As already discussed in operating principle of non-contact radar level sensors/gauges, radar level measurement can be done using: guided and unguarded radar waves. We have already covered unguided or non-contacting radar level measurement. Here the focus is on the operating principle of guided wave radar level measurement applications.
Guided wave radar (GWR) is also called time domain reflectometry (TDR) or micro-impulse radar (MIR). In guided wave radar installations, the guided wave radar sensor/gauge is mounted on the top of a tank or chamber, and the probe usually extends to the full depth of the vessel where level measurement is required. A typical guided wave radar installation in a vessel is shown below:
 |
| Guided Wave Radar Level Measurement Probe |
Low energy pulses of microwaves, traveling at the speed of light, are sent down the probe. At the point of the liquid level (air / liquid interface) on the probe, a significant proportion of the microwave energy is reflected back up the probe to the transmitter (The GWR installation always comprise the sensor and transmitter as a composite unit).
The transmitter measures the time delay between the transmitted and received echo signal and the on-board microprocessor in the transmitter calculates the distance to the liquid surface using the formula:
Distance = [Speed of light x time delay] / 2
Once the transmitter is programmed with the reference gauge height (Ho shown above) of the application – usually the bottom of the tank or chamber – the liquid level(H - Ho) is calculated by the microprocessor.
At the interface of air and liquid within the vessel, a proportion of the pulses will continue down the probe through low dielectric fluids and a second echo can be detected from an interface between two liquids at a point below the initial liquid level. This characteristic makes guided wave radar a good technique for measuring liquid/liquid interfaces such as oil and water and measuring through some types of foams. Guided wave radar can be used in vessels with tight geometry, in chambers, and in tanks of all sizes.
Advantages
- They can be used in low dielectric process fluids
- They can be applied to liquids in turbulent applications
- Because reflection of waves is dependent on a flat surface, they can be used with many powders and grains as well as liquids with slanted surfaces caused by vortices
- They provide accurate and reliable measurement for both level and interface applications
- They are used with liquids, sludges, slurries and some solids
- No compensation is necessary for changes in density, dielectric or conductivity of fluids under measurement.
- Changes in pressure, temperature and vapour space conditions have no impact whatsoever on the accuracy of radar level measurements
- Maintenance of Guided Wave Radar probes/sensors is minimal since there are no moving parts
- Guided Wave Radar probes/sensors are easy to install and can be used in place of other level measurement technologies such as displacer and capacitance.
Application Limitations
- Guided Wave Radar probes should not be in direct contact with metallic object (vessel wall) as this will impact the signal. An exception is the use of a coax-style probe.
- Material build up on probe tends to affect measurement accuracy. This can be remedied by purchasing probes with advanced diagnostics capabilities.
- Application of guided wave radar probes in chambers less than 3 inches creates a lot of problems – there tend to material build up and the metallic contact between probe and vessel wall cannot be guaranteed most times
