Examining Guided Wave Radar for Level Control
Measurement of liquid level can be very challenging, especially in water and wastewater applications.
Measurement of liquid level can be very challenging, especially in water and wastewater applications. While mechanical instruments are still widely used to control level in the municipal market, over time mechanical products have given way to newer electronic technologies. One such technology is Radar. There are two basic types of radar level devices in common use today. Through Air Radar (TAR) relies on through air propagation of the electromagnetic energy and is an excellent choice in applications where contacting technology is not desirable. The second type is Guided Wave Radar (GWR), which is the type that we will explore in more detail in this article. With technological advantages over other level measurement techniques, Radar transmitters hold great promise for the municipal marketplace.
The basis for Guided Wave Radar is TDR (Time Domain Reflectometry) and it works in much the same way as conventional through-air radar. TDR has been used for many years to find cable breaks in underground pipes and in-wall cables. In a typical GWR transmitter, more than 250K pulses of electromechanical energy are developed by a TDR generator and sent down the wave-guide (probe/rod). When the pulses meet a discontinuity (a change in impedance due to a dielectric change) a reflection is developed.
After an engineered impedance change or baseline reflection is detected, the primary level reflection is detected and the level of the media is then calculated as a time of flight. High dielectric liquids (e.g. water) develop a strong reflection while low dielectric media (e.g. hydrocarbons, grease) develop a weaker reflection. The surface of low dielectric media allows some of the energy to continue down the probe until it dissipates or is reflected by a higher dielectric media. This characteristic gives GWR the ability to do interface measurements where a lower dielectric material (oil for example) is riding on top of a higher dielectric material (water for example) and to handle applications where build-up on the probe can occur.
Radar (electromagnetic energy) has inherent advantages over ultrasonic transmitters due to the inherent limitations of mechanical energy (sound). Sound waves use the atmosphere to propagate. Changes in the makeup of the air column between the transducer and the target (temperature or vapor gradients for example) cause variations in the speed of sound, affecting accuracy and reliability.
Electromagnetic energy does not have these limitations. In addition, electromagnetic energy has the ability, depending on its transmitting frequency, to penetrate materials, such as foam or build-up, whereas mechanical energy will not. It is this nature of electromagnetic energy, and hence GWR, that makes it suitable for water and wastewater applications.
Many operators immediately begin considering the inherent limitations and liability of a contact technology in applications where build-up can occur. While in many cases non-contact technology is preferred, perception plays a bigger role than fact when dismissing GWR for these types of applications.
As discussed above, GWR has been successfully installed in many “build-up type” applications in the water and wastewater industry. Considering that many, if not most applications are water based and the transmitter is looking for the largest impedance change, the ability of the radar system to look beyond the buildup to the strongest reflection is well documented.
Guided Wave Radar transmitters are a viable alternative to other level measurement techniques in virtually any level application. The question often arises as to why it has not been mass adopted. The answer to that question is twofold - cost and need.
While the cost of radar systems in the past made it economically unjustifiable except in the most difficult applications, the cost has come down significantly in the past couple years to the point where it rivals the cost of a non-contact ultrasonic unit.
The other major factor is need. There are many applications in the water and wastewater industry currently served by older technologies that produce results that are “good enough.” In addition, the comfort level and experience of operators, engineers and integrators with these technologies, which are like familiar old friends, cannot be set aside as if it does not matter.
However, the overall advantages and competitive cost one can gain from the use of radar are strong enough to warrant careful, if not extremely strong consideration, especially in applications where other technology is being marginally applied.
About the Author:
Craig Carroll is the business manager for STI Controls, a subsidiary of Magnetrol International and a producer of level measurement devices for the Water and Wastewater industries. He has been involved in the water and wastewater markets for nearly 13 years. STI offers a range of level and flow control technologies for the municipal industry, including guided wave radar, pulse burst radar, thermal dispersion and ultrasonic technologies.