Flow & Level Measurement
the pros & cons of guided wave radar

Throughout the history of industrial instrumentation, there have been numerous times where a new technology comes onto the market and marks a new era in measurement due to its enhanced performance. Industrial level measurement has witnessed several: pressure-differential pressure, RF admittance-capacitance, ultrasonic, nuclear and magnetostrictive, to name a few. In recent years, new radar transmitters have been released into the process markets that hold great promise. To many, radar is the "be all and end all" level measurement technology- the answer to all level measurement applications- or is it?

Upon initial observation, radar level transmitters seem perfect. The antenna does not contact the liquid; the high frequency, electromagnetic signal travels easily over long ranges, and the measurement is not affected by changes in the process media. Initially, many manufacturers believed their only challenge was to reduce the price of radar to the point of being competitive with ultrasonic transmitters; at which point it would sweep the market. As more and more radar transmitters have been installed, flaws in their character are coming to light.
The output of electromagnetic energy at the antenna of a radar transmitter is typically around 1 mW- a very weak signal. After the energy is launched into free air it begins to weaken very rapidly. The signal reaches the level surface where it is reflected back. The reflection off a liquid surface is directly related to the dielectric value of the liquid - very low dielectrics, like hydrocarbon media, reflect very little of the signal. On the return path to the top of the tank this weakened signal loses more energy until what is received back at the transmitter may be 1-3% of what was initially transmitted. Turbulence and some foam types further complicate the matter by scattering the signal off its direct path or absorbing it leaving little or no return signal. In addition, with spurious signals due to mixers, piping, ladders, etc., it is a wonder radar transmitters are as effective as they are.
Remember, these units are line-powered devices that contain powerful microprocessors and some of the most sophisticated signal processing in the industry - just to measure the level. Even so, most radar manufacturers de-rate their transmitter’s performance in the presence of such "complications" or refuse to apply them at all.

Why A Wave Guide?
In a perfect world, all measurement would be non-contact to the media; even non-invasive to the vessel. However, in the real world, daily we sort through the various strengths and weaknesses of different measurement technologies searching for the one that best solves the application at hand.
Guided Wave Radar uses a wave-guide (probe) to yield solutions to these tough applications by offering the performance of conventional, through-air radar but with distinct advantages of its own:
1. The output into the wave-guide is extremely small- approximately 10% the output of conventional radar (0.1mW). This can be accomplished since the wave-guide offers a highly efficient path for the signal to travel down to the surface of the liquid and back. Degradation of the signal is kept to a minimum, therefore, extremely low dielectric media (>1.7) can be measured effectively. Further, the guided wave radar transmitter is loop powered. not line powered. Installation costs are reduced considerably.
2. Since the signal is "contained" within the wave-guide, turbulence, tank obstructions, etc. cause no problems.
3.Variations in media dielectric have little effect on performance. Guided wave radar, like its cousin conventional through-air radar, uses transit time to measure the media level. The signal reflecting from a surface of hydrocarbon (dielectric = 2 to 3) or water (dielectric = 80) is the same in transit time; only the amplitude of the signal changes.
4.Since the speed of light (electromagnetic energy) is a constant, no level movement is necessary to calibrate the device. In fact, there is no field calibration, per se. Field configuration is done by simply entering data related to the specific application. Numerous transmitters can be configured on an instrument bench in minutes- all that is need is a 24VDC power supply and the tag sheets for each tank.
5. Varying specific gravity has no effect.
6. Vapors and foam have no effect.
7. Media buildup and coating have little effect. Coating needs to be considered in two categories: film and bridging. A film coating is the effect of a viscous or light slurry when the liquid level drops. This type of coating has little effect. Bridging, however, can cause significant error. When a chunk or slug of media bridges between the elements of the probe, a level will be detected at that point.
8. The cost of guided wave radar transmitters are competitive to standard level measurement technologies - not the premium paid for conventional through-air radar.

Loop Powered Radar?
The new, low cost, loop-powered radar transmitters offer many of their own advantages. The hardware is much less expensive than conventional radar transmitters and installation costs are kept to a minimum. Loop power, of course, offers significant technical obstacles to developers- 4mA @24VDC offers very little in the power budget. The output at the antenna must be significantly reduced by launching less energy and processing is done with less-powerful microprocessors and averaging the return signal over a longer time period. Remember the application problems associated with through-air radar: low dielectric media, turbulence. foam, obstacles in the vessel. You must ask yourself, if expensive, line-powered transmitters that use powerful microprocessors and sophisticated signal processing have problems with these applications, what are the chances for a looppowered device?