Level Measurement Level Measurement in High Temperature and Abrasive Ambience

Author / Editor: Jürgen Skowaisa * / Marcel Dröttboom

Non-contact microwave sensors can be used in various industries. Through the use of simple mechanical tools and components, the sensors can be easily adapted to harsh operating environments with high temperatures, extreme dust generation or abrasion, ensuring maintenance-free, reliable operation.

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(Picturee: Vega Grieshaber)

Level measurement in abrasive media, high temperatures and harsh ambient conditions take big efforts on the implemented measurement technology. Mechanical measuring systems are often used in such applications, but are not ideal because they are subject to considerable wear and, thus, high maintenance costs. Through the use of simple mechanical tools and components, microwave-based sensors can be adapted to many different applications, ensuring maintenance-free, reliable operation.

A Call for Customisation

Nowadays, non-contact radar sensors are used in many different industries. Whether in the food industry, in wood and paper processing, in the construction material industry or in power plant technology – radar sensors are deployed in a wide variety of applications. Often used are standard devices that differ only with respect to their antenna system.

Applications with extremely high temperatures, extreme dust generation or abrasive materials call for individual customisation to optimise standard instruments for these challenging environments. The necessary adaptations – mostly mechanical in nature – are simple to implement compared to many previously used mechanical measuring techniques. Combining application knowledge and microwave know-how makes it possible to find an optimal solution for every task.

When adapting sensors to the various applications, two main characteristics of microwaves are utilised: their natural tendency to follow the inner surfaces of tubes (so-called waveguides) and their ability to penetrate non-conductive materials.

Properties of Microwaves

Since microwaves are capable of penetrating non-conductive materials such as glass, plastics or ceramics, measurement can in principle be carried out right through windows made of these materials. A portion of the microwaves is reflected by the window material and travels back to the transmitter.

In case of a microwave sensor for point level detection, only the signal attenuation caused by the measured medium is detected. That's why reflection of part of the signal doesn’t matter – the slight attenuation resulting from it can be compensated by an appropriate calibration.

With radar sensors for continuous level measurement, the reflection from the window material generates a false echo that is dependent on the window material as well as its thickness and orientation. Ceramic windows, for example, produce stronger reflections than plastic ones, and surfaces at right angles to signal direction generate significantly larger interfering signals than sloping surfaces – installing the window material with an slanted orientation is therefore a good idea when measuring with radar sensors.

Since thin slabs of material cause hardly any interference, simple covers of plastic or synthetic fabric can be mounted on the radar sensors for protection against dust.

To protect the sensors from high temperatures or extreme ambient conditions, the antenna can be distanced from the process by means of an extension tube, the microwaves are then fed into a waveguide, see Fig. 2. Here, too, there is a big difference between point level detection and continuous measurement. While microwave barriers impose no special requirements on the tube (because reflections cause no harmful interference), radar sensors for level measurement place stringent demands on the tube's dimensions and mechanical features.

The tube inner diameter must be adjusted to the frequency of the sensor and wrought in such a way that there are no disturbing features (weld beads, gaps, etc.) at the joints. To ensure optimal performance, antenna extensions should be provided by the sensor manufacturer and the sensors adapted to the respective antenna systems. Customized design of tubular extensions with any necessary bends and special high-temperature antenna systems is possible, see Fig. 3.