Level Measurement

The New Simplicity: Guided Radar for Level Measurement

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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.

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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.

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