Operating principle

Pulses of low power microwaves are sent along conductors. At the point where the waves meet the product surface they are reflected back.  The intensity of the reflection depends on the dielectric constant, the higher it is the stronger the reflection will be. The time measured between emission and reflection reception is proportional to the distance.

For TDR guided radar there are two different categories of product:

Products with a high dielectric constant > 1.4

These applications work in “Direct Mode”, which means that the reflection from the product surface is used directly for the measurement of the level.  Two different applications are possible: 

Level Measurement

The wave is reflected from the first product surface and is received by the processing unit on the top of the tank.

The wave travels along the conductor above the product at the speed of light and 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).

Interface measurement

Interface measurement can only be made if the upper layer is greater than 50mm and has a lower dielectric constant than the lower, and, if the difference between the two dielectric constants is greater than 10.

For measuring interface level we use the residual wave after the first reflection.  This part of the wave continues down the conductors through the first product layer until it is reflected by the interface level.  The speed of this wave depends on the dielectric constant of the upper product.  This means that we have to know the dielectric of this layer has to be known to determine the interface level. 

Products with a low dielectric constant < 1.4

To measure the level of low dielectric products we use the Tank Bottom Following principle (TBF).  The downward wave will first pass through the air at a known speed and then through the product at a speed depending on the dielectric constant of the product.  Since the return in air is calibrated and known; the difference in the two times is directly proportional to the product level in the tank.  As we are largely dependent on a stable dielectric constant, this method can be less accurate than the direct mode method