Guided Wave Radar Level Measurement Operating Principle

Radar level measurement operating principle
Radar level transmitters measure the distance from the transmitter to the fluid surface by measuring the time of flight of a high frequency electromagnetic radio wave, typically in the microwave frequency range - GHz. The distance from the transmitter to fluid surface is subtracted from the tank depth to give the liquid level.

Difference between guided wave radar and radar level measurement
Guided wave radar level measuring instruments use a probe to guide the electromagnetic waves to and from the process liquid, as shown in the diagram below:

Advantages of guided-wave radar over non-contact radar level transmitters
Non-contact radar devices experience more signal loss than guided-wave radar devices, due to dispersion of the electromagnetic waves. Waveguides combat this signal loss by channeling the radio energy along a straight-line path.

Guided wave radar wave guide types
There are various types of wave guide available commercially, including single metal rods, parallel pairs of metal rods, and coaxial metal rod and tube structure. Single rod probes exhibit the greatest energy losses, whilst coaxial probes suffer the least.
However, single rod probes are more tolerant of process fouling than two-rod or coaxial probes, where viscous liquid or solid matter may cling to the wave guide. Clinging liquid or solids can cause electromagnetic wave reflections that fool the transmitter into thinkinging it is seeing a reflection from a liquid level or liquid interface surface.
PTFE probes are recommended for viscous fluids.

Guided wave radar and liquid dielectric permittivity
With all radar level instruments, ingluding guided-wave radar transmitters, the necessary condition for electromagnetic wave reflection is a sudden change in dielectric permittivity. When an electromagnetic wave encounters a sudden change in dielectric permittivity, some of that wave’s energy will be reflected resulting in another wave traveling in the opposite direction, while the remainder of the wave’s energy continues forward to propagate into the new material. The strength of the reflected signal depends on how greatly the two materials’dielectric permittivity differs, i.e. their relative permattivity, or as it is sometimes called - dielectric constant. A strong reflected wave is required for good operation of the guided-wave radar measuring device, therefore radar level instruments function best when there is a large difference in relative permittivity between the two substances at the interface.
With air as the gas medium above the liquid we have realative permativities of approximately 2 for gasoline, 42 for glycerin, and 80 for water.

Compensating for changes in dielectric constant
Changes in dielectric constant can alter the accuracy of radar level measurement. Factors that can alter the dielectric constant of gases include pressure and temperature, therefore the accuracy of a radar level instrument will vary as the pressure and/or temperature of the gas above the liquid vary. Often, the variation is insufficient to cause concern.
In high accuracy applications it is common to compensate for the error by either
- external pressure and temperature measurement, and calculation. The calculation can be performed within the control system, or some manufacturers offer this capability within their guided wave radar transmitter.
- use of a reference signal. In this case the radar level instrument is furnished with a reference probe of fixed length who's entire length is always above the liquid level (i.e. it only senses gas).

Guided wave radar installation
Guided wave radar level measurement requires a relatively flat fluid surface. If the surface is turbulent then the use of a Stilling Well should be considered.
For long probes, the lower end of the probe should be be fixed to the bottom of the vessel.
Adequate headroom, i.e. clearance above the vessel is required for insertion and removal of the probe.
Nozzles used for guided wave radar instruemnts should be a minimum of 4 inch.



Further Reading

For those who want to delve further into the techniques employed in level measurement then the following may be of interest: