Time Domain Reflectometry (TDR)

Time Domain Reflectometry (TDR) is a technique based on the electromagnetic signal generation and its reflected echo analysis by a generic load. By measuring the reflection coefficient it is possible to detect the load characteristics, such as dielectric constant, electric conductivity of any materials or the equivalent impedance of a generic load.

In particular, the current research activity, that refers to the quantitative (levels/volumes of material) and qualitative (physic parameters such as conductibility , pH, etc..) control, focuses the following main application:

•  Environmental monitoring (water, soil, structural deformations);
•  Level control of rivers, lakes, wells, dams, etc.
•  Monitoring of tanks, vessels, silos, etc.
•  Transmission line control and fault detection;
•  Industrial process control involving liquids or granular substances.

The general measurement set-up based on TDR, generates a pulse that is transmitted through the coaxial cable to the probe . The probe is inserted in the material under analysis.

coaxial_cable

The propagating signal along the probe encounters an impedance break due to the variation of the dielectric constant; a part of the incident pulse is reflected, the ratio between the reflected signal amplitude and primary pulse amplitude gives the reflection coefficient

formula

It’s therefore possible to monitor the behavior of different interfaces placed in a container and to localize the different discontinuities of the material and the physical/chemical characteristics of material, (dielectric constant, electrical conductivity, eventual presence of emulsions, sediments, dispersions, etc…);

The electronic device TDR transmits a very short time pulse to the probe; the travel time, that represents the time to travel forth and back in the transmission line of length L , can be written:

formula_1

therefore the device stores the amplitude of the reflected signal returning from the material under examination because of the impedance variations; it converts the time-voltage into distance-voltage measurements in order to localize the discontinuity point. Then, it   calculates the reflection coefficient sending the data to the PC.

tdrbig

The probe can be of various kinds; for our application a coaxial probe is suitable. It is made up of a central, cylindrical conductor with a coaxial conductive shield, both stainless made. The shield is suitably perforated, in order to allow the fluid circulation.

The measurements are performed by user-friendly software control, that permits to visualize data.

The most important parameter to be set is   Vp (propagation velocity in the medium) , that is related to the relative dielectric permittivity ? by the following equation:

formula_2

The length of the transmission system up to the point where the mismatch occurred can be measured:

formula_3

where D is the distance of the signal trip up to the mismatch point in the new medium, Da  is the corresponding distance for an air dielectric.

The dielectric constant e is related to the reflection coefficient through the following equation:

formula_4In particular, our research is focused on an integrated system for measurement and monitoring of liquid and granular substances whose main applications field concerns the quantitative and qualitative monitoring of tanks, vessels, silos, etc., as well as the environmental monitoring.

The system can be also completely managed by an additional remote control device for data detection and two directional transmission , that can query the system, for example through GSM, GPRS or satellite networks , receive and process data.