output signal is often built. Konduktivsonden with an electrode can be used in metal containers as full or empty detector with two electrodes in metal containers as full and empty detectors and with three electrodes as full and empty detector in non-metallic containers. Single-rod in the container wall is electrically conductive as a counter electrode.

Conductivity measurement is a simple and inexpensive method of measurement for conductive liquids such as water, waste water and liquid foods for level detection.

Heat conduction

This measuring principle is mainly used for liquid helium. A vertically tensioned, current-carrying wire is in the area in which it is immersed in liquid helium, superconducting, also he is the current heated ( Joule heat ); at this higher temperature, the wire is normally conducting. The electrical resistance contributes only the portion of the wire above the liquid helium. By measuring the resistance can be determined to liquid helium is the height to which the tank. The wire is usually made of a niobium -titanium alloy.

Capacitive measurement

Continuous level measurement

Wherein the capacitive liquid level measurement, the change in the electrical capacitance between the electrodes is detected, if they are surrounded by a medium. This change depends on the dielectric constant ( in the measurement as DK hereinafter) of the medium. Is it constant, it can be inferred from the measured capacitance on how far the electrodes are immersed in the medium. In this case, therefore, can not be built only one limit switch, but also the continuous filling level can be determined.

Level switch

When measured with capacitive switches a change of the dielectric constant is detected and transferred to the change in the switching signal. The advantage of this technology lies in the fact that the medium can be detected to some extent behind the container walls. So there is no hole in the container necessary. The function depends on the ratio of dielectric constant to the dielectric constant of the medium of the container. A typical pairing is a water-based liquid behind a plastic or glass wall.

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A sensor detects the dielectric constant of the medium. The signal may then be converted into a switching signal. So, unwanted substances that are hidden adhere to the sensor. This concerns buildup on the sensor tip, foams or spillage when filling medium.

Optical measurement

It is detected, the absorption of light or the disappearance of the total reflection when the sensor is immersed in the medium. The opto- electronic level sensor comprises an infrared LED and a light receiver. Light from the LED is directed into a prism at the top of the sensor. While the tip is not immersed in liquid, the light is reflected within the prism to the receiver. When rising liquid immerses the prism, the light is refracted by the liquid and not more, or diminished the recipient. The transmitter converts this change into a switching operation. Problem with this measurement method is the sensitivity to contamination.

Ultrasound

The measurement is based on an ultrasonic transit time measurement. The light emitted by a sensor, the ultrasonic pulses are reflected from the surface of the medium and captured by the sensor. The required running time is a measure of the distance traveled in the empty container part. This value is subtracted from the total stand height and one obtains the level. Due to the decay of the sensor there is directly below the sensor a the blocking distance is called, in the area no pulses can be received. The blocking distance determines the minimum distance between the meter and the maximum level.

This echo-sounding method is a contactless and maintenance-free measurement without being affected by product characteristics such as dielectric constant, conductivity, density or moisture.

In addition to this widespread ultrasound measurement from the top through the gas chamber there are also sensors that measure the level of the bottom through the vessel wall through or serve on the side as a limit switch. Here the term is measured in the medium itself. Most important feature of this method is the requirement of Durchschallbarkeit wall and medium. The application is therefore limited to single-walled containers with liquids.

Microwaves

The measurement is based on a microwave transceiver principle. From a transmitter emitted microwave pulses are registered by an empty silo from the receiver, however, dampened by a full silo from the medium. With metallic container walls window must be installed. Applications: As the microwave barrier uses a contact procedure for detection they can be mounted in tanks, pipes, ducts, or in free-fall chutes. For non- metallic container materials measurement through the container from the outside is possible. Report microwave barriers blockages, indicate level limits, solve positioning and counting tasks, measure without contact from the outside and are therefore wear -free and maintenance -free. Typical applications are eg wood chips, paper, cardboard pulp, lime, gravel, sand or even whole bags and boxes. Process temperature and process pressure are arbitrary in direct installation, however, the limits of the manufacturer must be observed.

Radar

The radar sensor operates with high-frequency radar pulses which are emitted by an antenna and reflected from the product surface. The term of the reflected radar pulse is directly proportional to the distance covered. With the known geometry of the vessel can be calculated from the level.

Guided Radar

The radar sensor operates with high-frequency radar pulses are transmitted down a probe. Upon impact of the pulses on the media surface, the characteristic impedance and a part of the transmitted pulse changes is reflected. The measured and evaluated by the device length of time between the transmission and reception of the reflected pulse is a direct measure for the distance between Prozesseinkopplung and the media surface. Application is the maintenance-free measurement in liquids, even in turbulence and foam. The measurement is independent of density, temperature, conductivity, and is not affected by moisture and vapors above the liquid.

Radiometry

In the radiometric level measurement, the intensity of gamma rays is detected, which penetrate on the way from the radioactive source to the detector medium. A gamma source, generally cesium or cobalt isotope emits a radiation, undergoes an attenuation during penetration of materials. The measuring effect results from the absorption of radiation by the medium to be measured. Opening the container or tank internals are not required; the measurement is non-contact from the outside and is therefore suitable for extreme applications such as for highly corrosive, aggressive and abrasive media.