How it works
Guided Wave Radar is also known as the Time Domain Reflectometry (TDR) or in simpler terms, micro-impulse radar. When guided wave radar unit is being installed, the GWR sensor or gauge is usually mounted on top of a tank or chamber while the probe is extended to the full depth of the vessel where the level measurement is needed.
Once the chamber has been set up, the low energy microwave pulses are pushed down the probe while travelling at the speed of light. Once the microwave energy hits the surface of the liquid, at the point where air meets the liquid, a great percentage of the microwave energy is reflected back up the probe to the transmitter. It is important to note that a Guided Wave Radar (GWR) unit is always made up of a sensor and a transmitter. The transmitter is used to calculate the delay in time between the transmitted signal and the received echo. The microprocessor on board the transmitter is then used to calculate the distance to the liquid surface.
image source: http://us.magnetrol.com
While some pulses will be reflected back up the probe at the point of interface between the air and the liquid, some of the pulses will travel all the way down the probe through some low dielectric fluids and a second echo can then be recorded from an interface between two liquids at a point below the initial liquid level. This feature of GWR makes it one of the few best techniques for measuring liquid/liquid interfaces such as oil and water and also in measuring through certain interfaces of form. GWR is also used when measuring interfaces in vessels that have very tight geometries and in tanks of all shapes and sizes.
GWR is based mainly on microwave technology. Microwaves are usually affected by materials that reflect energy and as such, may not be affected by factors such as temperature variations, dust, pressure, and the viscosity of the liquid. In most processes, there are common varying conditions. The temperature, density and viscosity may change and under such conditions, there may be significant variations in the level measurements. However, the GWR will not be affected by any of these conditions. The device will not be required to compensate for changes in any of these conditions. This makes it a very robust top down measurement technique.
Knowing the Interface
GWR can be used to measure both the level and the interface of the fluid. When working on a separation process, it is important to acquire both the level reading and the interface level. A case example in this is when measuring level readings for both oil and water. Oil and water have distinct interfaces. When the microwaves from the transmitter hit the oil interface, some of these waves will be reflected back up the probe while some will continue down the probe. The level reading will be provided by the microwaves that are reflected back up the probe while the interface reading will be provided by the microwaves that continue down the probe because these are reflected on the water surface.
Why the reflection media is important
Level measurement using the Guided Wave Radar technology is based on the reflection of microwaves after they hit a surface media. All media have a dielectric constant. The dielectric constant will determine the strength of the reflection of the microwave. A higher dielectric constant means that the reflection will be stronger. On the other hand, a lower dielectric constant means that the signal will be weaker. With this in mind, it therefore follows that a vacuum will return no reflection at all because it has a dielectric constant of 1. In the case of oil and water, oil records a dielectric constant of 2 while water has a higher dielectric constant of 80.
Any dielectric constant that is below 1.5 is usually difficult to measure and may not return accurate values. However, this does not mean that the GWR will not be able to handle it.