A wildcard in the silo
Radar-based, non-contact level measurement designed for demanding bulk solids applications

Users have often had to live with compromises when they deployed continuous measurement systems for their bulk solids. Dust, material filling noise, mechanical loads and demanding installation conditions make measurements difficult or even impossible. A new bulk solids radar gauge acting as a type of wildcard in the field of measurement engineering has been developed to eliminate these vulnerabilities.

No doubt about it: level measurement in the world of bulk solids is a different ball game compared to liquid level measurement. There are significant differences in both the measurement principle and the measurement task. The vessel, installation conditions, process conditions and product characteristics all have a significant impact.
-Vessels: Different types of vessels are used in the bulk solids industry. Large, tall, narrow silos are typical, and they are often segmented, which means that space is at a premium in the vessel. This factor must be taken into consideration in level measurement design. Bunkers, mounds and stockpiles on the other hand tend to be wide, flat, large and open. The vessel outlet (flat or conical) and the roof construction also influence the level measurement. Depending on the measurement principle used, sensors mounted on the roof can be exposed to considerable mechanical loads.
-Installation raises a number of questions. Is the sensor mounted from the top or through the wall from the side? Is the correct distance maintained to the container wall, the inlet and the outlet (the sensor should not be continuously exposed to the material flow), to built-in objects and to other measurement devices? What are the installation conditions, for example how large is the vessel opening and when can the sensor be installed? Does the vessel have to be empty for installation, and can the sensor be installed at all in the existing vessel (available manhole if necessary)? Is product needed for calibration during installation?

-Process conditions. Temperature, pressure, material flow, dust generation, air turbulence, noise generated during filling and the formation of conical piles all have to be taken into consideration. During emptying, the formation of funnels and cornices (overhangs), which can cause discontinuous discharging (sudden sliding), can be significant.
-Product characteristics. Grain size, shape, weight, moisture and electrical properties (conductive or insulating) of the material are all important.
-Combined factors. This includes mechanical forces (tension and pressure), electrical influences on the measurement method and the measurement equipment, product deposits in the container and on the measurement equipment and safety regulations (dust or gas explosion hazards).
The most common methods of measuring the level of bulk solids are electro-mechanical (sounding), weighing, capacitive measurement, TDR (guided microwave), laser and ultrasonic. The table compares the advantages and disadvantages of each method. As you can see, the list of disadvantages is not exactly short. However, people in the bulk solids industry have learned to live with certain restrictions over the years. If you were to ask users whether they are satisfied with their measurement methods, nine out of ten would probably answer “yes”. But do we really have to put up with these disadvantages? The level measurement experts at Vega Grieshaber in Schiltach are convinced that there is a better way.
The arrival of a new era
An important measuring principle is missing in the above enumeration: radar
measurement. For years, Vega has held a
leadership position in the field of radar-based liquid level measurement. The company introduced its Vegapuls 68 bulk solids radar device last year, combining the advantages of radar technology for liquids with the special aspects of the bulk solids industry.
The processing algorithms were also optimized. What was originally intended as a “mere” logical enhancement of the product line has been surprisingly successful during the first year in the field. The only applications which are not suitable for the radar gauge are those where the dielectric constant of the bulk solids to too low (<1.4). Bulk solids radar has been one of the highlights at this year’s Interkama, and Managing Director Günter Kech proudly announced that “our Vegapuls 68 ushers in a new era of bulk solids level measurement. Our vision of a universal, radar-based gauge has now become a reality.” What is it that makes the new bulk solids radar so “revolutionary”?
Pinpoint measurement
The gauge delivers reliable filling levels even when exposed to high temperatures, and even extremely dusty surroundings don’t bother it. A feature that stands out in comparison with previous radar devices is the very high dynamic range of over 100 dB. This allows signals to be evaluated that are 1000 times smaller than those processed by conventional radar instruments. This can be clarified with an example: If a person with a mass of 100 kg were weighed, scales with corresponding precision would register the loss of the weight of a single hair! Rainer Waltersbacher, Director of Product Management and Marketing at Vega, expounds on this: “It’s exactly these performance reserves, gained through the high dynamic range, that allow us to still carry out reliable measurements under the most difficult conditions.” The precision was achieved through (among other things) the use of highly sensitive, low-noise receivers which, due to their low inherent noise, considerably amplify diminutive signals. Another innovation has to do with the electrical connection: a 2-wire version is available with which measurements up to 70 m can be carried out. This, for example, isn’t possible with the ultrasonic principle—for bulk solids measurement, 4-wire instruments are always required.
New antenna technology
Vegapuls 68 is available with two antenna versions:
-The familiar horn antenna was optimised for bulk solids—esp. with regard to its emission characteristics—and can even
be mounted in small vessel sockets. It is extremely sensitive, especially at close range.
-The parabolic antenna is new—with a
diameter of 250 mm, it boasts (acc. to Vega) a hitherto unachieved signal focusing performance. Because it is a separable antenna system, it can be mounted in very small process connections. Thanks to its recessed feed system, it is insensitive to fouling and damage and emits a steady signal unaffected by external factors.
Both antenna systems are swivel-mounted to enable optimum alignment of the sensor. They also have a scavenging air connection for cleaning and cooling.