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Level Measurement

Level Measurement in High Temperature and Abrasive Ambience

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Unique Applications

Complex applications often require a customisation of the sensor for an optimal technical solution – here is where a flexible sensor system is needed. Thanks to their modular concept, standard radar sensors from Vega can be adapted to widely different applications. Costs thus remain within reasonable limits because only small adaptions have to be carried out. The physical advantages of microwaves can be utilised without having to re-engineer the sensor completely.

One great advantage of radar technology is that microwaves propagate without being affected by dust. Reliable measurement is therefore possible even under conditions of extreme dust generation.

Depending on the process conditions, a certain amount of dust will inevitably be deposited on the sensor; whether this interferes with the measurement in the long term, however, depends on the particulars of the application. In most applications the sensors operate reliably and without maintenance despite heavy soiling (Fig. 4).

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Antenna covers of various materials are available to help avoid excess soiling of the antenna system. In addition to protective caps of PP or PTFE, there are flexible fabric covers that can be cleaned pneumatically with a short air pulse. This consumes considerably less air than continuous air rinsing and results in significant savings in the costs associated with air supply. The different covers are easy to retrofit and can be simply mounted when needed.

It is vitally important to protect the electronics of sensors, especially in applications with high temperatures. Antenna extensions allow the electronics to be detached and installed in a “safe” place. The extensions can be straight, curved or segmented to suit the specific application.

In applications with high temperatures, the usual antenna systems with PTFE components quickly reach their limits. Versions with high-temperature resistant plastics such as PEEK or even ceramic are available for such cases. They can be used in process temperatures up to 450 °C.

To ensure that the antennas function optimally even in the highest of process temperatures, high-temperature resistant materials as well as solid die-cast antennas are available.

High Temperature Examples

Monitoring layer thickness in a clinker cooler is one example of how radar sensors are used in applications with high temperatures (Fig. 5). After burning, the clinker, i.e. the unground cement, is cooled. This involves loading the material, which has a temperature of up to 1500 °C, onto a conveyor belt with continuous air flowthrough. Radar sensors measure the thickness of the layer and thus enable efficient cooling.

Another good example is level measurement in a lime kiln. Here, a microwave barrier can be used for non-contact measurement. The sensors measure the filling right through the firebrick lining (Fig. 6). The measuring system operates without any physical contact with the abrasive material, so it is completely wear free and keeps maintenance costs to a minimum.

An example for measurement in heavy dust environments is the continuous level measurement in crushers. Fig. 7 shows a radar sensor with a plastic encapsulated antenna which is used to monitor the filling of a crusher in a copper mine. Despite heavy dust deposits on the antenna system, the system – a Vegapuls 67 – detects the level reliably and accurately.

Effects on Costs

Using standard sensors and adapting them to the process via simple mechanical components keeps investment costs within reasonable limits. In many cases there is no need for costly, specially designed parts or setups. Non-contact measurement ensures reliable operation and a long service life of the equipment. Sensor maintenance is reduced to a minimum. Altogether, these advantages result in significant cost savings. n

* Jürgen Skowaisa, Product Manager Radar/Ultrasonics at Vega Grieshaber KG, Germany, Tel. +49 (0)7836/50-182, E-Mail: j.skowaisa@vega.com

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