01 July 2003
Advancements in radar level
By Chris Lamakul
The key to success in process level measurement instrumentation technology is matching the appropriate technology (pulse vs. frequency-modulated continuous wave [FMCW], high frequency vs. low frequency, and line-powered vs. loop-powered) with associated applications. Today end users, engineering firms, and original equipment manufacturers have more advantages than ever.
BENEFITS OF RADAR
Like other noncontacting technologies, radar is easy to install, requires low maintenance, and experiences no mechanical foul-up and no material contamination. Product reliability and the reduced costs incurred with the commissioning and troubleshooting of a radar level measurement system are the main benefits. Unlike the noncontacting technology of ultrasonics, radar is virtually unaffected by varying atmospheric conditions such as temperature and vapor density. Applications involving chemicals and fuels, such as kerosene, diesel, gasoline, and liquefied petroleum gas, have changing vapor densities depending on the material level and temperature in the vessel. In general, you can use radar in higher pressures, temperatures, and vacuums where other noncontacting technologies (ultrasonics) have limitations. Radar is not affected by changing material properties such as moisture content and dielectric.
Some radar systems now have +/– 1-millimeter (mm) accuracy, and you can use them for custody transfer measurement in liquid applications such as oil storage vessels. Noncustody transfer radar systems typically range from +/– 5 mm to +/– 15 mm in accuracy with a repeatability ranging from +/– 1 mm to +/– 5 mm. You can use other radar systems to measure the material level of a solid in very dusty applications—kiln dust and carbon black—where other noncontacting technologies have had problems penetrating the thick dust clouds.
Noncontacting radar technology uses electromagnetic waves in the microwave frequency band. These microwaves propagate at the speed of light—300 million meters or 186 thousand miles per second. The two methods of implementing this technology are pulse and FMCW. Take a look at some of the advantages:
- The noncontacting nature of free-space level measurement radar has major advantages for end users. There are no moving parts to wear or become mechanically fouled with material. Applications, such as drilling mud and liquid asphalt, have traditionally used mechanical contacting technologies such as floats and displacers.
- No material buildup occurs on the instrument. Buildup on contacting technologies can cause false or inaccurate measurement readings. Applications involving materials such as water-based paint, rubber, or glue are typically ideal for noncontacting technologies. Capacitance, probes, and guided-wave radar are contacting technologies, with no moving parts, that are susceptible to false measurements with materials that are conductive and tend to build up on the measurement probes.
- There's less risk of contamination. Noncontacting instruments require little or no cleaning in the event you have to remove them from an application. They are also easier to decontaminate if the application involves a hazardous material such as styrene or phosgene. With adverse process conditions, such as temperature extremes, high pressures, agitation, and turbulence, the amount of wear and torque placed on a contacting technology may damage it and contaminate the process.
- Users can vary the measurement range of their application using the exact same instrument.
- The instrument is easy to install. Noncontacting instruments are typically compact in size, making mounting efforts less cumbersome. Many contacting technologies require a process disruption during installation, such as emptying the vessel, attaching part of the device to the bottom of the vessel, or moving the material level for calibration.
The same advance echo processing techniques that have evolved with the long-proven technology of ultrasonics are now integrated into radar echo processing. The ability to learn where obstructions are in a vessel and to ignore them is illustrated in the figures below.
On the "Before learning" figure, there is an echo, from an obstruction, with a strength of about 50 dB at about 3.2 meters and another echo, from the material, with a strength of about 80 dB at about 9 meters. The unit only considers echoes with strength above the time-varying threshold curve to be possible valid echoes. If the unit were programmed with an algorithm to give a heavy weight to the first echo, the unit would choose the obstruction. If the algorithm were to give a heavy weight to the larger echo, the unit would display the correct material level. A problem that might occur with having the unit choose the larger echo is that it may choose a secondary echo, when the material is at a higher level, and display a false low material level.
The "After learning" figure shows how the unit shapes the threshold curve above the obstruction. Now the echo from the obstruction cannot be considered to be a valid echo no matter which algorithms are applied.
The y-axis represents echo strength in decibels (dB), and the x-axis represents distance from the flange in meters.