Level Measurement The New Simplicity: Guided Radar for Level Measurement

Author / Editor: Wolfgang Perenthaler / Dr. Jörg Kempf

“Just give me the level!” Even the simplest user requests cannot always be easily fulfilled. However, a new level gauge based on the guided radar principle meets exactly this requirement thanks to self-learning signal processing and innovative algorithms.

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The new Vegaflex 82 for level measurement of bulk solids materials
The new Vegaflex 82 for level measurement of bulk solids materials
(Picture: Vega)

Easier still! This basic requirement was the driving force behind the development of the new sensors of the Vegaflex 80 series. Besides the simple instrument selection and menu-guided setup, reliability and measurement certainty were the focus. The series consists of four different instrument types, each of which is tailored to a particular application. Vegaflex 81 is optimized for use in liquids, Vegaflex 82 is extraordinarily robust and ideal for bulk solids. When it comes to measurement of food or corrosive products, Vegaflex 83 meets the special requirements for hygiene and durability, with its resistant materials and high-quality surface finish. For extreme temperatures as well as high pressure applications, such as distillation columns, Vegaflex 86 is the optimal solution.

An example of this new development focus is the Vegaflex 82 for level measurement of bulk solids materials. The instrument can measure solids with a small dielectric constant, in measuring ranges up to 70 meters in metal or concrete silos, even during pneumatic filling. Vegaflex 82 comes in two versions: one with a 4 mm cable (tensile loads up to 500 kg) for free-flowing granules and heights of about 8 to 10 meters and one with a 6 mm cable (tensile loads up to 3 tons) for heavy powders and longer ranges.

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Studies have shown that more than 20 % of all production disturbances or stoppages result from incorrect instrument setup and commissioning. That’s why the top priority for Vega was to make setup and commissioning as simple as possible. The user is guided through the set up, so that after only six steps, measurement with the instrument can begin. They only need to specify whether the application is in a metal or concrete container. The instrument itself takes care of the rest, setting all operating parameters.

Once Vegaflex 82 is electrically connected, it measures the level immediately. Potential entry errors are thus completely avoided. This is thanks to the large number of processes integrated in the sensor software. These not only facilitate setup and commissioning, but also ensure reliable measurement data and allow for a wider range of applications.

The most important innovation for Vegaflex 82 you will find on the next page.

Automatic Probe End Tracking

The most important innovation for Vegaflex 82 is its automatic probe end tracking function. This allows absolutely reliable measurement of nearly all bulk solids. But how do you track something that does not move? After all, the probe in the silo does not get longer or shorter.

In non-conductive media, part of the microwave pulse penetrates the medium right down to the probe end. The pulse is reflected back and Vegaflex can detect this signal. If the container is empty, the signal from the end of the probe is the actual distance, i.e., the total length of the probe. The speed of propagation of the microwave pulse is dependent upon the surrounding dielectric.

Since the microwave pulse propagates through the bulk solid at a slower rate than in air, the signal from the probe end appears to be coming from a greater distance when the silo is full. The higher the level climbs on the probe, the greater the distance the probe end signal seems to be coming from. Because Vegaflex 82 knows its correct probe length and measures this shift, it can continuously determine the dielectric constant of the bulk material.

Particularly in bulk materials, the reflectivity (dielectric constant) of the medium is often very small. If the level echo itself is not detectable, Vegaflex 82 automatically switches over to probe end tracking (indirect level measurement). It then measures the distance to the end of the probe and calculates the level using the most recently determined dielectric constant. If the level echo becomes visible again, the instrument switches back to the direct measurement mode.

The Specific Limits of Guided Radar Are Extended

With this method, the specific limits of guided radar are greatly extended. Plastic granules or powders with very low reflectivity (small dielectric constant) can be measured without restrictions. One example is PE or PP powder. Thanks to its probe end tracking function, Vegaflex 82 can take the initiative to continue the measurement.

This method is also applied when the medium is fluidized with compressed air, which generally takes place in the conical part of a silo. Due to the injection of air, the dielectric constant of the medium is considerably reduced. Standard instruments often simply drop out in these conditions and report that the container is empty. Other methods, such as ultrasonic measurement, also fail in this application due to the huge amounts of dust generated.

Vegaflex 82 acts quite differently, because it has the ability to determine the dielectric constant at all times. If the instrument loses the level signal in this area of the silo, it automatically switches to the indirect mode and measures the level down to the end of the probe.

Additional Moisture Meter May Not Be Required

Since the dielectric constant is continuously detected, it can also be transmitted as an additional measurement and processed in the control system. This can be particularly useful as the dielectric constant is related to the moisture content of the bulk material, which allows the moisture content to be measured continuously as well. Normally, a separate moisture meter is used for this, but such devices can only measure at certain points. In contrast, Vegaflex 82 monitors the dielectric constant over the whole measuring range and is able to indicate an average moisture content and detect even extremely small traces of moisture. This means an additional moisture meter may not be required.

Another unique feature is the automatic length determination. Accurate knowledge of the probe length is essential for all TDR sensors. This is also true for Vegaflex 82. In practice, almost all probes are ordered longer, because the exact length can only be determined at the installation site. Once the instrument is installed, the technician often shortens the probe to the right length directly on the spot. The probe length then has to be measured again. Many users find this very inconvenient. The new Vegaflex 82 determines the length of the probe automatically and thereby accelerates the setup procedure immensely.

Learn on the next page how Vegaflex 82 always makes the correct echo decision.

Detecting Echo Movement

The intelligent algorithms also play a part in the higher reliability of the signal processing. To put it more graphically: previously, during signal detection and analysis, the largest echo usually won out. But such echoes might be caused by struts in the container or product buildup. Vegaflex 82 works differently, it creates an echo profile from the moment it starts operating.

It then uses this history to analyse and make the correct echo decision. It rates moving echoes higher than static signals that always occur in the same place. For the sensor, a moving signal that fits the echo history is much more likely to be the level echo than other signals. Intelligent motion detection thus effectively increases measurement reliability, resulting in continuous and maintenance-free operation.

High precision is also required for troubleshooting. The more accurate the information available to the technician or maintenance personnel is, the faster the instrument can be up and running and the plant operating at optimum level again. Many asset management functions, whether for preventive maintenance or immediate corrective action, have been implemented in the new series.

These are for example; the status messages according to NE 107 or VDI/VDE 2650, the plaintext display of device status, the documentation in the integrated event memory with real-time stamp, as well as the indication of measurement certainty and electronics temperature. All events, whether malfunctions or status and parameter changes, are logged in the sensor. Within seconds, the entire instrument history with real-time stamp is available to the technician for fast and reliable diagnosis. Malfunctions can thus be remedied quickly and the process brought back to normal operation in record time.

Conclusion

Thanks to intelligent algorithms and software, Vegaflex 82 scores highly in applications where measurement was previously very difficult, such as plastics with low dielectric constant. But even in standard applications, the new generation of level and interface transmitters delivers a convincing performance due to its simplicity, menu-driven setup and reliable data acquisition.

* The author is Product Manager at Vega Grieshaber KG, Schiltach.

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