Ask Dr. Level

Guided Reflex Microwave (or radar) is based on the TDR principle (Time Domain Reflectometry), which is has been traditionally used in the continuity measurement of communication cables.

Extremely short low power microwave (or radar) pulses are guided along conductors – rods or cables suspended in the vessel.  At the point where the pulse meets the surface of the material, the pulse is reflected back.  This reflection enables accurate level measurement in vessels, storage tanks, silos and hoppers.

The intensity of the reflection is dependent on the dielectric constant of the material. The higher the dielectric constant, the stronger the reflection will be, e. g. up to a 80% reflection for water.  The instrument measures the time between pulse emission and reception.

Guided Reflex Microwave has two different categories of level application dependent on dielectric constant K of the materials being monitored:

1 – Products with a dielectric constant K  > 2.0

With these materials, the "direct mode", is utilized, meaning that the reflection from the product surface is used directly for the measurement.  Using the "direct mode" both level and/or interface measurement are possible:

a) Level measurement

The wave is reflected from the first product surface and is received by the receiver on the top of the tank. As the wave only propagates through the air above the product at the speed of light, the return time of the wave pulse is directly proportional to the distance between the top of the tank and the surface of the product (level).

b) Interface measurement

Interface measurement can only be made if the first layer has a lower dielectric constant than the second and if the difference between the two dielectric constants is larger than 10.

For measuring the interface level we use the residual wave after the first reflection. This part of the wave moves further downwards along the conductors through the first product layer until reflected on the interface level. The speed of this wave depends fully on the dielectric constant of the first product. This means that we have to know the e r of this first layer to determine the interface level. The instrument measures time between emission and second reflection. Since we know the time between emission and first reflection (from level measurement) the difference between the two gives the transit time through the first layer.

The thickness of the layer is directly proportional to the time and dielectric constant .

If the dielectric constant of a product is not known, start the measurement with an approximate value for the dielectric constant and make adjustments after calibration.

2 – Products with a dielectric constant K < 2.0

To measure the level of low dielectric products a different procedure is needed due to the poor reflection of the wave on the surface of the product. This reflection is not strong enough to make a reliable level measurement. To achieve reliable and precise measurement we use the Tank Bottom Following Principle (TBF). 

The instrument has a short circuit at the bottom end at a precisely known length. The downward wave will first go down through the air at a known speed and then passes through the product at a speed depending on the dielectric constant of the product.

The instrument measures the time between emission and reception from the bottom. As the exact the return time in air when no product is in the tank is known, then the difference in time between the two is directly proportional to the product level in the tank. As the pulse speed through the product depends on the dielectric constant, this value has to be programmed into the unit in order to calculate the level. Since this measurement is largely dependent on the dielectric constant, its accuracy will be less than in the direct mode.

Copyright © 1999 - 2005 HiTECH Technologies, Inc.

All rights reserved.

Last revised - March 17, 2006

webmaster@hitechtech.com