Free-Air Radar Level
Figure 2. Using the distance between the device and the top level gives the level in the vessel.
Free-air radar works much better than ultrasonic level gauges and is significantly less costly than nuclear level gauges or laser level devices. It's substantially immune to vapor blanket variation in the vessel, to steam, dust and foam in the vessel, and can be easily removed for cleaning and calibration.
Free-air radar solves many of the problems of difficult level measurement applications. You're able to mount the device in many existing vessels using an existing connection, which is normally 4 in. to 12 in. Vessel nozzles on many vessels are unused and available.
However, what happens if you have a vessel where there's extreme agitation, vessel internals, granular materials or extreme coating of the vessel side walls? These all reduce the ability of the radar level gauge to receive the return signal. In the case of transit-time, free-air radar, signal loss can be total. The dielectric constant of the material being measured matters too. If the dielectric is low and there are other issues, free-air radar may not work well, or it may not work at all.
For decades, we've been installing capacitance or RF admittance devices in tanks to measure level. These devices work very well—if they can be installed to miss internal structures, have appropriate materials of construction and the tank isn't agitated much, if at all. The physical design is well-suited for tank level measurement, and these devices can often be inserted through a tank nozzle much smaller than the ones necessary for free-air radar level measurement. The problem is that radar works on applications where capacitance or RF admittance devices do not.
Enter a technology called time domain reflectometry (TDR). A probe, somewhat similar to an RF admittance probe in physical shape, is introduced into the vessel through a tank nozzle. Nozzles can be as small as 2 in. for this purpose. Generated pulses of microwave energy are transmitted down the probe. As soon as the energy pulse encounters a material, liquid or solid, that has a different dielectric constant from that of the vapor space in the vessel, a reflection is generated, and a return pulse travels back up the probe. The transmitter's circuitry creates the transmitted pulses, receives the reflected pulses, and uses the time differential between them to calculate the distance from the probe to the surface of the level to be measured. The difference between that measured distance and the bottom of the vessel is the actual level in the vessel. (Figure 3 shows a typical TDR setup.) Because the probe is used as a waveguide, the technology is usually called guided-wave radar.