E-Zine March 2013
Radar contact level measurement sensors are commonly referred to as guided-wave radar (GWR) or time domain reflectometry (TDR) transmitters. Their sensor assembly typically consists of two probes that are partially covered by material in the vessel. Rising level tends to cover more of probes inserted from the top of the vessel.
The transmitter sends a radar frequency pulse down one of the probes. The dielectric constant of gas above the material is close to 1, so the radar energy does not disperse and travels down the probe. When the pulse reaches the material, some of its energy is dispersed into the material. Some of the dispersed energy is sensed by a second probe in the assembly that is located parallel to the first probe. The sensed energy then travels on the second probe to the transmitter. In a sense, the energy traveling on the second probe could be considered a reflected signal from the surface of the material.
In effect, the transmitter sends a pulse down one of the probes and receives a reflected signal on the other probe. The reflected signal takes more time to return to the transmitter when the probe assembly is less covered. The transmitter measures the amount of time that the pulse takes to travel on the two probes and across the gap between the probes. The measured time, speed of the radar pulse, and probe geometry are used to calculate the covering of the probe. Mechanical dimensions can then be used to determine the level in the vessel.
The ability to utilize this technology is dependent upon the dielectric constant of the material. Materials with sufficiently high dielectric constants allow the energy to be absorbed into the material where it can reach the second (measurement) probe. In general, materials with higher dielectric constants tend to have stronger reflected signals that result in more reliable and more accurate measurements.
Materials with low dielectric constants do not absorb radar energy, so the radar energy tends to be confined to the first probe and cannot reach the second probe where the return signal is measured. Such materials are generally not appropriate applications for this technology.
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Excerpted from The Consumer Guide to Capacitance and Radar Contact Level Gauges
ISSN 1538-5280
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