Radar Level Transmitters

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:

1. 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.
2. 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 fluid's 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.

FMCW systems

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.1This 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.

Antenna Designs

Radar antennas employed for noninvasive measurement systems are available in following two major designs:

1. Parabolic dish antenna
2. 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.

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 radars energy enters or passes through. Water ( ?r = 80) produces an excellent reflection at the change or discontinuity in ?4Besides, 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.

Advantages

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).

Disadvantages

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.