Contactless “through-the-air” radar measurement of levels is an alternative for many liquid and solid applications that can be employed in place of time domain reflectometry (TDR, also known as guided-wave radar or guided radar). Frequency modulated continuous wave (FMCW) radar is the most advanced contactless radar technology available today.
FMCW radar emits a high-frequency signal which is reflected from the product surface and received back. (See the section “How Does FMCW Radar Work?” below for more.) FMCW meters allow for the continuous, contactless measurement of distance, level, volume, and mass of liquids, pastes, granulates, powders, and other solids in a wide variety of applications.
Typical FMCW radar-based level measurement applications include:
- Reaction vessels
- Leakage monitoring near pipelines/vessels
- Storage and production of toxic or corrosive liquids
- Storage of liquefied gases in high-pressure/low-temperature spheres
- Hygienic process applications
- Flow measurement in open channels with pre-shaped flumes and weirs
- Silos, bunkers, and stockpiles for solids
Compared to TDR/guided radar, FMCW is usually applied when the application meets one or more of these criteria:
- Vessels with complex internals or agitators, where the FMCW radar is not affected by disturbance reflections from installed equipment
- Large measurement ranges up to 80 m (262 ft) in high silos, for example, where a TDR probe would experience extreme mechanical traction (the mechanical stress that the TDR probe experiences when a medium moves in the tank or silo and the probe moves with it) and be very expensive due to the bigger diameter and length or special material due to corrosive liquids.
Needless to say, there are also applications where TDR has an edge over FMCW. For example, because the latter method measures in a contactless (through-the-air) mode, influences such as thick foam, vapor, or dust can disturb the emitted signal. Based on over 20 years of experience with FMCW technology, however, there are now signal converters with software filters that can effectively eliminate disturbances.
The biggest advantage of TDR over FMCW radar is price. TDR probes are approximately 30 percent lower in price. This means that whenever contact between probe and medium does not present a problem, TDR technology is nearly as good as contactless radar.
With FMCW radar, there is no problem with noise interference due to the fact that two-wire radar level measurement devices do not have any circuits in contact with the vessel or the process. The same goes for state-of-the-art two-wire TDR devices. All major suppliers of two-wire TDR offer devices that conform to SIL (Safety Integrity Level), FCC (U.S.), IC (Canada), CE (Europe), and NAMUR requirements and are immune to noise levels defined in these norms and standards.
Today, radar measurement devices from manufacturers differ markedly when it comes to antenna systems and operating frequencies. Radar measurement has the reputation of being complicated and often causing a disproportionately high amount of rework. This is because level measurement using radar is not that easy when compared to other measurement methods.
Application Knowledge Is Critical in Instrument Selection
Rarely is the case whereby two applications are the same; each measurement case should be looked at individually. There is now a great number of radar level measurement devices on the market that can be combined with a number of antennas and antenna options with various measuring frequencies. The device technology is generally good, but the problem lies in the application of knowledge: Two conditions must be met in order to select the right measurement principle and device size.
First, the user must possess sufficient knowledge of the application and the measurement requirements. The equipment supplier completes the picture with appropriate experience and advice. The quality of advice is crucial here: If the right questions are asked prior to selecting the instrument, then most of the typical application problems can be solved.
In practice, there are often cases in which users replace electromechanical systems or ultrasonic measurement devices with radar devices and then encounter problems with unstable measurement. The reason for the change is generally cited as wanting to increase plant availability in the long term and prevent the overfilling of tanks and thus production stoppages. For this situation, a non-contact measurement method such as radar is ideal since it can continue measuring levels during the filling process and is completely maintenance free.
However, if the user has not been properly advised or gets the impression that radar devices in their standard versions are always suitable for any application, there is a high probability that problems will occur. In this situation, device suppliers must invest more time: In the case of a properly designed and installed radar device that has been pre-parameterized by the supplier, the customer usually only needs to input a few parameters at start-up.
However, one should not get caught thinking that the limits of all radar level measurement devices are the same. There are major differences at times – if the radar device of one supplier fails in an application or the supplier turns down the job, it still makes sense to revisit the measurement task with another supplier. The other supplier may be able to offer alternatives that may be better suited to the individual application.
An example: For standard liquid applications with measurement ranges up to 30 m (98 ft), KROHNE recently introduced a new 10 GHz FMCW radar level meter, the Optiwave 5200 C/F. For larger measurement ranges up to 80 m (262 ft) and/or higher accuracy requirements up to 3 mm (0.12 in), the 24 GHz FMCW radar level meter, Optiwave 7300 C, is available as an alternative for the same application. Users should not be afraid to contact the manufacturers of radar devices to discuss measurement task needs in advance.
Know the Different Types of Antennae on the Market
Antenna design is crucial for radar level meters. When developing antennas, many factors have to be taken into consideration, including typical size, position and type of process connection, vessel size and shape, and the influence of medium properties of the targeted industry or area of application. For corrosive environments, for example, drop antennae featuring polyproplyene (PP) or polytetrafluoroethylene (PTFE) flange plate protection are available. In addition to the drop shape, a stainless steel horn antenna is available for higher temperatures. In this manner, the special requirements of various industries such as chemicals and paper are met.
Since 2013, wave horn antennae made of PP or PTFE have been available for liquid applications. This antenna type combines the advantages of a wavestick antenna in terms of corrosion resistance and a small process connection with the insensitivity of a horn antenna to condensate. Process connections are also specially designed: Wave horn antennae are process-sealed by their antenna material instead of a traditional process-seal construction with O-ring gaskets. These gasket-free antennae are therefore ideally suited for extreme corrosive environments.
The PP antenna can be mounted on process connections as small as 1.5 in. Metallic horn and wave-guide antennae are alternatives for use with highly toxic and explosive liquids. They use a dual-seal mechanism — a combination of O-ring gaskets and a Metaglas seal — to guarantee complete hermeticity.
Last but not least, flexibility is key for a radar level meter: Bayonet locking systems and antenna extensions ensure suitability for a variety of mounting positions and applications. A quick coupling system permits removal of the housing under process conditions (without opening the vessel) and rotation through 360° to make the display screen easy to read. Remote converters are also available for many level meters, offering full display and configuration capability up to 100 m (328 ft) away from the antenna to make operation convenient without having to climb to the top of the vessel.
How Does FMCW Radar Work?
In FMCW, a radar signal is emitted via an antenna, reflected on the product surface, and received after a time delay, t. The FMCW radar emits a high-frequency signal whose frequency increases linearly during the measurement phase (called the frequency sweep).
Delay time is t= 2d/c, where d is the distance to the product surface and c is the speed of light in the gas above the product. For further signal processing, the difference is calculated from the actual transmit frequency and the receive frequency. The difference is directly proportional to the distance. A large frequency difference corresponds to a large distance and vice versa.
The frequency difference is transformed via a Fourier transformation (FFT) into a frequency spectrum, and then the distance is calculated from the spectrum. The level results from the difference between tank height and measuring distance.
Volker Lenz is of Product Management Level Products at KROHNE Messtechnik GmbH based in Duisburg, Germany. He can be reached at firstname.lastname@example.org. KROHNE, www.krohne.com, is a world-leading manufacturer and supplier of solutions in industrial process instrumentation. Founded in 1921, the company has annual sales of nearly 450 million euros and employs more than 3,000 worldwide. The company has production facilities in the U.S., UK, China, India, France, Netherlands, Germany, Russia, Sweden, and Norway.