All about Radar Level Measurements.

What is the principle of operation of Guided Wave Radar ?

• Radar level measurement technology can be broken down into two different categories; Pulsed and Frequency Modulated Continuous Wave (FMCW). 
• An advantage with Pulsed Technology is that it requires less processing power. Therefore most two-wire gauges use this technology. 
• An advantage with FMCW is that higher accuracy can be achieved but more processing power is required and therefore FMCW-radars are typically four-wire. 
• In Pulsed transmitters the level measurement is a function of the time taken from the radar signal to travel to the surface and back. 
• In FMCW gauges the transmitter constantly emits a swept frequency and the distance is calculated by the difference in frequency of emitted and received signal.

Are radar gauges safe?

• Yes 
•  Radar waves are of no greater intensity than the constant radio, cellular and other communication waves that surround us every day. Furthermore the transmitter is normally placed in a metallic tank that acts as a Faraday’s cage and therefore the radar waves are isolated within the tank.

What is the difference in frequency between Pulsed Technology Guided Wave Radar and Non Contacting Radar?

• With Guided Wave Radar the pulsed microwave are guided down the tank by the probe, making it less sensitive to disturbances than free propagating microwaves. Pulsed Non Contacting Radar uses a carrier frequency, e.g. 6 Hz  or 26 Hz , to carry the microwave which are radiated into the tank with an antenna.

Why should we use radar technology?

• The radar signal is  unaffected by the tank content and tank atmosphere, temperature or pressure. The measurement is not influenced by changing material characteristics such as density, dielectric properties and viscosity. Since there are no moving parts the transmitters are  maintenance free. All of the characteristics above make radar a very useful and fast growing level measurement technology.

How does the frequency of the radar affect the measurement?

• A higher frequency provides a more concentrated narrow beam which can be useful in applications where there are obstacles present in the tank such as man-ways, agitators or heating coils. The downside of high frequency is that the measurement is more affected by vapours, dust and product build up on the antenna, Low frequency radar which have a longer wavelength and wider beam angle, tends to cope better with steam, dust, condensation, contamination and turbulent surfaces.

In what way does the dielectric constant (DC) of the media affect the measurement?

• Electromagnetic energy is emitted from all radar devices. When the emitted signal reaches a point where there is a change in DC, usually the media surface, some of the signal is reflected back to the transmitter. The amount of energy that is reflected back to the transmitter is proportional to the DC of the media. A rule-of-thumb is that the value of the dielectric constant represents the percentage of energy that is reflected. Thus a DC of eight means that eight percent of the emitted energy is reflected back to the transmitter. Fundamentally media with a higher DC provide stronger return signals and are therefore easier to measure.

Does radar work on foam?

• The effects of foam on a radar measurement can be difficult to predict. In some applications the foam may dampen out the signal completely while other types of foam may be transparent to the transmitter. The thickness, density and the dielectric constant are factors that need to be considered when evaluating an application with foam.
• On dry foam the microwaves typically passes through and detects the liquid surface below. On medium type foam the signal can be absorbed or scattered and the results are therefore hard to predict. If the foam is wet the microwaves are often reflected from the foam surface and thereby the foam surface level is measured.
• The frequency at which the radar operates also affects how foam is measured. Low frequency radar (5 GHz) in general penetrates foam to a larger extent than high frequency (20 GHz) radar. Guided Wave Radar is in general better suited to measure on applications where foam is present, since the radar uses a lower frequency pulse.

Will radio noise or other interference cause problems?

• The transmitters use a specific narrow frequency and are therefore not prone to disturbances from other sources. It is very uncommon with disturbances and it is rare that the disturbance source operates at precisely the same frequency as the transmitter. Furthermore, the transmitters are often installed in metallic tanks that provide a Faraday’s cage which prevents electromagnetic disturbances from the outside to enter the tank. With Guided Wave Radar, if disturbances are present in tank the coaxial probe are recommended, since the radar signal travels inside of the pipe undisturbed by the interference sources on the outside.

Are there any restrictions on the nozzle height?

• Since the nozzle, and especially the lower end of the nozzle, can create interfering echoes it is recommended that the height of the nozzle is kept within certain values depending on the type of probe / antenna and type of transmitter that is used. 

Are there any restrictions on the nozzle diameter?

• In conjunction with the above statement, the nozzle diameter also affects the measurement,since a diameter nozzle that is too small will create disturbance echoes. 

What is purpose of exotic materials?

• In some applications with high temperatures, or in highly corrosive environment, the probes or antennas need to be made out of exotic materials that can be stand the material stress. 

How viscous can the measured media be?

• The different probes have different max limits regarding the viscosity of the measured media. The single probes are more suitable for high viscosity media while the coax probe can be used on low viscosity media. The guidelines for the different probes and examples of viscosity are presented below:

How will coating affect the measurement?

• If coating forms on the probe the measured signal will be weaker. If the media itself has a high dielectric constant some coating is not much concern but if it is a low DC media coating can be a problem. If a twin probe or a coaxial probe is used the coating can cause bridging between the two leads and this will create false echoes that can lead the transmitter to interpret a bridge as the actual level. Single lead is recommended in coating applications. Coating can cause an accuracy influence. Maximum error due to coating is 1-10% depending on probe type, dielectric constant, coating thickness and coating height above product surface.

Will the Guided Wave Radar see sand in the bottom of a vessel?

• No. Since the sand will be embedded in water which is a high dielectric media (DC~80) the transmitter will only see the water. The same is true for all media that are embedded in water.

How close to the tank wall can the probe be mounted?

• Like the nozzle the tank wall can also affect the measurement through disturbance echoes. The minimum distance to the tank wall is the same as the distance to any disturbing object that may be present in the tank. If there are obstacles present in the tank the coaxial probe is the best probe to use. If the tank wall is metallic and smooth the probe can be mounted closer to the wall.

What measurement error will an incorrect dielectric constant cause in on an interface level?

• Errors due to changes in the medium’s dielectric values can be significant. It is calculated by:
• For example, if the physical thickness is 20” (51 cm) and dielectric varies from 2 to 4:
• E = ( 20/sqrt(2) ) – ( 20/sqrt(4) ) = 4.14″ = 11 cm

Non Contacting Radar Questions

What is the principle of operation for non contacting radars?

•  The transmitter constantly emits a swept frequency signal and the distance is calculated by the difference in frequency between the emitted and received signal at any point in time. One advantage with FMCW is that higher accuracy can be achieved. The level of the liquid is measured by short radar pulses which are transmitted from the antenna at the tank top towards the liquid. When a radar pulse reaches a media with a different dielectric constant, part of the energy is reflected back to the transmitter. The time difference between the transmitted and the reflected pulse is proportional to the distance, from which the level is calculated.

How close to the tank wall can the non contacting transmitter be mounted?

• The minimum distance to the tank wall depends on which antenna is used. With a more concentrated beam (larger antenna), the closer to the tank wall the device can be mounted. 

What is the difference between a cone and a horn antenna?

• It is just different names for the same type of antenna.
• I will soon be posting  technical information on how a radar level transmitter be mounted with problem regards entering the total range of the level to be measured.. Stay Tuned.