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Only two wires

Radar level control of low dielectrics
Only two wires

Only two wires
The Pegasus FMCW transmitter can be set to bottom tracking mode for low-dielectric liquids in order to measure liquid level using the echo signal from the bottom of a tank
According to some estimates, over 80 % of measurement devices sold today to process plants are two-wire, loop-powered instruments. This has also become a fact of life in radar level transmitters. Pegasus is an FMCW radar transmitter which operates with only two wires and is available in intrinsically safe and explosion-proof designs.

Unlike with pressure and temperature transmitters, when it comes to radar level gauges, reducing power to fit within the 4…20 mA budget has proved to be difficult. The greatest success so far has been achieved using pulse radar transmitters, which emit pulses that reflect from the surface of the media being measured and based on a time-of-flight measurement yield the distance to the surface. The other radar level technology is FMCW or Frequency Modulated Continuous Wave. FMCW requires more processing power than pulse radar, because it must continuously perform Fast Fourier Transforms on the received signal. The FMCW measures the frequency difference between transmit and echo signals.

Two-wire FMCW
Pegasus FMCW radar transmitters from Solartron Mobrey operate with only two wires. When the predecessor – the four-wire MRL700 transmitter – starts up, it draws 400 mA to handle all the initial echo recognition and calibration processing before settling down to normal operation in the 50 to 60 mA range. With Pegasus, all of this has been squeezed into a “less than 4 mA” budget. Pegasus comes with either a rod or a horn antenna for a wide range of applications. It is available in intrinsically safe and explosion-proof designs. Its Hart compatibility allows it to be controlled from anywhere in the two-wire loop. There are options to access it from a PC, a Palm PDA and a series of wall and panel mounted control units. All transmitters operate at temperatures up to 150 °C.
The MRL700 will be retained for applications that require the power of a four-wire transmitter, particularly where excessive turbulence or very low-dielectric liquids at long range are involved. Its range is 35 m, compared to 30 m for the Pegasus, and its reference accuracy is given as ±3 mm, as opposed to the Pegasus’ ±10 mm.
Bottom tracking
If a liquid has a very low dielectric constant (3.0 or less), conventional detection of the echo from the liquid surface may not be possible, because the signal penetrates the substance instead of reflecting from it. Water, for example, has a dielectric constant of 80 and reflects about 60 % of the radar wave. With a dielectric constant of 2.4, only 4.5 % is reflected. Among the typically troublesome dielectric constants are butane (1.4), gasoline (2.2), octane (2.1), vegetable oil (3.5) and petroleum (2.2). Since the low-dielectric liquid reflects little of the incident energy of the microwave, most of this wave travels through the liquid and hits the bottom of the tank. The reflection from the tank bottom may be stronger than the reflection from the top surface of the liquid. The signal is slowed down, however, as it passes through the low-dielectric liquid, making the bottom of the tank appear to be further away from the transmitter than it really is. Nevertheless, with low dielectrics, measuring the apparent distance to the bottom of the tank is a more reliable method of determining how much liquid there is inside the tank than measuring the distance between the transmitter and the top of the liquid. The Pegasus FMCW transmitter can be set to bottom tracking mode for low-dielectric liquids. Pegasus’ bottom tracking mode is enabled in a few simple programming steps. It is recommended to measure the distance from the instrument reference line to the bottom of the tank using the gauge itself in an empty tank. The user must input the dielectric of the liquid. If, for example, a 20 m tank is used for a fluid with a dielectric constant of 2.4, it takes 133.426 ns for the signal to bounce off the bottom and return to the transmitter in the empty tank, whereas when the tank is half filled the signal is slowed down so that the same path takes 170.064 ns. This is the equivalent time lapse for an empty tank of 25.4913 m depth. The radar transmitter, in other words, perceives the bottom as being an extra 5.4913 m away. This additional distance can be directly correlated to the depth of the fluid in the tank.
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More information on Pegasus
Supplier Directory – Level Measurement
Trade Fair Interkama+ 2005
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