Radar technology is mainly put into use for detection of level in continuous level measurement applications. Radar level transmitters provide non contact type of level measurement in case of liquids in a metal tank. They make use of EM i.e. electromagnetic waves usually in the microwave X-band range which is near about 10 GHz. Hence, they can be also known as microwave level measurement devices. However there are some differences between radar and microwave types. They are:
- Power levels in case of radar systems are about 0.01 mW/cm2 whereas in case of microwave systems, these levels range from 0.1 to 5 mW/cm2.
- Microwaves can work at higher energy levels; hence they are competent enough to endure extra coating as compared to radar level detectors.
A radar level detector basically includes:
- A transmitter with an inbuilt solid-state oscillator
- A radar antenna
- A receiver along with a signal processor and an operator interface
The operation of all radar level detectors involves sending microwave beams emitted by a sensor to the surface of liquid in a tank. The electromagnetic waves after hitting the fluids surface returns back to the sensor which is mounted at the top of the tank or vessel. The time taken by the signal to return back i.e. time of flight (TOF) is then determined to measure the level of fluid in the tank.
Types of Radar Level Measurement Systems
Radar level measurement technology has been primarily classified into following two systems:
- Noninvasive or Non-contact Systems
- Invasive or Contact Systems
Two types of noninvasive systems exist. One is the frequency-modulated continuous wave i.e. FMCW technology and the other one is Pulsed radar technology.
“From an electronic module on top of the tank, a sensor oscillator sends down a linear frequency sweep, at a fixed bandwidth and sweep time. The reflected radar signal is delayed in proportion to the distance to the level surface. Its frequency is different from that of the transmitted signal, and the two signals blend into a new frequency proportional to distance.”1
This new frequency can then be used for accurate determination of fluid level.
The major benefit of employing FMCW technique for level measurement in a tank is that the signals transmitted are frequency modulated i.e. FM instead of amplitude modulated i.e. AM signals. Now, the major part of noise in a tank falls in the AM range which does not influence the FM signals. Hence, FMCW happens to be the only system which can be suitably used for meeting high accuracy requirements of tank gauging.
Pulsed radar systems
They are also referred to as pulsed time-of-flight systems. Their working principle is very much like ultrasonic level transmitters. “Pulsed Wave systems emit a microwave burst towards the process material. This burst is reflected by the surface of the material and detected by the same sensor which now acts as a receiver. Level is inferred from the time of flight (transmission to reception) of the microwave signal.”2
The power range of pulse radar systems is very less as compared to FMCW systems. Hence, their performance gets largely influenced by tank obstructions and materials having low dielectric constants and foams.
Radar antennas employed for noninvasive measurement systems are available in following two major designs:
- Parabolic dish antenna
- Cone antenna
The figure below shows the schematic diagram of a parabolic dish antenna which has the tendency to transmit the signals over a larger area and the cone antenna which usually restrict the signals in a very narrow region.
One can select among above two antenna designs depending upon the application requirements and considering various factors like tank obstructions, presence of vapors or foam, surface turbulence and other physical properties of the liquid being measured.
Size of the radar antenna also matters in deciding its suitability for a particular application. If the diameter of the antenna is small, there will be higher beam divergence as well as greater risk of undesirable wave reflections from tank obstructions. However, the probability of directed wave going back to the sensor is greater in case of small antennas. Also, the alignment of sensor is not very significant in small size antennas.
On the other hand antennas having larger diameters tend to produce a more focused and strong signal since they cause smaller beam divergence. Besides, they are useful in eliminating noise disturbances emerging from flat and horizontal metallic surfaces. On the negative side, large antennas are more susceptible to multiple reflections from surface turbulence, tank obstructions and sloping surfaces.
In some applications, the antennas installed at the top of the tank are totally sealed and isolated for protection purpose.
The invasive method used for liquid level measurement is called Guided-wave radar i.e. GWR method. In this method, a cable or rod is employed which act as a wave guide and directs the microwave from the sensor to the surface of material in the tank and then straight to its bottom. “The basis for GWR is time-domain reflectometry (TDR), which has been used for years to locate breaks in long lengths of cable that are underground or in building walls. A TDR generator develops more than 200,000 pulses of electromagnetic energy that travel down the waveguide and back.”3
The dielectric constant of the process material will cause variation in impedance and reflects the wave back to the radar. Time taken by the pulses to go down and reflect back is determined to measure level of the fluid.
In this method, the degradation of the signal in use is very less since the waveguide offers extremely efficient course for signal travel. Hence, level measurement in case of materials having very low dielectric constant can be done effectively. Also in this invasive measurement method, pulses are directed via a guide; hence factors like surface turbulence, foams, vapors or tank obstructions do not influence the measurement.
GWR method is capable of working with different specific gravities and material coatings. However, there is always a danger that the probe or rod used as a waveguide may get impaired by the agitator blade or corrosiveness of the fluid under measurement. A typical guided wave radar system is shown in the figure below.
Through-air Radar Systems
Non-invasive systems of measurement are basically known as the through-air radar systems. They usually employ a horn antenna or a rod antenna for sending microwave beams onto the surface of the liquid being measured. These antennas mounted at the top of the tank then receive the reflected microwave signal back from the fluid surface. A timing circuit is incorporated in the systems which measures the time of flight and hence the distance between the antenna and the fluid level is determined.
These systems can pose measurement problems if the dielectric constant of the fluid being measured is very low. “The reason is that the amount of reflected energy at microwave (radar) frequencies is dependent on the dielectric constant i.e. ?r of the fluid, and if ?r is low, most of the radar’s energy enters or passes through. Water ( ?r = 80) produces an excellent reflection at the change or discontinuity in ?4
Besides, this radar level measurement technique faces the same beam divergence issues which affects ultrasonic level transmitters. Moreover, issues like internal piping, antenna deposits, and manifold wave reflections from tank coatings and obstructions may lead to inaccurate results. In order to get rid of these errors, advanced algorithms employing fuzzy logic should be integrated with these radar transmitters. However, these arrangements would make the transmitter setup very complicated.
Guided Wave Radar v/s Through-air Radar
To overcome the measurement problems faced by through-air radar systems, guided wave radar systems are generally employed since they offer following advantages over through-air radar systems:
- “As with through-air radar, a change from a lower to a higher ?r causes the reflection. Guided wave radar is 20 × more efficient than through-air radar because the guide provides a more focused energy path.”
- In GWR method, various antenna designs and configurations make it possible to determine level of fluids having dielectric constant less than 1.4.
- Also, these systems can be mounted in both vertical and horizontal positions depending upon the application.
- These systems offer and efficient and clear path for signal travel.
- The performance of GWR systems is not disturbed by vapors, foams, high temperature or pressure conditions.
- These systems can operate in vacuum too without requiring any recalibration.
- Beam divergence issues and false echoes resulting from tank walls and obstructions are not present in these guided wave radar systems.
Major advantages of radar level detectors include:
- Radar level measurement technique offer extremely accurate and reliable detection of level in storage tanks and process vessels.
- The performance of radar level transmitters remains unaffected by heavy vapors and mostly all other physical properties of the fluid under level measurement (except dielectric constant of the liquid).
Radar level measurement systems incorporate following drawbacks:
- Major disadvantage associated with radar level detectors is their high cost.
- Besides, these systems are not capable of detecting level between interfaces.
- Also their pressure ratings are very restricted.
- In case of pulse radar, one usually faces problem in getting accurate measurement results if the fluid being measured is very near to the radar antenna. Since, in that case the time taken by the signal to travel between sensor and process material will be very fast i.e. not adequate for accurate determination of level.
- These devices work well with light layer of dirt and dust only. In situations where the layer of dust or foam gets substantial, they cease to detect the fluid level. Therefore, in dirty applications the radar level detectors gets replaced by ultrasonic level detectors.